@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.", } @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.", } @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.", } @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.", } @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).", } @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.", } @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.", }