[ { "id": "authors:3kmwx-5yh25", "collection": "authors", "collection_id": "3kmwx-5yh25", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20121113-154436415", "type": "article", "title": "Assessment of (U-Th)/He thermochronometry: The low-temperature history of the San Jacinto mountains, California", "author": [ { "family_name": "Wolf", "given_name": "R. A.", "clpid": "Wolf-R-A" }, { "family_name": "Farley", "given_name": "K. A.", "orcid": "0000-0002-7846-7546", "clpid": "Farley-K-A" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" } ], "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 (\u223c7\u00b0) 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.", "doi": "10.1130/0091-7613(1997)025<0065:AOUTHT>2.3.CO;2", "issn": "0091-7613", "publisher": "Geological Society of America", "publication": "Geology", "publication_date": "1997-01", "series_number": "1", "volume": "25", "issue": "1", "pages": "65-68" }, { "id": "authors:3c4be-n8d38", "collection": "authors", "collection_id": "3c4be-n8d38", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140716-113219787", "type": "article", "title": "Isotopic mapping in the Peninsular Ranges Batholith", "author": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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.", "issn": "0016-7592", "publisher": "Geological Society of America", "publication": "Abstracts with Programs - Geological Society of America", "publication_date": "1997", "series_number": "6", "volume": "29", "issue": "6", "pages": "69-69" }, { "id": "authors:a2qze-vtt20", "collection": "authors", "collection_id": "a2qze-vtt20", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20121120-104629253", "type": "article", "title": "Helium diffusion and low-temperature thermochronometry of apatite", "author": [ { "family_name": "Wolf", "given_name": "R. A.", "clpid": "Wolf-R-A" }, { "family_name": "Farley", "given_name": "K. A.", "orcid": "0000-0002-7846-7546", "clpid": "Farley-K-A" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" } ], "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\u00b0C) 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 \u03bcm) which are nearly identical in size in all samples. At temperatures below 290\u00b0C, 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\u00b0C. Using the results from the low-temperature regime, all samples yield helium closure temperatures in the range 75 \u00b1 7\u00b0C. 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\u2013120 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.", "doi": "10.1016/S0016-7037(96)00192-5", "issn": "0016-7037", "publisher": "Elsevier", "publication": "Geochimica et Cosmochimica Acta", "publication_date": "1996-11", "series_number": "21", "volume": "60", "issue": "21", "pages": "4231-4240" }, { "id": "authors:evb39-kn857", "collection": "authors", "collection_id": "evb39-kn857", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20121120-143017682", "type": "article", "title": "The effects of long alpha-stopping distances on (U-Th)/He ages", "author": [ { "family_name": "Farley", "given_name": "K. A.", "orcid": "0000-0002-7846-7546", "clpid": "Farley-K-A" }, { "family_name": "Wolf", "given_name": "R. A.", "clpid": "Wolf-R-A" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" } ], "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 \u223c10 and \u223c30 \u03bcm, 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 \u03bcm radius retains only \u223c82% of its alphas. For a homogeneous distribution of parent nuclides, the fraction of alphas ejected is \u223c 1/4 of the mean alpha range multiplied by the crystal surface to volume ratio, independent of geometry. Removal of the outer 20 \u03bcm 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.", "doi": "10.1016/S0016-7037(96)00193-7", "issn": "0016-7037", "publisher": "Elsevier", "publication": "Geochimica et Cosmochimica Acta", "publication_date": "1996-11", "series_number": "21", "volume": "60", "issue": "21", "pages": "4223-4229" }, { "id": "authors:69wnk-0ee96", "collection": "authors", "collection_id": "69wnk-0ee96", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130510-154654100", "type": "article", "title": "Evidence for possible horizontal faulting in southern California from earthquake mechanisms", "author": [ { "family_name": "Huang", "given_name": "Weishi", "clpid": "Huang-Weishi" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" }, { "family_name": "Kanamori", "given_name": "H.", "orcid": "0000-0001-8219-9428", "clpid": "Kanamori-H" } ], "abstract": "We find that 36 of the 505 fault-plane solutions (M \u2265 3.0, 1981\u20131990) in southern California have a nodal plane dipping no more than 30 \u00b0. 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.", "doi": "10.1130/0091-7613(1996)024<0123:EFPHFI>2.3.CO;2", "issn": "0091-7613", "publisher": "Geological Society of America", "publication": "Geology", "publication_date": "1996-02", "series_number": "2", "volume": "24", "issue": "2", "pages": "123-126" }, { "id": "authors:kmg8k-y3r32", "collection": "authors", "collection_id": "kmg8k-y3r32", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140331-160851774", "type": "article", "title": "Tertiary metamorphic core complexes in Sonora, northwestern Mexico", "author": [ { "family_name": "Nourse", "given_name": "Jonathan A.", "clpid": "Nourse-J-A" }, { "family_name": "Anderson", "given_name": "Thomas H.", "clpid": "Anderson-T-H" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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.", "doi": "10.1029/93TC03324", "issn": "0278-7407", "publisher": "American Geophysical Union", "publication": "Tectonics", "publication_date": "1994-10", "series_number": "5", "volume": "13", "issue": "5", "pages": "1161-1182" }, { "id": "authors:00jtv-0w351", "collection": "authors", "collection_id": "00jtv-0w351", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130510-154746884", "type": "article", "title": "Emplacement and deformation history of the western margin\n of the Idaho batholith near McCall, Idaho: Influence of a major terrane boundary", "author": [ { "family_name": "Manduca", "given_name": "Cathryn A.", "clpid": "Manduca-C-A" }, { "family_name": "Kuntz", "given_name": "Mel A.", "clpid": "Kuntz-M-A" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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.", "doi": "10.1130/0016-7606(1993)105<0749:EADHOT>2.3.CO;2", "issn": "0016-7606", "publisher": "Geological Society of America", "publication": "Geological Society of America Bulletin", "publication_date": "1993-06", "series_number": "6", "volume": "105", "issue": "6", "pages": "749-765" }, { "id": "authors:h6w3a-vcr68", "collection": "authors", "collection_id": "h6w3a-vcr68", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140115-111655716", "type": "article", "title": "Aspects of the crustal structure of the western Mojave Desert, California, from seismic reflection and gravity data", "author": [ { "family_name": "Li", "given_name": "Yong-Gang", "clpid": "Li-Y-G" }, { "family_name": "Henyey", "given_name": "Thomas L.", "clpid": "Henyey-T-L" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "abstract": "Seismic and gravity data taken along line 1 of the 1982 Consortium for Continental Reflection Profiling (COCORP) Mojave Desert Survey (N-S profile, \u223c30 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 \u223c1.8 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.", "doi": "10.1029/91JB02119", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research B", "publication_date": "1992-06-10", "series_number": "B6", "volume": "97", "issue": "B6", "pages": "8805-8816" }, { "id": "authors:gh2yj-p3h43", "collection": "authors", "collection_id": "gh2yj-p3h43", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140312-075614994", "type": "article", "title": "Geochemical Logging in the Cajon Pass Drill Hole and Its Application to a New, Oxide, Igneous Rock Classification Scheme", "author": [ { "family_name": "Pratson", "given_name": "Elizabeth Lewis", "clpid": "Pratson-E-L" }, { "family_name": "Anderson", "given_name": "Roger N.", "clpid": "Anderson-R-N" }, { "family_name": "Dove", "given_name": "Roy E.", "clpid": "Dove-R-E" }, { "family_name": "Lyle", "given_name": "Mitchell", "clpid": "Lyle-M" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "James", "given_name": "Eric W.", "clpid": "James-E-W" }, { "family_name": "Chappell", "given_name": "Bruce W.", "clpid": "Chappell-B-W" } ], "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.", "doi": "10.1029/91JB02643", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research B", "publication_date": "1992-04-10", "series_number": "B4", "volume": "97", "issue": "B4", "pages": "5167-5180" }, { "id": "authors:z3mdd-jt753", "collection": "authors", "collection_id": "z3mdd-jt753", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140416-095344675", "type": "article", "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": [ { "family_name": "Manduca", "given_name": "Cathryn A.", "clpid": "Manduca-C-A" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Taylor", "given_name": "Hugh P.", "clpid": "Taylor-H-P-Jr" } ], "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 \u03b4^(18)O greater than 7.5, indicating little or no continental crust in its source region; however, elevated \u03b4^(18)O requires some incorporation of rocks formed or altered at the earth's surface. A large shift in Ri and \u03b4^(18)O is observed across the 5\u20138 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 \u03b4^(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 \u03b4^(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.", "doi": "10.1007/BF00283324", "issn": "0010-7999", "publisher": "Springer", "publication": "Contributions to Mineralogy and Petrology", "publication_date": "1992-01", "series_number": "3", "volume": "109", "issue": "3", "pages": "355-372" }, { "id": "authors:n2460-8sp45", "collection": "authors", "collection_id": "n2460-8sp45", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130514-103113195", "type": "article", "title": "The metamictization of zircon: Radiation dose-dependent structural characteristics", "author": [ { "family_name": "Woodhead", "given_name": "James A.", "clpid": "Woodhead-J-A" }, { "family_name": "Rossman", "given_name": "George R.", "orcid": "0000-0002-4571-6884", "clpid": "Rossman-G-R" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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.", "issn": "0003-004X", "publisher": "Mineralogical Society of America", "publication": "American Mineralogist", "publication_date": "1991-02", "series_number": "1-2", "volume": "76", "issue": "1-2", "pages": "74-82" }, { "id": "authors:c1ydn-s8052", "collection": "authors", "collection_id": "c1ydn-s8052", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140311-155520484", "type": "article", "title": "San Jacinto Intrusive Complex: 2. Geochemistry", "author": [ { "family_name": "Hill", "given_name": "R. I.", "clpid": "Hill-R-I" }, { "family_name": "Chappell", "given_name": "B. W.", "clpid": "Chappell-B-W" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" } ], "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\u224870 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.", "doi": "10.1029/JB093iB09p10349", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research B", "publication_date": "1988-09-10", "series_number": "B9", "volume": "93", "issue": "B9", "pages": "10349-10372" }, { "id": "authors:gjm0g-2vn50", "collection": "authors", "collection_id": "gjm0g-2vn50", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20131031-103755851", "type": "article", "title": "San Jacinto Intrusive Complex: 3. Constraints on crustal magma chamber processes from strontium isotope heterogeneity", "author": [ { "family_name": "Hill", "given_name": "R. I.", "clpid": "Hill-R-I" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" } ], "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\u20130.7076; unit II, 0.7060\u20130.7074; unit III, 0.7058\u20130.7073. 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\u20130.7084. 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.", "doi": "10.1029/JB093iB09p10373", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research B", "publication_date": "1988-09-10", "series_number": "B9", "volume": "93", "issue": "B9", "pages": "10373-10388" }, { "id": "authors:3axg6-bw088", "collection": "authors", "collection_id": "3axg6-bw088", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140310-152425705", "type": "article", "title": "Lithologic column of the \"Arkoma\" Drillhole and its relation to the Cajon Pass Deep Drillhole", "author": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "James", "given_name": "Eric W.", "clpid": "James-E-W" } ], "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.", "doi": "10.1029/GL015i009p00945", "issn": "0094-8276", "publisher": "American Geophysical Union", "publication": "Geophysical Research Letters", "publication_date": "1988-08", "series_number": "9", "volume": "15", "issue": "9", "pages": "945-948" }, { "id": "authors:tz794-62f72", "collection": "authors", "collection_id": "tz794-62f72", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130513-093549509", "type": "article", "title": "Implications of zeolites and their zonation in the Cajon Pass Deep Drillhole", "author": [ { "family_name": "James", "given_name": "Eric W.", "clpid": "James-E-W" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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.", "doi": "10.1029/GL015i009p00973", "issn": "0094-8276", "publisher": "American Geophysical Union", "publication": "Geophysical Research Letters", "publication_date": "1988-08", "series_number": "9", "volume": "15", "issue": "9", "pages": "973-976" }, { "id": "authors:w1rtz-52398", "collection": "authors", "collection_id": "w1rtz-52398", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130710-095829427", "type": "article", "title": "Geologic setting and lithologic column of the Cajon Pass Deep Drillhole", "author": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "James", "given_name": "Eric W.", "clpid": "James-E-W" } ], "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\u00b1calcite or chlorite-epidote. Fractures and faults decrease in abundance with depth.", "doi": "10.1029/GL015i009p00941", "issn": "0094-8276", "publisher": "American Geophysical Union", "publication": "Geophysical Research Letters", "publication_date": "1988-08", "series_number": "9", "volume": "15", "issue": "9", "pages": "941-944" }, { "id": "authors:ejt6s-2d544", "collection": "authors", "collection_id": "ejt6s-2d544", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130621-145700946", "type": "article", "title": "Elemental and mineralogical analyses using geochemical logs from the Cajon Pass Scientific Drillhole, California, and their preliminary comparison with core analyses", "author": [ { "family_name": "Anderson", "given_name": "Roger N.", "clpid": "Anderson-R-N" }, { "family_name": "Dove", "given_name": "Roy E.", "clpid": "Dove-R-E" }, { "family_name": "Broglia", "given_name": "Cristina", "clpid": "Broglia-C" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "James", "given_name": "Eric W.", "clpid": "James-E-W" }, { "family_name": "Chappell", "given_name": "B. W.", "clpid": "Chappell-B-W" } ], "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%.", "doi": "10.1029/GL015i009p00969", "issn": "0094-8276", "publisher": "American Geophysical Union", "publication": "Geophysical Research Letters", "publication_date": "1988-08", "series_number": "9", "volume": "15", "issue": "9", "pages": "969-972" }, { "id": "authors:czwqp-8wz13", "collection": "authors", "collection_id": "czwqp-8wz13", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140311-153804093", "type": "article", "title": "Petrological and geochemical investigations at the Cajon Pass Deep Drillhole", "author": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "James", "given_name": "Eric W.", "clpid": "James-E-W" }, { "family_name": "Chappell", "given_name": "Bruce W.", "clpid": "Chappell-B-W" } ], "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.", "doi": "10.1029/GL015i009p00961", "issn": "0094-8276", "publisher": "American Geophysical Union", "publication": "Geophysical Research Letters", "publication_date": "1988-08", "series_number": "9", "volume": "15", "issue": "9", "pages": "961-964" }, { "id": "authors:pyrsh-8pt24", "collection": "authors", "collection_id": "pyrsh-8pt24", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140311-151633689", "type": "article", "title": "The Cajon Pass Scientific Drilling Experiment: Overview of phase 1", "author": [ { "family_name": "Zoback", "given_name": "Mark D.", "orcid": "0000-0002-8851-2099", "clpid": "Zoback-M-D" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Henyey", "given_name": "Thomas", "clpid": "Henyey-T-L" }, { "family_name": "Thatcher", "given_name": "Wayne", "clpid": "Thatcher-W" } ], "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.", "doi": "10.1029/GL015i009p00933", "issn": "0094-8276", "publisher": "American Geophysical Union", "publication": "Geophysical Research Letters", "publication_date": "1988-08", "series_number": "9", "volume": "15", "issue": "9", "pages": "933-936" }, { "id": "authors:6jfyr-5hp53", "collection": "authors", "collection_id": "6jfyr-5hp53", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220729-945022900", "type": "article", "title": "The Peninsular Ranges Batholith: an insight into the evolution of the Cordilleran batholiths of southwestern North America", "author": [ { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" }, { "family_name": "Chappell", "given_name": "B. W.", "clpid": "Chappell-Bruce-W" } ], "abstract": "The Peninsular Ranges Batholith of southern and Baja California is the largest segment of a Cretaceous magmatic arc that was once continuous from northern California to southern Baja California. In this batholith, the emplacement of igneous rocks took place during a single sequence of magmatic activity, unlike many of the other components of the Cordilleran batholiths which formed during successive separate magmatic episodes. Detailed radiometric dating has shown that it is a composite of two batholiths. A western batholith, which was more heterogeneous in composition, formed as a static magmatic arc between 140 and 105 Ma and was intrusive in part into related volcanic rocks. The eastern batholith formed as a laterally transgressing arc which moved away from those older rocks between 105 and 80 Ma, intruding metasedimentary rocks. Rocks of the batholith range from undersaturated gabbros through to felsic granites, but tonalite is the most abundant rock throughout. Perhaps better than elsewhere in the Cordillera, the batholith shows beautifully developed asymmetries in chemical and isotopic properties. The main gradients in chemical composition from W to E are found among the trace elements, with Ba, Sr, Nb and the light rare earth elements increasing by more than a factor of two, and P, Rb, Pb, Th, Zn and Ga showing smaller increases. Mg and the transition metals decrease strongly towards the E, with Sc, V and Cu falling to less than half of their value in the most westerly rocks. Oxygen becomes very systematically more enriched in \u00b9\u2078O from W to E and the Sr, Nd and Pb isotopic systems change progressively from mantle values in the W to a more evolved character on the eastern side of the batholith. In detail the petrogenesis of the Peninsular Ranges Batholith is not completely understood, but many general aspects of the origin are clear. The exposed rocks, particularly in the western batholith, closely resemble those of present day island arcs, although the most typical and average tonalitic composition is distinctly more felsic than the mean quartz diorite or mafic andesite composition of arcs. Chemical and isotopic properties of the western part of the batholith indicate that it formed as the root of a primitive island arc on oceanic lithosphere at a convergent plate margin. Further E, the plutonic rocks appear to have been derived by partial melting from deeper sources of broadly basaltic composition at subcrustal levels. The compositional systematics of the batholith do not reflect a simple mixing of various end-members but are a reflection of the differing character of the source regions laterally and vertically away from the pre-Cretaceous continental margin.", "doi": "10.1017/s0263593300014152", "issn": "1755-6910", "publisher": "Cambridge University Press", "publication": "Earth and Environmental Science Transactions of the Royal Society of Edinburgh", "publication_date": "1988", "series_number": "2-3", "volume": "79", "issue": "2-3", "pages": "105-121" }, { "id": "authors:v0358-9jz10", "collection": "authors", "collection_id": "v0358-9jz10", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130508-152220795", "type": "article", "title": "Comment and Reply on \"Late Triassic paleogeography of the southern Cordillera:\n The problem of a source for voluminous volcanic detritus in the Chinle Formation\n of the Colorado Plateau region\"", "author": [ { "family_name": "Stewart", "given_name": "John H.", "clpid": "Stewart-J-H" }, { "family_name": "Anderson", "given_name": "Thomas H.", "clpid": "Anderson-T-H" }, { "family_name": "Haxel", "given_name": "Gordon B.", "clpid": "Haxel-G-B" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Wright", "given_name": "James E.", "clpid": "Wright-J-E" } ], "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\u2014namely, 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 \u00b11 Ma for uranium ores from the Chinle\nFormation in Lisbon Valley (Ludwig et al., 1986).", "doi": "10.1130/0091-7613(1987)15<578a:CAROLT>2.0.CO;2", "issn": "0091-7613", "publisher": "Geological Society of America", "publication": "Geology", "publication_date": "1987-06", "series_number": "6", "volume": "15", "issue": "6", "pages": "578-579" }, { "id": "authors:3zv2f-kzy68", "collection": "authors", "collection_id": "3zv2f-kzy68", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140415-090105736", "type": "article", "title": "REE Variations Across the Peninsular Ranges Batholith: Implications for Batholithic Petrogenesis and Crustal Growth in Magmatic Arcs", "author": [ { "family_name": "Gromet", "given_name": "L. Peter", "clpid": "Gromet-L-Peter" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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 \u03b4^(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, \u2078\u2077Sr/\u2078\u2076Sr and \u00b9\u2078O. 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\u2082O 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-\u00b9\u2078O 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.", "doi": "10.1093/petrology/28.1.75", "issn": "0022-3530", "publisher": "Oxford University Press", "publication": "Journal of Petrology", "publication_date": "1987-02", "series_number": "1", "volume": "28", "issue": "1", "pages": "75-125" }, { "id": "authors:2pw20-7w698", "collection": "authors", "collection_id": "2pw20-7w698", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130708-154037504", "type": "article", "title": "Comment and Reply on \"S-type granites and their probable absence in southwestern North America\"", "author": [ { "family_name": "White", "given_name": "A. J. R.", "clpid": "White-A-J-R" }, { "family_name": "Clemens", "given_name": "J. D.", "clpid": "Clemens-J-D" }, { "family_name": "Holloway", "given_name": "J. R.", "clpid": "Holloway-J-R" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" }, { "family_name": "Chappell", "given_name": "B. W.", "clpid": "Chappell-B-W" }, { "family_name": "Wall", "given_name": "V. J.", "clpid": "Wall-V-J" } ], "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.", "doi": "10.1130/0091-7613(1986)14<895:CAROSG>2.0.CO;2", "issn": "0091-7613", "publisher": "Geological Society of America", "publication": "Geology", "publication_date": "1986-10", "series_number": "10", "volume": "14", "issue": "10", "pages": "895-895" }, { "id": "authors:hevf7-hjt27", "collection": "authors", "collection_id": "hevf7-hjt27", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130513-090131521", "type": "article", "title": "Comment and Reply on \"S-type granites and their probable absence in southwestern North America\"", "author": [ { "family_name": "White", "given_name": "A. J. R.", "clpid": "White-A-J-R" }, { "family_name": "Clemens", "given_name": "J. D.", "clpid": "Clemens-J-D" }, { "family_name": "Holloway", "given_name": "J. R.", "clpid": "Holloway-J-R" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" }, { "family_name": "Chappell", "given_name": "B. W.", "clpid": "Chappell-B-W" }, { "family_name": "Wall", "given_name": "V. J.", "clpid": "Wall-V-J" } ], "abstract": "Miller's Comment highlights the necessity of clarifying the use of the\nterm S-type granite. We here make some further points.", "doi": "10.1130/0091-7613(1986)14<805:CAROSG>2.0.CO;2", "issn": "0091-7613", "publisher": "Geological Society of America", "publication": "Geology", "publication_date": "1986-09", "series_number": "9", "volume": "14", "issue": "9", "pages": "805-806" }, { "id": "authors:79zj7-bbr66", "collection": "authors", "collection_id": "79zj7-bbr66", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130508-152950143", "type": "article", "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": [ { "family_name": "Stewart", "given_name": "John H.", "clpid": "Stewart-J-H" }, { "family_name": "Anderson", "given_name": "Thomas H.", "clpid": "Anderson-T-H" }, { "family_name": "Haxel", "given_name": "Gordon B.", "clpid": "Haxel-G-B" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Wright", "given_name": "James E.", "clpid": "Wright-J-E" } ], "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.", "doi": "10.1130/0091-7613(1986)14<567:LTPOTS>2.0.CO;2", "issn": "0091-7613", "publisher": "Geological Society of America", "publication": "Geology", "publication_date": "1986-07", "series_number": "7", "volume": "14", "issue": "7", "pages": "567-570" }, { "id": "authors:dtg07-0bp21", "collection": "authors", "collection_id": "dtg07-0bp21", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130808-083358812", "type": "article", "title": "S-type granites and their probable absence in southwestern North America", "author": [ { "family_name": "White", "given_name": "A. J. R.", "clpid": "White-A-J-R" }, { "family_name": "Clemens", "given_name": "J. D.", "clpid": "Clemens-J-D" }, { "family_name": "Holloway", "given_name": "J. R.", "clpid": "Holloway-J-R" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" }, { "family_name": "Chappell", "given_name": "B. W.", "clpid": "Chappell-B-W" }, { "family_name": "Wall", "given_name": "V. J.", "clpid": "Wall-V-J" } ], "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.", "doi": "10.1130/0091-7613(1986)14<115:SGATPA>2.0.CO;2", "issn": "0091-7613", "publisher": "Geological Society of America", "publication": "Geology", "publication_date": "1986-02", "series_number": "2", "volume": "14", "issue": "2", "pages": "115-118" }, { "id": "authors:3y964-9s806", "collection": "authors", "collection_id": "3y964-9s806", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151022-132313827", "type": "article", "title": "The border connection; geological correlations and contrasts between Arizona and Sonora", "author": [ { "family_name": "Anderson", "given_name": "Thomas H.", "clpid": "Anderson-T-H" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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.", "issn": "0066-7412", "publisher": "Arizona Geological Society", "publication": "Arizona Geological Society Digest", "publication_date": "1986", "volume": "16", "pages": "72-73" }, { "id": "authors:b9jqz-pma80", "collection": "authors", "collection_id": "b9jqz-pma80", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20131120-102544794", "type": "article", "title": "Coupled Sr-O isotope variations as an indicator of source heterogeneity for the Northern Peninsular Ranges batholith", "author": [ { "family_name": "Hill", "given_name": "R. I.", "clpid": "Hill-R-I" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" }, { "family_name": "Taylor", "given_name": "H. P., Jr.", "clpid": "Taylor-H-P-Jr" } ], "abstract": "Primary \u03b4^(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 \u03b4^(18)O tend to have higher initial ^(87)Sr/^(86)Sr. The majority of samples have a limited range of \u03b4^(18)O values (+10.0\u2013+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 \u03b4^(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\u201335% (oxygen atom basis) contribution to the San Jacinto rocks from an old continental lithosphere source.", "doi": "10.1007/BF00572164", "issn": "0010-7999", "publisher": "Springer", "publication": "Contributions to Mineralogy and Petrology", "publication_date": "1986", "series_number": "3", "volume": "92", "issue": "3", "pages": "351-361" }, { "id": "authors:9s5r3-4mt78", "collection": "authors", "collection_id": "9s5r3-4mt78", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140311-101114151", "type": "article", "title": "Solidification and recharge of SiO_2-rich plutonic magma chambers", "author": [ { "family_name": "Hill", "given_name": "R. I.", "clpid": "Hill-R-I" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" }, { "family_name": "Chappell", "given_name": "B. W.", "clpid": "Chappell-B-W" }, { "family_name": "Taylor", "given_name": "H. P., Jr.", "clpid": "Taylor-H-P-Jr" } ], "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.", "doi": "10.1038/313643a0", "issn": "0028-0836", "publisher": "Nature Publishing Group", "publication": "Nature", "publication_date": "1985-02-21", "series_number": "6004", "volume": "313", "issue": "6004", "pages": "643-646" }, { "id": "authors:80k1g-3g949", "collection": "authors", "collection_id": "80k1g-3g949", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20131113-090618069", "type": "article", "title": "Rare earth element distributions among minerals in a granodiorite and their petrogenetic implications", "author": [ { "family_name": "Gromet", "given_name": "L. Peter", "clpid": "Gromet-L-P" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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\u00b0C) indicates the importance of mantle-derived components within the sources of batholithic magmas in the Peninsular Ranges.", "issn": "0016-7037", "publisher": "Elsevier", "publication": "Geochimica et Cosmochimica Acta", "publication_date": "1983-05", "series_number": "5", "volume": "47", "issue": "5", "pages": "925-939" }, { "id": "authors:1n67p-4mg26", "collection": "authors", "collection_id": "1n67p-4mg26", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151021-110115541", "type": "article", "title": "An overview of Precambrian rocks in Sonora", "author": [ { "family_name": "Anderson", "given_name": "Thomas H.", "clpid": "Anderson-T-H" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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 \u00b1 15 m.y. ago, concealing original lithologies and ages. In northeastern Sonora, a younger belt of eugeosynolinal strata, about 1,680 \u00b1 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.", "issn": "0185-0962", "publisher": "Universidad Nacional Aut\u00f3noma de Mexico", "publication": "Revista Mexicana de Ciencias Geol\u00f3gicas", "publication_date": "1981", "series_number": "2", "volume": "5", "issue": "2", "pages": "131-139" }, { "id": "authors:6eh48-b5e75", "collection": "authors", "collection_id": "6eh48-b5e75", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20131112-134631506", "type": "article", "title": "Lead-isotope inhomogeneity in Precambrian igneous K-feldspars", "author": [ { "family_name": "Ludwig", "given_name": "Kenneth R.", "clpid": "Ludwig-K-R" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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\u00b0C.\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.", "doi": "10.1016/0016-7037(77)90251-4", "issn": "0016-7037", "publisher": "Elsevier", "publication": "Geochimica et Cosmochimica Acta", "publication_date": "1977-10", "series_number": "10", "volume": "41", "issue": "10", "pages": "1457-1471" }, { "id": "authors:eetxt-05z27", "collection": "authors", "collection_id": "eetxt-05z27", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220718-988950100", "type": "article", "title": "U-Pb isotope ages of granitic plutons near Cananea, Sonora", "author": [ { "family_name": "Anderson", "given_name": "Thomas H.", "clpid": "Anderson-Thomas-H" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "abstract": "The Cananea \"granite\" crops out in the Cananea mining district, which includes some of the most important known copper deposits in Mexico. The area is in northern Sonora, approximately 100 km southeast of the twin cities of Nogales, Arizona-Sonera, on the international border.By means of isotopic analyses of U-Pb systems in cogenetic suites of zircon from two different phases of the pluton, its interpreted age has been firmly established at 1,440 + or - 15 m.y. Cananea granite, actually a quartz monzonite, is lithologically characteristic of a major Precambrian suite of anorogenic, consanguineous plutons of porphyritic granodiorite to granite. Constituents of this previously recognized series, which crop out throughout the southwestern U. S. and into northern Mexico, consistently yield zircon radiometric ages within the interval 1,425-1,475 m.y.Major younger intrusions in and nearby the Cananea district include Cuitaca granodiorite and Chivato monzodiorite. These bodies yield apparent zircon ages of 64 + or - 3 and 69 + or - 1 m.y., respectively. Although more precise geochronologic relationships between young plutons, the adjacent layered sequence, and cross-cutting, brecciated, quartz porphyry plugs are needed, correlation exists between major plutonism and mineralization.The Precambrian age of the Cananea granite precludes its chronological association with the younger mineralized plugs and should be taken into account in future planning of exploration.", "doi": "10.2113/gsecongeo.72.5.827", "issn": "1554-0774", "publisher": "Society of Economic Geologists", "publication": "Economic Geology", "publication_date": "1977-08-01", "series_number": "5", "volume": "72", "issue": "5", "pages": "827-836" }, { "id": "authors:15yne-hpn18", "collection": "authors", "collection_id": "15yne-hpn18", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130515-071510683", "type": "article", "title": "Rubidium-Strontium Fractionation Domains in the Peninsular Ranges Batholith and Their Implications for Magmatic Arc Evolution", "author": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Early", "given_name": "Thomas O.", "clpid": "Early-T-O" } ], "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.", "issn": "0002-8606", "publisher": "American Geophysical Union", "publication": "Transactions - American Geophysical Union", "publication_date": "1977", "series_number": "6", "volume": "58", "issue": "6", "pages": "532" }, { "id": "authors:05feg-cw329", "collection": "authors", "collection_id": "05feg-cw329", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130514-135353772", "type": "article", "title": "Geographic Variation of Rare Earth Fractionations in Plutonic Rocks Across the Peninsular Ranges Batholith, Southern California", "author": [ { "family_name": "Gromet", "given_name": "L. Peter", "clpid": "Gromet-L-P" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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.", "issn": "0002-8606", "publisher": "American Geophysical Union", "publication": "Transactions - American Geophysical Union", "publication_date": "1977", "series_number": "6", "volume": "58", "issue": "6", "pages": "532" }, { "id": "authors:w68n8-75565", "collection": "authors", "collection_id": "w68n8-75565", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130515-142928635", "type": "article", "title": "Thorium-Uranium fractionation as an indicator of petrogenetic processes", "author": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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.", "issn": "0002-8606", "publisher": "American Geophysical Union", "publication": "Transactions - American Geophysical Union", "publication_date": "1976", "series_number": "4", "volume": "57", "issue": "4", "pages": "351" }, { "id": "authors:1te6q-7mn23", "collection": "authors", "collection_id": "1te6q-7mn23", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151021-103915490", "type": "article", "title": "Potassic Granophyre Associated with Precambrian Diabase, Sierra Ancha, Central Arizona", "author": [ { "family_name": "Smith", "given_name": "Douglas", "clpid": "Smith-D" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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.", "doi": "10.1130/0016-7606(1975)86<503:PGAWPD>2.0.CO;2", "issn": "0016-7606", "publisher": "Geological Society of America", "publication": "Geological Society of America Bulletin", "publication_date": "1975-04", "series_number": "4", "volume": "86", "issue": "4", "pages": "503-513" }, { "id": "authors:6g885-w8469", "collection": "authors", "collection_id": "6g885-w8469", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150729-094139122", "type": "article", "title": "Apollo 16 Exploration of Descartes: A Geologic Summary", "author": [ { "family_name": "Ulrich", "given_name": "G. E.", "clpid": "Ulrich-G-E" }, { "family_name": "Rennilson", "given_name": "J. J.", "clpid": "Rennilson-J-J" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" }, { "literal": "Apollo Field Geology Investigation Team" } ], "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.", "doi": "10.1126/science.179.4068.62", "issn": "0036-8075", "publisher": "American Association for the Advancement of Science", "publication": "Science", "publication_date": "1973-01-05", "series_number": "4068", "volume": "179", "issue": "4068", "pages": "62-69" }, { "id": "authors:0dcf9-5pc21", "collection": "authors", "collection_id": "0dcf9-5pc21", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20141111-135550881", "type": "article", "title": "A crustal-upper-mantle model for the Colorado Plateau based on observations of crystalline rock fragments in the Moses Rock Dike", "author": [ { "family_name": "McGetchin", "given_name": "Thomas R.", "clpid": "McGetchin-T-R" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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.", "doi": "10.1029/JB077i035p07022", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research", "publication_date": "1972-12-10", "series_number": "35", "volume": "77", "issue": "35", "pages": "7022-7037" }, { "id": "authors:vfe2y-2jg42", "collection": "authors", "collection_id": "vfe2y-2jg42", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220729-282315900", "type": "article", "title": "Geologic Setting and Petrology of Apollo 15 Anorthosite (15415)", "author": [ { "family_name": "Wilshire", "given_name": "H. G.", "clpid": "Wilshire-H-G" }, { "family_name": "Schaber", "given_name": "G. G.", "clpid": "Schaber-G-G" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" }, { "family_name": "Phinney", "given_name": "W. C.", "clpid": "Phinney-W-C" }, { "family_name": "Jackson", "given_name": "E. D.", "clpid": "Jackson-E-D" } ], "abstract": "Apollo 15 sample number 15415, popularly called the \"Genesis Rock,\" is coarse-grained anorthosite composed largely of calcic plagioclase with small amounts of three pyroxene phases. The rock was found as a clast in a piece of friable soil breccia on the lip of Spur crater, a small young crater on the lower slopes of the Apennine Mountains. The mode of occurrence of sample 15415 indicates that it has undergone at least two, and possibly three or more, fragmentation events. These events are reflected in the texture of the rock by shattered and granulated minerals. An earlier thermal metamorphic event is represented by irregular bands of coarsely recrystallized plagioclase and minor pyroxene that cross larger plagioclase grains. Preliminary observations of textural relations of the large plagioclase grains are consistent either with accumulation of plagioclase followed by overgrowth of cumulus grains and post-cumulus crystallization of minor interstitial pyroxene, or with metamorphic recrystallization that eradicated original textures. Any of the events in the complex history of this rock may have affected apparent radiometric ages. Comparative abundance of similar, though smaller, pieces of anorthositic rock in the area and dominance of originally coarse-grained gabbroic-anorthositic clasts in breccia at Spur crater suggest that sample 15415 is the least-deformed member of a suite of similar rocks that were ejected from beneath the regolith at Spur crater.", "doi": "10.1130/0016-7606(1972)83[1083:gsapoa]2.0.co;2", "issn": "0016-7606", "publisher": "Geological Society of America", "publication": "Geological Society of America Bulletin", "publication_date": "1972-04", "series_number": "4", "volume": "83", "issue": "4", "pages": "1083-1092" }, { "id": "authors:qcpez-8x441", "collection": "authors", "collection_id": "qcpez-8x441", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151111-080003517", "type": "article", "title": "The Apollo 15 Lunar Samples: A Preliminary Description", "author": [ { "family_name": "Gast", "given_name": "P. W.", "clpid": "Gast-PW" }, { "family_name": "Phinney", "given_name": "W. C.", "clpid": "Phinney-W-C" }, { "family_name": "Duke", "given_name": "M. B.", "clpid": "Duke-M-B" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" } ], "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.", "doi": "10.1126/science.175.4020.363", "issn": "0036-8075", "publisher": "American Association for the Advancement of Science", "publication": "Science", "publication_date": "1972-01-28", "series_number": "4020", "volume": "175", "issue": "4020", "pages": "363-375" }, { "id": "authors:qsg92-ppk63", "collection": "authors", "collection_id": "qsg92-ppk63", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150729-123808643", "type": "article", "title": "Geologic Setting of the Apollo 15 Samples", "author": [ { "family_name": "Swann", "given_name": "G. A.", "clpid": "Swann-G-A" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" }, { "family_name": "Rennilson", "given_name": "J. J.", "clpid": "Rennilson-J-J" }, { "literal": "Apollo Lunar Geology Investigation Team" } ], "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.", "doi": "10.1126/science.175.4020.407", "issn": "0036-8075", "publisher": "American Association for the Advancement of Science", "publication": "Science", "publication_date": "1972-01-28", "series_number": "4020", "volume": "175", "issue": "4020", "pages": "407-415" }, { "id": "authors:9vte8-rvy24", "collection": "authors", "collection_id": "9vte8-rvy24", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151022-084358082", "type": "article", "title": "Observaciones geochronologicas sobre los complejos cristalinos de Sonora y Oaxaca, Mexico", "author": [ { "family_name": "Anderson", "given_name": "Thomas H.", "clpid": "Anderson-T-H" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "C\u00f3rdoba", "given_name": "Diego A.", "clpid": "C\u00f3rdoba-D-A" }, { "family_name": "Pearson", "given_name": "Mar\u00eda", "clpid": "Pearson-M" }, { "family_name": "Baenteli", "given_name": "Geraldine", "clpid": "Baenteli-G" } ], "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.", "issn": "0188-2686", "publisher": "Sociedad Geol\u00f3gica Mexicana", "publication": "Memoria - Sociedad Geologica Mexicana", "publication_date": "1972", "pages": "115-122" }, { "id": "authors:wz2xg-99x69", "collection": "authors", "collection_id": "wz2xg-99x69", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140812-151015768", "type": "article", "title": "Preliminary seismological and geological studies of the San Fernando, California, earthquake of February 9 1971", "author": [ { "family_name": "Abrams", "given_name": "M.", "clpid": "Abrams-M" }, { "family_name": "Allen", "given_name": "C.", "clpid": "Allen-C-R" }, { "family_name": "Anderson", "given_name": "D.", "clpid": "Anderson-D-L" }, { "family_name": "Berkey", "given_name": "N.", "clpid": "Berkey-N" }, { "family_name": "Carey", "given_name": "D.", "clpid": "Carey-D" }, { "family_name": "Carter", "given_name": "B.", "clpid": "Carter-B" }, { "family_name": "Davies", "given_name": "G.", "clpid": "Davies-G" }, { "family_name": "Engen", "given_name": "G.", "clpid": "Engen-G-R" }, { "family_name": "Foley", "given_name": "M.", "clpid": "Foley-M-G" }, { "family_name": "Hanks", "given_name": "T.", "clpid": "Hanks-T-C" }, { "family_name": "Helmberger", "given_name": "D.", "clpid": "Helmberger-D-V" }, { "family_name": "Hileman", "given_name": "J.", "clpid": "Hileman-J-A" }, { "family_name": "Jordan", "given_name": "T.", "clpid": "Jordan-T" }, { "family_name": "Jungels", "given_name": "P.", "clpid": "Jungels-P" }, { "family_name": "Kamb", "given_name": "B.", "clpid": "Kamb-B" }, { "family_name": "Liu", "given_name": "H.", "clpid": "Liu-H" }, { "family_name": "Minster", "given_name": "B.", "clpid": "Minster-B" }, { "family_name": "Nordquist", "given_name": "J.", "clpid": "Nordquist-J-M" }, { "family_name": "Penrose", "given_name": "B.", "clpid": "Penrose-B" }, { "family_name": "Silver", "given_name": "L.", "clpid": "Silver-L-T" }, { "family_name": "Smith", "given_name": "R.", "clpid": "Smith-R" }, { "family_name": "Thatcher", "given_name": "W.", "clpid": "Thatcher-W" }, { "family_name": "Thomsen", "given_name": "L.", "clpid": "Thomsen-L" }, { "family_name": "Whitcomb", "given_name": "J.", "clpid": "Whitcomb-J-H" }, { "family_name": "Wood", "given_name": "S.", "clpid": "Wood-S" }, { "literal": "Caltech Division of Geological and Planetary Sciences" } ], "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\u00b024.0'N,\n118\u00b023.7'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 \u00b0 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.", "issn": "0037-1106", "publisher": "Seismological Society of America", "publication": "Bulletin of the Seismological Society of America", "publication_date": "1971-04", "series_number": "2", "volume": "61", "issue": "2", "pages": "491-495" }, { "id": "authors:4hmq6-xbm38", "collection": "authors", "collection_id": "4hmq6-xbm38", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151022-112739977", "type": "article", "title": "Compositional relations in minerals from kimberlite and related rocks in the Moses Rock dike, San Juan County, Utah", "author": [ { "family_name": "McGetchin", "given_name": "Thomas R.", "clpid": "McGetchin-T-R" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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.", "issn": "0003-004X", "publisher": "Mineralogical Society of America", "publication": "American Mineralogist", "publication_date": "1970-09", "series_number": "9-10", "volume": "55", "issue": "9-10", "pages": "1738-1771" }, { "id": "authors:cekvw-jgn98", "collection": "authors", "collection_id": "cekvw-jgn98", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150217-155839306", "type": "article", "title": "Uranium-Thorium-Lead Isotope Relations in Lunar Materials", "author": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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.", "doi": "10.1126/science.167.3918.468", "issn": "0036-8075", "publisher": "American Association for the Advancement of Science", "publication": "Science", "publication_date": "1970-01-30", "series_number": "3918", "volume": "167", "issue": "3918", "pages": "468-471" }, { "id": "authors:b1x6x-rk259", "collection": "authors", "collection_id": "b1x6x-rk259", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20141111-141125063", "type": "article", "title": "Titanoclinohumite: A possible mineralogical site for water in the upper mantle", "author": [ { "family_name": "McGetchin", "given_name": "T. R.", "clpid": "McGetchin-T-R" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" }, { "family_name": "Chodos", "given_name": "A. A.", "clpid": "Chodos-A-A" } ], "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\u00b0C. 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.", "doi": "10.1029/JB075i002p00255", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research", "publication_date": "1970-01-10", "series_number": "2", "volume": "75", "issue": "2", "pages": "255-259" }, { "id": "authors:y7e10-1z039", "collection": "authors", "collection_id": "y7e10-1z039", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151022-082640004", "type": "article", "title": "A geochronologic investigation of the anorthosite complex, Adirondack mountains, New York", "author": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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 \u00b1 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.", "issn": "0548-8265", "publisher": "New York State Museum", "publication": "Memoir - New York State Museum and Science Service", "publication_date": "1968", "pages": "233-251" }, { "id": "authors:5zrsj-j9v13", "collection": "authors", "collection_id": "5zrsj-j9v13", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151029-074635750", "type": "article", "title": "Pre-Cretaceous basement rocks and their bearing on large-scale displacements in the San Andreas fault system", "author": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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.", "issn": "0081-4350", "publisher": "Stanford University", "publication": "Stanford University Publications. Geological Sciences", "publication_date": "1968", "volume": "11", "pages": "279-280" }, { "id": "authors:10802-4h432", "collection": "authors", "collection_id": "10802-4h432", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151021-104842616", "type": "article", "title": "Petrology of eucrites, howardites and mesosiderites", "author": [ { "family_name": "Duke", "given_name": "Michael B.", "clpid": "Duke-M-B" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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\u2014unbrecciated and monomict brecciated achondrites; howardites\u2014polymict 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.", "doi": "10.1016/0016-7037(67)90112-3", "issn": "0016-7037", "publisher": "Elsevier", "publication": "Geochimica et Cosmochimica Acta", "publication_date": "1967-10", "series_number": "10", "volume": "31", "issue": "10", "pages": "1637-1665" }, { "id": "authors:c2w0n-9c540", "collection": "authors", "collection_id": "c2w0n-9c540", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151021-101038760", "type": "article", "title": "Uranium and Lead Isotopic Stability in a Metamict Zircon under Experimental Hydrothermal Conditions", "author": [ { "family_name": "Pidgeon", "given_name": "R. T.", "clpid": "Pidgeon-R-T" }, { "family_name": "O'Neil", "given_name": "J. R.", "clpid": "O'Neil-J-R" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" } ], "abstract": "Disturbance of the uranium-lead isotopic system in a metamict Ceylon zircon has been produced in a 2 molal NaCI solution at 500\u00b0C 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.", "doi": "10.1126/science.154.3756.1538", "issn": "0036-8075", "publisher": "American Association for the Advancement of Science", "publication": "Science", "publication_date": "1966-12-23", "series_number": "3756", "volume": "154", "issue": "3756", "pages": "1538-1540" }, { "id": "authors:vy92p-fjb78", "collection": "authors", "collection_id": "vy92p-fjb78", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20141111-142013966", "type": "article", "title": "Evaluation of the decay constant of uranium 235 from lead isotope ratios", "author": [ { "family_name": "Banks", "given_name": "Philip O.", "clpid": "Banks-P-O" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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 \u03bb_(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.", "doi": "10.1029/JZ071i016p04037", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research", "publication_date": "1966-08-15", "series_number": "16", "volume": "71", "issue": "16", "pages": "4037-4046" }, { "id": "authors:tnknb-gqj76", "collection": "authors", "collection_id": "tnknb-gqj76", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151021-100249965", "type": "article", "title": "Oxygen isotope studies of minerals in stony meteorites", "author": [ { "family_name": "Taylor", "given_name": "Hugh P., Jr.", "clpid": "Taylor-H-P-Jr" }, { "family_name": "Duke", "given_name": "Michael B.", "clpid": "Duke-M-B" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Epstein", "given_name": "Samuel", "clpid": "Epstein-S" } ], "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 \u03b4 = \u22120.5 per mil to \u03b4 = + 8.6 per mil (relative to SMOW), whereas pyroxenes in 8 terrestrial igneous rocks have \u03b4-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\u2014with pyroxene \u03b4-values of 3.7 to 4.4.\n2.\nII. Hypersthene-olivine chondrites, bronzite-olivine chondrites, enstatite chondrites, enstatite achondrites, and nakhlites\u2014with pyroxene \u03b4-values of 5.3 to 6.3.\n3.\nIII. Types I, II and III carbonaceous chondrites, and ureilites\u2014with highly variable olivine and pyroxene \u03b4-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.", "doi": "10.1016/0016-7037(65)90043-8", "issn": "0016-7037", "publisher": "Elsevier", "publication": "Geochimica et Cosmochimica Acta", "publication_date": "1965-05", "series_number": "5", "volume": "29", "issue": "5", "pages": "489-512" }, { "id": "authors:qk6v9-zrr53", "collection": "authors", "collection_id": "qk6v9-zrr53", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140801-125831579", "type": "article", "title": "Re-examination of isotopic relationships in Colorado Front Range uranium ores", "author": [ { "family_name": "Banks", "given_name": "P. O.", "clpid": "Banks-P-O" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "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.", "doi": "10.1130/0016-7606(1964)75[469:ROIRIC]2.0.CO;2", "issn": "0016-7606", "publisher": "Geological Society of America", "publication": "Geological Society of America Bulletin", "publication_date": "1964-05", "series_number": "5", "volume": "75", "issue": "5", "pages": "469-476" }, { "id": "authors:jj2yy-qvx96", "collection": "authors", "collection_id": "jj2yy-qvx96", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190924-083419112", "type": "article", "title": "Lower Cretaceous Pre-Batholithic Rocks of Northern Baja California, Mexico", "author": [ { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" }, { "family_name": "Stehli", "given_name": "F. G.", "clpid": "Stehli-F-G" }, { "family_name": "Allen", "given_name": "C. R.", "clpid": "Allen-C-R" } ], "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.", "issn": "0149-1423", "publisher": "American Association of Petroleum Geologists", "publication": "AAPG Bulletin", "publication_date": "1963-12", "series_number": "12", "volume": "47", "issue": "12", "pages": "2054-2059" }, { "id": "authors:c1y57-3g587", "collection": "authors", "collection_id": "c1y57-3g587", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151022-075922766", "type": "article", "title": "Uranium-Lead Isotopic Variations in Zircons \u2013 A Case Study", "author": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Deutsch", "given_name": "Sarah", "clpid": "Deutsch-S" } ], "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 \u00b1 20 million years on the orogeny called Mazatzal Revolution.", "issn": "0022-1376", "publisher": "University of Chicago Press", "publication": "Journal of Geology", "publication_date": "1963-11", "series_number": "6", "volume": "71", "issue": "6", "pages": "721-758" }, { "id": "authors:j6sfj-z3g54", "collection": "authors", "collection_id": "j6sfj-z3g54", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20170822-102946036", "type": "article", "title": "Precambrian Age Determinations in the Western San Gabriel Mountains, California", "author": [ { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" }, { "family_name": "McKinney", "given_name": "C. R.", "clpid": "McKinney-C-R" }, { "family_name": "Deutsch", "given_name": "S.", "clpid": "Deutsch-S" }, { "family_name": "Bolinger", "given_name": "J.", "clpid": "Bolinger-J" } ], "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.", "doi": "10.1086/626893", "issn": "0022-1376", "publisher": "University of Chicago Press", "publication": "Journal of Geology", "publication_date": "1963-03", "series_number": "2", "volume": "71", "issue": "2", "pages": "196-214" }, { "id": "authors:tdn6g-6y385", "collection": "authors", "collection_id": "tdn6g-6y385", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20131211-090759937", "type": "article", "title": "A study of the ages of the Precambrian of Texas", "author": [ { "family_name": "Wasserburg", "given_name": "G. J.", "orcid": "0000-0002-7957-8029", "clpid": "Wasserburg-G-J" }, { "family_name": "Wetherill", "given_name": "G. W.", "clpid": "Wetherill-G-W" }, { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" }, { "family_name": "Flawn", "given_name": "P. T.", "clpid": "Flawn-P-T" } ], "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.", "doi": "10.1029/JZ067i010p04021", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research", "publication_date": "1962-09", "series_number": "10", "volume": "67", "issue": "10", "pages": "4021-4047" }, { "id": "authors:hcnwr-41p64", "collection": "authors", "collection_id": "hcnwr-41p64", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220718-524934900", "type": "article", "title": "Uranium-Lead Method On Zircons", "author": [ { "family_name": "Silver", "given_name": "L. T.", "clpid": "Silver-L-T" }, { "family_name": "Deutsch", "given_name": "Sarah", "clpid": "Deutsch-Sarah" } ], "abstract": "In this paper I report some data obtained by a study of zircons from a single block of granite approximately 1.5 cu. ft. in volume. The purpose of this investigation was to better our understanding of geological, mineralogical, and physical-chemical controls on the behavior of U-Pb equilibria in these systems. Along with such parameters as zircon grain size, zoning, and morphology, my associates and I hoped to couple the study with a fair understanding of the geologic history of this particular granitic mass; this understanding is based upon a five-year field-mapping and laboratory-investigation of the area in which the mass is found.", "doi": "10.1111/j.1749-6632.1961.tb35460.x", "issn": "0077-8923", "publisher": "New York Academy of Sciences", "publication": "Annals of the New York Academy of Sciences", "publication_date": "1961-04", "series_number": "2", "volume": "91", "issue": "2", "pages": "279-283" }, { "id": "authors:mr3we-re792", "collection": "authors", "collection_id": "mr3we-re792", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140801-124917356", "type": "article", "title": "Agua Blanca fault -- a major transverse structure of northern Baja California, Mexico", "author": [ { "family_name": "Allen", "given_name": "Clarence R.", "clpid": "Allen-C-R" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Stehli", "given_name": "Francis G.", "clpid": "Stehli-F-G" } ], "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 Mat\u00edas, Valle de La Trinidad, Ca\u00f1on de Dolores, Valle de Agua Blanca, Valle de Santo Tom\u00e1s, Bah\u00eda 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.", "doi": "10.1130/0016-7606(1960)71[467:ABFMTS]2.0.CO;2", "issn": "0016-7606", "publisher": "Geological Society of America", "publication": "Geological Society of America Bulletin", "publication_date": "1960-04", "series_number": "4", "volume": "71", "issue": "4", "pages": "467-482" }, { "id": "authors:swdkn-nbz96", "collection": "authors", "collection_id": "swdkn-nbz96", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220812-144834119", "type": "article", "title": "A joint-free sample splitter", "author": [ { "family_name": "McKinney", "given_name": "Charles R.", "clpid": "McKinney-Charles-R" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "abstract": "A sample splitter based on the Jones riffle design has been devised which incorporates certain features not found in other splitters heretofore available. The splitter was folded from one piece of sheet metal and as such has neither cracks or joints to hold up material nor joining metal such as solder to contaminate the sample. It is possible to form the splitter from such materials as plastic and paper or to line metal chutes with these materials. Several sizes of 16-chute splitters have been made in this laboratory; the smallest being made from 0.015 inch thick pure aluminum, each chute opening being \u00bd inch long and \u215b inch wide. This is a microsplitter comparable to the Otto microsplitter in size. The largest splitter was made from 1/32 inch thick pure aluminum, each chute opening being 3 inches long and \u00bd inch wide. An isometric view of a folded 45-degree splitter is shown in Fig. 1. The term \"45-degree\" refers to the angle between the chute bottom and the horizontal. While 45 degrees is greater than the angle of repose, some hang-up is still experienced and cleaning is required. A 60 degree slope should be much better.", "issn": "0003-004X", "publisher": "Mineralogical Society of America", "publication": "American Mineralogist", "publication_date": "1956-06", "series_number": "5-6", "volume": "41", "issue": "5-6", "pages": "521-523" } ]