@book_section {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/54375, title ="Jurassic rocks in Sonora, Mexico: Relations to the Mojave-Sonora megashear and its inferred northwestward extension", author = "Anderson, Thomas H. and Rodríguez-Castañeda, José Luis", number = "393", pages = "51-95", month = "January", year = "2005", doi = "10.1130/0-8137-2393-0.51", isbn = "9780813723938", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150204-124834957", note = "© 2005 Geological Society of America.\n\nAccepted 24 March 2005. \n\nAnderson is very grateful to Leon T. Silver, who initiated and\nsupported the first six years of the work reported here. Silver,\nwho served as mentor and colleague, conceived the idea of a\ngreat fault to explain the patterns described above. His contributions\nto the development of the tectonostratigraphy of\nsouthwestern North America and to the recognition and interpretation\nof regional structures have been seminal. Anderson\nhas benefited from the hard work of University of Pittsburgh\nstudents, who commonly conducted extensive field mapping\nprojects of largely geologically unknown areas. These stalwarts\ninclude D. Bajek, J. Chepega, K. Cohen, C. Connors, F.\nCorona, J. Dembosky, K. Ichikawa, M.B. McKee, J.L. Rodríguez-Castañeda, and W. Stephens. Bruce Carter, George Davis,\nKees De Jong, Gordon Haxel, John Marzolf, Jon Nourse, Bob\nPowell, Vic Schmidt, Jack Stewart, Dick Tosdal, D.J. Wood\nand Claude Rangin all have been generous in helping in one\nway or another with the development of Anderson's ideas with\nrespect to Jurassic rocks in Sonora. Clark Blake pointed out the\npotential importance of the faults near the Yolly Bolly junction.\nGeologists of the Instituto de Geología (Universidad Nacional\nAutónoma de México), the Universidad de Sonora, and the\nConsejo de Recursos Minerales have been patient, friendly,\nand very supportive. Jesus Nájera, Jaime Roldán-Quintana,\nand Guillermo Salas, former directors of the Estación Regional\ndel Noroeste, lnstituto de Geología in Hermosillo were most\ninvolved in a formal or semi-formal administrative manner.\nIn addition to them, Juan Carlos Garcia y Barragán, Carlos\nGonzález-León, and César Jacques-Ayala led field trips, talked\nover data, and facilitated the work. Jim McKee and Mary\nBeth McKee and Jon Nourse have spared no effort to provide\nAnderson with a base firmly tied to field relationships for\nmany of the ideas discussed. Research monies from National\nScience Foundation provided support for much of Anderson's\nand Rodríguez's work. The lnstituto de Geología, Universidad\nNacional Autónoma de México (UNAM) provided support to\nJ.L. Rodríguez-Castañeda during his work in cooperation with\nthe Estación Regional del Noroeste. Special thanks to Jose\nGuerrero and Fernando Ortega-Gutierrez, former Directors of\nthe Instituto de Geología, UNAM, whose encouragement and\nsupport were very important during the course of this work.\nScott Davidson carefully crafted accurate maps and Paul Coyle\ncollated the data tables. Ken Ludwig generously provided suggestions\npertaining to the interpretation of the apparent U-Pb\nages of zircon fractions.\nCarlos González-León, Karl Karlstrom, S.T. Kitinyrear,\nJon Nourse, and Nancy Riggs reviewed preliminary versions\nof the manuscript, whereas Gordon Haxel, Carl Jacobson, and\nJon Nourse reviewed the final version. The constructive assessments\nincluded many beneficial suggestions and improvements.\nHaxel 's remarkably thorough reviews were especially helpful,\nas were the multiple reviews by Nourse. Research grants from\nthe National Science Foundation to Silver funded the early\nphases of this work.", revision_no = "13", abstract = "The Mojave-Sonora megashear constitutes a regional boundary between lithologically distinct Jurassic assemblages of different ages. North of the Mojave-Sonora megashear, arc-related volcanic, volcaniclastic, and clastic rocks, intruded by plutons (175–160 Ma) compose part of the Middle Jurassic (commonly ca. 175 Ma) igneous province, previously recognized in Arizona and California. Distinct domains among Jurassic igneous rocks in northern Sonora are: (1) southern Papago, a region where pre-Jurassic rocks are unknown, (2) Nogales-Cananea-Nacozari, where Jurassic rocks are underlain by 1.7–1.4 Ga crystalline basement, and (3) Mojave-Sonora, where strata, including Oxfordian beds, along the north side of the Mojave-Sonora megashear are commonly strongly deformed, as recorded by thrust faults, mylonitic foliation, and recumbent folds. The Mojave-Sonora domain extends across the southwestern margins of the southern Papago and the Nogales-Cananea-Nacozari domains. Strong deformation that distinguishes the zone markedly declines within a few tens of kilometers northward. South of the Mojave-Sonora megashear, in central and southern Sonora, Lower Jurassic clastic and volcaniclastic rocks distinguish the Caborca domain. Upper Jurassic sedimentary rocks, commonly conglomeratic, are abundant north of Mojave-Sonora megashear; a single occurrence is known south of the Mojave-Sonora megashear. \n\nWaning of subduction-related Middle Jurassic magmatism was followed by the abrupt formation, ca. 165 Ma, of Coast Range, Josephine, Great Valley, and Devil's Elbow ophiolites and the Smartville Complex within oceanic pull-aparts west of the margin of the North America plate. The formation of ophiolitic rocks signaled the beginning of transtensional faulting. \n\nAlmost contemporaneously (ca. 163 Ma) the lowest volcanic units and overlying coarse sedimentary beds began to accumulate in fault-bounded continental pull-apart basins such as the McCoy Mountains basin. Other transtensional basins, formed at releasing steps where pull-aparts formed, are well developed within the Papago domain and other parts of southwestern United States and northern Mexico. \n\nFrom Sonora northward into California the Mojave-Sonora megashear fault zone, developed generally within the Middle Jurassic arc-parallel to the former continental margin, is inferred to link with strands of the Melones and Bear Mountain faults of the Foothills fault system, the Wolf Creek fault, and the Big Bend fault. A protuberance of Proterozoic basement (the Caborca block) that was truncated from the continental margin records ∼800–1000 km of left-lateral offset. The displacement of the Caborca block took place south of a major releasing step along the Big Bend fault with the result that a regional pull-apart that coincides with the Great Valley of California developed. \n\nInboard of the Mojave-Sonora megashear Late Jurassic magmatic rocks crop out near faults at some releasing steps and within floors of some pull-apart structures. The distribution suggests that magma rose along faults and into areas of thin crust. In southern Arizona these igneous rocks are included as part of the Artesa layered sequence and the Ko Vaya plutonic suite. \n\nOxfordian and younger beds, which crop out north of the Mojave-Sonora megashear may contain exotic blocks and contractional structures that are contemporaneous with the Nevadan orogeny. The variation in the style and intensity of deformation of Middle and Upper Jurassic strata, and Upper Jurassic conglomerate rich in clasts derived from rocks of the Caborca domain, are postulated to record transpression near the Mojave-Sonora megashear that locally overlapped the more widespread transtensional structures in time and space. \n\nThe cessation of strike-slip faulting locally began ca. 150 Ma, as shown by undeformed intrusive bodies that cut older deformed Middle Jurassic rocks. By the time that the Independence dikes and correlative rocks were emplaced at 148 Ma, scant evidence of lateral faulting is known. \n\nIntrusions, young volcanic cover, transecting strike-slip faults, and multiple generations of low-angle extensional and contractional faults obscure Jurassic structures in Sonora and southern California. Despite these complications, removal of the effects of superposed structures reveals a viable trace for an inferred Late Jurassic left-lateral fault linking the Mojave-Sonora megashear and more northerly fault segments. The position of this major inferred fault is constrained by distinctive tectonostratigraphic domains. \n\nThe Middle and Late Jurassic and earliest Cretaceous plate tectonic history includes (1) subduction (175–165 Ma), (2) coupling (ca. 165 Ma), (3) rifting, transtension, lateral faulting, transpression, and contraction (165–145 Ma), and (4) renewed subduction (ca. 135 Ma) along the western margin of the North America plate and terranes (e.g., Wrangellia) to the west. The structures that record the diverse plate processes and that are preserved best in the overriding North America plate are compatible with a consistently maintained easterly directed maximum compressive stress.", } @book_section {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/54379, title ="The Mojave-Sonora megashear — Field and analytical studies leading to the conception and evolution of the hypothesis", author = "Anderson, Thomas H. and Silver, Leon T.", number = "393", pages = "1-50", month = "January", year = "2005", doi = "10.1130/0-8137-2393-0.1", isbn = "9780813723938", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150204-135028772", note = "© 2005 Geological Society of America.\n\nAccepted 24 March 2005. \n\nWe benefited from cooperation offered generously by Guillermo\nSalas, Diego Cordoba, Jose Guererro, and Fernando Ortega-Gutierrez,\neach of whom served as Director of the Instituto de\nGeología of Universidad Nacional Autónoma de México. In\nSonora, we had constructive interactions with former chiefs of\nthe northwest regional office of the Instituto, including Jesus\nNajera, Jaime Roldán-Quintana, César Jacques Ayala, and Carlos\nGonzález-León. Claude Rangin, who conducted extensive\nmapping principally in northeastern Sonora, generously shared\nknowledge of stratigraphy and structure gained from study of\nfossiliferous strata. Anderson was aided in the field by Jaime\nAlvarez, Arturo Cisneros, Clay Conway, Tom Early, Ken Ludwig,\nJay Murray, Robert Powell, David Wenner, and especially\nJose Luis Rodríguez-Castañeda. Patti Campbell, Joe Chepega,\nKaren Cohen, Chris Connors, Franco Corona, John Dembosky,\nMary Beth McKee, Jose Luis Rodríguez-Castañeda, and William\nStephens, each of whom pursued graduate degrees under\nAnderson's direction, undertook daunting mapping projects that\ncontributed greatly to our understanding of geological relationships\nin Sonora. Gordon Haxel and Dick Tosdal aided Anderson\nin numerous ways, providing logistical support as well as\na backboard for ideas. After a field trip to Sonora in 1979, Jim\nMcKee and Norris Jones initiated cooperative work with Anderson\nin eastern Mexico. For two decades Anderson benefited from\nknowledge, logistical support, and patient shepherding provided\nby these two long-suffering gentlemen. Anderson, who began\nthe project with no first-hand knowledge of isotope geochronology,\nowes a great debt of gratitude to Silver's laboratory staff at\nCaltech, where the U-Pb isotopic analyses of zircon were performed.\nThe instruction and patient mentoring provided by Jaime\nAlvarez, chief rock processor, and Gerri Baenteli Silver, lab technician,\nwere essential to any success attributed to Anderson's role\nin this project. Ken Ludwig and Ed Lidiak were instrumental in\nthe interpretation and presentation of the U-Pb isotopic results.\nCarlos Aiken, Peter Coney, John Marzolf, Bill Muehlberger, Grover\nMurray, Jon Nourse, Pete Palmer, Norm Silberling, and Jack\nStewart contributed good thinking and good data that they generously\nshared with Anderson. S.T. Kitinyrere, Bill Muehlberger,\nCal Stevens, and Jack Stewart read early versions of the manuscript\nand offered helpful comments and constructive criticisms.\nScott Davidson prepared the illustrations with skill and patient\nforbearance; Paul Coyle collated the data tables. W.R. Van Schmus\nand Wayne Premo provided useful reviews of the final draft.\nSpecial thanks to Jon Nourse who reviewed the manuscript multiple\ntimes with great care and substantially improved it.", revision_no = "12", abstract = "The megashear hypothesis is based upon reconnaissance geologic and geochronologic studies conducted principally from 1968 until 1974 in northwestern Sonora, Mexico. Our research incorporated U-Pb isotopic analyses of more than 70 zircon populations separated from 33 Precambrian rock samples with field relations and maps based upon structural and stratigraphic measurements. The results delineate a region known as the Caborca block and further reveal that the block is a principal element of an unexpected, discordant pattern of Proterozoic basement provinces. The Mojave-Sonora megashear was conceived in an effort to explain: (1) the unexpected pattern of two Proterozoic crystalline provinces with distinct chronologic histories of crust formation (1.8–1.7 Ga, Caborca block versus 1.7–1.6 Ga, Pinal Province); (2) the distribution of contrasting cover rocks overlying these basement blocks, (3) the abrupt northeastern limit of the Caborca block (terrane) against which volcanic and plutonic rocks of mid-Jurassic (mainly 180–160 Ma) age are juxtaposed, and (4) the distribution of Jurassic magmatic units that intervene between the provinces of Proterozoic crust. The similarities that exist between crystalline crust and overlying pre-Jurassic cover in northwestern Sonora, Mexico, and units in the Inyo Mountains–Death Valley region are attributed to the offset of correlative units along a Late Jurassic left-lateral strike-slip fault postulated to extend from the Gulf of Mexico to California and beyond. This large fault or megashear is a principal structure that accommodated 800–1000 km of left-lateral displacement among a set of transforms related to the opening of the Gulf of Mexico. The fault is compatible with Late Jurassic plate motion.\n\nThe inferred trace of the Mojave-Sonora megashear is obscured by contractional and extensional deformation and extensive plutonism. These processes, concentrated along the fault, commonly obfuscate and displace fault zone rocks along the inferred trace as well as the rocks adjacent to it. However, the fault zone is exposed in Sierra de Los Tanques near the international boundary between Mexico and the United States, where mylonitic rocks that comprise three aligned, discontinuous, segments crop out 1 for ∼25 km. The zone of mylonitic rocks, which crosses Route 8, 13 km SW of Sonoita, is locally almost 5 km wide and separates Triassic granitoids and Precambrian gneiss from Jurassic volcanic and clastic rocks.\n\nThe limited exposure of the fault zone is a principal concern of those who object to the Mojave-Sonora megashear hypothesis. Studies of paleomagnetism, structure, stratigraphy, crustal geochemistry, and detrital zircons do not refute the megashear concept; commonly they reinforce existing evidence in support of the hypothesis.", } @book_section {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/54488, title ="Geology and geochronology of granitic batholith complex, Sinaloa, México: Implications for Cordilleran magmatism and tectonics", author = "Henry, Christopher D. and McDowell, Fred W.", number = "374", pages = "237-273", month = "January", year = "2003", doi = "10.1130/0-8137-2374-4.237", isbn = "9780813723747", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150206-123717397", note = "© 2003 by Geological Society of America.\n\nAccepted by the Society June 2, 2003.\n\nWe thank Goran Fredrikson, Steve Clabaugh, Michel Bonneau,\nand Ken Clark for discussions about the geology of Sinaloa and\nJim Faulds for discussions about the character and significance of\ndynamic recrystallization textures. Geologists and staff of Minas\nde San Luis, now Luismin, provided hospitality, a place to stay,\nand a thorough knowledge of the geology around Tayoltita, in the\nnortheastern part of the area. Paul Guenther provided instruction\nand guidance during K-Ar dating; Thomas Anderson, Jaime Alvarez,\nand Gerri Silver did the same for U-Pb work. The opportunity\nfor this study arose through a project of the Instituto de Geologia,\nUniversidad Nacional Autónoma de México, funded by the state\nof Sinaloa. The project provided expenses, vehicles, and other\nsupplies during 1971 and 1972. Funds were also provided by a\nPenrose Research Grant from the Geological Society of America\nand by National Science Foundation Grant GA-16080 to Steve\nClabaugh. We also thank Stan Keith for providing many of the\nchemical analyses reported here and Mark Barton and Joaquin\nRuiz for providing the data to construct the map of granitic rocks\nin western Mexico. Finally, we thank Jim Mattinson and David\nKimbrough for helpful, constructive reviews.", revision_no = "15", abstract = "Most of southern Sinaloa is underlain by a large, composite batholith, a continuation\nof the better-known Cordilleran batholiths of California and Baja California. Field\nrelations and extensive K-Ar and U-Pb dating within a 120-km-wide and 120-km-deep\ntransect show that the Sinaloa batholith formed in several stages. Early layered gabbros\nhave hornblende K-Ar ages of 139 and 134 Ma, although whether these record\nemplacement age, cooling from metamorphism, or excess Ar is unresolved. A group of\nrelatively mafic tonalites and granodiorites were emplaced before or during an episode\nof deformation and are restricted to within 50 km of the coast. These plutons, referred\nto here as syntectonic, show dynamic recrystallization textures that suggest deformation\nbetween 300° and 450 °C. A U-Pb zircon date on a probable syntectonic intrusion\nis 101 Ma. Hornblende K-Ar ages on definite syntectonic intrusions range between 98\nand 90 Ma; these may record cooling soon after emplacement or following regional\nmetamorphism.\nNumerous posttectonic intrusions crop out from within ~20 km of the coast, where\nthey intrude syntectonic rocks, to the eastern edge of the Sierra Madre Occidental,\nwhere they are covered by middle Cenozoic ash-flow tuffs. Posttectonic rocks are dominantly\nmore leucocratic granodiorites and granites. Three samples were analyzed by\nboth U-Pb and K-Ar methods. Their biotite and hornblende ages are concordant at 64,\n46, and 19 Ma and agree within analytical uncertainties with U-Pb zircon ages of 66.8,\n47.8, and 20 Ma. These data and field relations demonstrate that posttectonic intrusions\nwere emplaced at shallow depths and cooled rapidly. Therefore, concordant K-Ar age\npairs and hornblende ages in discordant samples approximate the time of emplacement.\nDiscordance of biotite-hornblende age pairs is largely if not entirely a result of reheating\nby younger plutons. The combined age data indicate that posttectonic intrusions were\nemplaced nearly continuously between 90 and 45 Ma. One intrusion is 20 Ma. Based on\noutcrop area, volumes of intrusions were relatively constant through time.\nThe combined geochronological data indicate that posttectonic magmatism\nshifted eastward between 1 and 1.5 km/Ma. Whether syntectonic magmatism also migrated is uncertain. Ages of middle and late Tertiary volcanic rocks indicate that\nmagmatism shifted rapidly (10-15 km/Ma) westward from the Sierra Madre Occidental\nafter ca. 30 Ma.\nThe Sinaloa batholith is borderline calc-alkalic to calcic. SiO_2 contents of analyzed\nrocks range from 47 to 74%; the lower limit excluding two gabbros is 54%. Syntectonic\nrocks are more mafic on average than posttectonic rocks. SiO_2 contents of seven out of\nnine analyzed syntectonic rocks range narrowly between 59 and 62%, with one each at\n65 and 67%. The posttectonic rocks show a wider range from 54 to 74% SiO_2, but only\nborder phases and small intrusions have SiO_2 less than ~63%. Combined with their\ndistribution, these data indicate that intrusions become more silicic eastward. The fact\nthat the 20 Ma intrusion is relatively mafic (61 % SiO_2\n) and lies near the coast with syntectonic\nrocks indicates that composition is related to location rather than to age.\nThe Sinaloa batholith shows both marked similarities and differences from\nbatholiths of the Peninsular Ranges, Sonora, Caho San Lucas (Baja California Sur),\nand Jalisco. The greatest similarities are in types of intrusions, a common sequence\nfrom early gabbro through syntectonic to posttectonic rocks, and general eastward\nmigration of magmatism. However, the end of deformation recorded by syntectonic\nrocks may be different in each area. Sinaloa rocks show a similar wide range of compositions\nas rocks of the Peninsular Ranges and Sonora but are more potassic than\nthe calcic Peninsular Ranges. Rare earth element patterns are most like those of the\neastern part of the Peninsular Ranges and central part of Sonora, both areas that\nare underlain by Proterozoic crust or crust with a substantial Proterozoic detrital\ncomponent. However, southern Sinaloa lies within the Guerrero terrane, which is\ninterpreted to be underlain by accreted Mesozoic crust. The greatest differences are\nin distance and rate of eastward migration. Published data show that magmatism\nmigrated eastward at ~10 km/Ma across the Peninsular Ranges and Sonora and from\nJalisco southeast along the southwestern México coast. The area of slower eastward\nmigration roughly correlates with the location of the Guerrero terrane and of possibly\naccreted oceanic crust that is no older than Jurassic.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/35443, title ="Assessment of (U-Th)/He thermochronometry: The low-temperature history of the San Jacinto mountains, California", author = "Wolf, R. A. and Farley, K. A.", journal = "Geology", volume = "25", number = "1", pages = "65-68", month = "January", year = "1997", issn = "0091-7613", url = "https://resolver.caltech.edu/CaltechAUTHORS:20121113-154436415", note = "© 1997 by the Geological Society of America. Manuscript received June 19, 1996.\nRevised manuscript received September 26, 1996.\nManuscript accepted October 21, 1996.\nSupported by the National Science Foundation. Detailed\nreviews by Trevor Dumitru and Mark Brandon\ngreatly improved this paper. We thank Brian Wernicke\nfor helpful comments throughout the course of this work.", revision_no = "12", abstract = "(U-Th)/He ages have been measured on igneous apatites from the San Jacinto mountains, a high region at the junction of the Peninsular and Transverse Ranges, to investigate the potential of this technique for thermochronometry of slowly cooled rocks. Helium ages from 79 to 17 Ma are younger than ages obtained by other dating techniques, including apatite fission-track counting, and are consistent with laboratory experiments that indicate this system has a uniquely low closure temperature. Helium ages are strongly correlated with elevation and record the latest low-temperature thermal evolution of the range. They suggest relative tectonic quiescence in the latest Cretaceous through mid-Tertiary and provide no evidence for rapid unroofing of the block during this period. Nor do they obviously require a large degree of uplift associated with convergence between the Transverse and Peninsular ranges in the last few million years. Helium ages document modest westward tilting of the block (∼7°) and a significant vertical component of motion on the block's bounding faults after helium retention began. This work suggests that apatite helium ages record low-temperature tectonic and thermal histories that are not apparent from other dating techniques.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47266, title ="Isotopic mapping in the Peninsular Ranges Batholith", author = "Silver, Leon T.", journal = "Abstracts with Programs - Geological Society of America", volume = "29", number = "6", pages = "69-69", month = "January", year = "1997", issn = "0016-7592", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140716-113219787", note = "© 1997 Geological Society of America. ", revision_no = "13", abstract = "The 1973 study of regional variations in the Sr isotopic composition and correlated trace elements in the Mesozoic granitic rocks of the central Sierra Nevada by R.W. Kistler and Z.E. Peterman was a first order contribution to the geochemistry and petrogenesis of Cordilleran batholiths. It\nestablished standards for isotopic mapping and for thoughtful interpretations of isotopic systematics which have guided all subsequent related work.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/35563, title ="Helium diffusion and low-temperature thermochronometry of apatite", author = "Wolf, R. A. and Farley, K. A.", journal = "Geochimica et Cosmochimica Acta", volume = "60", number = "21", pages = "4231-4240", month = "November", year = "1996", issn = "0016-7037", url = "https://resolver.caltech.edu/CaltechAUTHORS:20121120-104629253", note = "© 1996 Elsevier Science Ltd. Received January 10, 1996; accepted in revised form June 8, 1996. Editorial handling: D. E. Fisher. This work was supported by the National Science Foundation. We thank M. Harrison and M. Grove for helpful suggestions and P. Damon and an anonymous reviewer for their comments on the manuscript.", revision_no = "10", abstract = "To investigate the potential of the (U-Th)/He system for low-temperature thermochronometry, we have studied helium diffusion and have measured helium ages on Durango fluorapatite and on apatites from a gabbro and two tonalites from the Peninsular Ranges Batholith. Diffusivity at moderate to very low temperatures (as low as 80°C) was measured to high analytical precision using long duration incremental outgassing experiments. All four apatites displayed remarkably similar helium diffusion behavior. Helium loss apparently occurs via volume diffusion from subgrain domains (<60 μm) which are nearly identical in size in all samples. At temperatures below 290°C, diffusivity obeys a highly linear Arrhenius relationship with an implied activation energy of about 36 kcal/mol. Above this temperature, diffusivity deviates from linearity toward lower activation energies. This transition does not arise from multiple diffusion domains, but rather from a reversible change in the physical mechanism of helium diffusion. For thermochronometric purposes the high-temperature diffusion behavior is largely irrelevant because essentially no helium is retained over geologic time at temperatures above 290°C. Using the results from the low-temperature regime, all samples yield helium closure temperatures in the range 75 ± 7°C. This value is independent of chemical composition and grain size of the apatites, suggesting that a single closure temperature may apply to a wide range of samples. The (U-Th)/He ages of these apatites (17–120 Ma) range from a small fraction to nearly 100% of the crystallization age of their host rocks, and are consistent with a low-temperature thermochronometric interpretation. These results strongly support previous suggestions that (U-Th)/He dating of apatite can provide high precision chronometry of very low temperature geological events.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/35583, title ="The effects of long alpha-stopping distances on (U-Th)/He ages", author = "Farley, K. A. and Wolf, R. A.", journal = "Geochimica et Cosmochimica Acta", volume = "60", number = "21", pages = "4223-4229", month = "November", year = "1996", issn = "0016-7037", url = "https://resolver.caltech.edu/CaltechAUTHORS:20121120-143017682", note = "© 1996 Elsevier Science Ltd. Received January 10, 1996; accepted in revised form June 8, 1996. Editorial handling: D. E. Fisher. This work was funded by the National Science\nFoundation. We thank P. Damon and an anonymous reviewer for\nhelpful comments on the manuscript.", revision_no = "10", abstract = "A mathematical framework for quantitative evaluation of alpha-stopping effects on (U-Th)/He ages has been developed. Alpha stopping ranges in the ^(238)U, ^(235)U, and ^(232)Th chains vary between ∼10 and ∼30 μm, depending on decay energy and density/composition of the stopping medium. In the case of U- and Th-rich accessory minerals (e.g. apatite, zircon, titanite), the dominant effect of long stopping distances is alpha ejection to adjacent minerals. For grains smaller than a few hundred microns in minimum dimension, ejection effects will cause measured helium ages to substantially underestimate true ages. For example, a sphere of 100 μm radius retains only ∼82% of its alphas. For a homogeneous distribution of parent nuclides, the fraction of alphas ejected is ∼ 1/4 of the mean alpha range multiplied by the crystal surface to volume ratio, independent of geometry. Removal of the outer 20 μm of a crystal prior to dating eliminates the region which has experienced alpha loss, but may lead to erroneous ages when crystals are strongly zoned with respect to uranium and thorium. By careful characterization of four sieved apatite separates from a single sample, we show that it is possible to accurately correct (U-Th)/He ages for alpha ejection even when ejection exceeds 35% of total decays. Our results are useful for identifying the size and shape of grains which are best suited for (U-Th)/He dating and provide the basis for correcting ages when ejection effects are significant. This work underscores that meaningful (U-Th)/He ages require either large crystals, or correction of measured ages for alpha ejection.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/38435, title ="Evidence for possible horizontal faulting in southern California from earthquake mechanisms", author = "Huang, Weishi and Silver, L. T.", journal = "Geology", volume = "24", number = "2", pages = "123-126", month = "February", year = "1996", doi = "10.1130/0091-7613(1996)024<0123:EFPHFI>2.3.CO;2", issn = "0091-7613", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130510-154654100", note = "© 1996 Geological Society of America. Manuscript received May 12, 1995. Revised manuscript received October 10, 1995.\nManuscript accepted October 23, 1995. Supported by U.S. Geological Survey grant 14-08-0001-G1774. We thank Tom Parsons and an anonymous reviewer for constructive reviews of\nthe manuscript, and Don Anderson and Joann Stock for helpful discussions. Seismological Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology contribution 5542.", revision_no = "12", abstract = "We find that 36 of the 505 fault-plane solutions (M ≥ 3.0, 1981–1990) in southern California have a nodal plane dipping no more than 30 °. With the assumption of the low-angle nodal planes being the fault planes, four cross sections are constructed to show the possible horizontal faults in the middle and upper crust. More than half of these low-angle faults are located within or adjacent to the Transverse Ranges. The focal depths vary from 1 km in the southern end of the Sierra Nevada and the southwestern Mojave Desert to 20 km in the Transverse Ranges. The slip directions are also diverse. In general, east-west extensional movements are dominant in the boundary between the southern Sierra Nevada extending to the San Emigdio Mountains and the western Mojave Desert, whereas north-south compressional movements are dominant in the Transverse Ranges. In the Peninsular Ranges and the Salton Trough, both the slip directions and focal depths vary. These features suggest that seismically active low-angle faults in southern California may exist at different depths and slip in various directions. Our data do not support the existence of a regional-scale seismically active detachment in southern California. Only in the western Transverse Ranges is there some suggestion of a large detachment surface at a depth of about 13 to 14 km. ", } @book_section {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/55194, title ="Jurassic volcanic rocks in northeastern Mexico: A possible remnant of a Cordilleran magmatic arc", author = "Jones, Norris W. and McKee, James W.", number = "301", pages = "179-190", month = "January", year = "1995", doi = "10.1130/0-8137-2301-9.179", isbn = "9780813723013", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150225-125536778", note = "© 1995 Geological Society of America.\n\nManuscript accepted by the Society January 24, 1995.\n\nWe wish to thank M. Megan Miller and Joaquin Ruíz for their reviews of an early version of this paper, and Fred W. McDowell, José M. Grajales-Nishimura, and Cesar Jacques-Ayala for their most helpful final reviews. This work was supported by National Science Foundation Grant EAR-8705717, by the Faculty Development Fund at the University of Wisconsin, Oshkosh, and by the personal funds of L. T. Silver. Acknowledgement is also made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. This is Caltech Division\nof Geological and Planetary Sciences Contribution No. 5087.", revision_no = "13", abstract = "Pre-Oxfordian Mesozoic subaerial volcanogenic rocks occur in a band extending\nnorthwest from Ciudad Victoria, Tamaulipas, to Santa Maria del Oro, Durango.\nThese strata include Nazas, Rodeo, and Caopas Formations in Durango, Coahuila,\nand Zacatecas; La Boca Formation and its underlying volcanic basement at Canon\nde Huizachal, Tamaulipas; and volcanic units below La Joya Formation at Real de\nCatorce and Charcas, San Luis Potosi. Rocks at these localities have similar lithologies,\nstratigraphic positions, and paleontologic and isotopic ages. Field mapping in\nthe Caopas-Pico de Teyra area, northern Zacatecas, and ancillary research provide\ninsight into the nature of this suite.\nAt least 3 km of abundant airfall tuff, tuffaceous siltstone, and uncommon ashflow\ntuff are present near Pico de Teyra; this sequence appears to belong to a more\ndistal facies than the flows, breccias, and laharic conglomerates of the Nazas and\nRodeo Formations exposed 25 km to the north. Porphyritic rhyolite (Caopas Formation)\noccurs within these volcanogenic rocks as a fault-bounded block and is interpreted\nas a cogenetic, subvolcanic pluton. A relatively undeformed portion of the\nCaopas has yielded a zircon U-Pb age of 158 ± 4 Ma. Petrographic and limited chemical\ndata from these formations show that calc-alkaline andesite, dacite, and rhyolite\nare the most common compositions.\nThe volcanogenic rocks in northeastern Mexico are south of the inferred trace of\nthe Mojave-Sonora megashear. Their large volume, their lithologic and chemical characteristics,\nand their age suggest that these rocks may be a component of the Jurassic\narc of western North America that was translated southeastward along the megashear.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44558, title ="Tertiary metamorphic core complexes in Sonora, northwestern Mexico", author = "Nourse, Jonathan A. and Anderson, Thomas H.", journal = "Tectonics", volume = "13", number = "5", pages = "1161-1182", month = "October", year = "1994", doi = "10.1029/93TC03324", issn = "0278-7407", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140331-160851774", note = "© 1994 by the American Geophysical Union.\n\nReceived April 15, 1993; revised November 16, 1993; accepted November 26, 1993.\n\n\nThe distinct character of the Tertiary metamorphic rocks described above was initially recognized by Leon T. Silver, who during a field trip in 1970 noted the similarity in structural style between gneiss in northern Sonora and some of the forerange rocks of the Santa Catalina Mountains near Tucson. The experience and perspicuity of L. T. Silver and the insight into structural style of George Davis were of cmcial importance to T. H. A. and J. A. N. during this research.\nAnderson benefited from appropriate intellectual bludgeoning conducted by Dick Tosdal and Gordon Haxel during many stimulating, enjoyable, and fruitful exchanges. Discussions with Peter Coney and Doug Shakel were significant. Guillermo Salas, formerly of the Universidad de Sonora, and Jesus Najera and Jaime Roldan of the Instituto de Geologia provided logistical support and helpful advice. Franco Corona assisted during some of the field work. Much research was funded by a National Science Foundation grant EAR 76-84167 awarded to the University of Arizona as part of a cooperative project with George Davis. Gordon Haxel recognized and mapped the detachment fault at Sierra Mezquital during a cooperative field project sponsored by the United States Geological Survey and the Cosejo de Recursos Naturales de Mexico. Anderson also benefited from this program in that considerable structural information was obtained during his part of the mapping. Thesis mapping by J. A. Nourse in the Magdalena-Maderdao main and nearby areas was funded in part by two Sigma Xi grants and an NSF grant EAR 8519297 awarded to L. T. Silver. Nourse also gained insight into the geology of Sonora from discussions and field trips with geologists of the Institutod e Geologia. Use of the library and air-conditioned facilities at the Instituto is gratefully acknowledged. Reviews by Jaime Roldan, Steve Reynolds, and an anonomous person substantially improved the original manuscript.", revision_no = "19", abstract = "Several ranges encompassing more than 35,000 km^2 of Sonora, Mexico, contain distinctly lineated and foliated granitic and metamorphic rocks that constitute the lower plates of metamorphic core complexes. Penetrative deformation is characterized by gently dipping mylonitic foliation across which northeast trending stretching lineation is everywhere developed. Prominent northwest trending fractures, dikes, and normal faults are orthogonal to the lineation. Most kinematic indicators in lower plate mylonitic rocks record top-to-the-southwest sense of shear. Upper plate stratigraphic sequences include Mesozoic supracrustal rocks, Tertiary volcanic and sedimentary rocks, and allochthonous Precambrian basement. Tilted blocks of upper plate strata generally overlie the mylonites along gently dipping detachment faults. Previously published U-Pb and K-Ar ages from lower plate granitic orthogneisses, upper plate volcanic sequences, and crosscutting dikes constrain the time of mylonitic deformation and detachment faulting in several of these areas to late Oligocene-early Miocene. Partitioning of extensional strain in Sonora was influenced by pre-Tertiary crustal structure. The belt of core complexes developed across two contrasting blocks of continental crust separated by the N60 degrees W striking Mojave-Sonora megashear. Portions of the southern Papago block (northeast of the megashear) consisting of Jurassic magmatic are rocks and Upper Jurassic-Cretaceous siliciclastic and carbonate strata resting upon a concealed, tectonically fragmented Precambrian basement were especially susceptible to crustal attenuation. Some core complexes of the southern Papago block occur within zones trending northwest that may coincide with Late Jurassic lineaments. In the Caborca block (southwest of the megashear), core complex-related rocks and structures have not been identified where surface exposures of Middle Proterozoic basement and overlying Upper Proterozoic-Paleozoic platform strata are common. However, extensional mylonitic fabrics are locally developed along the margins of a Tertiary two-mica granite batholith. Core complexes on both sides of the megashear appear to be preferentially developed where Tertiary granites have intruded regions of crust with basement disrupted by pre-Tertiary structures. Sonoran core complexes preserve an extensional tectonic history comparable with that described from core complexes farther north in the United States and Canadian Cordillera. The timing of mid crustal extension in Sonora (25-18 Ma) is contemporaneous with the timing of core complex development in Arizona, Nevada, and Utah. Extension occurred later in these areas than in the Pacific Northwest-British Columbia region but earlier than in the Mojave Desert-Death Valley region. Middle Tertiary mylonitic fabrics of similar style and orientation have not been recognized farther south in Mexico. The southern terminus of the mid-Tertiary Cordilleran core complex belt appears to be in Sonora.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/38436, title ="Emplacement and deformation history of the western margin\nof the Idaho batholith near McCall, Idaho: Influence of a major terrane boundary", author = "Manduca, Cathryn A. and Kuntz, Mel A.", journal = "Geological Society of America Bulletin", volume = "105", number = "6", pages = "749-765", month = "June", year = "1993", issn = "0016-7606", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130510-154746884", note = "© 1993 Geological Society of America. Manuscript received by the Society May 1, 1992. Revised Manuscript received November 18, 1992. Manuscript accepted December 1, 1992. California Institute of Technology contribution no. 5088 approved by the director of the U.S. Geological Survey February 9, 1992. Field work for this study was funded by the U.S. Geological Survey. The paper presents some of the results from Manduca's doctoral dissertation conducted under the supervision of L. T. Silver at the California Institute of Technology. Her doctoral work was funded\nin part by the generous support of the Danforth Foundation, the Achievement Rewards for College Scientists, the Conoco Foundation, and the National Science Foundation (Grant EAR-86-16401). We thank J. Selverstone, B. Wernicke, K. Lund, L. Snee, and A. Hoover for helpful discussions of various aspects of this project; and Martha House, Kip Hodges, Fred Barker, j. Michael O'Neill, and Jane Selverstone for thorough reviews of the manuscript.", revision_no = "18", abstract = "Cretaceous plutons of the western margin of the Idaho batholith were emplaced along and to the west of the major terrane boundary separating middle Proterozoic and Paleozoic continental rocks from mostly Mesozoic accreted oceanic-arc terranes of the Blue Mountain Province. This boundary is marked by a change in the lithology of pendants and inclusions within the batholith. Plutons form two newly named complexes of igneous and metamorphosed igneous rocks. The Hazard Creek Complex, emplaced west of the boundary between the oceanic arc and the continental margin, consists primarily of a series of variably deformed and metamorphosed quartz diorite to trondhjemite plutons. The Little Goose Creek Complex, which intruded the boundary between the oceanic arc and the continental margin, is primarily porphyritic granodiorite to granite orthogneiss. A preliminary U-Pb age of 111 Ma for this porphyritic orthogneiss is a minimum age for the formation of the oceanic-arc-continent boundary.\nThe plutonic rocks were deformed both during and after emplacement in response to east-west compressive stresses. Cretaceous deformation was localized along the boundary between the accreted terranes and the continental margin and is interpreted to have occurred after the formation of this boundary. The major deformation of the Hazard Creek Complex occurred during its emplacement. The dominant fabric in the Little Goose Creek Complex is due to subsolidus ductile deformation. The localization of two deformation events along the pre-existing boundary between the accreted terranes and the continental margin suggests that a terrane boundary may form a long-lasting, crustal flaw. ", } @book_section {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/87923, title ="Transcontinental Proterozoic provinces", author = "Anderson, J. Lawford and Bender, E. Erik", number = "C-2", pages = "171-334", month = "January", year = "1993", doi = "10.1130/DNAG-GNA-C2.171", isbn = "9780813752181", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180717-105639238", note = "© 1993 Geological Society of America.", revision_no = "92", abstract = "Research on the Precambrian basement of North America over the past two decades has shown that Archean and earliest Proterozoic evolution culminated in suturing of Archean cratonic elements and pre-1.80-Ga Proterozoic terranes to form the Canadian Shield at about 1.80 Ga (Hoffman, 1988,1989a, b). We will refer to this part of Laurentia as the Hudsonian craton (Fig. 1) because it was fused together about 1.80 to 1.85 Ga during the Trans-Hudson and Penokean orogenies (Hoffman, 1988). The Hudsonian craton, including its extensions into the United States (Chapters 2 and 3, this volume), formed the foreland against which 1.8- to 1.6-Ga continental growth occurred, forming the larger Laurentia (Hoffman, 1989a, b). Geologic and geochronologic studies over the past three decades have shown that most of the Precambrian in the United States south of the Hudsonian craton and west of the Grenville province (Chapter 5) consists of a broad northeast to east-northeast-trending zone of orogenic provinces that formed between 1.8 and 1.6 Ga. This zone, including extensions into eastern Canada, comprises or hosts most rock units of this age in North America as well as extensive suites of 1.35- to 1.50-Ga granite and rhyolite. This addition to the Hudsonian Craton is referred to in this chapter as the Transcontinental Proterozoic provinces (Fig. 1); the plural form is used to denote the composite nature of this broad region. \n\nThe Transcontinental Proterozoic provinces consist of many distinct lithotectonic entities that can be defined on the basis of regional lithology, regional structure, U-Pb ages from zircons, Sr-Nd-Pb isotopic signatures, and regional geophysical anomalies.", } @book_section {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/51544, title ="Non-SAF type focal mechanisms adjacent to the SAF, Mojave Segment: Implications for Blind Thrust Beneath the San Gabriel Mountains, Southern California", author = "Huang, Weishi and Silver, Leon T.", number = "3", pages = "404-416", month = "January", year = "1993", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141111-080640739", note = "© 1993 Beijing Seismological Press.\n\nWe thank Ding Guoyu for his enthusiastic discussions and for his help to make\nthis paper published. Discussions with Gao Weiming, Ma Zonjing, and Paul Tapponnier during this\nmeeting arc also appreciated. We thank Steve Bryant for his help in checking the first motion data,\nClarence Allen and Egill Hauksson for helpful discussions. This work is supported by the U.S. Geol.\nSurvey Grant 14-08-0001-G1774. Contribution Number 5239, Seismological Laboratory, Division\nof Geological and Planetary Sciences, California Institute of Technology.", revision_no = "12", abstract = "Focal mechanisms of earthquakes with 2.5 ≤ M ≤ 4.1 from 1978 to 1990 were analyzed within\na 15 km-wide belt of the San Andreas fault (SAF) zone, in the Mojave segment, southern\nCalifornia. Of the 29 events, 41% are strike-slip, 28% oblique--slip, 24% thrust, and 7% are normal\nfault types. Most of the thrust events are located in the central section of the studied zone,\nwhere the fault geometry is relatively simple. In contrast, most of the strike-slip events arc at the\nintersection between the SAF and the San Jacinto fault. Both stress tensor inverted from slip\nvectors and strain tensor calculated from earthquake moment tensors produce similar directions of\nmaximum principal compressional axes that arc oriented in the direction between 351° and 5°,\nwhich is at an angle of about 60° to the strike of the SAF. These non-SAF type faulting events obviously\ncan not be explained by simple shear motion on the SAF. Instead, they accrued on the\nstructures that were activated in response to the regional N compression at the latitude of the\nTransverse Ranges. For the thrust events, we interpret that a back thrust (northward thrust) exists\nbeneath the northern side of the San Gabriel Mountains. This back thrust is located at the depth of\n8-13 km, within or beneath the Pelona schist. It is apparently associated with aseismic slip and uplifts\nthe San Gabriel Mountains on the northern side.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/43384, title ="Aspects of the crustal structure of the western Mojave Desert, California, from seismic reflection and gravity data", author = "Li, Yong-Gang and Henyey, Thomas L.", journal = "Journal of Geophysical Research B", volume = "97", number = "B6", pages = "8805-8816", month = "June", year = "1992", doi = "10.1029/91JB02119 ", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140115-111655716", note = "© 1992 by the American Geophysical Union.\n\nReceived December 27, 1990; revised July 18, 1991; accepted August 12, 1991.\n\nThe Mojave seismic reflection data reprocessed in this study were kindly provided by COCORP.\nWe would like to acknowledge discussions regarding the processing and interpretation with David Okaya, Peter Malin,\nEric Frost, and Gregory Davis. A. Levander, J. Namson, and an associate editor provided helpful comments during the review\nprocess. In particular we are indebted to J. Namson for his suggestion that the Hitchbrook is an older, down-to-the north\nnormal fault. The gravity data were obtained with the aid of John Scott. Susan Turnbow and Desser Motion typed the final\nversion of the manuscript. The study was supported by NSF grants EAR-8319254 and EAR-8609247 to CALCRUST.", revision_no = "8", abstract = "Seismic and gravity data taken along line 1 of the 1982 Consortium for Continental Reflection Profiling (COCORP) Mojave Desert Survey (N-S profile, ∼30 km long) have been used to characterize the upper crust north of the San Andreas fault in the western Mojave block of southern California. Consortium for Continental Reflection Profiling seismic reflection data were reprocessed to emphasize the upper 5 seconds (two-way travel time). The resultant common depth point (CDP) sections provided starting models for generating a refined geologic cross-section using a combination of ray tracing (forward modeling) and gravity interpretation. The forward modeling was used to validate the existence of faults and constrain their dips. The gravity data were used to refine the overall model, particularly in poor data areas on the CDP sections. Gravity data, taken along three nearby profiles parallel to primary line of section, were also used to determine the structural trend. Results from the first two seconds indicate the presence of a series of ENE striking reverse faults beneath the late Tertiary and Quaternary sedimentary cover of the western Mojave. The faults dip northward and offset the sediment-basement interface. The largest such feature has an apparent throw of ∼1.8 km and exhibits a subtle scarp at the Earth's surface suggesting Holocene displacement. The orientation of these faults, although not an instantaneous representation of the present-day stress field, is consistent with NNW compression across the western Mojave block and WNW striking San Andreas fault, as determined from nearby focal mechanisms and in situ stress measurements. The faults also appear to be closing small sedimentary basins in the Mojave block, which may have formed during an earlier extensional phase, similar to what is happening on a much larger scale in the Los Angeles basin to the south of the San Andreas fault. Reflections between 2 and 5 s, coupled with the local geology and gravity modeling, are consistent with the presence of the Pelona/Rand schist in the subsurface beneath the western Mojave. The upper surface of the schist (i.e., Vincent/Rand thrust equivalent) rises southward toward the San Andreas fault where it is displaced vertically (up to the south) at least 5 km along the E-W trending Hitchbrook fault, such that the schist crops out between the Hitchbrook and subparallel San Andreas to the south. The same structure may exist beneath the Tehachapi mountains, with the roles of the Hitchbrook and San Andreas faults played by the north and south branches of the Garlock fault, respectively. The rising or arching of the basement toward the San Andreas fault (and toward the Garlock) is not only reflected in the geology and topography local to these faults in many places but is also generally observed on seismic reflection profiles in the vicinity of these faults in the western Mojave. Furthermore, the arching is also consistent with a strong component of fault normal compression.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44270, title ="Geochemical Logging in the Cajon Pass Drill Hole and Its Application to a New, Oxide, Igneous Rock Classification Scheme", author = "Pratson, Elizabeth Lewis and Anderson, Roger N.", journal = "Journal of Geophysical Research B", volume = "97", number = "B4", pages = "5167-5180", month = "April", year = "1992", doi = "10.1029/91JB02643", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140312-075614994", note = "© 1992 American Geophysical Union. Received May 20, 1991;\nrevised October 3, 1991; accepted October 17, 1991. \n\nThe first author would like to thank L. Pratson, J. Tivy, W. R. Van Schmus, and F. E. Goff for their input on the manuscript and critical review. M. Herron provided useful input on the ideas of the IgClass system. S. Cotkin contributed to the petrographic\ncutting and thin-section observation and the petrological classification of the geologic column. Lamont-Doherty Geological Observatory contribution number 4878.", revision_no = "11", abstract = "A new elemental oxide classification scheme for crystalline rocks is developed and applied to geochemical well logs from the Cajon Pass drill hole. This classification scheme takes advantage of measurements of elements taken by a geochemical logging tool string. It uses K_2O versus SiO_2/Al_2O_3 to distinguish between granites, granodiorites, tonalites, syenites, monzonites, diorites, and gabbros. Oxide measurements from cores are used to calibrate the elemental abundances determined from the well logs. From these logs, a detailed lithologic column of the core is generated. The lithologic column derived from the well log classification scheme is compared with a lithologic column constructed from core samples and well cuttings. In the upper 1295 m of the well, agreement between the two columns is good. Discrepancies occur from 1295 to 2073 m and are believed to be caused by the occurrence of rock types not distinguished by the classification scheme and/or the occurrence of secondary minerals. Despite these discrepancies, the well log-based classification scheme helps to distinguish changes in rock type and shows potential as an aid to the construction of lithologic columns in boreholes of crystalline rocks.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44974, title ="^(87)Sr/^(86)Sr and ^(18)O/^(16)O isotopic systematics and geochemistry of granitoid plutons across a steeply-dipping boundary between contrasting lithospheric blocks in western Idaho", author = "Manduca, Cathryn A. and Silver, Leon T.", journal = "Contributions to Mineralogy and Petrology", volume = "109", number = "3", pages = "355-372", month = "January", year = "1992", doi = "10.1007/BF00283324", issn = "0010-7999", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140416-095344675", note = "© 1992 Springer-Verlag. Received December 17, 1990. Accepted June 18, 1991. Contribution No. 4926. Publication of Division of Geologic and Planetary Sciences, California Institute of Technology. This work was funded in large part by the National Science Foundation (Grants No.EAR-86-16401, EAR-83-13106, and EAR-88-16413). The paper presents results from Manduca's doctoral dissertation done under the supervision of L.T. Silver at the California Institute of Technology. Her doctoral work was funded in part by the generous support of the Danforth Foundation, the Achievement Rewards for College Scientists, and the Conoco Foundation. The field work for this study was funded by the U.S. Geological Survey and done in conjunction with Dr. Mel Kuntz. Jim LaFortune was of great assistance during the mapping. Helpful reviews were provided by Robert E. Criss and Zell E. Peterman and conversations with Robert J. Fleck provided additional insight. Editorial responsibility: T. Grove.", revision_no = "14", abstract = "A spatially abrupt geochemical boundary is preserved within four plutonic complexes along the western margin of the Cretaceous Idaho Batholith near McCall, Idaho. These intrusives ranging in composition from tonalite to granite were emplaced across a regional boundary between accreted oceanic-arc terranes and the continental margin, and their isotopic, major-element, and trace-element geochemistry provide detailed information about this change in crustal characteristics at depth, indicating that the boundary is nearly vertical and extends deep into the lithosphere. The Hazard Creek complex, emplaced west of the transition in wall-rock lithology, has initial ^(87)Sr/^(86)Sr (Ri) less than 0.7045 and δ^(18)O greater than 7.5, indicating little or no continental crust in its source region; however, elevated δ^(18)O requires some incorporation of rocks formed or altered at the earth's surface. A large shift in Ri and δ^(18)O is observed across the 5–8 km wide Little Goose Creek complex, which was emplaced across the wall-rock boundary. This is interpreted as mixing between: (1) a basaltic or andesitic magma with low K_2O and high Na_2O, Al_2O_3, and Sr, similar to that forming the Hazard Creek complex; and (2) materials similar to Precambrian sedimentary sedimentary rocks with low Sr, high δ^(18)O (+15) and high Ri (0.83 at 100 Ma). The Payette River complex, emplaced east of the wall-rock boundary, exhibits at least one additional component with low δ^(18)O (+6), moderate R_i (0.708) and mafic composition. This component is inferred to be old basaltic material in the lower crust or upper mantle similar to that inferred to be a minor part of the Peninsular Ranges Batholith in SE California (Silver et al. 1979; Hill et al. 1986). The easternmost complex in the Idaho transect is made up of granites that may contain a component of granitic cratonal basement. The entire west-to-east geochemical transition from oceanic-arc magmas to cratonal magmas takes place over a lateral distance of less than 20 km. Although the zone of transitional protolith dominated by metasedimentary rocks is unusually narrow and may have been in part tectonically removed, the striking geochemical similarities between this traverse and several other transects across much broader areas of Nevada and California suggest that the craton itself was not rifted apart, but that juxtaposition of the accreted oceanic-arc terranes occurred along the preexisting craton margin. The data confirm that the isotopic geochemistry of granitoid plutons can be used as a probe of deep lithospheric character, and that major lateral variations in the lithosphere on the order of one to two kilometers in width can be recognized in favorable circumstances.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/38487, title ="The metamictization of zircon: Radiation dose-dependent structural characteristics", author = "Woodhead, James A. and Rossman, George R.", journal = "American Mineralogist", volume = "76", number = "1-2", pages = "74-82", month = "February", year = "1991", issn = "0003-004X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130514-103113195", note = "© 1991 Mineralogical Society of America. Manuscript received May 21, 1990. Manuscript accepted November 14, 1990. This research was funded in part by National Science Foundation grants EAR-7919987, 8618200, and 8916064. We thank David Gottfried, Heinrich Holland, and Frank Senftle (USGS, Reston, Virginia) for samples included in this work. We thank Rodney Ewing, Franois Farges, and John Hughes for their helpful comments on the manuscript. This study is Caltech contribution no. 4606.", revision_no = "13", abstract = "A suite of gem zircon samples from Sri Lanka has been studied using infrared (IR) spectroscopy, X-ray diffraction, and chemical analysis. The degree of metamictization of the zircon, as indicated by unit-cell parameters, increases systematically with U-Th content\nup to the point of total metamictization. The appearance of IR spectra also varies as a function of metamictization; band widths increase and intensities decrease with increasing U-Th contents. Persistence of bands related to Si-O bonds and disappearance of bands related to Zr-O bonds indicates that the structure of metamict zircon consists of distorted and disoriented isolated silica tetrahedra with few if any undisplaced Zr cations. All spectroscopic\nindicators of crystalline order show that total metamictization is reached at an accumulated radiation dosage of ~4.5 x 10^(15) alpha decay events per mg. Hydrous components enter the structure only after total metamictization, but the amounts are not correlated with U-Th content. In all cases OH was the only hydrous species detected. ", } @book_section {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/48379, title ="Geochemical modeling of a steeply dipping boundary between continental and oceanic-arc lithosphere, west-central Idaho", author = "Manduca, Cathryn A. and Silver, Leon T.", pages = "63", month = "May", year = "1990", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140812-110611222", note = "© 1990 Geochemical Society.", revision_no = "15", abstract = "The western margin of the Cretaceous Idaho Batholith intruded the boundary between accreted oceanic and terranes and the continental margin. The geochemistry of the plutonic rocks indicates that it is an abrupt, steeply dipping boundary which\nextends through the lithosphere.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44269, title ="San Jacinto Intrusive Complex: 2. Geochemistry", author = "Hill, R. I. and Chappell, B. W.", journal = "Journal of Geophysical Research B", volume = "93", number = "B9", pages = "10349-10372", month = "September", year = "1988", doi = "10.1029/JB093iB09p10349", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140311-155520484", note = "© 1988 American Geophysical Union. Received November 26, 1984; revised March 3, 1986; accepted March 4, 1986.\n\nThis work was conducted as part of a Ph.D. project supervised by L.T.S.\nFunding was provided by a Conoco Fellowship, by the Division of Geological and Planetary Sciences, California Institute of Technology, and by the Australian Research Grants Committee. This work has benefited from discussion with many people, but particularly Dave Walker, Steve\nSparks, Alex McBirney, Wes Hildreth and Mike Dungan. Dungan pointed out the need for a model that explained the similarity of (inferred) liquid and solid Na/(Na+Ca) and Mg(Mg/Fe) (\"Dungan's Dilemna\"). Journal reviews by Calvin\nBarnes and Lawford Anderson led to significant improvements in clarity. Division of Geological and Planetary Sciences, California Institute of Technology, contribution 4161. Paper number 4B5361.", revision_no = "12", abstract = "Rocks from three large (>100^2 km) tonalitic intrusions exposed in the San Jacinto Mountains of southern California show a restricted compositional range of between 63 and 68 wt % SiO_2 for all but volumetrically minor felsic differentiates (with Si0_2≈70 wt %). All rocks with less than 65.5 wt % SiO_2 show linear element-element covariation. Felsic differentiates have characteristics (higher SiO_2, K_2O, Rb, Ba, U; higher and variable rare earth elements) consistent with derivation by in situ fractionation; rocks with between 65.5 and 70 wt % SiO_2 have intermediate characteristics and are interpreted as derived from liquids formed by mixing “primitive” liquids with fractionated liquids within an intermittently recharged, continuously solidifying magma chamber. Mafic inclusions extend the compositional trends of the mafic tonalites to 55 wt % SiO_2. The chemical variations of both inclusions and more mafic tonalites are interpreted as resulting from processes acting before injection of their parental liquids into the observed crustal magma chambers. Effects of chamber processes are minor for all but the most felsic rocks. The major effect of recharge is to buffer the thermal and chemical properties of liquids within the magma chambers, yielding large volumes of relatively homogeneous tonalite. For those elements where the bulk distribution coefficient is between about 0.5 and 2, concurrent recharge and solidification produces rocks that closely approximate the composition of the added liquids. Estimated Rayleigh numbers for these liquids are high (>10^(10)), implying convection throughout much of the solidification history of each chamber. Existence of trace element variations within analyzed rocks imply that convection was not totally efficient at homogenizing the various batches of liquid added to each chamber.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/42153, title ="San Jacinto Intrusive Complex: 3. Constraints on crustal magma chamber processes from strontium isotope heterogeneity", author = "Hill, R. I. and Silver, L. T.", journal = "Journal of Geophysical Research B", volume = "93", number = "B9", pages = "10373-10388", month = "September", year = "1988", doi = "10.1029/JB093iB09p10373 ", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20131031-103755851", note = "© 1988 by the American Geophysical Union.\n\nReceived November 26, 1984; revised October 23, 1986; accepted February 27, 1987.\n\nFinancial support was provided\nby National Science Foundation grants EAR\n76-23153 and EAR 77-23507 to L.T.S., by Department\nof Energy contract DE-AC13-76 GJO 1664,\nBendix Field Engineering Corp. subcontract 79-\n384-E to L.T.S, and by a Conoco Fellowship and\nthe Division of Geological and Planetary Sciences,\nCalifornia Institute of Technology for\nR.I.H. Discussions with Steve Sparks have materially\nassisted in the writing of this report, as\nhave detailed reviews, and comments from Mike\nDungan and Wes Hildreth. Contribution 4162 of\nthe Division of Geological and Planetary\nSciences, California Institute of Technology.", revision_no = "9", abstract = "Strontium isotope data from three large plutons interpreted as derived from dynamic magma chambers (Hill, this issue; Hill et al., this issue) show the existence of pronounced isotopic heterogeneity within each unit. Ranges of calculated initial ^(87)Sr/^(86)Sr are unit I, 0.7060–0.7076; unit II, 0.7060–0.7074; unit III, 0.7058–0.7073. A limited sample of mafic inclusions and dyke rocks, interpreted as samples of liquid added to the various chambers during inflation, have initial ^(87)Sr/^(86)Sr of 0.7068–0.7084. These variations are regular at the kilometer scale within each pluton and show no identified correlation with any other observed geochemical or petrological parameter. Slightly older small intrusions that span the compositional range olivine gabbro to granite show a systematic increase in initial ^(87)Sr/^(86)Sr from 0.7057 in the southwest to 0.7077 in the northeast, expressing the pattern previously reported for the northern 600 km of the Peninsular Ranges batholith. The Sr isotope data indicate that melt production, aggregation, transport, and crystallization processes were far from capable of completely homogenizing initial variations in initial ^(87)Sr/^(86)Sr within the liquids from which these rocks crystallized. Whole chamber convection is apparently ruled out by these data; more complex patterns, including double-diffusively induced horizontal stratification, are permitted.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/39033, title ="Elemental and mineralogical analyses using geochemical logs from the Cajon Pass Scientific Drillhole, California, and their preliminary comparison with core analyses", author = "Anderson, Roger N. and Dove, Roy E.", journal = "Geophysical Research Letters", volume = "15", number = "9", pages = "969-972", month = "August", year = "1988", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130621-145700946", note = "© 1988 by the American Geophysical Union.\n\nReceived March 11, 1988; accepted: May 4, 1988.\n\nWe wish to thank Honor O'Malley, Colin Williams, and Philippe Pezard for helpful advice during the preparation of this paper. Elizabeth Pratson developed the\nsilicate elemental weight percent transform. We especially thank Mark Zoback for conceiving of and carrying out the drilling of this magnificent scientific laboratory into the workings of the San Andreas Fault, and DOSECC and the National Science Foundation for supporting the effort.\n", revision_no = "10", abstract = "Estimates of elemental and mineralogical abundances from geochemical logs are compared to preliminary chemical and modal analyses from cores in the Cajon Pass Scientific Drillhole. Accuracies of log-computed weight percent oxide and mineralogy determinations range from 10 to 30%.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/39281, title ="Geologic setting and lithologic column of the Cajon Pass Deep Drillhole", author = "Silver, Leon T. and James, Eric W.", journal = "Geophysical Research Letters", volume = "15", number = "9", pages = "941-944", month = "August", year = "1988", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130710-095829427", note = "© 1988 by the American Geophysical Union.\n\nReceived April 6, 1988;\nrevised July 14, 1988; accepted July 14, 1988.\n\nSynthesis of the column involved major\nContributions from Epoch, Inc, especially D. Milham and R. Behl. Assistance by the U.S.G.S. curatorial facility staff, especially\nW. Campbell, F. Raup, D. Volturno, and W. Linenburger was invaluable. Meticulous reviews by K.\nMeisling and R. Weldon are appreciated. This work supported\nby the National Science Foundation and DOSECC.\nContribution No. 4635 of the Division of Geological and\nPlanetary Sciences, California Institute of Technology.\n", revision_no = "9", abstract = "The Cajon Pass Deep Drillhole penetrates a late Tertiary basin developed on crystalline basement in the SW Mojave Desert, 4 km from the San Andreas fault. Cores, continuous cuttings and geophysical logs from phase I show great diversity in rock types, structure, and alteration. The hole encountered Cajon formation arkoses; granodiorite/tonalite; unusual megacrystic granite and augen gneiss; granitic and pelitic gneisses with quartzite; quartzofeldspathic orthogneiss cut by sheets of hornblende gabbro; and interlayered mafic and quartzofeldspathic orthogneisses with rare calcsilicate intervals. Foliation and compositional layering have low dips throughout the column and layered gneisses contain 10-cm-scale recumbent folds. Faults and alteration zones bound several rock units with low apparent dips. Basement cores are typically cut by steep fractures, <1 mm wide, that contain zeolites±calcite or chlorite-epidote. Fractures and faults decrease in abundance with depth.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/38441, title ="Implications of zeolites and their zonation in the Cajon Pass Deep Drillhole", author = "James, Eric W. and Silver, Leon T.", journal = "Geophysical Research Letters", volume = "15", number = "9", pages = "973-976", month = "August", year = "1988", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130513-093549509", note = "© 1988 American Geophysical Union.\n\nReceived April 15, 1988; accepted June 16, 1988.\n\nWe appreciate assistance from Manuel\nBass of Chevron Oil Field Research Co. and discussions with\nM. Vincent, P. Ehlig, Y. Kharaka and C.-Y. Wang. This\nwork supported by the National Science Foundation and\nDOSECC. Contribution No. 4634, Division of Geological and\nPlanetary Sciences, California Institute of Technology.\n\n", revision_no = "10", abstract = "Zeolites occur in all cores and most cuttings samples of plutonic and gneissic rock from the Cajon Pass Deep Drillhole. Laumontite or stilbite replace plagioclase and fill fractures <1 mm to several cms in width. Zeolitic alteration is most intense in faulted and fractured zones. Zeolite species are zoned vertically. Laumontite occurs to a depth of 1885 m, stilbite from 1885 m to 2080 m and laumontite again to at least 2524 m. The transition from stilbite to laumontite at 2080 m fits both extrapolated equilibria and the CPDDH P-T gradient but laumontite occurrences above 1885 m are metastable apparently reflecting rapid uplift. Radioactive accessory minerals from the wall rock surrounded by zeolites in some fractures exhibit red-brown haloes. Halo intensity correlates to mineral radioactivity and may allow dating of mineraliztion. Observation in the CPDDH suggest a potential role for zeolites in determining chemical and physical properties such as pore water composition, seismic velocity, and gravity contrasts in faults of the San Andreas system.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44231, title ="Lithologic column of the “Arkoma” Drillhole and its relation to the Cajon Pass Deep Drillhole", author = "Silver, Leon T. and James, Eric W.", journal = "Geophysical Research Letters", volume = "15", number = "9", pages = "945-948", month = "August", year = "1988", doi = "10.1029/GL015i009p00945 ", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140310-152425705", note = "© 1988 by the American Geophysical Union.\n\n\nReceived April 6, 1988; revised July 14, 1988; accepted July 14, 1988.\n\nRobert Johnson, DOSECC engineer, Supplied the well-survey information presented in Figure1 and incorporated in Figure 2. R. Weldon, P. Pezard and K. Meisling have been generous in sharing their ideas and experience as well as providing preprints of important papers. M. Zoback provided geophysical logs for the Arkoma hole as well as extensive discussions. Weldon and Meisling provided thoughtful reviews. This work supported by the National Science Foundation and DOSECC. Contribution No. 4636, Division of Geological and Planetary Sciences California Institute of Technology.\n", revision_no = "8", abstract = "The 1795 m \"Arkoma\" Federal 1-26 well, 48.5 m from the Cajon Pass Deep Drillhole (CPDDH), provides additional lithologic and structural data pertinent to that project. Basement surface was encountered 158 m deeper than in the CPDDH. Rock units correlate well between the holes; the offset persists to 1128 m. Below this depth both lithologic unit thicknesses and fault zones correlate between holes on subhorizontal projections. A combination of previously unrecognized high-angle and low-angle faults of several ages are required to explain the structures. Blind low angle faults may be regionally important.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44264, title ="Petrological and geochemical investigations at the Cajon Pass Deep Drillhole", author = "Silver, Leon T. and James, Eric W.", journal = "Geophysical Research Letters", volume = "15", number = "9", pages = "961-964", month = "August", year = "1988", doi = "10.1029/GL015i009p00961", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140311-153804093", note = "© 1988 American Geophysical Union. Received April 15, 1988;\nrevised July 14, 1988; accepted July 14, 1988. \n\nWe wish to acknowledge the U.S.G.S. staff at Cajon Pass, particularly W. Campbell, F. Raup, D. Volturno and W. Linenburger, for measurements of core\ngamma ray activity. R. Anderson, of Lamont-Doherty Geological\nObservatory, and I. Sass and A. Lachenbruch, of the\nU.S.G.S., freely exchanged data on downhole measurements\nof gamma-ray activity and temperature gradients. Spencer\nCotkin provided editorial review. This work supported by the\nNational Science Foundation and DOSECC. Contribution\nnumber 4663, Division of Geological and Planetary Sciences,\nCalifornia Institute of Technology. ", revision_no = "12", abstract = "Chemical analyses of cores from the Cajon Pass Deep Drillhole confirm vertical lithologic diversity of the basement rocks. They are metaluminous and calcalkaline; weak major element correlations with wide-ranging SiO_2 suggest they are not a simple cogenetic suite. Quartz contents correlate with the thermal gradient profile, supporting conduction as the principal thermal transport mechanism. Abundances of heat producing elements are variable and somewhat lower than average crustal values. Cross-calibration with other measures of Th, U and K are underway. Ages of surface granitoids and core 17 are late Cretaceous; other plutonic rocks appear older but late Mesozoic. Initial lead isotopic signatures of the plutonic rocks change dramatically with depth in the hole.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44263, title ="The Cajon Pass Scientific Drilling Experiment: Overview of phase 1", author = "Zoback, Mark D. and Silver, Leon T.", journal = "Geophysical Research Letters", volume = "15", number = "9", pages = "933-936", month = "August", year = "1988", doi = "10.1029/GL015i009p00933", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140311-151633689", note = "© 1988 American Geophysical Union. Received: May 5, 1988;\nAccepted: June 27, 1988. \n\nWe wish to thank the National Science Foundation for providing financial support through DOSECC, Inc. for the Cajon Pass project and the U.S. Geological Survey for providing on site core-handling facilities and personnel and financial support for USGS scientists involved in the project We owe a considerable debt to Robert Johnson, DOSECC project supervisor, for providing engineering talent and advice for the project as a whole and for many members of the science team. We wish to thank Wayne Campbell, Francis Raup, Deborah Volturno and William Linenburger of the USGS core-handling team for their help. Many of the contractors associated with this project have provided assistance and services far beyond the terms and conditions of their contracts. We wish to thank in particular Steven Pittillo, Mark Andrews and Barney Thompson of Parker\nDrilling Co. and Bill Anderson, Doug Milham and Rick Behl of\nEpoch Mud Logging Services for providing considerable\nassistance to the scientists involved in this project", revision_no = "12", abstract = "The Cajon Pass Scientific Drilling Project is a broad, interdisciplinary experiment involving over two dozen principal scientists. Phase I of drilling, coring and downhole experimentation began Dec. 8, 1986 and ended April 2, 1987 with the hole at a depth of 2115 m, 82 m of core recovered and a wide range of downhole experiments successfully completed. In this paper we briefly outline the scientific motivation for the project and provide an overview of the scientific program. We also indicate some of the varied research areas in the earth sciences where data obtained in this experiment will provide unique and important insight into active in situ processes, regional geologic structure, and rock and fluid composition and origin.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/38366, title ="Comment and Reply on \"Late Triassic paleogeography of the southern Cordillera:\nThe problem of a source for voluminous volcanic detritus in the Chinle Formation\nof the Colorado Plateau region\"", author = "Stewart, John H. and Anderson, Thomas H.", journal = "Geology", volume = "15", number = "6", pages = "578-579", month = "June", year = "1987", issn = "0091-7613", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130508-152220795", note = "© 1987 Geological Society of America.\n", revision_no = "14", abstract = "On page 569 of our article (Stewart et al., 1986) we emphasized the\ncritical dependence of our ideas upon the validity of the isotopic and\npaleontologic geochronology, and on the calibration between the two\nmethods. Wyman also emphasizes this same point but introduces other\ninformation—namely, isotopic dates on uranium ores in the Chinle Formation.\nHe indicates, citing the work of Miller and Kulp (1963), that the\naverage date for uranium ores in the Chinle Formation in the Cameron\narea, Arizona, is 175 Ma and in Lisbon Valley, Utah, is 150 Ma. These\naverages, however, seem meaningless because they incorporate individual\ndates that range from 101 to 218 Ma at Cameron, and from 95 to 207 Ma at Lisbon Valley. In any case, recent data (not cited by Wyman) gives a\nconcordant U-Pb age of 206 ±1 Ma for uranium ores from the Chinle\nFormation in Lisbon Valley (Ludwig et al., 1986).\n", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44949, title ="REE Variations Across the Peninsular Ranges Batholith: Implications for Batholithic Petrogenesis and Crustal Growth in Magmatic Arcs", author = "Gromet, L. Peter and Silver, Leon T.", journal = "Journal of Petrology", volume = "28", number = "1", pages = "75-125", month = "February", year = "1987", doi = "10.1093/petrology/28.1.75", issn = "0022-3530", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140415-090105736", note = "© 1987 Oxford University Press.\n\nReceived 22 September 1985; revised typescript accepted 3 July 1986.\n\nThis work was drawn from a portion of the senior author's doctoral dissertation done at the California Institute of Technology under NSF grants EAR74-00155 and EAR76-23153\nand DOE grant EY-76-G-03-1305 to L. T. Silver. The senior author acknowledges the National Science Foundation for support under a Graduate Traineeship and an Energy\nRelated Graduate Fellowship. Preparation of this manuscript was partially supported by NSF grant EAR82-06659 and by Brown University. We thank B. W. Chappell, H. P. Taylor, Jr., the late A. K. Baird, T. O. Early, J.D. Murray\nand R. I. Hill for many discussions during the course of this study. We are indebted to Chappell for kindly providing some major element analyses and to Baird for providing early access to his extensive set of chemical analyses. The manuscript has benefited from early\nreviews by Hill, Paul Hess, Mac Rutherford, and Bruno Giletti. Journal reviews by F. A. Frey and M. A. Lanphere are sincerely appreciated. Lisa Sheehan's skill on the word processor was invaluable. Contribution 4349, Division of Geological and Planetary Sciences, California Institute of Technology.", revision_no = "14", abstract = "Rare earth element (REE) patterns of plutonic rocks across the Cretaceous Peninsular Ranges batholith vary systematically west to east, transverse to its long axis and structural trends and generally parallel to asymmetries in petrologic, geochronologic and isotopic properties. The batholith can be divided into three distinct parallel longitudinal regions, each defined by distinct REE pattern types. An abrupt transition occurs between rocks with slightly fractionated REE patterns in the western (coastal) region and rocks with middle to heavy REE fractionated and depleted patterns in the central region. Further to the east a second transition to strongly light REE enriched rocks occurs. The slopes of the REE patterns within each of these regions are largely independent of rock type. The first REE transition is closely coupled to regional discontinuities in other parameters: elimination of negative Eu anomalies, an increase in Sr content, and a marked restriction in petrologic diversity. This transition occurs over a range of initial ^(87)Sr/^(86)Sr ratios and δ^(18)O values, but approximately correlates to a major shift in the emplacement style of the batholith from a stationary arc to a rapidly eastward-migrating (cratonward) arc.\n\nThe sense of the regionally consistent REE trends cannot be explained by crystallization, assimilation, combined crystallization-assimilation, or mixing processes. The consequences of assimilation and high-level differentiation are not observed generally, despite the sensitivity of the REE to these processes. Geochemical and petrological features argue that the partial melting of mafic source rocks in which plagioclase-rich (gabbroic) residual assemblages abruptly gave way laterally and downward to garnet-bearing (eclogitic) residual assemblages produced all the changes associated with the first REE transition. The change in residual assemblages from gabbroic to eclogitic was superimposed on source regions already zoned in light REE abundances, ^(87)Sr/^(86)Sr and ^(18)O. Temperature and pressure constraints on the source regions place them in a subcrustal location. The calcic nature of the batholith and the dominance of tonalite and low-K_2O granodiorite in all its regions argue that the source materials are broadly basaltic in composition. Experimental studies are consistent with the generation of the abundant tonalitic magmas by the partial melting of basalt under both low and high pressure conditions. Arc basalts such as those commonly erupted in modern island arcs and continental margins are inferred to have provided much of the source material and the heat. Additional high-^(18)O components are needed in the more easterly source regions. These materials must be distributed so as to contribute equally to the range of magmas that occur in a given local region, and must preserve the calcic nature of batholithic sources. Altered basalts of ancient oceanic crust and possibly their associated metasediments, previously incorporated into the lithosphere beneath the continental margin during earlier cycles of subduction, most readily satisfy these constraints.\n\nThe REE geochemistry of the central and eastern regions of the batholith differs from that of oceanic island arcs in the presence of strongly heavy REE depleted and fractionated magmas. A model is proposed in which arc basalts accumulate beneath a crustal layer. Melting of accumulated material at low pressure produces magmas of the western region. Where thickening of the basaltic underplate is sufficient to form eclogitic assemblages, eclogite-derived magmas of the central and eastern region are produced. The abrupt transition to eclogite-derived magmas that suggests a process driven by a density instability is responsible for their origin.\n\nThe Peninsular Ranges batholith appears to be representative of a major differentiation process in which mantle-derived basalt is remelted, contributing its more sialic fractions to the continental crust and leaving its mafic to ultramafic residues in the mantle. This process preserves the sialic character of the continental crust and may play a significant role in its growth and evolution. The batholith and the processes that produced it may be a more appropriate basis than immature oceanic island arcs on which to construct models of continental growth and evolution. ", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/39254, title ="Comment and Reply on “S-type granites and their probable absence in southwestern North America”", author = "White, A. J. R. and Clemens, J. D.", journal = "Geology", volume = "14", number = "10", pages = "895-895", month = "October", year = "1986", issn = "0091-7613", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130708-154037504", note = "© 1986 Geological Society of America.\n\n", revision_no = "12", abstract = "The problem of muscovite in granites is also raised by Dickson.\nMuscovite is common in felsic S-type granites, as stated by Chappell and\nWhite (1974), but how much is primary and how much is secondary are\ndebatable. We currently think that very little is primary (above the solidus).\nWe are not arguing that muscovite (whether primary or secondary),\ngarnet, and andalusite are not found in S-types; we are arguing that these\nminerals may also be found in some I-types. Dickson is quite correct in\nsaying that replacement of cordierite by micas results from hydration,\nwhich may also affect the alkali/aluminum ratio of the rock. However,\nhe is not necessarily correct in his assertion that water must be introduced\ninto the system; it may be magmatic water released from the\nmagma near the solidus during cooling and crystallization. More systematic\ngoechemical and isotopic work is required on petrographically well-documented\ngranite suites before this question is resolved.\n", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/38439, title ="Comment and Reply on \"S-type granites and their probable absence in southwestern North America\"", author = "White, A. J. R. and Clemens, J. D.", journal = "Geology", volume = "14", number = "9", pages = "805-806", month = "September", year = "1986", issn = "0091-7613", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130513-090131521", note = "© 1986 Geological Society of America.\n", revision_no = "14", abstract = "Miller's Comment highlights the necessity of clarifying the use of the\nterm S-type granite. We here make some further points.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/38367, title ="Late Triassic paleogeography of the southern Cordillera: The problem of a source for voluminous volcanic detritus in the Chinle Formation of the Colorado Plateau region", author = "Stewart, John H. and Anderson, Thomas H.", journal = "Geology", volume = "14", number = "7", pages = "567-570", month = "July", year = "1986", issn = "0091-7613", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130508-152950143", note = "© 1986 Geological Society of America.\n\nManuscript received November 4, 1985;\nRevised manuscript received March 17, 1986;\nManuscript accepted March 28, 1986.\n\nWe thank N. J. Silberling and Paul Stone for their\nreviews of the manuscript and C. M. Conway for his\nhelp in collecting and describing the volcanic pebbles\nand cobbles from the Chinle Formation.", revision_no = "12", abstract = "The Upper Triassic Chinle Formation of the Colorado Plateau contains voluminous volcanic detritus evidently derived from a source to the south. Volcanic rocks exposed in southern Arizona and northern Sonora have been assumed to represent this source terrane, but U-Pb isotopic geochronology and regional stratigraphic correlations indicate that these volcanic rocks are distinctly younger than the Chinle, and thus not a source for the volcanic detritus in the Chinle. Igneous rocks of known or possible Late Triassic age in Nevada, California, or northeastern Mexico are possible sources, but a clearly defined source terrane for the volcanic detritus in the Chinle has not been identified. Tectonic removal of the source terrane by rifting or strike-slip offset, though not proven, is a possibility.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/39813, title ="S-type granites and their probable absence in southwestern North America", author = "White, A. J. R. and Clemens, J. D.", journal = "Geology", volume = "14", number = "2", pages = "115-118", month = "February", year = "1986", doi = "10.1130/0091-7613(1986)14<115:SGATPA>2.0.CO;2", issn = "0091-7613", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130808-083358812", note = "© 1986 Geological Society of America.\n\nManuscript received June 27, 1985;\nRevised manuscript received October 7, 1985;\nManuscript accepted October 29, 1985.\n\n", revision_no = "13", abstract = "Criteria that have been (and still are) used to characterize S-type granites of the Lachlan Fold Belt (LFB) of southeastern Australia are reviewed, and comparisons are made with various peraluminous granites of southwestern North America, some of which have been classified as S-types on the basis of insufficient data. \n\nVirtually all of the vast volume of S-type granites in the LFB are near-surface, batholithic granites that are commonly associated with S-type volcanics and are not associated with regional metamorphic rocks and migmatites. They are strongly peraluminous, as shown by the presence of cordierite. Granites with primary muscovite are rare. All are low in Na, Ca, and Sr as a result of chemical weathering during formation of the sedimentary sources. Peraluminous granites of various ages in southwestern North America are distinctly different. They rarely contain cordierite (a mineral characteristic of LFB S-types), but some are highly evolved such that Fe-Mn-rich garnet has crystallized. They are dominantly two-mica granites, indicating crystallization at higher water fugacities and greater depths than most peraluminous granites of the LFB. Cordierite-bearing volcanics (S-types) have not been reported. Sodium is generally high in the peraluminous granites of southwestern North America. Some of these rocks have trondhjemitic affinities; the parent magmas seem more likely to have been produced by partial melting of altered basaltic rocks. Locally, some peraluminous rocks (marginal to metaluminous types) may owe their compositions to high-level contamination of I-types; these are not S-type rocks. No compelling evidence has been presented that any of the peraluminous granites of southwestern North America are S-types. \n", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/42591, title ="Coupled Sr-O isotope variations as an indicator of source heterogeneity for the Northern Peninsular Ranges batholith", author = "Hill, R. I. and Silver, L. T.", journal = "Contributions to Mineralogy and Petrology", volume = "92", number = "3", pages = "351-361", month = "January", year = "1986", doi = "10.1007/BF00572164", issn = "0010-7999", url = "https://resolver.caltech.edu/CaltechAUTHORS:20131120-102544794", note = "© 1986 Springer-Verlag. \n\nReceived January 21, 1985; accepted October 28, 1985.\n\nFinancial support for field and laboratory work was provided by NSF Grants EAR 76-23153 and EAR 77-23507 to LTS and EAR 78-16874 to HPT; by DOE Contract DE-AC13-76 GJO 1664, Bendix Field Engineering Corp. subcontract No. 79-384-E to LTS; and by a grant from the Atlantic Richfield\nCorporation to L.T. Silver. Support for RIH was also provided by a Conoco Fellowship and by the Division of Geological and Planetary Sciences, California Institute of Technology. The California Dept. of Parks and Recreation kindly allowed sample collecting within Mt. San Jacinto State Park; the U.S. Forest Service and the Lake Hemet Municipal Water District allowed access to lands normally closed to public entry.", revision_no = "19", abstract = "Primary δ^(18)O values for tonalitic rocks from the San Jacinto Intrusive Complex range from +9.0 to +10.6; initial ^(87)Sr/^(86)Sr for the same samples varies from 0.7058 to 0.7076. Rocks with low δ^(18)O tend to have higher initial ^(87)Sr/^(86)Sr. The majority of samples have a limited range of δ^(18)O values (+10.0–+10.4) that is anticorrelated with colour index; this variation is compatible with crystallization of rocks with varying mineral abundances from an homogeneous (for oxygen) reservoir. More mafic rocks show a large range of δ^(18)O values, reflecting original variations in source material compositions. The combined Sr-O isotope data demonstrate that three or more distinct components were involved in the genesis of these rocks; these three components are also seen in batholithic rocks from the Mojave Block and Sierra Nevada to the north. These data are interpreted as indicating a 0–35% (oxygen atom basis) contribution to the San Jacinto rocks from an old continental lithosphere source.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61428, title ="The border connection; geological correlations and contrasts between Arizona and Sonora", author = "Anderson, Thomas H. and Silver, Leon T.", journal = "Arizona Geological Society Digest", volume = "16", pages = "72-73", month = "January", year = "1986", issn = "0066-7412", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151022-132313827", note = "© 1986 Arizona Geological Society.\n\nOur research in Sonora would not have been\npossible without the cooperation and support of\ncolleagues and institutions on both sides of the\nborder. Diego Cordoba and Jose Guerrero, past and\npresent Directors of the Instituto de Geologia de\nMexico have co-operated in our investigations for\nmany years. In Sonora, organizations such as the\nEstacion Regional del Noroeste del Instituto de\nGeologia directed by Jaime Roldan, the Consejo de\nRecursos Minerales and the Departamento de Geologia\nde la Universidad de Sonora have each contributed in\nsubstantive ways. Jack Stewart and Gordon Haxel\nkindly included Anderson in their recent research\nefforts. Exchanges with these geologists as well as\nwith many others including Claude Rangin, George\nDavis, Carlos Gonzalez and Guillermo Salas have been\nhelpful and informative. Anderson has been aided by\na willing and eager group of graduate students. We\nacknowledge National Science Foundation grants which\nhave financed many years of study.", revision_no = "9", abstract = "The same tectonostratigraphic evolution characterizes a broad belt of rocks centered along the border between Arizona and Sonora. Differences are evident in areas tens of kilometers north and south of the border region. No counterpart of the stable Colorado Plateau north of the border belt exists in Sonora. Terranes, south of the border belt, are separated by faults or less obvious discontinuities in structural, stratigraphic and isotopic patterns. Directions of movement on postulated faults are hotly debated. Locally, where terrane borders are strongly obscured by subsequent tectonism, volcanism, intrusion and/or sedimentation, they are almost mystical or mythical. Tests of the hypothesis that some of the terranes record hundreds of kilometers of left-lateral offset have been undertaken. Although the results are not\nprobative, they are very supportive.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44239, title ="Solidification and recharge of SiO_2-rich plutonic magma chambers", author = "Hill, R. I. and Silver, L. T.", journal = "Nature", volume = "313", number = "6004", pages = "643-646", month = "February", year = "1985", doi = "10.1038/313643a0", issn = "0028-0836", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140311-101114151", note = "© 1985 Nature Publishing Group. Received 17 September: accepted 27 December 1984. We acknowledge support from NSF Grants EAR 76-23153 and 77-23507 to L.T.S. and 78-16874 to H.P.T.; from DOE Contract DE-AC13-76 GJO 1664, Bendix Field Engineering Corp. subcontract 79-384-E, to L.T.S.; and from a Conoco Fellowship and the Division of Geological and Planetary Sciences, California Institute of Technology, for R.I. H. We thank R. S. J. Sparks for discussions.", revision_no = "14", abstract = "There exists substantial variation in initial strontium isotopic composition in rocks from three otherwise homogeneous tonalite plutons. We interpret disrupted dykes as conduits through which heterogeneous liquids were added periodically to the inflating magma chambers. Recharge buffered the physicochemical but not the strontium isotope properties of the crystallizing liquids.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/42410, title ="Rare earth element distributions among minerals in a granodiorite and their petrogenetic implications", author = "Gromet, L. Peter and Silver, Leon T.", journal = "Geochimica et Cosmochimica Acta", volume = "47", number = "5", pages = "925-939", month = "May", year = "1983", issn = "0016-7037", url = "https://resolver.caltech.edu/CaltechAUTHORS:20131113-090618069", note = "© 1983 Pergamon Press Ltd.\nReceived January 5, 1982; accepted in revised form February 3, 1983.\n\nThis work represents a portion of the\nsenior author’s doctoral dissertation done at the California\nInstitute of Technology under NSF grants EAR74-00155\nand EAR76-23150 and DOE grant EY-76-G-03-1305. The\nsenior author acknowledges the National Science Foundation\nfor providing financial support under a Graduate Traineeship\nand an Energy Related Graduate Fellowship. Preparation\nof this manuscript partially supported by NSF Grant\nEAR79-11184.\nWe thank Bruce Chappell for making available an unpublished\nchemical analysis. Reviews of an early draft by\nP. Hess and M. Rutherford and journal reviews by M. Loiselle,\nC. Miller and R. Price are appreciated.\nContribution 3719, Division of Geological and Planetary\nSciences, California Institute of Technology.", revision_no = "9", abstract = "A study of the distribution of lanthanide rare earths in a granodiorite from the eastern Peninsular Ranges batholith, southern California, reveals that a large fraction of the REE in this rock resides in the accessory phases sphene and allanite. The minerals plagioclase, alkali feldspar, biotite, epidote and apatite each contribute approximately 1% or less of each REE to the whole rock, with the exception of Eu for which plagioclase contributes 7%. Sphene and allanite together contain 80% to 95% of each REE. Each of these phases is zoned in REE concentration with substantial decreases from core to margin. Textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma. REE zoning trends in sphene and allanite, and unexpectedly low REE concentrations in largely later crystallizing minerals such as feldspar, indicate that the precipitation of sphene and allanite significantly reduced REE concentrations in residual melts. These results illustrate the potential that sphene and allanite have for controlling the behavior of REE in granitic magmas.\n\nAvailable information collectively suggest that the sampled granodiorite existed as a complete melt, that the REE contained in the assemblage of phases were derived by direct crystallization from the melt, and that the melt behaved essentially as a closed system once crystallization of the phases now present began. Close correspondences between the major and trace element chemistries of the granodiorite and phenocryst-poor lavas from similar tectonic settings support these conclusions. The REE pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism. The high liquidus temperature of a granodiorite melt (~ 1000°C) indicates the importance of mantle-derived components within the sources of batholithic magmas in the Peninsular Ranges.", } @book_section {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/92950, title ="Primary mineral distribution and secondary mobilization of uranium and thorium in radioactive granites", author = "Silver, Leon T. and Woodhead, James A.", journal = "Proceedings; uranium exploration methods; Nuclear Energy Agency and International Atomic Energy Agency R & D program.", pages = "355-367", month = "June", year = "1982", isbn = "978-92-64-02350-5", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190214-151508769", note = "© 1982 OECD.", revision_no = "12", abstract = "Radioactive granites in the southwestern United States tend to form distinct geochemical provinces, for which the resistant mineral zircon provides useful indices of primary igneous endowments. A large fraction of the uranium and thorium in these granites is contained in trace minerals, stoichiometric for the actinides (e.g. brannerite, coffinite, uranothorite), present at levels up to tens of ppm. Secondary mobilization of uranium in radioactive rocks seems most dependent on the geochemical stability of these rare phases, especially after their modification by radiation damage. U-Th-Pb isotope systematics provide several independent indications of the magnitude and timing of uranium transfers (1) within the granitic systems in what we believe are important preparation processes, and (2) to sites outside the granites including potential secondary uranium ore deposits.", } @book_section {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/88233, title ="Precambrian of the Conterminous United States", author = "Reed, John C., Jr. and Sims, Paul K.", number = "1", pages = "7-14", month = "January", year = "1982", doi = "10.1130/DNAG-SPEC-v1.7", isbn = "9780813752013", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180725-085833699", note = "© 1982 Geological Society of America.", revision_no = "9", abstract = "Precambrian rocks are at or near the surface in only about 10 percent of the conterminous United States, but it can reasonably be inferred that they comprise the continetal crust beneath about 90 percent. They are missing or unrecognized in the exotic terranes along the Pacific margin of North America, but they probably form significant parts of the crust in exotic terranes or continental fragments accreted to the eastern part of the continent during Paleozoic time. Thus, the total area of Precambrian rocks to be considered in this volume is comparable to that of the exposed Precambrian of the Canadian Shield. It is important to remember that in spite of the enormous lateral extent of the craton the volume of the continental crust is almost insignificant. The width of the North American craton is more than half the radius of the planet, but the thickness of the continental crust is less than one hundredth of the planetary radius (fig. 1). The volume of the crust is less than 2 percent of the volume of the mantle beneath the U.S. part of North America.\n\nPrecambrian rocks contain the only available record of the assembly and evolution of the fragile continental raft that we know as North America during more than four fifths of geologic time. Of the areas of exposed Precambrian rocks in the conterminous United States, about half have been covered by modern reconnaissance geologic mapping (scale 1:250,000 or larger); less than a quarter have been covered by detailed modern mapping (scale 1:62,500 or larger).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61374, title ="An overview of Precambrian rocks in Sonora", author = "Anderson, Thomas H. and Silver, Leon T.", journal = "Revista Mexicana de Ciencias Geológicas", volume = "5", number = "2", pages = "131-139", month = "January", year = "1981", issn = "0185-0962", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151021-110115541", note = "© 1981 Universidad Nacional Autónoma de Mexico. \n\nOur investigations have benefited from logistical support at times from the Instituto de Geologia, Universidad Nacional Autónoma de Mexico, Universidad de Sonora, Consejo de Recursos Naturales No Renovables, and Instituto Nacional de Energia Nuclear. Conversations with Jaime Roldán-Quintana, Guillermo A. Salas, Claude Rangin, Richard Merriam, and Jack Eells have contributed to our efforts to decipher the history of Precambrian rocks. Mineral separations by Jaime Alvarez, Rico Dagonel and O. Shields and invaluable efforts by Gerri Silver and Maria Pearson toward polishing off great batches of chemistry and mass spectrometry are most kindly acknowledged. This work was supported by NSF grants: GA-15989 and EAR 74-00155 A01 (formerly GA-40858) awarded to Caltech and EAR 76-84138 awarded to the University of Pittsburgh.", revision_no = "13", abstract = "The oldest stratified rocks recognized in NW Sonora (and in Mexico) are deformed muscovite-quartz schists, quartzites, and biotite-quartzofeldspathic gneisses near Caborca, which are cut by calcalkaline intrusives ranging from 1,710 to 1,750 m.y. in age. Southwest of Caborca, upper amphibolite facies layered quattzofeldspathic and amphibolitic gneisses were apparently deformed and metamorphosed at about 1,660 ± 15 m.y. ago, concealing original lithologies and ages. In northeastern Sonora, a younger belt of eugeosynolinal strata, about 1,680 ± 20 m. y. old was tightly\nfolded and metamorphosed to greenschist facies about 1,650 m.y. ago. Numerous granitic plutons intruded into the older Precambrian crust about 1,410 to 1,440 m.y. ago. These major intrusive masses are not known to have been accompanied by regional sedimentation or deformation. Rare, small plutons of micrographic granite added to the Precambrian crystalline complexes about 1,100 m.y. ago, are the youngest Precambrian igneous rocks recognized. They limit the age of a thick miogeoclinal sequence of unmetamorphosed quartzose sandstones, carbonates with numerous stromatolite horizons, and shales which rest nonconformably on them. The sequence is overlain without unconformity by a fossiliferous Lower Cambrian section. The northwestern and northeastern Precambrian suits appear to be separated by a Jurassic magmatic arc and a postulated shear structure of large lateral displacement. Both suites correlate northward into related belts in the SW United States. To the east they are concealed by Phanerozoic cover. Abrupt\ntermination of Precambrian exposures south and west suggests major younger tectonic features which we suspect played important but undefined roles in the apparent absence of Precambrian basement under much of northern and west-central Mexico.", } @misc {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/79553, title ="Uranium in granites from the Southwestern United States: actinide parent-daughter systems, sites and mobilization. First year report", author = "Silver, L. T. and Williams, I. S.", number = "GJBX-45(81)", month = "October", year = "1980", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170728-143702956", note = "Prepared for the U.S. Department of Energy, Assistant Secretary for Resource Applications, Grand Junction Office, Colorado, under Contract No. DE-AC13-76GJ01664, and Bendix Field Engineering Corporation Subcontract No. 79-384-E.", revision_no = "17", abstract = "Some of the principal findings of the study on the Lawler Peak Granite are: the granite is dated precisely by this work at 1411 ± 3 m.y., confirming its synchroneity with a great regional terrane of granites. Uranium is presently 8-10 times crustal abundance and thorium 2-3 times in this granite. Uranium is found to be enriched in at least eight, possibly ten, primary igneous mineral species over the whole-rock values. Individual mineral species show distinct levels in, and characteristics ranges of, uranium concentration. It appears that in a uraniferous granite such as this, conventional accuracy mineral suites probably cannot account for most of the uranium in the rock, and more rare, high U-concentration phases also are present and are significant uranium hosts. It appears that at least two different geological episodes have contributed to the disturbance of the U-Th-Pb isotope systems. Studies of various sites for transient dispersal of uranium, thorium, and radiogenic lead isotopes indicate a non-uniform dispersal of these components. It appears that the bulk rock has lost at least 24 percent of its original uranium endowment, accepting limited or no radiogenic lead or thorium migration from the sample.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/42395, title ="Lead-isotope inhomogeneity in Precambrian igneous K-feldspars", author = "Ludwig, Kenneth R. and Silver, Leon T.", journal = "Geochimica et Cosmochimica Acta", volume = "41", number = "10", pages = "1457-1471", month = "October", year = "1977", doi = "10.1016/0016-7037(77)90251-4", issn = "0016-7037", url = "https://resolver.caltech.edu/CaltechAUTHORS:20131112-134631506", note = "© 1977 Pergamon Press. Received 8 February 1977; accepted in revised form 21 June 1977. The study was part of a Ph.D. thesis by Ludwig, and was supported by N.S.F. grants GA 15989 and EAR 74-00155A01.", revision_no = "11", abstract = "Stepwise Pb-removal experiments, using both vacuum volatilization and HF-leaching techniques, on acid-washed K-feldspar concentrates from Precambrian igneous rocks show that all contain some unsupported radiogenic Pb. Two types of radiogenic Pb were recognized. One has a “normal” isotopic composition, with relative abundances of ^(206)Pb, ^(207)Pb and ^(208)Pb consistent with the age and U-Th contents of the rocks. The other type of unsupported radiogenic lead in the feldspars is apparently pure ^(206)Pb, derived from long-term migration and accumulation of radioactive daughter(s) of ^(238)U. This “pure ^(206)pb” lead occupies different sites from the “normal” radiogenic lead, and tends to show a release maximum during vacuum volatilization at about 1150°C.\nThe usefulness of stepwise vacuum volatilization may be limited by the tendency of a small amount of radiogenic lead to concentrate in the least volatile fraction. Stepwise partial HF attacks appear to give at least as good separation of radiogenic from original feldspar lead, and are recommended as a routine procedure for isotopic analyses of Precambrian feldspars. The fact that most of the five K-feldspars examined contained unsupported radiogenic lead implies that caution must be used in applying total-sample lead analyses of Precambrian feldspars to problems of lead-isotope evolution in crustal rocks.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/38504, title ="Geographic Variation of Rare Earth Fractionations in Plutonic Rocks Across the Peninsular Ranges Batholith, Southern California", author = "Gromet, L. Peter and Silver, Leon T.", journal = "Transactions - American Geophysical Union", volume = "58", number = "6", pages = "532", month = "January", year = "1977", issn = "0002-8606", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130514-135353772", note = "© 1977 American Geophysical Union.\n", revision_no = "15", abstract = "Rare earth element patterns of plutonic rocks (chondrite-\nnormolized) across the Peninsular Ranges batholith vary systematically west to east, transverse to the long axis and structural trends and parallel to known asymmetries\nin petrologic, geochronologic, and Sr isotopic properties. The abundant tonolites in the western region are characterized by slight light REE enrichment relative to heavy REE. Eastward, patterns become more highly\nfractionated, showing light REE enrichment and heavy\nREE depletion. The slope of patterns at any geographic\nlocation are largely independent of rock type. However,\nlocally, certain silica- saturated and undersaturated\ngabbros may show divergent trends. The relative fractionation among the middle and heavy REE indicates\ndifferentiation processes involving the observed major\nphases such as hornblende and plagioclase are not important\nin producing the basic geographic trends. They may\nbe reflected in more local lithologic variations. The\npronounced geographic zonation in REE character correlates\nstrongly with Sr concentration, suggesting a common\nmechanism for their origin. The systematic nature of\nthese fractionations, their correlation with previously\nreported Sr isotopic data, and their lock of correlation\nwith lithology, argues against significant upper-crustal\ncontamination with old cratonic components. These features appear to originate in heterogeneous deep-seated sources whose sampling has been selectively activated at a convergent plate boundary.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/38512, title ="Rubidium-Strontium Fractionation Domains in the Peninsular Ranges Batholith and Their Implications for Magmatic Arc Evolution", author = "Silver, Leon T. and Early, Thomas O.", journal = "Transactions - American Geophysical Union", volume = "58", number = "6", pages = "532", month = "January", year = "1977", issn = "0002-8606", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130515-071510683", note = "© 1977 American Geophysical Union.", revision_no = "10", abstract = "The northern 600 km of this Cretaceous batholith,\ncomprised of hundreds of diverse plutons, appears to\nconsist of 12-15 domains. Each domain is a region\nwhere rocks of all types (within our limited sampling)\nshow a regular linear covariation of Rb and Sr concentrations. This regularity displays different concentration levels and a different slope and is accompanied by a distinct set of isotopic systematics in each domain. The domains do not disrupt the remarkable zonation of initial Sr ratios in the batholith (Early and Silver, 1973). No independent field or petrographic recognition of the Rb-Sr characterized entities has been mode. Very similar rocks occur in adjacent domains. Domains are equant or elongate NNW, up to 200 km long and 30 km\nwide, parallel to regional tectonic grain. Each domain\nis interpreted as a region of magmas sampled or differentiated from a mantle source reservoir possessing characteristic trace element levels and Sr isotopic properties. An apparent age of reservoir formation and isolation prior to fractionation and crustal emplacement and on apparent initial Sr ratio at the time of reservoir isolation can be derived for each domain. There is no strong correlation between reservoir apparent ages and initial ratios. Mixing systems involving older granitic crust with primitive mantle seem precluded. We identify\nsimilar domains characterized by R-Sr systematics in\nother batholithic and volcanic complexes. We infer these domains to be fundamental loci of chemistry and energy from which magmatic arcs are constructed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/38529, title ="Thorium-Uranium fractionation as an indicator of petrogenetic processes", author = "Silver, Leon T.", journal = "Transactions - American Geophysical Union", volume = "57", number = "4", pages = "351", month = "January", year = "1976", issn = "0002-8606", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130515-142928635", note = "© 1976 American Geophysical Union. ", revision_no = "12", abstract = "A mean Th/U ratio, ~4, seems to characterize most\nterrestrial, lunar, and meteoritic igneous materials and\nmajor patterns of lead isotopic evolution develop principally in systems with Th/U about 3.7-4.0. Some\nimportant crustal igneous subsystems show systematic\ndeviations from these values. Such fractionation generally\nis attributed to the geochemical behavior of the large\nlithophile actinide ions in various enriched minor phases.\nThis probably is true in highly differentiated series with\nhigher Th and U levels (>8 and 2 ppm). A different\nfractionation mechanism may be more important in some\nlower concentration systems. Isotope dilution mass spectrometric studies of U and Th in diverse igneous feldspar separates reveal significant U and Th partitioning into them with drastic fractionation of Th/U (values 0.2- 3.0). Alkali feldspars in granitic rocks display partition coefficients (feldspar/total rock) of about 0.005-0.02 for U\nand 0.001-0.004 for Th; Th/U~0.5-3.0. In plagioclase\nin gabbros, tonalites, and granodiorites with lower Th and\nU, the coefficients appear larger (U~0.1-0.4, Th~0.04-0.2) but Th/U appears lower (Th/U~0.2-2.0) than in K-feldspar. Limited data suggests some pyraxenes and other major minerals may also fractionate Th/U to lower ratios. Differences in ionic radius and uranium oxidation states may contribute to the undefined fractionation mechanism.\nProcesses of primary differentiation (fractional\ncrystallization, partial melting) involving gabbroic systems\nseem to reflect this fractionation. Oceanic tholeiites, end\nmassive gabbroic complexes display generally low Th/U\nvalues. Their lead isotopes reflect reservoirs with more\nnormal Th/U ratios. This suggests limits on the number of\nfractionation cycles, mantle mixing, and/or reservoir dimensions for primary basaltic systems. Th and U and their\nassociated lead isotope systems can be used with lanthanide\nR.E.E. to assist in developmet of petrogenetic models.", } @misc {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61576, title ="Skylab-4 visual observations project: Geological features of southwestern North America", author = "Silver, Leon T. and Anderson, T. H.", number = "2694", month = "November", year = "1975", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151027-140622202", note = "NASA Technical Report NASA-CR-147392.\n\nN76 -15537.\n(NASA-CR-147392 ) SKYLAB-4 VISUAL OBSERVATIONS PROJECT: GEOLOGICAL FEATURES OF SOUTHWESTERN NORTH AMERICA. Final Report\n(Cal1fornia lnst. of Tech .) 189 p. \nCSCL 0 8G G3/43.\nUnclas 08458.\n\nThe Skylab 4 mission started on November 16, 1973.", revision_no = "18", abstract = "Visual observations conducted by Skylab-4 crewmen on seven designated geological target areas and other targets of opportunity in parts of southwestern United States and northwestern Mexico were described. The experiments were designed to learn how effectively geologic features could be observed from orbit and what research information could be obtained from the observations when supported by ground studies. For the limited preparation they received, the crewmen demonstrated exceptional observational ability and produced outstanding photographic studies. They also formulated cogent opinions on how to improve future observational and photo-documentation techniques. From the photographs and other observations, it was possible to obtain significant research contributions to on-going field investigations. These contributions were integrated into other aspects of the ground investigations to the following topics: major faults, regional stratigraphy, occurrence of Precambrian crystalline rocks, mapping of Mesozoic volcanic rocks, regional geology.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61372, title ="Potassic Granophyre Associated with Precambrian Diabase, Sierra Ancha, Central Arizona", author = "Smith, Douglas and Silver, Leon T.", journal = "Geological Society of America Bulletin", volume = "86", number = "4", pages = "503", month = "April", year = "1975", doi = "10.1130/0016-7606(1975)86<503:PGAWPD>2.0.CO;2 ", issn = "0016-7606", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151021-103915490", note = "© 1975 Geological Society of America. \n\nReceived March 4 1974; Revised manuscript received September 16, 1974. \n\nMuch of the data presented here was gathered for a Ph.D. thesis by Smith at the California Institute of Technology. Work there was supported by National Science Foundation Grants GA-287 and GA-1324, with additional support from U.S. Atomic Energy Commission contract AT(04-3)-427, Calt-767-56 . Andrew Shride provided valuable information on the geology of the area. Gordon Gastil participated in the initial discovery of the granophyre masses of Reynolds Creek. We appreciate careful reviews of the manuscript by D. S. Barker, G. M. Boone, and W. Hamilton.", revision_no = "10", abstract = "Lenses of granitic and syenitic granophyre unusually rich in potassium crop out at and near the roof of a Precambrian sill complex of olivine diabase intruded into the Apache Group. The lenses locally are 45 m thick; the diabase is 210 to 240 m thick in the same area. Granophyre contains 7 to 11 wt percent K2O, whereas intruded rocks of the Dripping Spring Quartzite contain 9 to 14 wt percent K_2O. The sedimentary rocks were enriched in potassium before emplacement of diabase, probably during diagenesis while saturated with saline water. \n\nThe granophyre apparently was mostly liquid when emplaced. Chemical data, together with arguments based on mineralogy, textures, and field relations, indicate that most of the granophyre was derived from Dripping Spring Quartzite. The granophyre, however, contains more Na, Ca, and P and less K than the sedimentary rocks. Rocks representing a differentiated fraction of diabase magma are concentrated below the granophyre at the roof of the complex. The unusual crystallization profile may reflect an influx of water from the overlying sedimentary rocks into the magma. The influx of water may have been responsible for the extensive interaction between magma and country rock. Country rock fused during the interaction appears to have migrated to local structural highs. \n\nBecause the evidence that the granophyre formed largely from country-rock material is unusually clearcut, this granophyre may serve as a petrogenic model for some other silica-rich igneous rocks associated with intrusions of basaltic magma.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/59052, title ="Apollo 16 Exploration of Descartes: A Geologic Summary", author = "Ulrich, G. E. and Rennilson, J. J.", journal = "Science", volume = "179", number = "4068", pages = "62-69", month = "January", year = "1973", doi = "10.1126/science.179.4068.62", issn = "0036-8075", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150729-094139122", note = "© 1973 American Association for the Advancement of Science.\n\nReceived 6 September 1972; revised 17 October 1972.\n\nWork done under NASA contract T-5874A. Publication authorized by the director, U.S. Geological Survey.", revision_no = "11", abstract = "The Cayley Plains at the Apollo 16 landing site consist of crudely stratified breccias to a depth of at least 200 meters, overlain by a regolith 10 to 15 meters thick. Samples, photographs, and observations by the astronauts indicate that most of the rocks are impact breccias derived from an anorthosite-gabbro complex. The least brecciated members of the suite include coarse-grained anorthosite and finer-grained, more mafic rocks, some with igneous and some with metamorphic textures. Much of the traverse area is covered by ejecta from North Ray and South Ray craters, but the abundance of rock fragments increases to the south toward the younger South Ray crater. The Descartes highlands, a distinct morphologic entity, differ from the adjacent Cayley formation more in physiographic expression than in lithologic character.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/51593, title ="A crustal-upper-mantle model for the Colorado Plateau based on observations of crystalline rock fragments in the Moses Rock Dike", author = "McGetchin, Thomas R. and Silver, Leon T.", journal = "Journal of Geophysical Research", volume = "77", number = "35", pages = "7022-7037", month = "December", year = "1972", doi = "10.1029/JB077i035p07022", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141111-135550881", note = "Copyright 1972 by the American Geophysical Union. \n\n(Received February 25, 1972; revised August 8, 1972.) \n\nWe are grateful for reviews by E. Wolfe and D. Stuart-Alexander of the U.S. Geological Survey, and Herwart Helmstaedt of McGill University, and for helpful discussions with many colleagues. In particular, we thank E. M. Shoemaker for advice, stimulation, and support over many years. \n\nThis report summarizes research carried on from 1965 to the present. It was supported by the National Aeronautics and Space Administration through grants to M.I.T. (NGR-22-009-637) and to the U.S. Geological Survey's Center for Astrogeology (R-66), by the Geological Society of America (Penrose grant 1040-65), by the Atomic Energy Commission under contract to the California Institute of Technology (AT (04-3)-767, CALT-767P7-61), by the U.S. Air Force (under faculty research project AFIT-68-14, Air Force Institute of Technology and AFIT-OAR contract F-33601-69-C-0568), and by the National Science Foundation (GA-31728). \n\nThe senior author held a NASA graduate traineeship at Cal Tech from 1965 to 1967 at the inception of the project; from 1967 to 1969, he was an active duty captain in the U.S. Air Force assigned as assistant professor in the School of Engineering, AFIT, Wright-Patterson Air Force Base, Ohio; he is currently assistant professor, Department of Earth and Planetary Sciences, M.I.T. \n\nDivision of Geological and Planetary Sciences, California Institute of Technology contribution 1919.", revision_no = "11", abstract = "On the basis of the size, the abundance, and the petrographic character of xenoliths in the Moses Rock dike, a model for the vertical stratigraphy of crystalline rocks beneath the dike is proposed extending from near the surface to a depth of about 200 km. Sedimentary clasts, whose original position in the undisturbed vent walls is known but which are now within the intrusive breccia of the Moses Rock dike, show a decrease in size with distance of upward transport from their original position in the vent walls. This inverse relationship between fragment size and known depth of origin provides an empirical basis for a reconstructed model for the distribution of rocks on the basis of the particle size of fragments in the intrusive breccia. Metabasalt, granite, and granite gneiss are abundant in the upper part of the crust along the dike walls; diorite, gabbro, and amphibole schists of basic composition constitute intermediate layers, and garnet-bearing metagabbro (basic granulite gneiss) and serpentine schist are present in the lower crust. The crustal rock suite is predominantly metavolcanic and metaplutonic and basic in composition. Dense ultramafic rocks, possibly derived from the mantle, constitute about 0.3% of the breccia filling the dike and include jadeite-rich clinopyroxenite, eclogite, spinel-websterite, spinel-lherzolite, and garnet-lherzolite. The M discontinuity appraently occurs within a petrologically complex region and may coincide with phase and compositional transitions, which include hydration. A compositional transition within the upper mantle between spinel- and garnet-peridotite (lherzolite) is inferred. The variety and the abundance of ultramafic and dense types, together with the complexity of their textures, suggest that the mantle may be as complicated as the crust in composition and history.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/59063, title ="Geologic Setting of the Apollo 15 Samples", author = "Swann, G. A. and Silver, L. T.", journal = "Science", volume = "175", number = "4020", pages = "407-415", month = "January", year = "1972", doi = "10.1126/science.175.4020.407", issn = "0036-8075", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150729-123808643", note = "© 1972 American Association for the Advancement of Science.\n\nReceived 9 November 1971.\n\nWork done under NASA contract T-65253-G. Published by permission of the director, U.S. Geological Survey. \n\nThis paper is based on the report submitted by the Apollo Lunar Geology Investigation Team to the National Aeronautics and Space Administration for inclusion in the Preliminary Science Report for Apollo 15. Contributors to the report are G. A. Swann, N. G. Bailey, R. M. Batson, V. L. Freeman, M. H. Hait,\nH. E. Holt, K. B. Larson, V. S. Reed, G. G. Schaber, R. L. Sutton, and E. W. Wolfe\n(U.S. Geological Survey, Flagstaff, Arizona); K. A. Howard and H. G. Wilshire (U.S. Geological Survey, Menlo Park, California); J. W. Head (Bellcomm, Inc., Washington, D.C.); J. B. Irwin and D. R. Scott (NASA Manned Spacecraft Center, Houston, Texas); W. R. Muehlberger (University of Texas, Austin); and L. T. Silver and J. J. Rennilson (California Institute of Technology, Pasadena). Information on rock types was supplied by the Lunar Sample Preliminary Examination Team [see Science 175, 363 (1972)]. The authors assume full responsibility for the categories of rock types as presented here and for all\ninterpretations concerning the samples.", revision_no = "12", abstract = "The samples and photographs returned from the Apollo 15 site show that Hadley Delta is largely underlain by breccias whose clasts are mainly fragments of coarse-grained feldspathic rocks and nonmare-type basalt. Conspicuous sets of lineaments, visible in surface and orbital photographs of Mount Hadley and Hadley Delta, may represent systematic layering or fracture sets. The mare surface, with regolith about 5 meters thick, is underlain by two major basalt types, at least one of which has extensive lateral continuity and is exposed in the upper wall of Hadley Rille. Gradual erosional recession of the edges and filing of the interior of the rille by talus have contributed to the present cross sectional profile.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/62047, title ="The Apollo 15 Lunar Samples: A Preliminary Description", author = "Gast, P. W. and Phinney, W. C.", journal = "Science", volume = "175", number = "4020", pages = "363-375", month = "January", year = "1972", doi = "10.1126/science.175.4020.363", issn = "0036-8075", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151111-080003517", note = "© 1972 American Association for the Advancement of Science.", revision_no = "11", abstract = "Samples returned from the Apollo 15 site consist of mare basalts and breccias with a variety of premare igneous rocks. The mare basalts are from at least two different lava flows. The bulk chemical compositions and textures of these rocks confirm the previous conclusion that the lunar maria consist of a series of extrusive volcanic rocks that are rich in iron and poor in sodium. The breccias contain abundant clasts of anorthositic fragments along with clasts of basaltic rocks much richer in plagioclase than the mare basalts. These two rock types also occur as common components in soil samples from this site. The rocks and soils from both the front and mare region exhibit a variety of shock characteristics that can best be ascribed to ray material from the craters Aristillus or Autolycus.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61410, title ="Observaciones geochronologicas sobre los complejos cristalinos de Sonora y Oaxaca, Mexico", author = "Anderson, Thomas H. and Silver, Leon T.", journal = "Memoria - Sociedad Geologica Mexicana", pages = "115-122", month = "January", year = "1972", issn = "0188-2686", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151022-084358082", note = "© 1972 Sociedad Geológica Mexicana.", revision_no = "9", abstract = "Reconnaissance geologic mapping and careful study of lead-uranium isotope relationships in zircons from crystalline rocks have resulted in the identification of Precambrian basement from the Mexico-U. S. border, southeast to the known localities near Caborca and from there to Rt. 15 north of Hermosillo. Recent studies have corroborated the presence of Precambrian in Oaxaca. No Precambrian has been identified between Sonora and Oaxaca, but almost no work has been done in these intervening areas.", } @book_section {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61706, title ="Structure and Petrology of the San Gabriel Anorthosite-Syenite Body, California", author = "Carter, Bruce and Silver, Leon T.", pages = "303-311", month = "January", year = "1972", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151029-075342154", note = "© 1972 Micropaleontology Press. \n\nThe authors have profited from discussions in the field with L. R. Wager, D. H. Lindsley and R. B. Hargraves. This work was supported by National Science Foundation Grant No. GA-15989 and by Project Agreement #7 under Atomic Energy Commission Contract No. AT(04-3)-767, CALT-767P7-72.", revision_no = "10", abstract = "The San Gabriel anorthosite \"massif\" is part of a large layered intrusive originally ≥ 10 km in thickness, part of which covers about 250 km^2 in the western San Gabriel Mountains between the San Gabriel and San Andreas fault zones 30 km north of Los Angeles. Although not subjected to post-emplacement regional metamorphism, the Precambian anorthosite is intruded by granitic rocks of Permo-Triassic and late Cretaceous ages, and is deformed by broad folds of at least two ages (Permo-Triassic (?) and mid-Cenozoic), which have produced several km of structural relief within the body. Complex Cenozoic faulting (normal and strike-slip) of several ages has strongly deformed the anorthosite, which is tectonically floored by a thick zone of mylonitized gneiss along which important post-late Cretaceous (?) thrust movement probably occurred, and at least the western part of which is underlain by a major thrust fault, as shown by the distribution of the main shock and aftershocks of the February 9, 1971 San Fernando earthquake.\nAbundant textural and structural evidence suggests that bottom crystal accumulation has produced the classic anorthosite-gabbro-syenite differentiation suite (andesine anorthosite-leuconorite-norite-jotunite-mangerite-syenite-quartz syenite) making up this body. All the rocks of this suite appear to be relatively iron-enriched; extensive deuteric uralitization of pyroxene in all but the last intruded rocks suggests that the magma probably was also relatively water-rich. In the lower part of the intrusion, extensive \"annealing\" recrystallization, especially of anorthosite, has strongly modified the original fabric of the rock. Some rocks near the top of the body (jotunite and syenite) contain small amounts of pigeonite, both in the cores of strongly zoned pigeonite-augite crystals and as uninverted remnants in hypersthene inverted from original pigeonite, suggesting a relatively shallow depth of crystallization.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/48443, title ="Preliminary seismological and geological studies of the San Fernando, California, earthquake of February 9 1971", author = "Abrams, M. and Allen, C.", journal = "Bulletin of the Seismological Society of America", volume = "61", number = "2", pages = "491-495", month = "April", year = "1971", issn = "0037-1106", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140812-151015768", note = "Copyright © 1971, by the Seismological Society of America.\n\nMany aspects of this study have been made possible by contributions over the years by the\nCaltech Earthquake Research Affiliates, a group of private sponsors whose help is particularly\nappreciated at times such as this. Postearthquake aerial photographic coverage was kindly supplied\nby the U.S. Geological Survey and Woodward, Lundgren and Associates. We also appreciate\nthe generosity and the spirit of free communication that has typified all the groups and individuals\ninvestigating this earthquake.", revision_no = "13", abstract = "The San Fernando earthquake was the largest earthquake to occur in the metropolitan Los\nAngeles area in more than 50 years. It has tentatively been assigned a magnitude, M_L of 6.6, a focal\ndepth of 13.0 km, and an epicentral location about 12 km east of Newhall, California, at 34°24.0'N,\n118°23.7'W (Figure 1), but these figures undoubtedly will be modified as further data become available.\nAlthough the focal depth is not as well defined as the epicenter, it is consistent with other\nobservations suggesting thrusting on a fault plane dipping north about 45 ° and breaking the surface\nin the Sylmar-San Fernando area (Figure 1). It should be emphasized that the hypocenter of\nthe main shock represents only the point of initial rupture. Breaking, presumably, then propagated\nsouthward and upward from this point, so that the main geological and engineering effects\nwere observed farther south where the fault was shallower and the displacement greater. The location\nof the main shock is based on readings from permanent stations of the Caltech network, as\nwell as the U. S. Geological Survey station at Point Mugu (SBLG) and the California Department\nof Water Resources stations at Pyramid (PYR) and Cedar Springs (CSP). Portable Caltech seismographs\nwere installed in the epicentral area as early as 3 hr following the main shock, and,\nwithin a few days, there were at least 30 portable units in the region operated by various groups\nand agencies.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61418, title ="Compositional relations in minerals from kimberlite and related rocks in the Moses Rock dike, San Juan County, Utah", author = "McGetchin, Thomas R. and Silver, Leon T.", journal = "American Mineralogist", volume = "55", number = "9-10", pages = "1738-1771", month = "September", year = "1970", issn = "0003-004X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151022-112739977", note = "© 1970 Mineralogical Society of America.\n\nElectron microprobe data were obtained with the able assistance of A. A. Chodos. Drs. A. E. Bence, R. S. Naylor and A. L. Albee provided valuable advice and Mrs. Lily Ray assisted in the computer reduction of the microprobe data. We have benefited greatly by discussions with staff and students at the California Institute of Technology and members of the U.S. Geological Survey in Flagstaff, Menlo Park and Washington, D. C. An early version of the manuscript was completed while the senior author was a visitor at the Australian National University, Canberra. Hospitality of Drs. John Lovering, David Green and A. E. Ringwood is gratefully acknowledged. W. G. Ernst, L. S. Hollister and K. D. Watson, U.C.L.A., and Howard Wilshire, U.S.G.S., Menlo Park, California, made criticisms which greatly improved an early version of the manuscript. \n\nThis report summarizes part of a Ph.D. thesis at the California Institute of Technology. Special thanks are due to Dr. E. M. Shoemaker for orignally suggesting the Moses Rock dike as a thesis topic. Financial support for this research was provided by a Penrose Grant by the Geological Society of America (No. 1040-65), the Atomic Energy Commission Contract AT (04-3)-427, CALT-427-52, and U.S. Geological Survey (Branch of Astrogeology, Flagstaff, Arizona, under contract R-66 from NASA, and Analytical Branch, Denver, Colorado). The senior author had a National Aeronautics and Space Administration Graduate Traineeship from 1965-1967. From 1967 to 1969, he was an active duty Captain, U.S. Air Force, assigned as Assistant Professor in the School of Engineering, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio. Funds from AFIT-OAR contract number F-33601-69-C-0568 were used in completion of the research and report. An earlier version appeared as an AFIT Technical Report (AFIT TR 69-12) Publication authorized by the director, U. S. Geological Survey.", revision_no = "10", abstract = "The Moses Rock dike, a well-exposed, kimberlite-bearing breccia intrusion, crops out in gently dipping beds of the Permian Cutler Formation, in eastern Monument Valley, Utah. \n\nPetrographic, bulk chemical, and electron microprobe analyses of kimberlite and its constituent minerals reveal this highly serpentinized microbreccia contains a primary mineral assemblage consisting of olivine (Mg/Mg+ Fe), 87 to 93), orthopyroxene and clinopyroxene (falling into two compositional ranges after correction for Na-pyroxene molecules-one with Al_2O_3 between 0.5 and 1 percent another, 2 to 5 percent), spinel, chrome-rich pyrope garnet, ilmenite-geikielite, titanoclinohumite and one or more micas. Diamonds are not known. \n\nWe conclude (1) mineral grains in kimberlite are unlike associated dense rock fragments, except rare lherzolite: (2) kimberlite was emplaced as discrete angular mineral clasts, not a silicate melt; (3) P-T assignments based on clinopyroxenes compositions suggest derivation over a depth range in the upper mantle extending to 150 km or more, at temperatures near or below the experimentally determined garnet-lherzolite solidus: (4) the kimberlite was derived by physical disaggregation of both Al-poor and Al-rich pyroxene bearing peridotite in the mantle (garnet- and spinel-lherzolite, respectively); (5) titanoclinohumite is present in both assemblages and may be an important mineralogical site for volatiles in the upper mantle: (6) dense rock fragments (except lherzolite) are unrelated to the kimberlite and are chunks of the vent wall from the crust and possibly the upper mantle sampled during the eruption.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/54896, title ="Uranium-Thorium-Lead Isotope Relations in Lunar Materials", author = "Silver, Leon T.", journal = "Science", volume = "167", number = "3918", pages = "468-471", month = "January", year = "1970", doi = "10.1126/science.167.3918.468", issn = "0036-8075", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150217-155839306", note = "© 1970 American Association for the Advancement of Science.\n\nReceived 4 January 1970.\n\nEssential to this work was the dedicated laboratory participation and cheer of Geraldine Baenteli and Dolly Pearson. E. V. Nenow and C. Bauman provided important technical support. E. Brown wrestled untiringly with NASA, which supported the major part of this research under contract NAS 9-7963. California Institute of Technology Contribution No. 1707.", revision_no = "11", abstract = "The lead isotopic compositions and uranium, thorium, and lead concentrations have been measured on six samples of material from the Sea of Tranquillity. The leads are moderately to very radiogenic; the initial lead concentrations are very low; the uranium and thorium levels are 0.26 to 0.88 and 0.87 to 3.35 parts per million, respectively. The Th/U ratios cluster about a 3.6 value. Apparent ages calculated for four rocks are 4.1 to 4.2 x 10^9 years. Dust and breccia yield apparent ages of 4.60 to 4.63 x 10^9 years. The uranium-lead ages are concordant, or nearly so, in all cases. The lunar surface is an ancient region with an extended record of events in the early history of the solar system. The discrepancy between the rock ages and dust ages poses a fundamental question about rock genesis on the moon.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/51595, title ="Titanoclinohumite: A possible mineralogical site for water in the upper mantle", author = "McGetchin, T. R. and Silver, L. T.", journal = "Journal of Geophysical Research", volume = "75", number = "2", pages = "255-259", month = "January", year = "1970", doi = "10.1029/JB075i002p00255", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141111-141125063", note = "Copyright 1970 by the American Geophysical Union. \n\n(Received July 11, 1969.) \n\nMicroprobe data were reduced with the help of Mrs. Lily Ray, with a computer program by A. L. Albee and A. E. Bence. The help of these individuals is gratefully acknowledged. \n\nThis research was supported by a Penrose grant (1940-65) from the Geological Society of America and the U.S. Atomic Energy Commission, contract AT(04-3)-427, CALT-427-40. \n\nContribution 1647, Division of Geological Sciences, California Institute of Technology.", revision_no = "11", abstract = "Titanium-rich clinohumite and layered structure minerals are observed in kimberlite and as inclusions in pyropic garnets from the Moses Rock dike, a kimberlite-bearing breccia dike in San Juan County, Utah. Associated clinopyroxenes observed as inclusions within similar pyropes and also in kimberlite are estimated to have equilibrated at depths ranging from about 50 to 150 km at modest temperatures, generally less than 1000°C. The presence of titanoclinohumite, a high-density hydrous phase, is of considerable interest as a possible site for volatiles in the earth's upper mantle. The dehydration of hydrous phases such as titanoclinohumite within the upper mantle (1) may provide water as a free phase, (2) could be important in the genesis of kimberlite and alkali-basalt magma, and (3) may be one means of producing a low-velocity zone in the upper mantle.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61403, title ="A geochronologic investigation of the anorthosite complex, Adirondack mountains, New York", author = "Silver, Leon T.", journal = "Memoir - New York State Museum and Science Service", pages = "233-251", month = "January", year = "1968", issn = "0548-8265", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151022-082640004", note = "© 1968 New York State Museum.\n\nThe author is indebted to numerous individuals for geologic guidance and assistance in the field aspects of this work, particularly Matt Walton in the eastern Adirondacks, Dirk de Waard and W. D. Romey in the central highlands, the late Brian Davis in the northern highlands, Robert Hargraves in the northwest highlands, A. E. J. Engel in the northwest lowlands, and Y. W. Isachsen throughout the entire Adirondack Mountains. For helping the writer focus on the problems of the age and origin of the anorthosite massif, each of these individuals and especially my colleague, H. P. Taylor, Jr., together with the writings of A. F. Buddington and R. Balk, deserve commendation without the onus of responsibility. Professor Donald Potter and enthusiastic students from Hamilton College cheerfully helped the author move mountains of rock, as did the New York State Geological Survey. Joseph Mayer, Ollie Shields, and Mrs. Cathy Savin helped with the laborious mineral separations at various stages, and Mrs. Geraldine Baenteli was a mainstay in the chemistry and mass spectrometry throughout this research. This work was supported by NSF grant\nNo. GP 931, and AEC research contract AT (04-3)-427 for which this is publication CALT 427-17.", revision_no = "10", abstract = "Cogenetic uranium-lead isotope systems in zircons have been analyzed for five rock samples from the anorthositic domes of the Adirondack highlands and compared with data from a cogenetic suite in a charnockitic gneiss from the Ticonderoga dome in the eastern Adirondacks. Textural and structural arguments applied to the zircons and rocks from the anorthosite suite indicate that some of the zircon are metamorphic and reveal that conditions producing granulite facies metamorphism exited in different parts of the anorthosite complex at discretely different times during the interval from 1020 to 1100 m.y. ago. The Ticonderoga gneiss sample yields an interpreted zircon age of 1130 ± 10 m.y. Zircon characteristics argue this to be the age of magmatic crystallization of the host rock. Extensive studies have confirmed this age as the prevailing age in many large bodies of charnockite (or syenite) gneiss which are peripheral to the anorthosite domes. Petrological arguments\nof persistent spatial association and compositional gradation between the anort11osite and a suite ranging from norite to mangerite, syenite, quartz syenite and granite, imply that all of these rock are comagmatic; this leads to\nthe conclusion that the age of the anorthosite itself is 1130 m.y. No evidence has been found for any older ages in any of the Adirondack orthogneisses to support the hypothesis of a pre-\"Grenville\" basement. The mounting evidence for almost precise age contemporaneity between the Adirondack anorthosite-syenite complex and the Duluth anorthositic gabbro-granophyre complex should provoke a close comparative petrologic and structural scrutiny of these two great stratiform masses.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61704, title ="Pre-Cretaceous basement rocks and their bearing on large-scale displacements in the San Andreas fault system", author = "Silver, Leon T.", journal = "Stanford University Publications. Geological Sciences", volume = "11", pages = "279-280", month = "January", year = "1968", issn = "0081-4350", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151029-074635750", note = "© 1968 Stanford University.", revision_no = "9", abstract = "Investigations of the history of the pre-Cretaceous crystalline rocks of the Transverse Ranges have indicated a complex history from Permo-Triassic times back to 1700 million years or more. Some of the major historical elements are known to be shared with much larger regions in the southwest; others appear unique to the area, at this time. The close juxtaposition to each other and to the San Andreas fault zone of diverse plutonic and metamorphic rocks of widely different ages provides important possibilities for contributing to the nature and history of displacement on the zone. The most effective evaluation will be possible when the crystalline rocks close to the San Andreas fault can be correctly placed into the regional \"basement\" patterns and history. Unfortunately, the necessary field and laboratory investigations are proceeding very slowly.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61373, title ="Petrology of eucrites, howardites and mesosiderites", author = "Duke, Michael B. and Silver, Leon T.", journal = "Geochimica et Cosmochimica Acta", volume = "31", number = "10", pages = "1637-1665", month = "October", year = "1967", doi = "10.1016/0016-7037(67)90112-3 ", issn = "0016-7037", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151021-104842616", note = "© 1967 Pergamon Press Ltd. \n\nReceived 8 May 1967; accepted in revised form 13 June 1967. \n\nWe wish to acknowledge the kind co-operation of the following people who\ngenerously supplied specimens and thin sections for this work: M. H. HEY; C. B. MOORE; W. V. ENGELHARDT; B. MASON; E. P. HENDERSON; J. F. LOVERING; P. W. GAST; E. ANDERS; K. KEIL; K. TUREKIAN; E. OLSEN. A. CHODOS, A. D. MAYNES and E. BINGHAM, staff analysts of the Division of Geological Sciences, California Institute of Technology, were extremely helpful in discussing analytical techniques. R. V. HUENE made the excellent thin and polished thin-sections and assisted on several problems. Professor HARRISON BROWN gave his continuing interest and co-operation to the project and provided the analysis of the Pasamonte meteorite which had been made by Dr. MAYNES. E. M. SHOEMAKER provided stimulating ideas and discussion in regard to the postmagmatic brecciation of the meteorites. Financial support for materials, chemical analyses and photographic equipment and materials was provided from the research program of L. T. SILVER under the U.S. Atomic Energy Commission contract At(04-3)-427. Travel funds were provided from National Aeronautics and Space Administration contract NSG 56-60. A grant from the Geological Society of America covered the cost of several chemical analyses. The senior author had a National Science Foundation Graduate Fellowship from 1960-1962. A critical review by T. L. WRIGHT, U.S. Geological Survey, is gratefully acknowledged.", revision_no = "10", abstract = "The eucrite and howardite calcium-rich achondrites and many mesosiderites are considered as a coherent meteorite assemblage, their silicates consisting essentially of calciumpoor monoclinic and orthorhombic pyroxenes and calcium-rich plagioclase feldspar. The achondrites can be grouped according to their brecciated structure as follows: eucrites—unbrecciated and monomict brecciated achondrites; howardites—polymict brecciated achondrites. Many mesosiderites contain brecciated structures; they are distinguished from the achondrites by their large metallic fraction. The structure and composition of rock fragments in the breccias indicate a complicated sequence of events including magmatic differentiation, brecciation, recrystallization and refragmentation, and ejection from the parent body. Detailed mineralogical and chemical data suggest that the magmatic differentiation proceeded primarily by the separation of pyroxene from an ultrabasic parent material that had a much lower alkali content than ordinary chondrites. Magmatic crystallization took place in environments ranging from extrusive to deep-seated intrusive. Polymict breccias contain fragments with a wide variety of magmatic and recrystallization textures, which suggests that the breccias were formed either in very large or repeated fragmentation events. Monomict breccias contain fragments with a small range of similar magmatic textures, which suggests that these breccias were formed by small or single events. Petrographic evidence suggests that many of the breccias are impact breccias. Either in their original magmatic crystallization sites or in the sites of breccia accumulation, most of these meteorites apparently had a near-surface location prior to ejection from the parent body.\n\nEvidence obtained from eucrites, howardites and mesosiderites forms an important part of our understanding of the early evolution of the surface regions of their parent body. Chemical and oxidation conditions were different from those presently found in the Earth's crust and upper mantle, but the necessary conditions may have been present in the early history of the Earth. A lunar origin for eucrites, howardites and mesosiderites is proposed, but an asteroidal origin can not be presently excluded.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61371, title ="Uranium and Lead Isotopic Stability in a Metamict Zircon under Experimental Hydrothermal Conditions", author = "Pidgeon, R. T. and O'Neil, J. R.", journal = "Science", volume = "154", number = "3756", pages = "1538-1540", month = "December", year = "1966", doi = "10.1126/science.154.3756.1538", issn = "0036-8075", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151021-101038760", note = "© 1966 American Association for the Advancement of Science. \n\nReceived 11 July 1966. \n\nSupport for this work is derived from AEC research contract AT(04-3)-427 (Calt.-427-5); from NSF grant GP 5486; and from funds provided by the Division of Geological Sciences, California Institute of Technology (contr. No. 1416).", revision_no = "11", abstract = "Disturbance of the uranium-lead isotopic system in a metamict Ceylon zircon has been produced in a 2 molal NaCI solution at 500°C and 1000 bars fluid pressure. Loss of radiogenic lead to the extent of 61 percent in 13 days was the most significant effect. The experimental results support the episodic rather than continuous lead-loss interpretation of natural zircon systems utilized in geochronology.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/51597, title ="Evaluation of the decay constant of uranium 235 from lead isotope ratios", author = "Banks, Philip O. and Silver, Leon T.", journal = "Journal of Geophysical Research", volume = "71", number = "16", pages = "4037-4046", month = "August", year = "1966", doi = "10.1029/JZ071i016p04037", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141111-142013966", note = "Copyright 1966 by the American Geophysical Union. \n\n(Manuscript received December 11, 1965; revised May 4, 1966.) \n\nWe express our thanks to G. Baenteli, C. R. McKinney, V. Renew, and C. Baumann for advice and assistance throughout the work. \n\nThis work was performed while Banks was a graduate assistant and N.S.F. Fellow at the California Institute of Technology and was supported by U.S. Atomic Energy Commission contract AT (04-3)-427, supplemented by a grant-in-aid from the Society of the Sigma Xi for special equipment. A.E.C. contribution CALT427-4. \n\nContribution 1404, Division of Geological Sciences, California Institute of Technology.", revision_no = "10", abstract = "The decay constant of U^(235) has been evaluated from the radiogenic Pb^(207)/Pb^(206) ratios of several cogenetic fractions of zircon and uranothorite, analyzed by conventional mass spectrometric methods. Variation of the Pb^(206)/U^(238) apparent ages among the mineral fractions demonstrates that some form of isotopic disturbance has occurred during their geologic history. Arguments are presented to show that the method of calculating λ_(235) is relatively insensitive to such disturbance because of the young geologic age of the samples and the probability that zircon and uranothorite experience isotopic disturbance by partial loss of Pb. Inaccuracy in the calculated value is further reduced by deducing, from geologic and isotopic considerations, the most likely limits to the apparent ages to which the Pb^(207)/Pb^(206) ratios should correspond. Although the uncertainties in the method do not permit a unique derivation of the decay constant, the results suggest that the currently accepted value is probably accurate to within 1%, relative to the accepted values for the decay constant of U^(238) and the isotopic composition of natural U.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61365, title ="Oxygen isotope studies of minerals in stony meteorites", author = "Taylor, Hugh P., Jr. and Duke, Michael B.", journal = "Geochimica et Cosmochimica Acta", volume = "29", number = "5", pages = "489-512", month = "May", year = "1965", doi = "10.1016/0016-7037(65)90043-8", issn = "0016-7037", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151021-100249965", note = "© 1965 Pergamon Press Ltd. \n\nReceived 28 August 1964. \n\nThe authors are indebted to the following persons who kindly provided samples of meteorites for analysis: H. S. BROWN, R. E. FOLINSBEE, G. GOLES, E. P. HENDERSON, I. KAPLAN, J. F. LOVERING, B. MASON, C. B. MOORE, V. R. MURTHY, B. ROY, R. SCHMITT, H. SUESS, H. C. UREY, W. VON ENGELHARDT and J. ZÄHRINGER. Financial support for this research was provided by the National Science Foundation (Grant no. G23992), and by the Atomic Energy Commission Contract AT(04-3)-427, CALT-427-1.", revision_no = "14", abstract = "Oxygen isotope analyses demonstrate the following sequence (as in terrestrial igneous rocks) of increasing O^(18) content for coexisting minerals of stony meteorites: olivine, pyroxene, plagioclase, free silica. Except for the carbonaceous chondrites, the O^(18)/O^(16) ratio of a given mineral is quite uniform in each meteorite class. Differences exist between classes of meteorites, however, as well as between certain meteorites and terrestrial igneous rocks. For example, the O^(18)/O^(16) ratios of meteoritic pyroxenes vary from δ = −0.5 per mil to δ = + 8.6 per mil (relative to SMOW), whereas pyroxenes in 8 terrestrial igneous rocks have δ-values which range only from + 5.5 to +6.6. The oxygen isotope data suggest a separation of the stony meteorites into three groups, as follows:\n1.\nI. Basaltic achondrites, hypersthene achondrites, and mesosiderites—with pyroxene δ-values of 3.7 to 4.4.\n2.\nII. Hypersthene-olivine chondrites, bronzite-olivine chondrites, enstatite chondrites, enstatite achondrites, and nakhlites—with pyroxene δ-values of 5.3 to 6.3.\n3.\nIII. Types I, II and III carbonaceous chondrites, and ureilites—with highly variable olivine and pyroxene δ-values.\nIt is suggested that the meteorites within each of these three groups are genetically related to one another, but that the three groups may themselves be only distantly related or unrelated. The chondrites have O^(18)/O^(16) ratios similar to their terrestrial analogs, the ultramafic rocks. However, whereas terrestrial basaltic rocks are 1 to 2 per mil richer in O^(18) than ultramafic rocks, the basaltic meteorites are 0.5 to 1.5 per mil lower than chondrites; thus there are serious restrictions placed on any postulated derivation of the basaltic meteorites from chondrites. In addition, the carbonaceous chondrites cannot be simply normal chondrites which have suffered a low-temperature alteration. The olivine in carbonaceous meteorites is lower in O^(18)/O^(16) and isotopically much more variable than chondrite olivine, implying that it is derived from a distinctly different source material.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47815, title ="Re-examination of isotopic relationships in Colorado Front Range uranium ores", author = "Banks, P. O. and Silver, Leon T.", journal = "Geological Society of America Bulletin", volume = "75", number = "5", pages = "469-476", month = "May", year = "1964", doi = "10.1130/0016-7606(1964)75[469:ROIRIC]2.0.CO;2", issn = "0016-7606", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140801-125831579", note = "Copyright © 1964, The Geological Society of America, Inc. \n\nReceived October 14, 1963. \n\nWe are indebted to Dr. George Phair, U. S.\nGeological Survey, for providing us with\nselected sample materials, for discussions of the\nproblems, and for critical reading of the manuscript,\nwithout necessarily concurring in all\nthat we have said. Dr. P. K. Sims has provided\ninformation on the geologic setting of the\nores. This work was supported by the U. S.\nAtomic Energy Commission, Contract AT\n(04-3)-427, as part of an investigation of\ngeologically young U-Pb isotopic systems.", revision_no = "12", abstract = "Some new isotopic analyses of Front Range pitchblende samples and comparisons with published data indicate a complex history of isotopic disturbance in these ores. Relationships in the U^(238)-Pb^(206) and U^(235)-Pb^(207) systems indicate extensive and protracted mobilizations of parents and daughters, and particularly intermediate daughters, in both isotopic series. These relationships preclude a simple, much less unique interpretation of the absolute age of the uranium mineralization in the Front Range. Considering the lack of close definition in the stratigraphic age assignment for this mineralization, it is suggested that the common use of the inferred age in the construction of geologic time scales is not justified.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/98821, title ="Lower Cretaceous Pre-Batholithic Rocks of Northern Baja California, Mexico", author = "Silver, L. T. and Stehli, F. G.", journal = "AAPG Bulletin", volume = "47", number = "12", pages = "2054-2059", month = "December", year = "1963", issn = "0149-1423", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190924-083419112", note = "© 1956 American Association of Petroleum Geologists. \n\nPresented at the XX international Geological Congress (Mexico, 1956). This paper was submitted in September, 1956, for inclusion in Volume 3 of the Cretaceous Symposium, which it now appears will not be printed. The manuscript presented here in essentially unmodified form with the kind advice and permission of Ing. Jenaro Gonzalez Reyna, secretary-general of the XX Session. Contribution No. 799, Division of Geological Sciences, California Institute of Technology. Manuscript received, July 29, 1963. \n\nIn this study the writers have had the opportunity of friendly scientific collaboration with the Institute of Geology, National University of Mexico, Ing. Guillermo P. Salas, Director. They also acknowledge the advice of Professor A. O. Woodford, and field consultations with E. C. Allison, Professor C. E. Weaver, C. R. McKinney, and with David Gottfried and Normal Silberling of the U.S. Geological Survey.", revision_no = "10", abstract = "Cretaceous fossils have been found at scattered localities in the pre-batholithic metamorphic rocks of northern Baja California by investigators during the past half-century. The resulting information has been inadequate, however, for the explanation of regional stratigraphic and structural relations, particularly those correlations between the less metamorphosed coastal sections and the more deformed rocks of the mountainous interior.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61402, title ="Uranium-Lead Isotopic Variations in Zircons – A Case Study", author = "Silver, Leon T. and Deutsch, Sarah", journal = "Journal of Geology", volume = "71", number = "6", pages = "721-758", month = "November", year = "1963", issn = "0022-1376", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151022-075922766", note = "© 1963 University of Chicago Press. \n\n\nManuscript received February 21, 1962; revised May 25, 1962. \n\nThe authors wish to acknowledge the continued critical advice and assistance of C. R. McKinney. J. Kawafuchi, D. Maynes, C. C. Patterson, and D. Ledent discussed analytical problems. H. P. Schwarcz and J. Bolinger assisted in mineral separations. V. Nenow constructed a very satisfactory furnace for control of fusion rates. Arthur Chodos provided emission spectrographic data on zirconium in the mineral separates. R. von Huene prepared the special ground zircon grain mounts for autoradiography studies. Professors Arden Albee and Barclay Kamb, colleagues at the California Institute of Technology, read the manuscript and discussed some significant points. This investigation was carried out as part of a program, \"A Study of the Fundamental Geochemistry of Critical Materials,\" supported by the United States Atomic Energy Commission under contract AT(11-1)-208.", revision_no = "9", abstract = "Zircons in a single 250-pound block of Precambrian Johnny Lyon granodiorite from the Dragoon Quadrangle in Cochise County, Arizona, have been concentrated with special attention to yield and nature of impurities. Morphology, zoning, color, inclusions, size distribution, radioactivity, refractive indexes, cell dimensions, and other properties have been compared with the isotopic properties in the U-Pb system (and to a less precise degree in the Th-Pb system). It has been observed: (1) Uranothorite impurities, while less than 1 per cent in abundance, contribute much more than 50 per cent of the activity in conventionally prepared concentrates. The presence of uranothorite drastically affects the apparent ages in the zirons. (2) The uranothorite can be satisfactorily removed by an appropriate acid-washing procedure. (3) The uranothorite-free zircon concentrates are not homogeneous and show systematic variations in radioactivity and various isotopic properties as a function of average crystal size. (4) Individual zircons show internal variation in radioactivity and may differ in specific activity from other individuals by as much as an order of magnitude. (5) It is possible to strip outer layers experimentally from an aggregate of zircons to determine variations in composite internal isotopic properties. (6) The family of uranium-lead systems distinguished in the inhomogeneous zircon suite may be utilized to establish patterns of isotopic ratio variations that yield much more useful geochronological information than any single system. (7) All systems in this rock appear to have formed 1,655 million years ago and to have been profoundly disturbed by an event 90 million years ago. There is no evidence of any type of disturbance other than this simple episodic pattern. (8) While the mechanism of disturbance is not directly established, it is evident that radioactivity and radiation damage strongly influence susceptibility of the systems to disturbance. (9) It is possible to offer reasonable explanations for some of the puzzling discrepancies between uranium-lead and thorium-lead ages determined on a single mineral concentrate. (10) Recognition of the existence of families of uranium-lead systems among the variable members of a single mineral species, or in associated mineral species, in a single typical granitic rock provides a powerful tool for investigation of the processes and conditions that have influenced the age-dating systems. (11) The systematic variations in U and Th provide interesting information on the role of some trace elements and accessory minerals in the crystallization history of the rock. (12) The Johnny Lyon granodiorite is the oldest igneous rock dated in Arizona thus far and places a minimum age of 1,655 ± 20 million years on the orogeny called Mazatzal Revolution.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/80672, title ="Precambrian Age Determinations in the Western San Gabriel Mountains, California", author = "Silver, L. T. and McKinney, C. R.", journal = "Journal of Geology", volume = "71", number = "2", pages = "196-214", month = "March", year = "1963", doi = "10.1086/626893", issn = "0022-1376", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170822-102946036", note = "© 1963 The University of Chicago.", revision_no = "6", abstract = "Isotopic age determinations (U-Th-Pb) on zircons from pegmatite and granophyre dikes in the western San Gabriel Mountains have yielded concordant ages of about 1,200 million years. From structural and petrological relations, this is both a minimum and a probable age for the San Gabriel Mountains anorthosite complex. It is also a minimum age for the Mendenhall gneiss, a retrograded \"charnockitic\" granulite which has been intruded by the anorthosite complex. Several zircon fractions from the gneiss yield consistent but moderately discordant apparent ages as follows: Pb^(206)/U^(238) = 1.295m.y., Pb^(207)/U^(235) = 1.355m.y., Pb^(207)/Pb^(206) = 1.440m.y.\n The Pb^(207)/Pb^(206) age is considered minimal for the zircon and, considering genetic uncertainties, a possible minimum age for the parental rock of the gneiss. These age determinations provide the first compelling evidence of Precambrian rocks on the west coast of North America. Their intimate involvement in the great fault systems of southern California complicates our understanding of the history of this part of the continental crust. It is indicated that a significant point on the Pacific margin of the continent originated more than 1,200 million years ago and has persisted without destructive modification to the present.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/42946, title ="A study of the ages of the Precambrian of Texas", author = "Wasserburg, G. J. and Wetherill, G. W.", journal = "Journal of Geophysical Research", volume = "67", number = "10", pages = "4021-4047", month = "September", year = "1962", doi = "10.1029/JZ067i010p04021", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20131211-090759937", note = "© 1962 by the American Geophysical Union.\n\nManuscript received May 7, 1962; revised June 19, 1962.\nWe should like to acknowledge\nthe help of Robert L. Harbour of the U.S.\nGeological Survey, who generously aided us in our\ncollecting trip to the Franklin Mountain area.\nSome of the core samples were made available\nto us through the kindness of Dr. R. G. Swanson\nof the Shell Oil Company, Dr. Milton Williams\nof the Humble Oil and Refining Company, and\nDr. James F. Johnson of the Sinclair Oil Company.\nWe should particularly like to thank Professor\nManuel Bass, whose enthusiastic interest encouraged\nthis work and who gave freely of his time\nand knowledge in discussing the problems.\nThis work was supported by the National Science\nFoundation grants NSF-G19084 and NSF-G15945,\nand the Atomic Energy Commission contract\nAT(11-1)-208.\n", revision_no = "8", abstract = "Age determinations using the Sr^(87)-Rb^(87), Ar^(40)-K^(40), and Pb-U methods were made on samples of muscovite, biotite, amphibole, microcline, and zircon from igneous and metamorphic rocks from the Franklin Mountains, Hueco Mountains, Pump Station Hills, and Carrizo and Van Horn Mountains. In addition ages were determined on a number of basement cores from Texas and New Mexico. The results show that a belt of rocks of varied lithology extending from El Paso to east of the Llano uplift are all of the same age. The general age by the strontium and argon methods is 1000 to 1090 m.y.; and by the lead-uranium method on zircons it is 1150 to 1200 m.y. This event is in the same time band as the ‘Grenville’ orogeny in Canada and the northeastern United States and possibly should be considered part of the general ‘Grenville’ episode. All the data now available indicate that the orogenic event at about 1000 to 1200 m.y. is the most widespread and pervasive episode of Precambrian orogeny on the North American continent for which adequate evidence has been presented. At least one and probably two older periods of igneous activity and metamorphism occurring at 1250 and 1400 m.y. are found in the northern regions of the Texas Precambrian basement. No evidence was found for any igneous event between the early Paleozoic and the 1000-m.y. episode.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47814, title ="Agua Blanca fault -- a major transverse structure of northern Baja California, Mexico", author = "Allen, Clarence R. and Silver, Leon T.", journal = "Geological Society of America Bulletin", volume = "71", number = "4", pages = "467-482", month = "April", year = "1960", doi = "10.1130/0016-7606(1960)71[467:ABFMTS]2.0.CO;2", issn = "0016-7606", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140801-124917356", note = "Copyright © 1960, The Geological Society of America, Inc. \n\nReceived June 5, 1959.\n", revision_no = "11", abstract = "Agua Blanca fault is a major right-handed strike-slip fault at least 80 miles in length that cuts transversely across the peninsula of Baja California about 70 miles south of the international border. Its trend is anomalous in being more nearly parallel to the Transverse Ranges of southern California than to the San Andres fault system that elsewhere dominates the tectonic grain of the peninsula. Geographic features delineating the fault trace are, from east to west: Paso San Matías, Valle de La Trinidad, Cañon de Dolores, Valle de Agua Blanca, Valle de Santo Tomás, Bahía Soledad (south branch of fault), Punta Banda (north branch). Farther west, both branches of the fault control submarine topography, and possibly the fault system is continuous with the northwest-trending San Clemente fault off the southern California coast.\n\nPhysiographic expression of the Agua Blanca fault is remarkably similar to that of the San Andreas. Typical features are Recent scarps, offset streams, shutterridges, fault sags and saddles, side-hill ridges, and fault-controlled valleys. Most of these features are particularly well exhibited in Valle de Agua Blanca, which is designated as the type locality.\n\nRocks cut by the fault are mainly Cretaceous plutonic rocks of the southern California batholith and Lower Cretaceous (Albian) metavolcanic rocks. Along the Pacific Coast, the fault cuts Upper Cretaceous (Maestrichtian) post-batholithic sedimentary rocks that are otherwise surprisingly little deformed as compared to rocks of similar age in most of California.\n\nAgua Blanca fault shows a history of right-lateral displacement throughout its length: Recent stream offsets occur from Valle de La Trinidad nearly to the Pacific Ocean; distinctive Quaternary(?) fan gravels in Valle de Agua Blanca are offset laterally 3 miles from their most likely source area across the fault; in the same area, a fault slice of distinctive antiperthitic granodiorite is best explained by 7 miles of lateral displacement, and a nearby slice of quartz diorite may indicate displacement as great as 14 miles. In general, evidence of both Recent activity and amount of total displacement appear to increase westward. The fault coincides in gross aspect with a broad east-west zone of seismic activity in a region elsewhere characterized by relative quiescence, but no large historical earthquakes can be positively correlated with this fault.\n\nDespite its orientation athwart the regional tectonic grain, Agua Blanca fault does not appear to represent a deep-seated structural feature analogous to those of the Transverse Ranges. Instead, it is probably one of several paths by which the San Andreas fault tends to break around the “knot” caused by the great bend of the San Andreas in southern California.", } @book_section {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61364, title ="The possibilities of obtaining long-range supplies of uranium, thorium, and other substances from igneous rocks", author = "Brown, Harrison and Silver, Leon T.", number = "300", pages = "91-95", month = "January", year = "1956", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151021-094312456", note = "© 1956 USGS.", revision_no = "13", abstract = "Most uranium and thorium which have been produced in the world thus far have been obtained from ores of relatively high grade. Such deposits are not very extensive and are found infrequently. When the high-grade deposits of these substances approach exhaustion, it may be that material containing these elements in low concentrations will be the ultimate source. An average granite contains about 4 ppm uranium and 12 ppm thorium. If all the uranium and thorium in 1 ton of average granite could be extracted and utilized by means of nuclear breeding, the energy output would be equivalent to that obtained by burning 50 tons of coal. Means are now available for relatively easy extraction of about one quarter of the uranium and thorium from average granite, with an energy profit per ton of rock processed equivalent to that obtained by burning 10 tons of coal. \n\nResults also indicate that a variety of both major and minor substances of industrial importance can be obtained as byproducts of uranium and thorium production from igneous rocks. The techniques required for extracting uranium and thorium from igneous rocks are wen within the realm of present mineral-dressing and industrial-chemical experience.", }