The oxygen isotope data in this study delineate 2 major episodes of water-rock interaction related to the metamorphic, plutonic, and tectonic development of the metamorphic core complexes in the southern Omineca belt. Episode 1 is a Paleocene pre\u00ad-extensional metamorphic/magmatic-hydrothermal event. The occurrence of isotopically uniform quartz (\u03b4\u00b9\u2078O = 12.5 \u00b1 0.5\u2030) and feldspar (10.9 \u00b1 0.7\u2030) throughout different rock types indicates that much of a 6-km-thick section of the mid-crustal Selkirk allochthon underwent internally buffered \u00b9\u2078O/\u00b9\u2076O homogenization during Paleocene melting and decompression as it moved up the Monashee decollement thrust ramp. Areas of uniform \u03b4\u00b9\u2078O are those with the most leucogranite or those subjected to severe anatexis. Only locally, in the most impermeable (or refractory) zones did 180 exchange among the rocks, leucogranite melts, and aqueous fluids fail to go to completion (i.e., in the deepest parts of the section, in a marble-rich zone, around some thick amphibolites, and in most garnets). Evidence for \u00b9\u2078O/\u00b9\u2076O heterogeneity in the protoliths of these rocks is observed in stratigraphically correlative lower-grade units elsewhere in British Columbia, as well as in garnets that coexist with isotopically homogeneous quartz. A model is introduced utilizing water as a petrologic catalyst: fluids evolved during muscovite breakdown and partial melting of pelites produce \u00b9\u2078O/\u00b9\u2076O homogenization with only minor influx of external H\u2082O; this is followed by release of magmatic H\u2082O from these melts as they crystallize (triggering further melting of adjacent feldspathic assemblages) during and after the ~20 km uplift that occurred in the thrusting event that took place just prior to detachment faulting.
\r\n \r\nEpisode 2 is a series of Eocene synextensional meteoric-hydrothermal events affecting the shallow crust along all of the major detachment faults in the region, and along some parts of the Monashee decollement; these effects were locally enhanced by added heat from some synextensional alkaline intrusions (the Coryell plutons). Very large quartz-feldspar \u00b9\u2078O/\u00b9\u2076O disequilibrium effects were imprinted upon the rocks during exchange with hot meteoric waters (initial \u03b4\u00b9\u2078O ~ -15); the mineral most affected was feldspar (\u03b4\u00b9\u2078O down to -5.0). In the Valhalla core complex, the hanging wall rocks above the Slocan Lake fault are sufficiently uniform to allow us to apply open-system kinetic oxygen isotope exchange modeling, thereby placing constraints on the duration (1-3 Ma) and integrated fluid flux (\u2265 10\u2077 cm\u00b3H\u2082O/cm\u2082rock) for this hydrothermal metamorphism.
", "doi": "10.7907/pkn3-p364", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "id": "thesis:4033", "collection": "thesis", "collection_id": "4033", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10112005-111913", "primary_object_url": { "basename": "Wolf_ra_1997.pdf", "content": "final", "filesize": 8989655, "license": "other", "mime_type": "application/pdf", "url": "/4033/1/Wolf_ra_1997.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "The development of the (U-Th)/He thermochronometer", "author": [ { "family_name": "Wolf", "given_name": "Richmond Andrew", "clpid": "Wolf-Richmond-Andrew" } ], "thesis_advisor": [ { "family_name": "Farley", "given_name": "Kenneth A.", "clpid": "Farley-K-A" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Farley", "given_name": "Kenneth A.", "clpid": "Farley-K-A" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Wernicke", "given_name": "Brian P.", "clpid": "Wernicke-B-P" }, { "family_name": "Burnett", "given_name": "Donald S.", "clpid": "Burnett-D-S" }, { "family_name": "Taylor", "given_name": "Hugh P.", "clpid": "Taylor-H-P" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "(U-Th)/He dating of apatite provides a tool for recording the low temperature (<100\u00b0C) history of the crust. A model based on stopping distances in apatite relates the fraction of alpha particles emitted from the crystal during U and Th decay to crystal size. Helium ages for different sized apatite aliquots are indistinguishable when corrected for the effects of alpha emission. Diffusion coefficients were measured by the incremental outgassing of helium from apatite. The measured range of diffusion parameters is nearly identical for apatites of different chemical composition, grain size, and grain morphology. Isothermal experiments are consistent with spherical diffusion domains which are smaller than the physical grain size.
\r\n\r\nHelium ages may reflect complex thermal histories where samples spend considerable amounts of time in the region where helium is only partially retained. Therefore, the solution to the full radiogenic helium diffusion/production equation is used to interpret helium ages instead of Dodson's (1973) closure temperature formulation. The time required to achieve a steady state between helium production and diffusion at various temperatures can be determined, as well as the range of temperatures defining the helium partial retention zone (the region where helium retentivity is most sensitive to temperature). In general, this zone resides at ~40-80\u00b0C (~2±1 km depth for typical continental geothermal gradients). This is ~35\u00b0C cooler than the analogous apatite fission track partial annealing zone.
\r\n\r\nApplication of the (U-Th)/He method to natural systems has provided consistent results and useful geologic information. Helium ages from the Cajon Pass Drillhole decline from 41.1 to 0.3 Ma between 526 and 2018 m depth, and appear to be in equilibrium with the present thermal gradient. This is in contrast to the previous assertion that the region is in a thermal transient resulting from recent erosion. Helium ages from Mt. San Jacinto, California, decrease monotonically from 79 to 17 Ma with sample elevation, and suggest a modest (~7\u00b0) westward tilting of the block with no evidence of rapid exhumation during this period. Helium ages from Mt. San Jacinto and Cajon Pass are younger than other available thermochronometric techniques, consistent with predictions from laboratory diffusion data.
", "doi": "10.7907/kz3w-5t57", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "id": "thesis:4372", "collection": "thesis", "collection_id": "4372", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11022007-093001", "type": "thesis", "title": "Seismic Strain Rates and the State of Tectonic Stress in the Southern California Region", "author": [ { "family_name": "Huang", "given_name": "Weishi", "clpid": "Huang-Weishi" } ], "thesis_advisor": [ { "family_name": "Kanamori", "given_name": "Hiroo", "orcid": "0000-0001-8219-9428", "clpid": "Kanamori-H" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Anderson", "given_name": "Donald L.", "clpid": "Anderson-D-L" }, { "family_name": "Allen", "given_name": "Clarence R.", "clpid": "Allen-C-R" }, { "family_name": "Kanamori", "given_name": "Hiroo", "orcid": "0000-0001-8219-9428", "clpid": "Kanamori-H" }, { "family_name": "Stock", "given_name": "Joann M.", "orcid": "0000-0003-4816-7865", "clpid": "Stock-J-M" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Clayton", "given_name": "Robert W.", "orcid": "0000-0003-3323-3508", "clpid": "Clayton-R-W" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "I determine 505 fault plane solutions from the first motions of P-waves for the\r\nbackground seismicity (3.0≤ M≤6.0, 1981-1991) and collect mechanisms of major\r\nearthquakes (M≥6.0, 1927-1994) from the literature in the southern California region.\r\nThen I study the seismic strain and tectonic stress fields in individual domains (ten in\r\ntotal) by analyzing these mechanism data. The seismic strain tensors are obtained by\r\ntensorial summation of individual seismic moment tensors. The tectonic stress tensors\r\nare determined by performing numerical inversions of the slip vector data, using\r\nAngelier's (1990) method. The findings are summarized as follows:
\r\n\r\n(1) Of the 505 fault plane solutions for the 1981-1991 background seismicity, 54%\r\nare strike-slip (SF), 21 % reverse (RF), 17% normal (NF), and 8% oblique-slip\r\nfaulting (OS) events. The catalog of the major earthquakes for the period 1927-\r\n1994 also displays similar proportions of the faulting mechanisms;
\r\n\r\n(2) The similarity of the focal mechanisms can be measured by a parameter, seismic\r\nconsistency (Sc) introduced by Apperson (1991). It is defined as the ratio of the\r\nscalar moment of the total moment tensor to the sum of the scalar moments of\r\nindividual moment tensors. In southern California, the Brawley fault (BYF)\r\ndomain shows the highest Sc (0.70), whereas the White Wolf fault (WWF)\r\ndomain displays the lowest Sc (0.44). Sc values in other domains vary between\r\nthe above two values;
\r\n\r\n(3) The depths of possible low-angle faults inferred from the fault plane solutions\r\nvary from 20 km in the Transverse ranges where N-S convergence dominates, to\r\nonly 1 km in the southern Sierra Nevada fault (SSNF) domain where E-W divergence\r\ndominates. Our current data do not show the existence of a sigle unified\r\nseismically-active master detachment in the seismogenic zone;
\r\n\r\n(4) The axes of the maximum principal stress, \u03b41. are oriented N6\u00b0E \u00b111\u00b0, whereas\r\nthose of the maximum principal strain, \u03b51 are oriented N5\u00b0E \u00b121\u00b0;
\r\n\r\n(5) The strain and stress tensors are similar to each other in the Mojave (MVE), San\r\nJacinto (SJF), Elsinore (ESF), BYF, western and eastern Transverse Ranges\r\n(WTR, ETR) domains, but dissimilar in the central Transverse Ranges (CTR),\r\nNewport-Inglewood fault (NIF), WWF, and SSNF domains. Areas with small\r\nvalues of \u03a6 = (\u03b42 - \u03b43)/(\u03b41 - \u03b43) (<0.35) such as the WTR, CTR, and NIF domains are associated with more than 40% of RF events. Areas with \u03a6 values around 0.5\r\nsuch as the SJF, ETR, WWF, ESF, BYF, and MVE domains are associated with\r\nmore than 47% of SF events. The SSNF domain has a large \u03a6 (>0.65) and\r\nshows 49% of NF events, Variation of the state of stress appears to be in the\r\nTransverse Ranges where hypocenters are generally deep. Other areas show a\r\nrelatively stable state of stress throughout the seismogenic depth;
\r\n\r\n(6) Seismic fraction of deformation, \u03b7, is a measure of the deformation mode. It is\r\ndefined as the ratio of seismic strain rate to the total deformation rate. Because of\r\nthe limited seismic data, we can usually estimate the apparent instead of the real\r\nseismic fraction of deformation. Therefore, caution must be exercised in applying\r\nthe values of \u03b7 to evaluations of seismic potential, In southern California, there\r\nare some indications that areas in which seismic deformation nearly accounts for\r\nthe total deformation are typically associated with cold and rigid batholithic rocks\r\nor high seismic velocity anomalies such as in the SJF, south central MVE, WWF,\r\nand possibly the ETR domains. However, areas with low seismic velocity\r\nanomalies are not free of earthquakes as seen, for example, in the BYF domain,\r\nwhich shows \u03b7 = 0.6-1.0. Other domains show \u03b7 < 0.4. The problem of whether the\r\nmissing deformation is being released aseismically or has accumulated elastically\r\nremains to be resolved.
", "doi": "10.7907/P2W6-3155", "publication_date": "1995", "thesis_type": "phd", "thesis_year": "1995" }, { "id": "thesis:7373", "collection": "thesis", "collection_id": "7373", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01072013-092342447", "primary_object_url": { "basename": "Grant_lb_1993.pdf", "content": "final", "filesize": 31001841, "license": "other", "mime_type": "application/pdf", "url": "/7373/1/Grant_lb_1993.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Characterization of Large Earthquakes on the San Andreas Fault in the Carrizo Plain: Implications for Fault Mechanics and Seismic Hazard", "author": [ { "family_name": "Grant", "given_name": "Lisa Baugh", "clpid": "Grant-Lisa-Baugh" } ], "thesis_advisor": [ { "family_name": "Sieh", "given_name": "Kerry E.", "clpid": "Sieh-K-E" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Brooks", "given_name": "Norman H.", "clpid": "Brooks-N-H" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Despite the widespread use of geomorphic offset measurements for\r\ncalculating earthquake probabilities, little attention has been paid to either the\r\nuncertainties in the interpretation of offset geomorphic features, or the effects\r\nof these uncertainties on fault models and estimates of seismic hazard.\r\nInterpretation of offsets along the San Andreas fault in the Carrizo Plain have\r\nbeen the basis of hypotheses of a strong Carrizo fault segment which regularly\r\nbreaks in great earthquakes several centuries apart with dextral surface slip on\r\nthe order of 10 m per event.
\r\n\r\nThe smallest geomorphic offset measurements along a 6 km stretch of\r\nthe fault southeast of Wallace Creek vary between ~6.5 m and ~10m. A 3-D\r\nexcavation of alluvial deposits at the Phelan fan shows that at least 6.6 to 6.9\r\nm of dextral slip occurred during the 1857 Fort Tejon earthquake, and that the\r\npenultimate earthquake occurred several centuries prior to 1857. Thus, either\r\nthe amount of surface slip varied several meters over a 2-3 km stretch of the\r\nfault in 1857, or 2 to 3 meters of slip in a penultimate earthquake was\r\nfollowed by ~7 m of slip in 1857.
\r\n\r\nTwo monuments from an 1855 survey which spans the San Andreas\r\nfault in the Carrizo Plain have been displaced 11.0 \u00b1 2.5m right-laterally by the\r\ngreat earthquake of 1857. This magnitude of offset is consistent with\r\ngeomorphic indications that slip across the fault during the 1857 earthquake\r\nand associated foreshocks and aftershocks varied from 6.6 to 10 m over 2.6 km\r\nalong this section. Comparison of recent geodetic measurements with the\r\nlate Holocene slip rate at Wallace Creek shows that fault slip rates determined\r\nfrom short-term wide aperture measurements are indistinguishable from\r\nrates determined from long-term narrow aperture measurements. Using\r\nradiocarbon dates of the penultimate large earthquake and measurements of\r\nslip in 1857, we calculate an average slip rate for the last complete earthquake\r\ncycle that is at least 25% lower than the late Holocene slip rate on the main\r\nfault trace. This suggests that variation in fault slip during the 1857\r\nearthquake left a slip deficit in at least the upper 1km of crust at Wallace\r\nCreek. Slip in future earthquakes may compensate this deficit.
\r\n\r\nThree trenches across the San Andreas fault on the Bidart fan in the\r\nCarrizo Plain record evidence of 7 previous earthquakes. Radiocarbon dating\r\nindicates five earthquakes, including the 1857 earthquake, have occurred\r\nsince A.D. 1218. The penultimate earthquake, event B, occurred between 1405\r\nand 1510 A.D. Several centuries before 1857, events B, C, D and E occurred in\r\na temporal cluster after approximately 1218 A.D. and prior to 1510 A.D. The\r\naverage recurrence interval within this cluster ranges from 73 to 116 years,\r\ndepending on assumptions. Events B and D may correlate with prehistoric\r\nearthquakes recorded in sediments elsewhere along the southern San\r\nAndreas fault. Events C and E appear to have ruptured locally in smaller\r\nmagnitude earthquakes. Surface slip from either event B, or events B and C\r\ncombined, totals 7 to 11 m.
\r\n\r\nIf fault strength is defined by long earthquake repeat time, then the\r\nCarrizo segment of the San Andreas is not inherently stronger than the\r\nMojave segment. The temporal and spatial distribution of large earthquakes\r\non the San Andreas fault is difficult to reconcile with slip-based theories of\r\nsegmentation of strike-slip faults. Temporal patterns of seismicity may be\r\nmore robust than spatial trends. Clusters of large earthquakes analogous to\r\nsequences of foreshocks, mainshocks and aftershocks may occur on longer\r\ntime scales of seismic \"supercycles.\"
\r\n", "doi": "10.7907/zs8s-9j51", "publication_date": "1993", "thesis_type": "phd", "thesis_year": "1993" }, { "id": "thesis:2759", "collection": "thesis", "collection_id": "2759", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06282007-082748", "primary_object_url": { "basename": "maher-ka_1989 .pdf", "content": "final", "filesize": 18356906, "license": "other", "mime_type": "application/pdf", "url": "/2759/43/maher-ka_1989 .pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Geology of the Jackson Mountains, Northwest Nevada", "author": [ { "family_name": "Maher", "given_name": "Kevin A.", "clpid": "Maher-Kevin-A" } ], "thesis_advisor": [ { "family_name": "Saleeby", "given_name": "Jason B.", "clpid": "Saleeby-J-B" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Kirschvink", "given_name": "Joseph L.", "orcid": "0000-0001-9486-6689", "clpid": "Kirschvink-J-L" }, { "family_name": "Murray", "given_name": "Bruce C.", "clpid": "Murray-B-C" }, { "family_name": "Saleeby", "given_name": "Jason B.", "clpid": "Saleeby-J-B" }, { "family_name": "Sieh", "given_name": "Kerry E.", "orcid": "0000-0002-7311-2447", "clpid": "Sieh-K-E" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The Jackson Mountains are located in the western Great Basin in Humboldt County, northwest Nevada. The range contains a late Paleozoic to Mesozoic depositional sequence. This sequence records sedimentation, volcanism and deformation in a back-arc setting. The Mississippian to late Early Permian McGill Canyon Formation was deposited in basinal to slope to distal shelf environments, dominated by hemipelagic and turbiditic facies. In the Permian there was an volcanic arc andesite component, and a nearby contemporaneous carbonate platform shed olistostromes into the unit. The McGill Canyon was laid down in an area between the McCloud arc and the Havallah back-arc basin. The late Middle Triassic to middle Norian Bliss Canyon Formation was laid down in basinal to fore-reef to carbonate platform to lagoonal to terrigenous littoral environments. Both of these formations are of flap sequences deposited on an east-facing, back-arc margin. The Bliss Canyon represents the western margin of the Early Mesozoic marine basin of the western Great Basin. From the late Norian to the Bathonian, several stages of subearial volcanism and alluvial epiclastic sedimentation laid down the Happy Creek Formation, a thick arc andesite volcanic pile. The Happy Creek is part of the Early Mesozoic Cordilleran magmatic arc province. In the Bathonian, this volcanic pile was cut by a conjugate sinistral high-angle wrench fault system as volcanism waned. During the Callovian, sediments of the King Lear Formation filled in and then overlapped the wrench basins. These sediments were derived from the east, where a west-vergent thrust system was active. This phase of thrusting ceased by the Oxfordian. Arc-related silicic volcanism and alluvial to fluvial sedimentation within the King Tear continued into the Aptian, when the thrusts were reactivated during a second phase. Both phases of thrusting verged both east and west. Stocks, dikes and sills of the Early Mesozoic Intrusive suite are comagmatic with the volcanism in the Happy Creek and King Lear, and intrude the sedimentary units. This suite both plugs and is truncated by the wrench faults and the first phase of thrusting, but is cut by the second phase. The Jackson Mountains are part of the Black Rock terrane in northwest Nevada. Within this terrane, the rocks share a common tectonic history and stratigraphy distinct from the neighboring terranes, and are separated from them by Mesozoic thrust and strike-slip faults.
", "doi": "10.7907/DE6N-4051", "publication_date": "1989", "thesis_type": "phd", "thesis_year": "1989" }, { "id": "thesis:8067", "collection": "thesis", "collection_id": "8067", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02112014-112136459", "primary_object_url": { "basename": "Nourse_ja_1989.pdf", "content": "final", "filesize": 60754614, "license": "other", "mime_type": "application/pdf", "url": "/8067/2/Nourse_ja_1989.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Geological Evolution of Two Crustal Scale Shear Zones. Part I: The Rand Thrust Complex, Northwestern Mojave Desert, California. Part II: The Magdalena Metamorphic Core Complex, North Central Sonora, Mexico", "author": [ { "family_name": "Nourse", "given_name": "Jonathan Alan", "clpid": "Nourse-Jonathan-Alan" } ], "thesis_advisor": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Sieh", "given_name": "Kerry E.", "orcid": "0000-0002-7311-2447", "clpid": "Sieh-K-E" }, { "family_name": "Saleeby", "given_name": "Jason B.", "clpid": "Saleeby-J-B" }, { "family_name": "Allen", "given_name": "Clarence R.", "clpid": "Allen-C-R" }, { "family_name": "Kirschvink", "given_name": "Joseph L.", "orcid": "0000-0001-9486-6689", "clpid": "Kirschvink-J-L" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The geology and structure of two crustal scale shear zones were studied to understand the partitioning of strain within intracontinental orogenic belts. Movement histories and regional tectonic implications are deduced from observational data. The two widely separated study areas bear the imprint of intense Late Mesozoic through Middle Cenozoic tectonic activity. A regional transition from Late Cretaceous-Early Tertiary plutonism, metamorphism, and shortening strain to Middle Tertiary extension and magmatism is preserved in each area, with contrasting environments and mechanisms. Compressional phases of this tectonic history are better displayed in the Rand Mountains, whereas younger extensional structures dominate rock fabrics in the Magdalena area.
\r\n\r\nIn the northwestern Mojave desert, the Rand Thrust Complex reveals a stack of four distinctive tectonic plates offset along the Garlock Fault. The lowermost plate, Rand Schist, is composed of greenschist facies metagraywacke, metachert, and metabasalt. Rand Schist is structurally overlain by Johannesburg Gneiss (= garnet-amphibolite grade orthogneisses, marbles and quartzites), which in turn is overlain by a Late Cretaceous hornblende-biotite granodiorite. Biotite granite forms the fourth and highest plate. Initial assembly of the tectonic stack involved a Late Cretaceous? south or southwest vergent overthrusting event in which Johannesburg Gneiss was imbricated and attenuated between Rand Schist and hornblende-biotite granodiorite. Thrusting postdated metamorphism and deformation of the lower two plates in separate environments. A post-kinematic stock, the Late Cretaceous Randsburg Granodiorite, intrudes deep levels of the complex and contains xenoliths of both Rand Schist and mylonitized Johannesburg? gneiss. Minimum shortening implied by the map patterns is 20 kilometers.
\r\n\r\nSome low angle faults of the Rand Thrust Complex formed or were reactivated between Late Cretaceous and Early Miocene time. South-southwest directed mylonites derived from Johannesburg Gneiss are commonly overprinted by less penetrative north-northeast vergent structures. Available kinematic information at shallower structural levels indicates that late disturbance(s) culminated in northward transport of the uppermost plate. Persistence of brittle fabrics along certain structural horizons suggests a possible association of late movement(s) with regionally known detachment faults. The four plates were juxtaposed and significant intraplate movements had ceased prior to Early Miocene emplacement of rhyolite porphyry dikes.
\r\n\r\nIn the Magdalena region of north central Sonora, components of a pre-Middle Cretaceous stratigraphy are used as strain markers in tracking the evolution of a long lived orogenic belt. Important elements of the tectonic history include: (1) Compression during the Late Cretaceous and Early Tertiary, accompanied by plutonism, metamorphism, and ductile strain at depth, and thrust driven? syntectonic sedimentation at the surface. (2) Middle Tertiary transition to crustal extension, initially recorded by intrusion of leucogranites, inflation of the previously shortened middle and upper crustal section, and surface volcanism. (3) Gravity induced development of a normal sense ductile shear zone at mid crustal levels, with eventual detachment and southwestward displacement of the upper crustal stratigraphy by Early Miocene time.
\r\n\r\nElucidation of the metamorphic core complex evolution just described was facilitated by fortuitous preservation of a unique assemblage of rocks and structures. The \"type\" stratigraphy utilized for regional correlation and strain analysis includes a Jurassic volcanic arc assemblage overlain by an Upper Jurassic-Lower Cretaceous quartz pebble conglomerate, in turn overlain by marine strata with fossiliferous Aptian-Albian limestones. The Jurassic strata, comprised of (a) rhyolite porphyries interstratified with quartz arenites, (b) rhyolite cobble conglomerate, and (c) intrusive granite porphyries, are known to rest on Precambrian basement north and east of the study area. The quartz pebble conglomerate is correlated with the Glance Conglomerate of southeastern Arizona and northeastern Sonora. The marine sequence represents part of an isolated arm? of the Bisbee Basin.
\r\n\r\nCrosscutting structural relationships between the pre-Middle Cretaceous supracrustal section, younger plutons, and deformational fabrics allow the tectonic sequence to be determined. Earliest phases of a Late Cretaceous-Early Tertiary orogeny are marked by emplacement of the 78 \u00b1 3 Ma Guacomea Granodiorite (U/Pb zircon, Anderson et al., 1980) as a sill into deep levels of the layered Jurassic series. Subsequent regional metamorphism and ductile strain is recorded by a penetrative schistosity and lineation, and east-west trending folds. These fabrics are intruded by post-kinematic Early Tertiary? two mica granites. At shallower crustal levels, the orogeny is represented by north directed thrust faulting, formation of a large intermontane basin, and development of a pronounced unconformity. A second important phase of ductile strain followed Middle Tertiary? emplacement of leucogranites as sills and northwest trending dikes into intermediate levels of the deformed section (surficial volcanism was also active during this transitional period to regional extension). Gravitational instabilities resulting from crustal swelling via intrusion and thermal expansion led to development of a ductile shear zone within the stratigraphic horizon occupied by a laterally extensive leucogranite sill. With continued extension, upper crustal brittle normal faults (detachment faults) enhanced the uplift and tectonic denudation of this mylonite zone, ultimately resulting in southwestward displacement of the upper crustal stratigraphy.
\r\n\r\nStrains associated with the two ductile deformation events have been successfully partitioned through a multifaceted analysis. Rf/\u00d8 measurements on various markers from the \"type\" stratigraphy allow a gradient representing cumulative strain since Middle Cretaceous time to be determined. From this gradient, noncoaxial strains accrued since emplacement of the leucogranites may be removed. Irrotational components of the postleucogranite strain are measured from quartz grain shapes in deformed granites; rotational components (shear strains) are determined from S-C fabrics and from restoration of rotated dike and vein networks. Structural observations and strain data are compatable with a deformation path of: (1) coaxial strain (pure shear?), followed by (2) injection of leucogranites as dikes (perpendicular to the minimum principle stress) and sills (parallel to the minimum principle stress), then (3) southwest directed simple shear. Modeling the late strain gradient as a simple shear zone permits a minimum displacement of 10 kilometers on the Magdalena mylonite zone/detachment fault system. Removal of the Middle Tertiary noncoaxial strains yields a residual (or pre-existing) strain gradient representative of the Late Cretaceous-Early Tertiary deformation. Several partially destrained cross sections, restored to the time of leucogranite emplacement, illustrate the idea that the upper plate of the core complex bas been detached from a region of significant topographic relief. 50% to 100% bulk extension across a 50 kilometer wide corridor is demonstrated.
\r\n\r\nLate Cenozoic tectonics of the Magdalena region are dominated by Basin and Range style faulting. Northeast and north-northwest trending high angle normal faults have interacted to extend the crust in an east-west direction. Net extension for this period is minor (10% to 15%) in comparison to the Middle Tertiary detachment related extensional episode.
", "doi": "10.7907/S427-2704", "publication_date": "1989", "thesis_type": "phd", "thesis_year": "1989" }, { "id": "thesis:490", "collection": "thesis", "collection_id": "490", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02042005-111245", "primary_object_url": { "basename": "Manduca_cca_1988.pdf", "content": "final", "filesize": 22647149, "license": "other", "mime_type": "application/pdf", "url": "/490/21/Manduca_cca_1988.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Geology and Geochemistry of the Oceanic Arc-Continent Boundary in the Western Idaho Batholith near McCall", "author": [ { "family_name": "Manduca", "given_name": "Cathryn Clement Allen", "clpid": "Manduca-Cathryn-Clement-Allen" } ], "thesis_advisor": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Taylor", "given_name": "Hugh P.", "clpid": "Taylor-H-P" } ], "thesis_committee": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Taylor", "given_name": "Hugh P.", "clpid": "Taylor-H-P" }, { "family_name": "Saleeby", "given_name": "Jason B.", "clpid": "Saleeby-J-B" }, { "family_name": "Stevenson", "given_name": "David John", "clpid": "Stevenson-D-J" }, { "family_name": "Albee", "given_name": "Arden Leroy", "clpid": "Albee-A-L" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "A major lithospheric boundary is preserved within the western Idaho Batholith. The juxtaposition of two suites of supracrustal rocks, exposed as sheets within intrusive rocks, is the expression of this boundary at the level of exposure. The western suite of mafic layered gneisses are inferred to be metamorphosed oceanic arc rocks; the eastern suite of biotite schist, quartzite and calc-silicate gneiss are inferred to be metamorphosed continental sedimentary rocks. Three broadly Cretaceous, plutonic and meta-plutonic complexes record the presence of the boundary at greater depth. The Hazard Creek complex, west of the supracrustal boundary, is comprised of epidote-bearing intrusives. The Little Goose Creek complex is comprised primarily of the porphyritic orthogneiss that intruded the supracrustal boundary. The Payette River complex, east of the supracrustal boundary, is comprised of large bodies of tonalite and granite.
\r\n\r\nEach complex has a distinct geochemical character. The Hazard Creek complex is dominantly a tonalite-trondhjemite suite characterized by Ri less than .7045, \u03b4\u00b9\u2078O less than 8.4, high Sr, Na\u2082O and Al\u2082O\u2083 concentrations and low MgO, Rb and K\u2082O concentrations. Porphyritic orthogneiss in the Little Goose Creek complex has a remarkable range in Ri and \u03b4\u00b9\u2078O (.7042-.7097, 8.0-10.7). The porphyritic orthogneiss is interpreted as dominated by two components: one similar in composition to the Hazard Creek complex and a second, modeled as Precambrian sedimentary material, with high Ri, \u03b4\u00b9\u2078O and K\u2082O concentrations and lower Sr concentration. The Payette River complex has generally high Ri (.7076-.7100) and variable \u03b4\u00b9\u2078O (7.2-10.4). The geochemical changes indicate that the supracrustal boundary is the surface expression of a steeply-dipping structure which juxtaposes oceanic-arc lithosphere against continental lithosphere. An abrupt geochemical discontinuity preserved within the porphyritic orthogneiss, near the change in supracrustal rocks, may reflect an abrupt discontinuity at depth or may be due to the juxtapositon of portions of a stratified pluton. The juxtaposition of lithospheric blocks must have occured prior to intrusion of the porphyritic orthogneiss approximately 111 Ma, and most probably, occured before 118 Ma, prior to the beginning of plutonism. No structural evidence for the initial formation of the boundary is recognized; it is proposed to form by transform faulting or by rifting followed by convergence.
\r\n\r\nEpisodic or continuous deformation along the boundary began prior to 118 Ma and produced four sets of structures. The oldest structures are foliation and isoclinal folding of crystalloblastic gneisses which may have formed during rapid burial of oceanic-arc rocks west of the boundary. Compressive deformation, forming north-south striking steeply-dipping foliations and steeply-plunging lineations in the eastern portion of the Hazard Creek complex, was broadly coeval with its emplacement. Igneous foliation and lineation with similar orientation formed during emplacement of the Payette River complex around 90 Ma. The youngest penetrative deformation formed similarly oriented, mylonitic fabrics in a 10 km wide zone centered on the boundary. All but the oldest structures are inferred to have formed by flattening and vertical flow in response to east-west compression. Deformation is interpreted to represent the response of a preexisting lithospheric boundary to compressive stresses related to subduction of material to the west.
", "doi": "10.7907/JP15-KC07", "publication_date": "1988", "thesis_type": "phd", "thesis_year": "1988" }, { "id": "thesis:2981", "collection": "thesis", "collection_id": "2981", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-07252007-135803", "primary_object_url": { "basename": "Powell_re_1981.pdf", "content": "final", "filesize": 18615907, "license": "other", "mime_type": "application/pdf", "url": "/2981/16/Powell_re_1981.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Geology of the Crystalline Basement Complex, Eastern Transverse Ranges, Southern California: Constraints on Regional Tectonic Interpretation", "author": [ { "family_name": "Powell", "given_name": "Robert Edward", "clpid": "Powell-Robert-Edward" } ], "thesis_advisor": [ { "family_name": "Allen", "given_name": "Clarence R.", "clpid": "Allen-C-R" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Albee", "given_name": "Arden Leroy", "clpid": "Albee-A-L" }, { "family_name": "Kamb", "given_name": "W. Barclay", "clpid": "Kamb-W-B" }, { "family_name": "Saleeby", "given_name": "Jason B.", "clpid": "Saleeby-J-B" }, { "family_name": "Sieh", "given_name": "Kerry E.", "orcid": "0000-0002-7311-2447", "clpid": "Sieh-K-E" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" }, { "family_name": "Allen", "given_name": "Clarence R.", "clpid": "Allen-C-R" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "About 3000 km2 within the crystalline basement complex of the Eastern Transverse Ranges in the Chuckwalla, Orocopia, Eagle, Cottonwood, Hexie, Little San Bernardino, and Pinto Mountains of Riverside County, California were mapped at scales of 1:36,000 and 1:62,500 and compiled at 1:125,000 (Plate I). Pre-Jurassic(?) (i.e., older than the Mesozoic batholiths) rocks of the crystalline complex comprise two lithologically distinct terranes. These terranes are called the Joshua Tree and San Gabriel terranes for regions of southern California in which their lithologies were initially characterized. The two terranes are superposed along a previously unrecognized low-angle fault system of regional extent, the Red Cloud thrust.
\r\n\r\nDuring the course of this study, the structurally lower Joshua Tree terrane has been defined as a stratigraphically coherent group of crystalline rocks that consists of Precambrian granite capped by a paleo-weathered zone and overlain nonconformably by orthoquartzite that interfingers westward with pelitic and feldspathic granofelses. The quartzite contains near-basal quartz/quartzite clast conglomerates, and has well-preserved cross-bedding that appears upright wherever it has been observed. Pelitic and feldspathic granofelses crop out to the west of the quartzite exposures in four lithologically different belts that trend northnorthwest throughout the area mapped. These lithologic belts are interpreted to have been derived from stratigraphically interfingering sedimentary protoliths deposited in a basin offshore from a quartzose beach-sand protolith. In proximity to the early Red Cloud thrust, this whole stratigraphic package was pervasively deformed to granite gneiss, stretched pebble conglomerate, lineated quartzite, and schist.
\r\n\r\nA northeast-trending pattern of metamorphic isograds was orthogonally superimposed on the northnorthwest-trending protoliths of the Pinto gneiss. A central andalusite zone, located in the southern Little San Bernardino and Hexie, and northern Eagle Mountains, is flanked to the northwest and southeast by sillimanite zones. Coincident with this symmetrical distribution of aluminosilicates is an asymmetrical distribution of other pelitic mineral zones, with prograde cordierite-aluminosilicate-biotite- and K-feldspar-aluminosilicate-bearing assemblages to the northwest in the northern Little San Bernardino and Pinto Mountains, staurolite-bearing assemblages in a narrow zone in the southern Little San Bernardino-Hexie and northern Eagle Mountains, and retrograde chlorite-muscovite-bearing assemblages in the southernmost Little San Bernardino, Cottonwood, southern Eagle, Orocopia, and Chuckwalla Mountains. One occurrence of chloritoid-sillimanite in the central Eagle Mountains is apparently also retrograde. The crossing isograds are interpreted to result from a temporal increase in PH2O relative to PT from south to north through the field area. Comparison of the pelitic assemblages with experimental studies suggests peak conditions of PT \u2248 3.5 to 4 kb, T \u2248 525 to 625\u00b0C. The early prograde metamorphism pre-dated the thrusting event; the retrograde stage may have overlapped in time with the emplacement of the San Gabriel terrane allochthon. Cordierite-orthoamphibole-bearing assemblages are present in one stratigraphic zone of the Pinto gneiss.
\r\n\r\nIn this study, the Precambrian lithologies of the San Gabriel terrane are viewed as a three-part deep crustal section, with uppermost amphibolite grade pelitic (Hexie) gneiss intruded by granodioritic (Soledad) augen gneiss at the highest level, retrograded granulite (Augustine) gneiss at an intermediate level, and syenite-mangerite-jotunite at the lowest level exposed in the Eastern Transverse Ranges. The Hexie gneiss, characterized by sillimanite-garnet-biotite-bearing assemblages, is thrust over andalusite-bearing granofels of the Pinto gneiss.
\r\n\r\nThe Red Cloud thrust system is inferred to have developed in three or four sequential structural events: 1) early thrusting that probably moved parallel to the ENE mineral lineations recorded in both plates; 2) regional folding of the initial thrust surface around NNE-trending axes; 3) later thrusting that broke with some component of westward movement across a fold in the older thrust surface to produce a stacking of crystalline thrust plates of the two terranes; 4) continued or renewed folding of both thrust faults with eventual overturning toward the SW. It is consistent with all observations to date to link these structural events into a single regional tectonic episode that resulted in westward-vergent allochthonous emplacement of the San Gabriel terrane over Joshua Tree terrane. The thrust timing can only be loosely bracketed in time between 1195 m.y. and 165 m.y. ago.
\r\n\r\nThe pre-batholithic terranes and the westward-vergent Red Cloud thrust are considered to be exotic with respect to the pre-batholithic rocks and structures exposed to the north and east of the field area. The bounding discontinuity has been obliterated by intrusion of both suites of Mesozoic batholithic rocks.
\r\n\r\nThe Mesozoic plutonic rocks comprise two batholithic suites, both of which intrude the Joshua Tree and San Gabriel terranes and the Red Cloud thrust system. NW-SE trending belts of plutonic lithologies have been mapped within each suite: the oldest lithology of the younger suite intrudes the youngest lithology of the older suite. The older suite, Jurassic(?), lying to the NE, appears to have an alkalic character; the younger suite, Cretaceous(?), appears calc-alkaline. The older suite consists of biotite- and K-feldspar-bearing gabbro-diorites intruded by low-quartz monzogranites. The younger suite includes hornblende-biotite-sphene granodiorite intruded by porphyritic monzogranites, intruded in turn by nonporphyritic monzogranite.
\r\n\r\nThe Eastern Transverse Ranges south of the Pinto Mountain fault are defined by several Cenozoic E-W left-lateral strike-slip faults that have a cumulative westward displacement from S to N of about 50 km. The left-lateral faults are interpreted to form part of a conjugate fault set with complementary right-lateral faults in the Mojave and Colorado Deserts. Along the western boundary of the Eastern Transverse Ranges in the Little San Bernardino Mountains, the crystalline rocks have been pervasively cataclasized by an event that post-dates intrusion of the Cretaceous(?) plutonic rocks. The cataclasis is attributed to the Vincent-Orocopia-Chocolate Mountain thrust that is thought to superpose the diverse pre-batholithic and batholithic rocks of the Eastern Transverse Ranges above Pelona-type schist. The cataclastic foliation is folded along the length of the Little San Bernardino Mountains in an antiform that is inferred to be cored with Pelona-type schist. This fold may have formed a single antiformal feature comprising all the crystalline-rock antiforms now recognized along the San Andreas fault that are cored by Pelona-type schist. Displacements of the piercing points formed by the antiformal axis apparently indicate 220 km of right-lateral offset on the present San Andreas strand and about 80 km of right-lateral offset along a fragmented older San Andreas strand that consisted of the San Francisquito, Fenner, and Clemens Well faults and a buried extension of this fault beneath the alluvial fill of the valley between the Chocolate and Chuckwalla Mountains.
\r\n", "doi": "10.7907/F22F-YX04", "publication_date": "1981", "thesis_type": "phd", "thesis_year": "1981" }, { "id": "thesis:108", "collection": "thesis", "collection_id": "108", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01102008-082801", "primary_object_url": { "basename": "Carter_ba_1980.pdf", "content": "final", "filesize": 38302391, "license": "other", "mime_type": "application/pdf", "url": "/108/1/Carter_ba_1980.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Structure and Petrology of the San Gabriel Anorthosite-Syenite Body, Los Angeles County, California", "author": [ { "family_name": "Carter", "given_name": "Bruce Alan", "clpid": "Carter-Bruce-Alan" } ], "thesis_advisor": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The San Gabriel anorthosite-syenite body is part of large layered intrusive, part of which underlies about 250 square kilometers in the western San Gabriel Mountains between the San Gabriel and San Andreas fault zones 30 kilometers north of Los Angeles. Although not subjected to post-emplacement regional metamorphism, the Precambrian anorthosite is intruded by the early Triassic Mt. Lowe granodiorite and the late Cretaceous Mt. Josephine granodiorite, and is deformed by broad folds of at least two ages (Triassic(?) and mid-Cenozoic), which have produced several kilometers of structural relief within the body.
\r\n\r\nSeveral sets of faults in the area were active in Tertiary time, but none of them show evidence of Holocene activity. From oldest to youngest, these faults include: (1) several more or less east-west faults with major apparent right-lateral or left-lateral and dip-slip displacements; (2) a NE-trending fault set with important left-lateral displacements; and (3) younger NW-trending faults with small dip-slip and right-lateral displacements. The San Gabriel fault, which lies 1-4 kilometers southwest of its margin, must cut the anorthosite-syenite body at depth and may offset anorthositic rocks at least 28-46 kilometers in a right-lateral sense.
\r\n\r\nThe San Gabriel anorthosite-syenite body is part of a large allochthonous sheet which is floored by a zone of unusual cataclastic gneisses, exposed in the northeastern part of the area in and near to Mill Canyon. Slickensides, lineations and minor folds within this zone suggest latest movement parallel to a N-S or NE-SW direction. Strongly deformed and mylonitized gneisses below the anorthosite include lineated granodioritic gneiss possibly equivalent to the late-Cretaceous Mt. Josephine granodiorite, gabbroic to anorthositic gneisses, and layered amphibolitic gneisses unlike any others seen in the western San Gabriel Mountains. Large thrust displacements probably occurred within this zone as well as along the Vincent thrust 45 kilometers to the east, which resembles this zone and may be related to it.
\r\n\r\nThe anorthosite-syenite body was intruded into previously metamorphosed, granulite-grade Mendenhall gneiss about 1200 million years ago. The body is a large, layered intrusive in which bottom crystal accumulation produced the observed anorthosite-gabbro-syenite differentiation suite. The part of this body now exposed was at least 10 kilometers in thickness and about 15 kilometers in diameter. It probably had the form of an inverted cone, with a sub-horizontal, concordant upper contact. Primary quartz is rare in rocks of this suite; the following lithologies have been distinguished on the basis of the percentage and composition of their constituent feldspars: anorthosite, leucogabbro, gabbro, ferrogabbro (all with calcic andesine), ultramafite (olivine, augite, ilmenite and apatite), jotunite (predominantly antiperthitic sodic andesine), mangerite (antiperthite and mesoperthite) and syenite (predominantly mesoperthite).
\r\n\r\nThe San Gabriel anorthosite-syenite body has been subdivided into three main stratigraphic units (from oldest to youngest): (1) The lowest, largest unit consists of thick sequences of massive anorthosite alternating with layered leucogabbro. This unit is at least 7 kilometers in thickness and becomes more mafic near its top. (2) The overlying syenite unit locally attains thickness of at least 3-5 kilometers, but is commonly much thinner or absent. In some areas, the basal 100-1000 meters of this unit is extremely mafic, but otherwise it is fairly homogeneous and massive, with no cyclic or cryptic layering. (3) The uppermost jotunite unit is a highly compositionally variable unit which intruded overlying granulite gneiss and has been subdivided into 5 subunits. This unit is at least 3-4 kilometers in maximum thickness. It is apparently younger than the syenite, and in places grades downward into syenite. Several masses of hornblende-bytownite gabbro within anorthosite are probably not directly related to the anorthosite-syenite body.
\r\n\r\nPrimary cumulate structures and textures in these rocks have greatly aided the structural interpretation of this body and provide strong evidence of its origin by bottom crystal accumulation. Large, 6 to 25-centimeter hypersthene crystals, which ophitically enclose numerous well-oriented 1 to 3-centimeter plagioclase tablets, indicate a cumulate origin of the leucogabbro. Occasional crescumulate layers in leucogabbro, especially near the margin of the body, formed when first plagioclase and then hypersthene grew from the floor upward into the magma. Slump structures are poorly defined in leucogabbro, but are excellently developed in the mafic lower part of the syenite unit and in some parts of the jotunite unit, and include 1 to 20-meter slump blocks and deformed compositional layers. Many 3-centimeter to 3-meter layers in the mafic lower part of the syenite unit and in the jotunite unit are both size- and density-graded, with coarser, ferromagnesian-mineral-enriched bases, and are extremely useful structural indicators.
\r\n\r\nLarge angular blocks of anorthosite (to 20 meters) are abundant in layered mafic rocks at the base of the syenite unit and in some parts of the jotunite unit. These are slump blocks, which indicate that parts of the syenite and jotunite units accumulated at the base of major tectonic scarps which developed in rocks of the anorthosite-leucogabbro unit which formed the floor of the chambers of the later magmas.
\r\n\r\nAll of the 20 chemical analyses of rocks from this body are iron-rich; the lowest FeO+Fe\u2082O\u2083/FeO+Fe\u2082O\u2083+MgO ratio (wt. %) is 0.61 and most are between 0.70 and 0.95, which suggest that the original magma was probably similarly iron-rich.
\r\n\r\nChemical analyses of these rocks define general trends with substantial scatter on variation diagrams, and in detail those from each of the three units define individual fields with little or no overlap. Thus a common line of descent for all rocks of the body seems somewhat unlikely. The estimated makeup of the entire body is about 46% anorthosite, 23% leucogabbro, 4% gabbro, 12% syenite, 11% jotunite and 4% ultramafite, giving the following suggested average composition: SiO\u2082, 53.73%, TiO\u2082, 1.17%, Al\u2082O\u2083, 22.33%, Fe\u2082O\u2083, 1.66%, FeO, 4.54%, MgO, 1.72%, CaO, 8.08%, Na\u2082O, 5.05%, K\u2082, 1.18%, P\u2082O\u2085, 0.42%.
\r\n\r\nLimited mineral composition data show that: (1) Plagioclase in anorthosite and leucogabbro ranges between about An\u2083\u2085 and An\u2085\u2085 with most between An\u2084\u2080 and An\u2085\u2080. Plagioclase in not concentrically zoned, but is inhomogeneous with a range of about 3-4% anorthite in individual crystals. (2) There are cyclic compositional variations of plagioclase in the anorthosite-leucogabbro unit, with more albite-rich and more anorthite-rich compositons alternating over hundreds of meters of the stratigraphic section. (3) There is no apparent consistent cryptic variation of proxene and olivine compositions within the syenite and jotunite units.
\r\n\r\nPostcumulous recrystallization has drastically altered the fabric of most anorthositic and leucogabbroic rocks and has produced extremely coarse grained textures. Pervasive deuteric uralitic alteration of the primary ferromagnesian minerals in all but a few rocks of the syenite and jotunite units suggests that the magma probably had a relatively high water content.
\r\n\r\nIt is possible that all of the rocks of the San Gabriel anorthosite-syenite body could have been produced by differentiation by fractional crystallization of a trachyandesitic parent magma, successive fractions of which were intruded into the magma chamber. However, the detailed sequence of lithologies, mineral compositions and the compositions of the three units suggest that at least two independently generated magmas may have combined to produce the San Gabriel anorthosite-syenite body.
\r\n\r\nSome important contributions of this study are: (1) the detailed geologic map of the anorthosite-syenite body and determination of its post-emplacement structural history; (2) the description of the complete suite of lithologies and their contact relationships; (3) the description of the mineralogical compositions of each lithology and the recognition of cryptic variation of plagioclase in the anorthosite-leucogabbro unit; (4) the description of the sequence of post-accumulation processes including recrystallization and the hydration of most primary ferromagnesian minerals; (5) the recognition of uninverted pigeonite in rocks of the jotunite and syenite units; and (6) recognition that this andesine anorthosite massif is actually a large deformed stratiform pluton.
", "doi": "10.7907/Q9N7-MS19", "publication_date": "1980", "thesis_type": "phd", "thesis_year": "1980" }, { "id": "thesis:13676", "collection": "thesis", "collection_id": "13676", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04152020-165415853", "type": "thesis", "title": "Regional Variations in the Lead Isotopic Characteristics of Late Cenozoic Basalts from the Southwestern United States", "author": [ { "family_name": "Everson", "given_name": "Joel Earl", "clpid": "Everson-Joel-Earl" } ], "thesis_advisor": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Over a hundred new analyses on late Cenozoic basalts characterize and delimit regional patterns of lead isotopic behavior in the southwestern United States. The lead isotopic systematics of these volcanic rocks can be divided into three broad regional groups: one characteristic of Colorado Plateau (CP) volcanic fields, a second found in many areas of the Basin and Range (BR) province, and in neighboring areas along the Pacific coast, and a third peculiar to an area in southern Nevada (SN) Colorado Plateau volcanic rocks have generally unradiogenic leads (\u00b2\u2070\u2076Pb/\u00b2\u2070\u2074Pb \u2264 18.7), locally exhibit large variations in isotopic ratio, and define linear arrays (secondary isochrons) on a \u00b2\u2070\u2076Pb/\u00b2\u2070\u2074Pb-\u00b2\u2070\u2077Pb/\u00b2\u2070\u2074Pb diagram. Basin and Range type samples are characterized by relatively radiogenic leads (18.7 \u2264 \u00b2\u2070\u2076Pb/\u00b2\u2070\u2074Pb \u2264 19.6) and, comparatively, a uniformity of isotopic composition in local areas. Southern Nevada leads are somewhat unradiogenic (18.2 \u2264 \u00b2\u2070\u2076Pb/\u00b2\u2070\u2074Pb \u2264 18.5), appear to display a local uniformity in isotopic composition, and lie off the Colorado Plateau secondary isochrons. Literature sources indicate that southern Nevada basalts are also characterized by relatively high \u2078\u2077Sr/\u2078\u2076Sr ratios (.7060-.7080).
\r\n\r\nThe isotopic properties of volcanic rocks from each of these regions do not appear to record crustal contamination. The general uniformity of isotopic characteristics over large areas (hundreds to thousands of kilometers in size) of the Southwest suggests a fundamental consistency in volcanic source region character over comparable dimensions. Isotopic variations within each province seem to record smaller scale source region heterogeneities.
\r\n\r\nIsotopic variability observed within individual Colorado Plateau volcanic fields may be interpreted in terms of kilometer-sized mantle heterogeneities developed approximately 1.4-1.8 by ago. These heterogeneities may record the effects of magma extraction from mantle sources during the primary generation of Precambrian crystalline basement and the formation of the continental lithosphere. A rough but apparently significant correlation between the chemical and lead isotopic compositions of CP basalts suggests that the mantle of the continental lithosphere may be crudely \"stratified\" in its lead isotopic composition, with the uppermost mantle generally less radiogenic in lead (and by inference more depleted in uranium relative to lead) than the deeper lithosphere. Isotopic differences distinguishable between different portions of the Colorado Plateau may relate to heterogeneities hundreds of kilometers in size which developed in different mantle domains prior to and in the course of continental lithosphere formation.
\r\n\r\nBasin and Range basalts have lead isotopic properties (and Sr and Nd isotopic compositions) similar to volcanic rocks from oceanic island and volcanic arc settings, suggesting a correspondence between BR volcanic source regions and \"oceanic\" mantle. Within the Basin and Range province \u00b2\u2070\u2076Pb/\u00b2\u2070\u2074Pb ratios tend to be rather uniform laterally for distances measured in hundreds of kilometers; this implies that the underlying mantle may be isotopically rather homogeneous for similar distances. One 200 km long section of the Rio Grande rift is characterized by CP type leads. This may possibly indicate that a large body of \"oceanic\" mantle has penetrated the continental lithosphere beneath this segment of the Rio Grande rift.
\r\n\r\nOlder (Pliocene and Miocene) calc-alkaline volcanic rocks from the Basin and Range province tend to display somewhat higher \u00b2\u2070\u2077Pb/\u00b2\u2070\u2074Pb ratios than the Quaternary alkaline basalts within each area of the Basin and Range province. This could suggest that adjustments in BR type source region characteristics continued very late into the Cenozoic era, perhaps in conjunction with changes in the tectonic environment.
\r\n\r\nSN type isotopic systematics resist a unique interpretation, but may reflect long term (of the order of a billion years or so) evolutionary characteristics of a source region in the mantle or the lower crust.
\r\n\r\nAlthough the different isotopic province boundaries appear to record discontinuities deep within the earth (most probably fundamental discontinuities in mantle character), they follow the trends of major geological and structural boundaries in the crust. The SN isotopic province, for example, appears to be associated geographically with a \"fundamental, west-trending, transverse crustal boundary\" (Eaton, 1975) which demarcates the northern and southern portions of the Basin and Range province. The Basin and Range - Colorado Plateau isotopic province boundary approximately follows the Mesozoic-early Tertiary Cordilleran foreland thrust belt and the limit of late Cenozoic normal faulting. Thus the BR-CP isotopic boundary appears to be geographically related to the boundary between tectonically unstable, \"orogenic\" areas of the Basin and Range province and Pacific border regions, and the more stable \"platform\" areas of the Colorado Plateau, Southern Rocky Mountains, and Great Plains.
\r\n\r\nBR type leads occur in some areas (notably southern Arizona and southeastern California) characterized by known Precambrian basement. These areas were presumably underlain originally by \"old\" mantle belonging to the continental lithosphere. The inferred presence of BR type, \"oceanic\" mantle beneath these regions during late Cenozoic times seems to imply a disruption of continental lithosphere and the emplacement of \"new\" oceanic mantle. The geographic association of Mesozoic and Cenozoic structural boundaries in the crust and the BR-CP isotopic boundary suggests that this mantle disruption may be related either to the compressional tectonism of Mesozoic-early Tertiary times or to the extensional tectonism of the late Cenozoic.
", "doi": "10.7907/2dp3-4b64", "publication_date": "1979", "thesis_type": "phd", "thesis_year": "1979" }, { "id": "thesis:11753", "collection": "thesis", "collection_id": "11753", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:07252019-083750670", "type": "thesis", "title": "Rare Earths Abundances and Fractionations and their Implications for Batholithic Petrogenesis in the Peninsular Ranges Batholith, California, USA, and Baja California, Mexico", "author": [ { "family_name": "Gromet", "given_name": "L. Peter", "clpid": "Gromet-L-Peter" } ], "thesis_advisor": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Rare Earth Element (REE) patterns of plutonic rocks across the Peninsular Ranges batholith vary systematically west to east, transverse to the long axis and structural trends of the batholith. Three major parallel elongate geographic regions are each defined by distinct REE pattern types. Rocks from the western region display slight light REE enrichment, flat heavy REE, and negative Eu anomalies. An abrupt transition to rocks with middle and heavy REE fractionated and depleted REE patterns with no or positive Eu anomalies occurs in the central region of the batholith. Further to the east a second transition to strongly light REE enriched rocks some of which have positive or negative Eu anomalies occurs. Some gabbros may show divergent patterns.
\r\n\r\nThese large variations are observed even in similar lithologies across the three regions and notably in tonalites, the major rock type of the batholith. The slopes of the REE patterns within rocks of each region are largely independent of rock type, and no consistent variations in REE abundances and Eu anomalies with lithology are noted with the exception of some gabbros. Most of the leucogranodiorites of the western region have larger negative Eu anomalies than nearby tonalites. Granodioritic rocks of the central and eastern regions may have positive, negative, or no Eu anomalies.
\r\n\r\nThese results are the first report of systematic variations in REE characteristics across a granitic batholith whose geologic setting at a convergent plate boundary has been established. Some similarities and contrasts to REE variations across modern volcanic arcs are noted. Along the westernmost margin of the batholith in northern Baja California, Mexico, leucotonalitic rocks of the San Telmo pluton display essentially flat REE patterns strongly resembling those observed for near-trench volcanic rocks. The REE patterns of quartz gabbros and tonalites of the western region correspond closely to those of circum-Pacific high-alumina basalts. The heavy REE depleted and fractionated patterns observed in the rocks of the central and eastern regions of the batholith do not have counterparts in oceanic island volcanic arcs, and few counterparts in continental margin volcanic arcs.
\r\n\r\nThe REE variations generally correlate with other transverse asymmetries in major petrologic and geochemical characteristics. The abrupt depletion and fractionation in the middle to heavy REE and elimination of negative Eu anomalies appear coupled to an increase in Sr concentration and a marked restriction in lithologic diversity. This transition occurs over a range of initial 87Sr/86Sr ratios. The light REE enriched rocks of the eastern region are distinguished from the central and western regions by higher initial ratios. Geographic discontinuities in \u03b418O and age distributions in the batholith correlate approximately with the REE discontinuities, but locally diverge by the dimensions of one or two plutons.
\r\n\r\nDeterminations of REE abundances in major and trace phases of a representative eastern region granodiorite indicate accessory sphene and allanite are the major reservoirs of REE in this rock. Hornblende is the only significant REE site in the major minerals, and in some batholithic lithologies it may be the dominant site. High-level crystal fractionations involving hornblende and accessory phases do not appear capable of producing the observed geographic characteristics. Contamination processes including upper crustal material also seem ruled out.
\r\n\r\nThe REE and other geochemical variations across the batholith appear to originate in deep-seated sources. Partial melting in source rocks in which assemblages rich in plagioclase give way laterally to garnet-bearing assemblages in source regions of broadly basaltic composition which are already zoned in light REE abundances, 87Sr/86Sr, \u03b418O, and possibly Sr content appears to account for most of the observed features. The geologic context of the source material remains largely undefined and may include mantle and crustal components. However, the source regions for all parts of the batholith must have bulk compositions and phase assemblages capable of producing the dominant tonalite and low-K2O granodiorite lithologies. This major constraint appears to strongly limit the amount of sialic crustal material permitted to be present in the source regions. The geometry of the convergent boundary appears to have determined the elongate form of the batholith, and, probably, the general alignment of all the geochemical variations along its length. The results of this study may be useful in comparing possibly related crust-forming processes and products in other orogenic-plutonic terrains.
\r\n\r\n", "doi": "10.7907/1X44-DN70", "publication_date": "1979", "thesis_type": "phd", "thesis_year": "1979" }, { "id": "thesis:5623", "collection": "thesis", "collection_id": "5623", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03282010-104553997", "primary_object_url": { "basename": "Murray_jd_1978.pdf", "content": "final", "filesize": 64732611, "license": "other", "mime_type": "application/pdf", "url": "/5623/1/Murray_jd_1978.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "The Structure and Petrology of the San Jose Pluton, Northern Baja California, Mexico", "author": [ { "family_name": "Murray", "given_name": "Jay Dennis", "clpid": "Murray-Jay-Dennis" } ], "thesis_advisor": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The San Jose pluton, a 117-km^2 body of medium-grained biotite-hornblende tonalite in the foothills of the Sierra San Pedro M\u00e1rtir, northern Baja California, Mexico, is one of the more westerly plutons in the Cretaceous Peninsular Ranges batholith. The pluton has a teardrop form in plan view, elongate north-south and outlined by commonly well-exposed contacts. To the north the outline is remarkably hemi-circular and concordant to isoclinally folded, highly stretched strata. Traced southward, the form becomes increasingly discordant and tapers irregularly to a point. The wall rocks consist of tuffs, tuffaceous sedimentary rocks, and lesser amounts of marble, volcanic flows, and hypabyssal intrusive rocks of primarily basaltic to andesitic composition. Aptian and Albian fossils place the strata in the lower Cretaceous. Hornblende-hornfels facies assemblages are developed near the pluton contacts, but lower greenschist facies assemblages prevail outside the contact aureole.\r\n\r\nDetailed field and laboratory studies indicate that the pluton is chemically and mineralogically very uniform. The mapping has revealed, however, three texturally distinguishable lithologies each of significant areal extent and coherent form. (1) An interior core of prismatic hornblende tonalite (PHbT) is characterized by elongate prismatic hornblende, thin platy biotite, and a seriate grain-size distribution. (2) An outer horseshoe-shaped partial shell of stubby hornblende tonalite (SHbT) characterized by equant, equigranular, and anhedral poikilitic hornblende and biotite surrounds the PHbT. Together these units comprise the northern two thirds of the body. (3) A seriate porphyritic tonalite (SPT) phase distinguished from the PHbT by the presence of sparse phenocrystic plagioclase grains 6-20 mm in size forms the discordant southern wedge.\r\n\r\nThese major lithologic types represent two, or possibly three, distinct intrusive pulses. Conclusive evidence of the relative ages of the SHbT and PHbT intrusive units is lacking. However, the relationships of both units to several narrow zones of diorite and inclusion-rich tonalite associated with their mutual contact in the northeast, together with the scattered occurrence throughout the interior PHbT unit of inclusions of wall-rock lithologies similar to those around the northern and eastern margins of the pluton, strongly suggest that the SHbT is the younger mass and was emplaced around the perimeter of a pre-existing PHbT pluton. The SPT is completely gradational, both structurally and petrographically, into the PHbT, but is not exposed in contact with the SHbT. The SPT appears to be the youngest of the three units and may be either a late phase of the PHbT intrusive pulse or a separate pulse intruded prior to complete solidification of the PHbT.\r\n\r\nThe SHbT and PHbT units appear to have been emplaced almost entirely by forceful shouldering aside of the wall rocks. The effects of the penetrative deformation and distension of the wall rocks accompanying intrusion (and enhanced by externally-imposed deformation) are intense around the northern contact. These effects decrease gradually southward, giving way to predominantly brittle deformation within the wallrocks around the southern and particularly the southwestern margins. Emplacement of the SPT involved large-scale stoping as well as lateral displacement of the strata. Altogether, stoping appears to have contributed no more than 5-10% to the exposed area of the pluton.\r\n\r\nMagmatic flow plus marginal protoclasis produced an unusually regular pattern of northward-convex, arcuate foliation in the northern two thirds of the pluton. Foliation is gneissose and conformable to the contact near the pluton margins and decreases to weak in the interior.\tThe dip of foliation changes systematically from steeply outward around the margins through vertical to inward dipping in the interior. Traced southward into the SPT, foliation becomes faint or unrecognizable, the dip increases gradually again to vertical, and the pattern becomes only vaguely concentric and not obviously related everywhere to the local contacts. There is a suggestion of closure (in plan view) of vertical foliation attitudes around the probable center of latest intrusion in the interior of the SPT.\r\n\r\nThis foliation pattern suggests a funnel-shaped geometry of intrusion of PHbT and SPT magmas rising through relatively narrow conduits and expanding primarily to the north and east at essentially the final level of emplacement. The center of intrusion apparently migrated slightly southward with time from the interior of the PHbT into the center of the SPT. The SHbT was emplaced separately around the margins of the PHbT prior to intrusion of much or all of the SPT. Again expansion was preferentially to the north and east.\r\n\r\nThe pattern of deflection of wall rocks around the pluton also indicates predominantly northward and eastward expansion; lesser southward expansion was limited to late fracture-controlled breaking out of the SPT magma from the concordant confines of the northern masses.\r\n\r\nPetrographic and structural observations indicate that preferred mineral orientation developed primarily by rotation of crystals in the magma rather than by solid-state deformation and recrystallization. However, flow deformation may have continued locally after the completion of magmatic crystallization, particularly in the protoclastic gneissose rocks.\r\n\r\nVariations in the intensity of foliation and protoclasis in the tonalite and of penetrative deformation and distension in the wall rocks are attributed in large part to the history of progressive intrusion and asymmetric expansion. However, the pluton was apparently syntectonic, and externally-imposed deformation not directly related to emplacement of the San Jose pluton also contributed to the deformation of the wall rocks and outer parts of the tonalite, particularly around the northern and eastern margins. Systematic lineation patterns developed in both the wall rocks and the marginal gneissose tonalite in the latter areas suggest two sources of externally-imposed deformation: (1) southward expansion or shoving of the nearby Las Cochas pluton towards the northern margin of the San Jose pluton, and (2) deformation directed apparently upward and southwestward from the northeast and east, possibly in response to plutonic emplacement or regional uplift in the Sierra San PedroMartir.\r\n\r\nAlthough the oval outline and striking concordance of the pluton may be superficially suggestive of en masse diapiric emplacement, as opposed to gradual rise of magma through narrow channelways and balloon-like expansion at the final level of emplacement, the asymmetry of both the foliation pattern in the tonalite and the deformation of the wall rocks appears inconsistent with such a mechanism.\r\n\r\nMineralogically, the tonalite consists on the average of 64% plagioclase, 16% quartz, 11% hornblende, 5-6% biotite, and <0.3% alkali feldspar. Plagioclase, zoned from cores of An_(40-45) to rims of An_(20-30), was the principal liquidus phase, accompanied by minor clino-pyroxene in the SHbT and by hornblende and locally minor clinopyroxene in the PHbT and SPT. Biotite crystallized as a late interstitial or poikilitic phase primarily in coexistence with hornblende rather than as a reaction product. The trace amounts of late interstitial alkali feldspar (sanidine-orthoclase) are unusually potassic (0r_(90-95)) and do not appear to have crystallized in equilibrium with plagioclase; much or all of the potassium feldspar and possibly some of the biotite may have crystallized from a late-stage exsolved vapor phase.\r\n\r\nThe assemblage of opaque minerals -- nearly pure magnetite coexisting with complex lamellar intergrowths of titano-hematite and ferrian-ilmenite --implies crystallization under unusually oxidizing conditions apparently approaching those of the Mn0-Mn_30_4 buffer during much of crystallization, and increasing to those of the hematite-magnetite buffer at near-solidus conditions. Low Fe/(Fe + Mg) ratios in biotite (0.40-0.44), hornblende (0.32-0.36), and relict clinopyroxene (0.23-0.26) and low whole-rock Fe0/(Fe) + Fe_2O_3) ratios (0.408-0.515) also record unusually oxidizing conditions. Textural characteristics distinguishing the SHbT phase, on the one hand, from the PHbT and SPT phases, on the other, appear to be due to slight differences in f_0_2, f_H_2O and possibly temperature throughout all but the late stages of crystallization (SHbT magma drier, less oxidizing, and possibly slightly hotter).\r\n\r\nBoth the modal and chemical data confirm the petrologic significance of the textural units defined in the field. Together the PHbT and SPT define one set of variation trends which, for a given Si0_2 or modal quartz content, are characterized by slightly higher normative or modal plagioclase content and slightly lower normative or modal color index than the combined trends of the SUbT and its gneissose border phase. The SPT samples show a much more restricted range of modal and chemical compositions than do the PHbT samples, supporting the identification of the SPT as a third unit closely related to the PHbT. The composition of the outer gneissose phase of the SHbT is also distinctive, being slightly more felsic rather than more mafic than the rest of the tonalite.\r\n\r\nDespite the systematic compositional distinctions and the evidence for at least two, possibly three separate pulses of intrusion, both the modal and chemical compositions are remarkably uniform. SiO_2 contents range from 59.4 to 65.2% (exclusive of the gneissose border rocks). Variations in other elements are correspondingly much smaller. The SHbT and SPT, in particular, are both exceptionally homogeneous, with Si0_2 contents consistently in the ranges 60.1-62.0 and 61.8-64.1%, respectively. Yet both the strong compositional zoning in plagioclase and the sequence of mineral crystallization indicate that the composition of the melt changed considerably as crystallization proceeded. The homogeneity of the tonalite, particularly within the individual textural units, therefore implies highly viscous magmas in which processes of segregation or differential migration of crystals relative to melt (e.g., gravity settling or flow differentiation) were generally ineffective except on a small scale (local schlieren). Multiple emplacement of such large volumes of homogeneous magmas, rising essentially through the same conduit, without visible association of materials with other compositions argues strongly that the tonalite magmas were generated as primary magmas.\r\n\r\nChemically, the tonalite is characterized by a calcic alkali-lime index (63); high Na_2O and low K_2O contents (average: 4.82 and 0.74%, respectively), hence high Na_2O/K_2O ratios (average: 6.51); high Al_2O_3 and normative plagioclase contents (18.17 and 66.37%, respectively); and low Fe0/(Fe0 + Fe_2O_3) ratios (average: 0.463). The pluton is depleted in Rb, U, Th, and rare earth elements, and the ^(87)Sr/^(86)Sr initial (0.7036) and Pb isotopic ratios (206/204 = 18.56, 207/204 = 15.58, and 208/204 = 38.19) are relatively unradiogenic. The low concentrations of K and other incompatible elements and the relatively unradiogenic Sr initial and Pb isotopic ratios rule out significant contribution from older sialic crust or related sedimentary rocks. Partial melting of a material chemically similar to ocean floor (ocean ridge or abyssal) basalt, whether subducted oceanic crust or basaltic areas in the overlying mantle or lower crust, appears to best satisfy all of the compositional constraints.\r\n", "doi": "10.7907/jrav-6990", "publication_date": "1978", "thesis_type": "phd", "thesis_year": "1978" }, { "id": "thesis:6794", "collection": "thesis", "collection_id": "6794", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01302012-140641318", "primary_object_url": { "basename": "Hadley_dm_1978.pdf", "content": "final", "filesize": 31294031, "license": "other", "mime_type": "application/pdf", "url": "/6794/1/Hadley_dm_1978.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Geophysical Investigations of the Structure and Tectonics of Southern California", "author": [ { "family_name": "Hadley", "given_name": "David Milton", "clpid": "Hadley-David-Milton" } ], "thesis_advisor": [ { "family_name": "Kanamori", "given_name": "Hiroo", "clpid": "Kanamori-H" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "Seismological Laboratory" }, { "literal": "div_gps" } ], "abstract": "Regional variations in the crustal structure of southern California are defined by travel-time data from natural and artificial events. We show that the crust of the Mojave, northeastern Peninsular Ranges, eastern Transverse Ranges and Colorado Desert is dominated by a velocity of 6.2\u00b10.1 km/sec. The western Transverse Ranges and the western portion of the Peninsular Ranges are typified by a crustal\r\nvelocity of 6.7 km/sec. The data indicate that the Transverse Ranges do not have a distinct crustal root. As the topography is not supported isostatically, the Range must be sustained by major north-south compression. A composite profile extending north from the southern end of the Salton Sea defines a crustal thickness for the\r\nCoachella Valley of less than 20 km. Through the inversion of Rayleigh wave dispersion data obtained from the analysis of teleseismic surface waves recorded across southern California, we have obtained average S-wave models for the southern Mojave-central Transverse Ranges and the Peninsular Ranges. The observed P-wave velocities and the calculated Poisson's ratio from both P- and S-wave data require a quartz rich crust for the Mojave and a more mafic crust for the Peninsular Ranges. All S-wave models suggest a slight mid- crustal velocity reversal that is approximately coincident with the bottom of the seismic\r\nzone.
\r\n\r\nRegional variations in P_n velocities are obtained from several reversed refraction profiles . These data show that P_n varies from 7.7 to 8.2 km/sec. The high P_n values, 8.2 km/sec, are observed in the eastern Mojave, the western Transverse Ranges and the Coast Ranges.\r\nThe 7.8 km/sec P_n velocity extends from the Imperial Valley, through the central Transverse Ranges, and across the western Mojave. P_n profiles indicate that the Moho beneath the eastern Transverse Ranges and the southeastern Mojave dips 2-3\u00b0 west.
\r\n\r\nP-delay studies of a vertically incident PKP phase indicate that a high velocity, 8.3 km/sec structure exists within the shallow upper mantle beneath much of the geomorphic Transverse Ranges. This feature is not offset by the San Andreas fault. We suggest that the continuity of this anomaly across the plate boundary indicates that if the upper mantle participates in plate motion, the mantle plate boundary must be laterally displaced from the crustal boundary. We suggest that the mantle boundary may extend northwest from the Salton Trough and across the eastern end of the velocity anomaly, in the vicinity of the active Helendale-Lenwood-Camprock faults. We propose that the\r\nhorizontal decoupling between the crust and mantle, required by the lateral displacement at depth of the plate boundary, is accommodated, in part, within the 7.8 km/sec layer.
\r\n", "doi": "10.7907/1GTN-2W64", "publication_date": "1978", "thesis_type": "phd", "thesis_year": "1978" }, { "id": "thesis:16287", "collection": "thesis", "collection_id": "16287", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02062024-221007119", "type": "thesis", "title": "Petrology, Structure, and Evolution of a Precambrian Volcanic and Plutonic Complex, Tonto Basin, Gila County, Arizona", "author": [ { "family_name": "Conway", "given_name": "Clay Michael", "clpid": "Conway-Clay-Michael" } ], "thesis_advisor": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Precambrian exposures in Tonto Basin, central Arizona, are among the best in the southwest and the rocks are superbly preserved. Stratigraphic, structural, and petrologic relations of the Tonto Basin rocks, as determined in this study, contribute to our understanding of an important interval of Precambrian history in the southwest, and to the petrogenesis of volcanic and plutonic rocks emplaced in a great silicic alkali magmatic event.
\r\n\r\nIn the only detailed field study within the Tonto Basin prior to the present work, Gordon Gastil defined a stratigraphic sequence fundamentally eugeosynclinal (wacke, slate) in lower parts becoming more miogeosynclinal upward (conglomerate, sandstone) and culminating with a great thickness of rhyolite. Current mapping shows that the extrusive rhyolite sequence is about 2 km thick and is overlain by a km of quartzite. Through joint efforts of L. T. Silver, K. R. Ludwig and the author, a correlation is apparent with corresponding eugeosynclinal, rhyolite and quartzite sequences in the Mazatzal Mountains. The names Alder Group, Haigler Group, and Mazatzal Group are proposed for the respective sequences in both areas. The Alder Group includes not only Gastil's lowermost Alder Formation (here renamed Breadpan Formation) but all of his overlying formations beneath the Haigler rhyolite. Haigler Group in Tonto Basin is composed of Winter Camp Formation, Haigler rhyolite undivided, and overlying Oxbow Rhyolite. The first was the basal part of Gastil's Haigler Formation, and the Oxbow Rhyolite remains as defined by Gastil. Haigler rhyolite undivided includes most of Gastil's Haigler Formation, much of his Hell's Gate Rhyolite (the remainder is intrusive), and extrusive rhyolite which is wide-spread in Tonto Basin outside Gastil's mapped area. The name Christopher Mountain Quartzite is proposed for quartzite of the Mazatzal Group overlying Haigler Group rhyolites in Tonto Basin.
\r\n\r\nThe folded sedimentary and volcanic strata of these three groups occur in a NE-SW belt flanked on both NW and SE by large granite bodies. The southeasterly granite, near Young, was shown by Gastil to be intrusive into Alder Group strata. The northwesterly granite (Payson Granite) was not mapped by him in detail. He hypothesized that both granites were part of a single widespread batholith and that the Payson Granite was gradational through granophyre and intrusive rhyolite into contemporaneous rhyolite (Haigler Group). L. T. Silver subsequently obtained U-Pb zircon ages of 1730 \u00b1 15 m.y. for the Payson Granite, 1650 \u00b1 15 m.y. for the granite at Young, and 1715 \u00b1 15 m.y. for a rhyolite flow in the Alder Group (Flying W Formation), in apparent disagreement with both aspects of Gastil's hypothesis. This apparently placed Payson Granite in a northern, older regional geochronologic province (volcanic rocks~ 1750 - 1820 m.y.; plutonic rocks~ 1720 - 1760 m.y.) and the granite at Young and associated stratified rocks in a southern, younger regional province (volcanic rocks~ 1700 - 1720 m.y.; plutonic rocks~ 1650 - 1700 m.y.), suggesting that Payson Granite might be part of a basement upon which the volcanic and sedimentary rocks were deposited.
\r\n\r\nMajor findings of the present study relating to this problem are that Payson Granite-granophyre, granophyre-intrusive rhyolite, and intrusive rhyolite..,.e:xtrusive rhyolfte contacts are non-gradational intrusive contacts and that Payson Granite has a smooth upper surface dipping gently and apparently concordantly southward beneath the sedimentary and volcanic strata. Enormous composite sills intruded, each beneath the preceding, in the sequence (1) rhyolite porphyry (Hell's Gate Rhyolite and King Ridge Rhyolite), (2) granophyre (Green Valley Granophyre), and (3) alaskite along the upper Payson Granite surface primarily between granite and Haigler rhyolite but locally into the folded stratified rocks (including Alder and Mazatzal Group rocks).
\r\n\r\nThe sills are widespread along the upper Payson Granite surface and no direct relationship between Payson Granite and the stratified rocks could be determined. However, at Gisela where the felsic sills are locally absent, Payson Granite intrudes a small body of distinctive finegrained rock characterized by megacrysts of plagioclase (mafite porphyry). Another of the small scattered masses of mafite porphyry intrudes Haigler rhyolite. It is hypothesized that all mafite porphyries are a single generation of igneous intrusive rock and that all occurrences are correlative. If this is so, Payson Granite must be younger than Haigler rhyolite. In support of this hypothesis, sedimentary and volcanic rocks of nearby pendants in diorite of the Gibson Complex (a differentiated gabbro-diorite body intruded by Payson Granite) contain lithologies similar to those in the Alder and Haigler Group strata.
\r\n\r\nFrom independent structural considerations an intrusive contact relation between Payson Granite and the stratified rocks is preferred over alternative basement and thrust contact hypotheses. This preference is based primarily on the apparent continuity of the sub-planar southwarddipping upper granite surface beneath the Gibson Complex. The intrusive hypothesis is also supported by an apparent distribution of more differentiated granite near the upper contact, suggesting a roof zone.
\r\n\r\nThe hypothesized intrusive relation of the Payson Granite to the stratified rocks provides the first suggestion of a structural relation between rocks of the northern and southern geochronologic provinces and places the Payson Granite, and probably the Gibson Complex, within the interval of a great silicic alkali magmatic event in Tonto Basin, intermediate in age between Haigler rhyolite and the felsic hypabyssal sills. The regional implication is that latest plutonism in the northern province overlapped (along the two-province boundary) with earliest volcanism of the southern province.
\r\n\r\nAll Precambrian X units in Tonto Basin have been folded and/or faulted. Primary textures in massive volcanic and plutonic bodies and quartzite are virtually unmodified and penetrative deformation is present only in less competent strata farthest from the large plutonic bodies. Earliest deformation was large-scale folding on NE-SW axes with shallow plunges. This was followed by thrusting and reverse faulting to the northwest under the same regime of NW-SE compression. These early structures were disrupted by left-lateral strike-slip and east-side-down normal offset on NE-SW to N-S faults. All deformation is probably older than Precambrian Y Apache Group rocks and presumably occurred in the interval 1715-1650 m.y. prior to intrusion of the granite at Young and possibly immediately subsequent to the earlier magmatic activity.
\r\n\r\nThe deformation sequence suggests that Tonto Basin was initially the site of foreland folding and thrusting as the southern, basin crustal block impinged northwestward upon the slightly older proto-cratonic mass. It appears that regional tectonic stresses then shifted to give rise to a left-lateral couple in which the southern block was translated northeastward with a strike-slip system developing along or near the two-province boundary. Similarity of this hypothetical tectonic evolution to some modern tectonic histories may imply similar crustal processes .
\r\n\r\nHaigler rhyolite and rhyolite, granophyre, and alaskite sills are all silicic alkali rocks of similar and distinctive chemical composition. These leucocratic rocks are generally characterized by late-stage alteration of mafic silicate phases to hematite, and by coarse exsolution and pervasive hematite clouding of feldspars. In rare gray (unoxidized) facies, sodic (?) pyroxene (and amphibole?) is preserved in granophyre and intrusive rhyolite. Biotite in alaskite is poorly preserved. Alteration is attributed to hydrothermal activity associated primarily with intrusion of Green Valley Granophyre. Indications of slight alkali depletions and enrichments in Haigler rhyolite and Green Valley Granophyre, respectively, suggest some alkali exchange during the hydrothermal event. From textural and mineralogical evidence in granophyre, alkali enrichment may have occurred in the magmatic state.
\r\n\r\nConsideration of normative data and feldspar character of the silicic alkali rocks in light of experimental work on the petrogeny's residua system leads to the interpretations that the magmas were water-undersaturated at the deep site of phenocryst formation and that certain magmas originated by partial melting, The enormous volume of silicic rocks and the near absence of intermediate rocks in Tonto Basin argues against an origin by differentiation and suggests that all magmas formed by partial melting.
\r\n\r\nIn a petrogenetic model based on these interpretations, on evidence for minimum depths of emplacement, and on comparative temperatures of formation (estimated by hypersolvus and subsolvus feldspar characteristics and mafic mineralogy), phenocrysts of the porphyritic rocks formed in an intermediate-level region of magma chambers where the various magmas began to crystallize at different temperatures depending on the degree of water-undersaturation. The Payson Granite magma may have been a high, early member. Upon ascension most magmas became water-saturated and were either extruded as ash-flow tuffs or crystallized as relatively coarse sill rocks (alaskite, granophyre, spherulitic rhyolite). Hottest, driest magmas perhaps did not become water-saturated and were extruded as viscous flows or emplaced as fine-grained porphyritic sills. Extreme textural variations and parallel variations in mineralogy in sills of comparable composition and emplacement level are best explained by variable water content.
\r\n\r\nThe Tonto Basin silicic alkali province is a well-preserved, deeply exposed analog of modern ash-flow tuff caldera complexes, notable in its remarkable similarity, particularly in chemical composition, to the Yellowstone rhyolite plateau of Wyoming. Aspects of the petrogenetic model above are compatible with new evidence for the existence of magmas beneath Yellowstone.
\r\n\r\nRhyolite and/or granophyre and granite of other silicic alkali provinces (including Yellowstone) commonly occur bimodally with basalt or with tholeiitic layered gabbro bodies. The predominantly silicic rocks in Tonto Basin are associated with small amounts of mafic volcanic and hypabyssal rocks and the large mafic Gibson Complex is thought to have differentia ted from a basaltic magma. The analogy with other bimodal terranes may break down, however, because the Gibson Complex has a calc-alkaline differentiation trend. The calcic character of the Gibson Complex is compatible with inclusion of the complex as part of the northern older province , but seems incompatible with the hypothesized contemporaneity of the Gibson Complex and the silicic alkali rocks. The apparent anomalous differentiation might be explained by assimilation of water by the magma upon intrusion into water-rich Alder Group sediments.
\r\n\r\nAnorogenic tectonic settings observed for other silicic alkali provinces imply that the Tonto Basin magmatism occurred as a post-orogenic event subsequent to orogeny of the northern province and prior to that of the southern province, or perhaps in a site of back-arc extension during the magmatic stage of the later orogenic cycle.
", "doi": "10.7907/hz23-6e04", "publication_date": "1976", "thesis_type": "phd", "thesis_year": "1976" }, { "id": "thesis:6025", "collection": "thesis", "collection_id": "6025", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09092010-080806480", "primary_object_url": { "basename": "Ludwig_kr_1974.pdf", "content": "final", "filesize": 54341163, "license": "other", "mime_type": "application/pdf", "url": "/6025/5/Ludwig_kr_1974.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Precambrian Geology of the Central Mazatzal Mountains, Arizona (part I). Lead Isotope Heterogeneity in Precambrian Igneous Feldspars (Part II)", "author": [ { "family_name": "Ludwig", "given_name": "Kenneth Raymond", "clpid": "Ludwig-Kenneth-Raymond" } ], "thesis_advisor": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Part I:\r\n \r\n The Mazatzal Mountains are located in Gila, Yavapai, and Maricopa counties in central Arizona. The rocks in the central part of this range are dominantly Precambrian, and the only younger stratified rocks are of Tertiary or younger age. The Precambrian rocks have undergone a Precambrian regional metamorphism to lower greenschist grade.\r\n\r\n Two main mappable units of stratified rocks are present: the Alder series and the Red Rock rhyolite. Previous work (Wilson, 1939) had indicated that the Alder series is a clastic sedimentary pile, and that it lies in fault contact with an older Red Rock rhyolite. This work shows that the Alder series consists of about one-third directly accumulated volcanic material, another third of slightly reworked volcanic material, and the remaining third of argillites, quartzose sandstones, and limestones. The total thickness of the section is unknown, but is at least 14000' (4200 meters). The overall character of the Alder series is eugeosynclinal, in view of its great thickness, the abundance of volcanics, the thick sections of volcanic sandstones and wackes, and an overall trend towards more shallow-water deposited rocks towards the top of the section. The volcanics of the Alder series, however, are predominantly felsic (dacites to rhyolites), and basalts and pillow lavas, though present, are not abundant.\r\n\r\n The Red Rock rhyolite is a thick (at least 1000 meters) pile of extrusive rhyolitic volcanics including abundant ash-flow tuffs. There is no evidence of a fault between the Red Rock rhyolite and the Alder series. The Red Rock rhyolite lies with depositional contact and without apparent unconformity on the uppermost Alder series beds. In the lower Gold Creek area, a feeder dike to the Red Rock rhyolite penetrates the uppermost Alder series, and broke the surface at the time of accumulation of the Red Rock rhyolite.\r\n\r\n The lithologies of the Alder series show a fairly continuous evolution from sediments and volcanics accumulated in relatively deep water to sediments and volcanics accumulated in progressively more shallow water, and finally to sediments and volcanics accumulated under intermittently subaerial conditions. The basin of accumulation of the Alder series became progressively more shallow, at least partly because of the large volumes of volcanic material which accumulated there, until the eruption of the tremendous amounts of volcanics of the Red Rock rhyolite obliterated the remaining vestiges.\r\n\r\n The Red Rock rhyolite is petrographically and chemically indistinguishable from some of the volcanics in the uppermost Alder series. It evidently accumulated from sources which were active in latest Alder series time and which contributed to the uppermost Alder series section. Thus at least locally, the Alder series - Red Rock rhyolite contact intertongues. The thickness of the Red Rock rhyolite and the abundance of associated intrusive increases to the northeast, suggesting a probable direction of the source area, perhaps the Precambrian alkali rhyolite complex of Tonto Basin described by Conway (1973).\r\n\r\n Several generations of shallow intrusive rocks penetrate the Alder series and the Red Rock rhyolite. Two types of felsic porphyries were emplaced before\tfolding, while one type of felsic porphyry was emplaced after folding. Mafic volcanic sheets and dikes which intruded in and near the Red Rock mass of the Red Rock rhyolite may have been emplaced during folding.\r\n\r\n Chemical analyses were obtained for nine samples of the Pre-cambrian rocks of the central Mazatzal Mountains. These analyses suggest that the volcanic rocks of the lower Alder series are dominantly dacitic, while the upper Alder series volcanics are dominantly rhyolitic. The chemical composition of the upper Alder series volcanics and the Red Rock rhyolite are generally quite similar to modern, unaltered alkalic rhyolites.\r\n\r\n The structural framework of the area is dominated by tight, northeastward-trending folds. The Red Rock rhyolite is exposed at the cores of synclines, while a faulted anticlinal structure cuts off the oldest Alder series beds to the southeast of Mt. Peeley. These large-scale structures are of Precambrian age.\r\n\r\n Isotopic age determinations using the U-Pb method on cogenetic zircon fractions were performed on upper Alder series volcanics. The apparent age indicated by the analyses is 1730 \u00b1 20 m.y. The upper Alder series volcanics are not distinguishable in apparent age from the Red Rock rhyolite as determined by Silver (1964). This apparent age is distinctly younger than the ages obtained for previously-correlated rocks in the Prescott-Jerome area, and confirms the suspicions of Anderson et al (1971) that the Yavapai schist of the Prescott-Jerome area is not correlable with the type Alder series.\r\n\r\nPart II:\r\n\r\n The lead of high-purity, acid-washed K-feldspar concentrates from five Precambrian granites was removed in a stepwise fashion by two different techniques. Both the technique of stepwise volatilization under vacuum and the technique of stepwise partial attack by hydrofluoric acid yielded leads from the same K-feldspar concentrate which varied significantly in their isotopic composition.\r\n\r\n The patterns of lead isotopic variation from the feldspar concentrates show that the lead isotopic heterogeneity is due to a variable component of radiogenic lead which was generated since the crystallization of the rock. Two classes of lead isotopic variation were observed:\r\n\r\n (I) Parallel and colinear variation in the 206/204, 207/204, and 208/204 ratios, suggesting that the feldspar incorporated lead derived from long-term uranium and thorium decay from a source with constant Th/U, and\r\n\r\n (II) Variation in the 206/204 ratio independent of any 207/204 and 208/204 variation. This suggests that the feldspar contains lead derived from the continuing separation and concentration of a U^(238) intermediate-daughter, probably Rn^ (222).\r\n\r\n The amount and type of radiogenic lead incorporated by the five feldspar concentrates correlates with the concentrations of uranium and thorium in the rocks and with the geologic age and history of the rocks. Thus the rock with the highest uranium content (the Lawler Peak granite) has the K-feldspar with the greatest amount of uranium-derived radiogenic lead, and the rock with the highest thorium content (the Marble Mts. granite) has the K-feldspar with the greatest amount of thorium-derived radiogenic lead. The calculated Th/U values for the sources of the radiogenic leads ranges from two to eleven. Th/U values in this range are reasonable for several common granitic accessory minerals of appreciable U and Th content, but are higher than the Th/U values of the zircons in the rock.\r\n\r\n For two of the rocks, the Payson granite of Arizona and the Giants' Range granite of Minnesota, the composition of the radiogenic lead of the K-feldspar concentrates suggests that the rocks were disturbed by nearby Precambrian intrusives. The apparent time of incorporation of the radiogenic lead of the K-feldspar of the Giants' Range granite is 1010 \u00b1 150 m.y., which correlates well with the time of intrusion of the Duluth gabbro complex at 1120-1140 m.y. (Silver and Green, 1972).\r\n\r\n Although the least radiogenic lead fractions derived from the K-feldspar concentrates were significantly closer to the composition of the original feldspar lead, in no case was the original feldspar lead isolated. Moreover, because the least volatile lead fractions of some of the feldspar concentrates were more radiogenic in character than fractions of lesser volatility, it appears that the technique of stepwise volatilization is not promising for the routine isolation of original feldspar leads. The technique of stepwise hydrofluoric acid attack, however, yielded a greater isolation of original feldspar leads in fewer and simpler steps. This technique may be useful for the routine removal of the greatest part of the radiogenic lead component of Precambrian feldspars, and with further development may lead to the complete isolation of the original leads of such feldspars.\r\n\r\n The isotopic compositions of the least radiogenic fractions of the three 1450 \u00b1 20 m.y. southwestern granites have almost no variation in 207/204 (15.39 \u00b1 .02) and less than 1% variation in 206/204 (16.32\u00b1 .08). Calculations using the patterns of lead isotopic heterogeneity of these feldspars suggest that the original 206/204 values of these rocks were close to 16.16. The observed least radiogenic 208/204 value of the high-thorium (142 ppm) Marble Mts. granite is distinctly higher than those of the other two 1450 \u00b1 20 m.y. granites. These patterns suggest that these rocks were derived from a common, long-lived source, which source underwent a differentiation so as to cause a change in Th/U and Th/Pb a few hundreds of millions of years before the emplacement of the granites.\r\n", "doi": "10.7907/A0JK-VJ62", "publication_date": "1974", "thesis_type": "phd", "thesis_year": "1974" }, { "id": "thesis:6117", "collection": "thesis", "collection_id": "6117", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10072010-111049115", "primary_object_url": { "basename": "Smith_d_1969.pdf", "content": "final", "filesize": 22752326, "license": "other", "mime_type": "application/pdf", "url": "/6117/1/Smith_d_1969.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Mineralogy and Petrology of an Olivine Diabase Sill Complex and Associated Unusually Potassic Granophyres, Sierra Ancha, Central Arizona", "author": [ { "family_name": "Smith", "given_name": "Douglas", "clpid": "Smith-Douglas" } ], "thesis_advisor": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The Precambrian Sierra Ancha sill complex, 700 to 800 feet thick, was intruded into flat-lying sedimentary rocks of the Apache Group in central Arizona. The bulk of the complex consists of a central layer of feldspathic olivine-rich diabase and upper and lower layers of olivine diabase. Diabasic rocks present in relatively minor quantity in the upper part of the complex include diabase pegmatite, albite diabase, and quartz diabase. Potassic granophyres locally form lenses up to two hundred feet thick near and at the roof of the complex. The intrusion was studied in the field and in the laboratory to determine the origins of the rock types and the conditions under which they formed. Extensive electron microprobe studies of mineral compositions and zoning are presented together with nineteen new whole rock chemical analyses.\r\n\r\nThe Sierra Ancha olivine diabase has a high-alumina olivine basalt composition. Olivine diabase and olivine-rich diabase display a differentiation pattern characterized by moderate iron enrichment. Diabase pegmatite is relatively enriched in alkalis.\r\n\r\nThe principle primary minerals in feldspathic olivine-rich diabase and olivine diabase include: plagioclase (An_(72)-An_(16)); augite (Wo_(43)En_(44)Fs_(13) to Wo_(40)En_(38)Fs_(22)); olivine (Fo_(74)-Fo_(54)); orthopyroxene (En_(77)-En_(44)); magnetite_ss (Mgt_(66)Usp_(34)-Mgt_(89)Usp_(11)); and ilmenite_(ss) (Ilm_(86)Hem_(14)-Ilm_(96)Hem_4). All of the orthopyroxene is primary. Fe- Mg fractionations between mafic mineral pairs increase with iron enrichment and declining crystallization temperatures. Ilmenite which formed by reaction-exsolution from magnetite was found to be consistently different in composition from primary ilmenite.\r\n\r\nThe late-crystallizing diabase pegmatites contain an assemblage including iron-rich chlorite together with calcic pyroxene; from textural evidence the two phses appear primary. The calcic pyroxene has a compositional range from Wo_(49)En_(28)Fs_(23) to Wo_(49) En_(14)Fs_(37); its compositions define an iron-enriched trend in the pyroxene quadrilateral more calcic (i.e., closer to the diopsidehedenburgite join) than other iron-enriched igneous pyroxene trends described in the literature.\r\n\r\nMost diabasic rocks in the sill display some deuteric alteration. The mineral assemblage seemingly stable in the most-altered rocks includes albite (An_(2-0)), prehnite, calcic pyroxene (saute), chlorite, sphene, and apatite. Albite diabase contains this assemblage and apparently formed by recrystallization of normal diabase under deuteric conditions. The alteration assemblages are similar to those found in spilites. They provide an important example of the development of a spilitic assemblage by autometamorphism.\r\n\r\nThe massive granophyres at and near the top of the sill appear to be igneous. The larger lenses occur at local high points in the roof of the complex near discordant contacts. The granophyres consist primarily of alkali feldspar with subordinate calcic pyroxene, iron-rich hornblende, biotite, and quartz and minor plagioclase and other phases. They have no relict sedimentary textural features, and they contain miarolitic cavities and rotated and displaced sedimentary rock inclusions. Locally, they occur as masses truncating overlying strata and as dikes in the overlying sedimentary rocks. Some of the dikes have apparent chilled contacts against the sedimentary rocks, suggesting that they were emplaced largely as melts.\r\n\r\nThe granophyres formed as a result of the interaction of diabase magma with stratified rocks of the overlying Dripping Spring Quartzite. The massive granophyres are generally similar in composition to the overlying sedimentary rocks; both rock types have very unusual and distinctive high potassium contents. Contact metamorphism by the diabase has produced layered metasedimentary rocks with granophyric textures and mineral assemblages comparable to those in some massive granophyres. Consistent compositional differences between granophyres and sedimentary rocks may have been caused by metasomatic processes or by mixing of diabase magma with the sedimentary rock material which constitutes most of the granophyres.\r\n\r\nThe interaction of diabase and sedimentary rocks may have occurred because magma in the upper part of the intrusion absorbed water from the overlying sedimentary rocks and solidified after magma in the central part of the intrusive. If this happened, the sedimentary rocks over the sill might have been melted to form the granophyres. No chilled facies of diabase occurs at the sill roof where granophyres are present. Compositional trends in mineral series indicate that the diabase magma in the upper part of the sill solidified towards the roof in at least one locality.\r\n\r\nNormal processes of magmatic differentiation produced feldspathic olivine-rich diabase, olivine diabase, and diabase pegmatite in the Sierra Ancha complex. The processes which produced the granophyres include recrystallization and fusion of rocks overlying the intrusion. The Sierra Ancha granophyres offer a superb opportunity to study these processes and others which may have produced many of the granitic rocks in the crust of the earth.\r\n", "doi": "10.7907/JRYS-CR97", "publication_date": "1969", "thesis_type": "phd", "thesis_year": "1969" }, { "id": "thesis:2645", "collection": "thesis", "collection_id": "2645", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06182009-081030", "primary_object_url": { "basename": "McGetchin_tr_1968.pdf", "content": "final", "filesize": 28570543, "license": "other", "mime_type": "application/pdf", "url": "/2645/1/McGetchin_tr_1968.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "The Moses Rock Dike: Geology, Petrology and Mode of Emplacement of a Kimberlite-Bearing Breccia Dike, San Juan County, Utah", "author": [ { "family_name": "McGetchin", "given_name": "Thomas Richard", "clpid": "McGetchin-Thomas-Richard" } ], "thesis_advisor": [ { "family_name": "Shoemaker", "given_name": "Eugene Merle", "clpid": "Shoemaker-E-M" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThe Moses Rock dike is a well-exposed, four-mile long, kimberlite-bearing breccia intrusion in the east central Colorado Plateau, one of eight known kimberlite-bearing diatremes in the province. The dike occurs in gently dipping beds of the Permian Cutler Formation, 2 miles west of the Comb Ridge monocline in eastern Monument Valley, Utah. Contacts are little altered and the wall rocks generally undeformed. The present erosion surface is probably about 5,000 feet below the surface at the time of emplacement. By volume, the breccia in the dike consists of Cutler formation blocks, 72%; limestone fragments from underlying Paleozoic formations, 13%; crystalline rock fragments, 3%;and kimberlite, 12%. Essentially undiluted kimberlite occurs only locally, and occupies only about l% of the exposed parts of the dike. Mineral constituents of kimberlite are generally dispersed through the unconsolidated breccias.\r\n\r\nThe breccias, including kimberlite, were probably emplaced as a fluidized solid-volatile system. This conclusion is based on the following observations: (1) No silicate melt was intruded at the level present erosion surface, (2) the breccias are particulate on all scales, (3) the particle size frequency distributions of the breccias are like those produced in comminution processes, (4) different types of breccias are intricately mixed, and (5) the mineral constituents of the kimberlite are commonly highly diluted with rock debris. Relationships of the breccia units suggests that flow of the fluidized system was concentrated in channels, now occupied by breccias that contain the largest upward displaced fragments and the largest crystalline rock fragments. Apparently the dike was emplaced along a fissure on which channels soon developed. A local joint system parallel to the contact, which cross-cuts regional joints, apparently played a key role in the dike formation and brecciation process.\r\n\r\nCrystalline rock and mineral fragments found in the dike range from acid to ultramafic types and are believed to represent rocks derived from the vent walls during the eruption. On the basis of the relative size and abundance of the xenoliths, it is inferred that metabasalt, granite and granite gneiss are abundant in the upper part of the crust, along the dikewalls; diorite, gabbro, mafic amphibolite constitute intermediate crystal layers; and mafic granulite and possibly hydrated ultramafic rocks constitute the lower crust. The suite of presumed crustal rocks is predominantly metavolcanic or metaplutonic, not metasedimentary.\r\n\r\nDense and ultramafic fragments possibly derived from the mantle include antigorite-tremolite schist, jadeite-rich clinopyroxenite, eclogite, spinel-websterite, and spinel-lherzolite. The presence of garnet-periodotite at depth is inferred from the suite of mineral inclusions observed within pyropic garnets.\r\n\r\nKimberlite of the Moses Rock dike is believed to be derived mechanically from physically disaggregated spinel and garnet peridotite in the mantle. All other rocks are believed to be accidental inclusions from the vent walls. Tentative P-T assignments to kimberlite clinopyroxenes based on their compositions suggests they are derived from various depths ranging from 50 to about 150 kilometers where the indicated temperatures are modest, about [...] C.\r\n\r\nTitanoclinohumite observed in kimberlite and as inclusions in pyropes may contain most of the water in the upper mantle.\r\n\r\nThe Mohorovicic discontinuity apparently occurs in a petrologically complex region and may coincide with phase and compositional transitions, including hydration. A compositional transition between spinel and garnet periodotite with increasing depth in the mantle is consistent with the observations. The variety of ultramafic types and the complexity of the textures in the xenoliths suggest the mantle may be as complicated as the crust in composition and history.\r\n\r\nNumerical hydrodynamic models of eruption show that flow velocities are probably controlled by viscous losses and expansion of a volatile phase near the surface. Field observations of the largest blocks transported upward in the dike suggest flow velocities of 10 to 50 m/sec at the level of the present surface. Upward extrapolation by use of theoretical models suggests velocities of about 400 m/sec for the erupting fluidized system as it reached the earth's surface.\r\n\r\nThe Moses Rock dike probably formed by eruption of kimberlite from a large reservoir in the mantle. The eruption was driven by volatiles, apparently mostly [...]. The kimberlite consists of physically disrupted rock from the reservoir environment.\r\n", "doi": "10.7907/km6d-5q43", "publication_date": "1968", "thesis_type": "phd", "thesis_year": "1968" }, { "id": "thesis:4951", "collection": "thesis", "collection_id": "4951", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12122003-091348", "primary_object_url": { "basename": "Sharp_rv_1965.pdf", "content": "final", "filesize": 9249059, "license": "other", "mime_type": "application/pdf", "url": "/4951/14/Sharp_rv_1965.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Geology of the San Jacinto Fault Zone in the Peninsular Ranges of Southern California", "author": [ { "family_name": "Sharp", "given_name": "Robert Victor", "clpid": "Sharp-Robert-Victor" } ], "thesis_advisor": [ { "family_name": "Allen", "given_name": "Clarence R.", "clpid": "Allen-C-R" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The San Jacinto fault zone is one of the major branches of the San Andreas fault system in southern California. The straightness, continuity, and high seismicity of the San Jacinto fault zone suggest that it may be currently the most important member of the system.
\r\n\r\nAlthough alluvium conceals much of the San Jacinto fault, intrusive rocks of the mid-Cretaceous southern California batholith are exposed together with prebatholithic metamorphic rocks along a 50-mile segment in the northeastern Peninsular Ranges. The prebatholithic terrane on both sides of the fault consists of migmatitic gneiss and minor amounts of amphibolite, quartzite, marble, and metaconglomerate. Between San Jacinto and Clark Valleys, various members of a distinctive sequence of metamorphic rocks and gabbroic, tonalitic, and adamellitic plutons are separated by the fault 13 1/2 to 15 miles in the right-lateral sense. A marker section of relatively marble-rich metamorphic rocks within and parallel to a regionally unique post-intrusion zone of cataclastic deformation exposed at the southern end of the Santa Rosa Mountains and at Coyote Mountain is separated between 8 and 13 miles.
\r\n\r\nGeometric extrapolation of the various contacts suggests the southwestern block has risen between 1/2 and possibly 8 miles near Anza and between 0 and 6 miles near Clark Valley. Small net vertical movement near Clark Valley may correspond to relatively large vertical offsets near Anza. The sense of vertical movements probably has reversed repeatedly throughout the history of the fault.
\r\n\r\nThe right-lateral component of the net displacement probably increases southeastward from about 14 miles near San Jacinto Valley to between 14 1/2 and 17 miles, but conceivably as much as 22 1/2 miles, near Clark Valley. North of Anza, Quaternary gravels are offset at least 2 miles, and stream courses are displaced at least 2300 feet and possibly 3200 feet. Drainage lines north of Clark Valley have been offset possibly 3 miles in Quaternary time.
\r\n\r\nThe displacement on the San Jacinto fault suggests that (1) the line of major displacement within the San Jacinto fault zone extends southeastward into the central part of Imperial Valley and may connect with the Imperial fault, (2) the Banning fault at the southern margin of the San Bernardino Mountains may be the offset continuation of the Sierra Madre fault on the southern flank of the San Gabriel Mountains, and (3) if the displacement on the San Andreas fault is as large as 160 miles, the San Jacinto fault has not always been as important a member of the larger system as its current activity suggests.
", "doi": "10.7907/B37R-MD73", "publication_date": "1965", "thesis_type": "phd", "thesis_year": "1965" }, { "id": "thesis:4028", "collection": "thesis", "collection_id": "4028", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10112002-152503", "primary_object_url": { "basename": "Jory_lt_1964.pdf", "content": "final", "filesize": 55574013, "license": "other", "mime_type": "application/pdf", "url": "/4028/1/Jory_lt_1964.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Mineralogical and Isotopic Relations in the Port Radium Pitchblende Deposit, Great Bear Lake, Canada", "author": [ { "family_name": "Jory", "given_name": "Lisle Thomas", "clpid": "Jory-Lisle-Thomas" } ], "thesis_advisor": [ { "family_name": "Patterson", "given_name": "Clair C.", "clpid": "Patterson-C-C" }, { "family_name": "Noble", "given_name": "James A.", "clpid": "Noble-J-A" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThis thesis integrates the data from field, petrographic, X-ray, and lead and uranium isotopic studies on the Port Radium pitchblende deposit, Great Bear Lake, Canada.\r\n\r\nThe oldest rocks exposed are andesitic-dacitic bedded tuffs and flows of the Echo Bay group. Overlying detrital rocks of the Cameron Bay group were accumulated before intrusion of hypabyssal, dacitic porphyry bodies and granitic rocks. Sandstones of the Hornby Bay group lie unconformably on the above groups and on denuded granitic rocks. Folds are generally open in the Echo Bay and Cameron Bay groups; the Hornby Bay group is flat-lying. The Echo Bay group was metamorphosed to hornblende hornfels facies rocks, probably during intrusion of granitic rocks.\r\n\r\nFault and fracture zones, most commonly striking northeasterly and dipping steeply north, were in places loci for the successive introduction of \"giant quartz veins\", diabase dykes, and complex vein mineralizations. In the development of the Port Radium deposit, six stages of metallic and non-metallic vein mineral deposition are recognized. Three periods of wall rock alteration are correlative with episodes of vein mineralization. Tuffs of the Lower Echo Bay subgroup are the predominant host rocks to the ore mineralization. Localization of the pitchblende is attributed to physical factors. Diabase sills, intruded after development of the Port Radium deposit, were fractured during late-stage movements on the veins.\r\n\r\nAn age of 1820[plus or minus] 30 million years is assigned to a granitic rock at Port Radium on the basis of the lead-uranium isotopic analyses of seven zircon fractions. This sets a lower limit on the age of the Echo Bay and Cameron Bay groups and an upper limit on the age of Hornby Bay group and diabase dykes. In the zircon fractions, uranium and radiogenic lead concentrations increase with decreasing average grain size. All fractions show the discordant age pattern Pb [superscript 206]/U[superscript 238] < Pb [superscript 207]/U[superscript 235] < Pb[superscript 207]/Pb[superscript 206]. On a \"Concordia\" diagram, points representing the different zircon fractions define a chord intersecting the Concordia curve at 1815 and 50 million years.\r\n\r\nMicroscopic and macroscopic samples of three pitchblende specimens from the Port Radium mine were analysed isotopically for lead and uranium. For a specimen of pitchblende in carbonate gangue, concordant lead-uranium ages were obtained on two microscopic samples. The age, 1445[plus or minus] 20 million years, is the only concordant age which has been measured for the Port Radium ores. It places a lower limit on the age of the \"giant quartz veins\", the diabase dykes, and possibly the Hornby Bay group, and an upper limit on the diabase sills. For a specimen of pitchblende in siliceous gangue, two analyses define a chord intersecting the Concordia curve at 1450 and 300 million years. In general, Port Radium pitchblende samples show evidence of past loss of lead; the lead was removed from the vein systems. Comparison of isotopic data on microscopic and macroscopic samples from the same specimen shows that the microscopic samples can be less discordant and have higher lead-lead ages. Analyses of a number of lead-uranium systems in the same specimen offers a possible means of determining the age of mineralization and the age of an episodic disturbance.\r\n\r\nOf eight samples of galena and one of chalcopyrite analysed isotopically for lead, three are ordinary lead, four J-type anomalous lead and two possibly mixtures of ordinary lead and radiogenic uranium lead derived from the pitchblende. The ordinary lead is finely disseminated through the other vein minerals and was probably introduced shortly after the pitchblende. The J-type lead occurs in late, lenticular veinlets and was not formed by mixing with radiogenic lead from the pitchblende ores. Because of the absence of mixing and because of considerations concerning the geochemical character of the source system for the J-type lead, this lead was probably introduced less than 300 million years ago. Disturbances to the lead-uranium systems of the zircon and pitchblende samples provide further isotopic evidence for processes probably active within the last 200-300 million years but not recognized on the basis of geological field data.\r\n\r\nAdditional isotopic work to confirm and elaborate on these interpretations is suggested.", "doi": "10.7907/W43G-ED67", "publication_date": "1964", "thesis_type": "phd", "thesis_year": "1964" }, { "id": "thesis:6529", "collection": "thesis", "collection_id": "6529", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06272011-104837127", "primary_object_url": { "basename": "Banks_po_1963.pdf", "content": "final", "filesize": 6779047, "license": "other", "mime_type": "application/pdf", "url": "/6529/1/Banks_po_1963.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Systematics of the Distribution of Uranium and Lead in Relation to the Petrology of the Mt. Rubidoux Granites, Riverside County, California.", "author": [ { "family_name": "Banks", "given_name": "Philip Oren", "clpid": "Banks-Philip-Oren" } ], "thesis_advisor": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Field and petrologic studies and uranium-lead isotope geochemistry investigations have been made of the Mt. Rubidoux granites, Riverside Co., Calif. The coarse-grained granite was emplaced earlier than the fine-grained granite, and both were intruded by later dikes of pyroxene granodiorite. The two rocks are very similar quartz monzonites with an unusual assemblage of iron-rich silicates including biotite, hornblende, hypersthene, and fayalite. The composition of the fine granite corresponds very nearly to a natural minimum in the Q-Or-Ab-water system. Data from published experimental studies suggest a temperature of 700-760\u00b0C and a water vapor pressure of 1000-1500 bars during crystallization. The presence of fayalite and hypersthene is attributed to late-stage loss of water pressure.\r\n\r\nU-Pb isotopic analyses of accessory minerals, potash feldspar, and whole rock samples were performed to investigate the behavior of these isotopes in the coarse granite. Zircon, uranothorite, allanite, and apatite constitute the principal sites of U and radiogenic Pb in the rock. Apatite was not analyzed. Systematic analyses of the other minerals have provided a criterion for recognizing discordance in young U-Pb systems. The granites are at least 116 m.y. old and possibly as old as 130 m.y. Variations in the degree of discordance for equivalent zircon fractions from different localities suggest that an unrecognized episodic event less than 100 m.y. ago may have caused the discordance.\r\n\r\nZircon fractions from other rocks of the southern California batholith gave the following minimum ages: Woodson Mtn. granodiorite 119 m.y.; pyroxene granodiorite 108 m.y.; Crestmore quartz monzonite porphyry 106 m.y. Zircons from a soil profile over the coarse granite contained older zircons which have a minimum age of 1730 m.y. and are thought to be from metasedimentary wall rocks.\r\n\r\nThe isotopic composition of common lead was determined by several analyses of potash feldspar. Analyses of whole rock material and acid leaches of the fresh granite indicate that the total rock is discordant to the same extent as the principal minerals, and that the abundance and leachability of U and radiogenic Pb in the rock are governed strongly by the mineral uranothorite. Similar analyses of C-zone weathered granite indicate that the chief effect of weathering is to make available to acid leaching a higher proportion of common lead from the feldspar and radiogenic Pb^(208) from allanite.\r\n\r\nIsotopic analyses of radiogenic Pb have been utilized to evaluate the decay constant of U^(235). The presently accepted constant is believed to be accurate to within 1%, relative to the accepted constant for U^(238).\r\n\r\nThe effects of intermediate daughter losses in contributing to discordance in natural U-Pb systems have been considered in a series of model calculations. It is suggested that such losses may be of significance where the degree of discordance is relatively small and/or where the apparent time of episodic disturbance is very young.", "doi": "10.7907/PB8W-1F55", "publication_date": "1963", "thesis_type": "phd", "thesis_year": "1963" }, { "id": "thesis:6962", "collection": "thesis", "collection_id": "6962", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04242012-095648459", "type": "thesis", "title": "Petrology of the Basaltic Achondrite Meteorites", "author": [ { "family_name": "Duke", "given_name": "Michael B.", "clpid": "Duke-Michael-B" } ], "thesis_advisor": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Mineralogical and textural evidence indicates that the basaltic\r\nachondrites originated in one or more magmatic episodes in a\r\nvariety of cooling environments that resulted in textures ranging\r\nfrom gabbroic to diabasic. Chemical compositional and mineralogical\r\nvariations are consistent with a common origin for the basaltic\r\nachondrites by magmatic differentiation. The characteristics of\r\nthe mineralogical, major element and trace element variations are\r\nsimilar to those of the Skaergaard intrusion, but the basaltic\r\nachondrite magmas started from different compositions and crystallized\r\nunder much lower partial pressures of oxygen(in the stability\r\nfield of metallic iron) than did the magmas of the Skaergaard\r\nintrusion. The differentiation trends shown by the basaltic achondrites\r\nindicate that the starting material had calcic plagioclase\r\nand was depleted in alkalis with respect to chondritic meteorites,\r\nin which the plagioclase is sodic.
\r\n\r\nBrecciation is a conspicuous feature of most basaltic achondrites,\r\nwhich can be most satisfactorily subgrouped on the basis of\r\nbreccia type as brecciated eucrites (monomict breccias), eucrites\r\n(unbrecciated), and howardites (polymict breccias). Petrographic\r\nevidence suggests that some textural metamorphism and brecciation were\r\nproduced by shock effects that accompanied impact events on the surface\r\nof the parent body. The abundances of basaltic achondrite falls are\r\nconsistent with a surface-sampling mechanism such as meteorite impact\r\nejection. A preponderance of near-surface samples and the distinct\r\ndifferences between the basaltic achondrites and chondrites suggest\r\nthat the moon is the probable parent body of the basaltic achondrites.
\r\n", "doi": "10.7907/YPP2-ME45", "publication_date": "1963", "thesis_type": "phd", "thesis_year": "1963" }, { "id": "thesis:6303", "collection": "thesis", "collection_id": "6303", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04112011-153508154", "type": "thesis", "title": "Geology of the Alta stock, Utah", "author": [ { "family_name": "Wilson", "given_name": "John Coe", "clpid": "Wilson-John-Coe" } ], "thesis_advisor": [ { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The Alta stock of granodiorite is closely associated in space and\r\ntime with many of the Pb, Zn and Ag ore deposits of the Park City-Cottonwood\r\nmining area in the central Wasatch Mountains, Utah. The purpose of\r\nthis study was to determine if fractional crystallization of the Alta magma\r\nultimately resulted in a metal-rich ore-forming fluid.\r\nThe Alta magma was intruded in two pulses into pre-Triassic\r\nsedimentary rocks at a depth of approximately 21,000 feet and at a temperature\r\nin excess of 720\u00b0C. Detailed study of texture indicates that the first\r\nphase of magma solidified with a nonporphyritic texture, crystallizing\r\nfrom the wall progressively toward the center of the stock. The rock at\r\nthe wall is rich in mafic minerals but grades inward within a few hundred\r\nfeet to light-gray medium-grained biotite-hornblende granodiorite. Early\r\nin the history of the first pulse of magma, convection currents carrying\r\ngrowing crystals of early minerals formed stratified gravity accumulations\r\nrich in hornblende, biotite, sphene, magnetite and apatite. These layered\r\nmasses were subsequently broken and displaced. They now form tabular\r\ninclusions of mafic layered granodiorite in the light-gray nonporphyritic\r\ngranodiorite.\r\nAt a late stage in the crystallization history, the Alta stock had an\r\nessentially solid shell of nonporphyritic granodiorite at least 1200 feet\r\nthick that surrounded a partially liquid but semirigid crystal mush at the\r\ncenter of the stock. Structural adjustments at this late stage re sulted in\r\nclose-space jointing and emplacement of aplite-pegmatite dikes in the nonporphyritic\r\ngranodiorite shell. After emplacement of the dikes but before\r\nthe interstitial liquid of the magma at the center of the stock had completely\r\ncrystallized, the semi-rigid crystal mush and solid shell were intruded by\r\na second pulse of the magma. The distribution of strongly porphyritic\r\ntexture indicates that loss of volatiles through fractures in the overlying\r\nmaterial caused this later phase of the magma to finish crystallization with\r\na fine-grained groundmass. Modal analyses supported by gravimetric\r\nchemical analyses show that the porphyritic granodiorite has a slightly\r\nmore silicic composition than the nonporphyritic granodiorite.\r\nThe Flagstaff-Emma ore zone in the Little Cottonwood district is\r\non the trend of a westward dike like projection of the porphyritic granodiorite.\r\nThe fluids that were released during the emplacement of this late\r\nphase of the magma conceivably may have been the same fluids that deposited\r\nthe ore minerals in the Flagstaff-Emma ore zone.\r\nIn order to determine the trend of the ore-metals during crystallization\r\nof the Alta magma, 76 mineral separates of orthoclase, plagioclase,\r\nbiotite, hornblende, magnetite, sphene, quartz and chlorite were analyzed\r\nby the emission spectrographic method for Cu, Pb, Zn, Ag, Co, Ni and\r\nSn. Comparison between samples of the same mineral show that Co and\r\nNi are slightly less abundant and Cu, Pb and Sn are more abundant in the\r\nlater phases of the Alta igneous suite. No trend is apparent for Ag and\r\nZn. Cu, Co, Ni, Ag and Zn were added to the solid phases of the rock\r\nduring chloritic alteration.\r\nThe release of volatiles associated with the late-stage intrusion of\r\nporphyritic granodiorite, the structural relation of the adjacent ore deposits\r\nto this porphyritic granodiorite and the trend in trace-element\r\nabundances suggest that an ore-forming fluid may have been produced at a\r\nlate stage in the crystallization of the Alta stock.\r\n", "doi": "10.7907/8775-NJ40", "publication_date": "1961", "thesis_type": "phd", "thesis_year": "1961" }, { "id": "thesis:1611", "collection": "thesis", "collection_id": "1611", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05042007-083758", "primary_object_url": { "basename": "Schwarcz_hp_1960.pdf", "content": "final", "filesize": 31062349, "license": "other", "mime_type": "application/pdf", "url": "/1611/1/Schwarcz_hp_1960.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "I. Geology of the Winchester-Hemet Area, Riverside County, California. II. Geochemical Investigations of an Arkosic Quartzite of the Winchester-Hemet Area, California", "author": [ { "family_name": "Schwarcz", "given_name": "Henry Philip", "clpid": "Schwarcz-Henry-Philip" } ], "thesis_advisor": [ { "family_name": "Engel", "given_name": "Albert Edward John", "clpid": "Engel-A-E-J" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "I. \r\n\r\nAn area of about 100 square miles located 20 miles southeast of Riverside, California, has been geologically mapped. The oldest rocks of the area are phyllites and quartzites correlated with the Bedford Canyon formation (Triassic). Conformably overlying these rocks is a 13,000 foot thick section of quartzite, schist and rare amphibolite, here named the French Valley formation. The metasedimentary rocks were formed from a series of shales, shale-clast conglomerates, poorly sorted feldspathic, calcareous and arkosic sandstones and rare basalt flows or tuffs. Relict sedimentary structures and textures are locally well preserved.\r\n\r\nThe sedimentary rocks were intruded with a series of basic and ultrabasic magmas and were tightly folded prior to late Cretaceous time. Late in the period of folding they were metamorphosed to the hornblende-hornfels facies (200\u00b0-600\u00b0C, 3-5 kilobars), producing andalusite, cordierite, sillimanite and garnet in the schists and hornblende-plagioclase (-diopside, -garnet) assemblages in the amphibolites. Zones of progressive metamorphism are mapped trending N-S and cutting the NW-SE structural trend of the folded rocks. In the late Cretaceous these rocks were intruded by basic to intermediate plutonic igneous rocks of the southern California batholith, with no appreciable contact metamorphism.\r\n\r\nII: \r\n\r\nA series of seven samples of arkosic quartzite from a single homogeneous member of the French Valley formation has been collected along a profile of increasing metamorphic rank. The component minerals and aliquots of the total rocks have been analysed by X-ray fluorescence, emission spectrographic and wet chemical procedures. For most elements studied, concentrations in the total rock, biotite and ilmenite appear to be constant through the series. These elements either did not vary beyond the experimental uncertainty or they varied without apparent trend over a small range slightly exceeding the uncertainty. With increasing degree of metamorphism the following changes were suggested but fell within or only slightly exceeded the assigned limits of error: a) the total rock gained CaO, Mn, Sr, Co, La, Y, and Ti(?) and lost K2O, Na2O, and V; b) the biotite gained Mn, Ti, Ba, and possibly Co, Cr, V, and La and lost CaO; c) ilmenite gained Mn and Ni and lost Cr, Ba, Zr(?), Y, and Pb. Other changes observed with increasing metamorphic rank were: a) coarsening and homogenization of the rock texture; b) decrease in the ratio Fe+++/Fe++ in both rock and biotite; c) decrease in 2V and basal spacing of the biotite; and d) variations in albite content of plagioclase and potash feldspar that suggest loss of Na2O from rock.\r\n\r\nFor most elements fractionations between biotite and rock appear to vary directly with the concentration in biotite. Fractionations between ilmenite and biotite are shown to be related to ionic size and charge.\r\n\r\nThe composition of the rock is such that partial fusion would yield a liquid with a composition close to that of the total rock. This may be the cause of the small variations in major element composition with metamorphic grade.", "doi": "10.7907/DP95-HX65", "publication_date": "1960", "thesis_type": "phd", "thesis_year": "1960" }, { "id": "thesis:665", "collection": "thesis", "collection_id": "665", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02172006-144353", "primary_object_url": { "basename": "Campbell_rb_1959.pdf", "content": "final", "filesize": 42136415, "license": "other", "mime_type": "application/pdf", "url": "/665/1/Campbell_rb_1959.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "The Texture, Origin, and Emplacement of the Granitic Rocks of Glenlyon Range, Yukon, Canada", "author": [ { "family_name": "Campbell", "given_name": "Richard Bradford", "clpid": "Campbell-Richard-Bradford" } ], "thesis_advisor": [ { "family_name": "Campbell", "given_name": "Ian", "clpid": "Campbell-I" }, { "family_name": "Silver", "given_name": "Leon T.", "clpid": "Silver-L-T" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "In Glenlyon range the older pre-intrusive rocks include quartzo-feldspathic schists (Ambibolite facies) with minor carbonate and lime-silicate rocks of the Yukon group overlain by a succession of limestones and slates and phyllites (green schist facies) of the Harvey Group. These rocks form the north limb of an east-west trending anticlinorium. The Drury quartz monzonite is intruded into the axial region of the anticlinorium. This mass grades continuously from a biotite granodiorite core to an outer zone of quartz monzonite in which the proportions of hydridized inclusions and septa of metamorphic rock increase with complete gradation to the granite-free metamorphic host terrane. To the north and east a second large mass, the Peak granodiorite, identical with the core of the Drury quartz monzonite, has been emplaced with clearly crosscutting intrusive relations. A pattern of large scale faulting associated with the contact of the Peak granodiorite suggests that fault block movement may have provided some of the intrusive space requirements. Smaller alaskite dikes cut both the metamorphic complex and the Drury quartz monzonite.\r\n\r\nDetailed petrographic studies of the textures and mineral modes of the granitic rocks argue for a similar crystallization history of the Peak granodiorite and the core of the Drury quartz monzonite. The textural evidence for paragenesis and the mineral composition trends of the entire intrusive complex are combined with discussions of the crystallization of a hypothetical granodiarite magma based on available experimental data of the system KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8 - SiO2 - H2O. All of the significant natural paragenetic relations, particularly the important role of potash feldspar (including replacement reactions) in the later crystallization stages, can be explained as resulting from magmatic crystallization. These effects are distinguished from metasomatic phenomena in the host rocks.\r\n\r\nThe validity of such arguments rests in part on a detailed analysis of the sampling of these granitic masses and problems of representation of modal data. The results of approximately 150 modal analyses are presented in four mineral-component tetrahedrons to support these discussions.\r\n", "doi": "10.7907/HM6J-8Y76", "publication_date": "1959", "thesis_type": "phd", "thesis_year": "1959" } ]