Dissertations / Theses on the topic 'Geology – Nevada – Ruby Mountains'

The Ordovician strata of the Lower Member of the Vinini Formation comprise a sequence of greenstone, sandstone, shale, and siltstone representing the prograding and retrograding of submarine fans along the continental margin. Although graptolites are normally preserved within shale beds in the Lower Member of the Vinini Formation, the greatest abundance of well preserved graptolites is found within the sandstone turbidite beds. These graptolites are uniquely preserved in full relief as opposed to being flattened on shale. It is interpreted based on fragmentation and species composition within the sandstone that the graptolites flourished in an upwelling zone on the continental margin and that as their remains accumulated on the underlying seafloor, were swept downslope in turbidity currents.

Graptolites were collected from 10 beds within the stratigraphic section and represent 33 taxa from 17 genera. There are no new taxa. All taxa are described, illustrated, and compared to other collections.

Major and trace metal loading to mountains in the western US depends on dust sources, intensity of storms and their availability for transport during snowmelt and runoff. Previous work has been conducted on dust production, composition, and its affect on solar radiation and timing of snow melt. This study was conducted to 1) examine temporal and spatial variability in dust chemistry; 2) evaluate form and availability of major and trace elements in dust; and 3) identify potential dust sources affecting mountains in Utah and Nevada. Spring and summertime dust was collected across northern Utah over the course of three years (2013-2015). Additional dust samples were collected from eastern Nevada for comparison. All samples were analyzed for mineralogy. The spring dust samples were also leached with 1 M acetic acid, 0.8 M nitric acid, and aqua regia and analyzed for 87Sr/86Sr ratios and concentrations of 40+ trace and major elements. Nearly all dust samples were enriched in playa-associated elements (U, Mg, Li, Ca, Sr, As) and anthropogenic elements (Sb, Mn, Zn, Cu, Pb, Se, Cd) relative to average upper continental crust. Leachate results showed that nearly 60% Ca, Sr, and Cd mass is potentially available for transport during snowmelt and that the rare earth elements could be mobilized under lower pH conditions in the soil zone. A major dust event on 17 March 2014 that was sampled across the study area showed spatially variable trace element concentrations and 87Sr/86Sr ratios, indicating that dust deposited to mountain snowpack originated from multiple upwind desert dust source areas. The NOAA HYSPLIT model was used to calculate back trajectories for this dust event and showed potential dust sources ranged from the Sevier, West and Great Salt Lake deserts in Utah and the Snake River Plain in Idaho. In contrast, multivariate statistical analysis showed that over the course of the study samples had unique geochemical signatures within each sample area. These findings suggest that spatial variability is more important than temporal variability in terms of the chemistry of dust deposition. With increasing populations and land use change in the western US, the short and long term effects of aeolian dust deposition to mountain environments need to continual monitored and constrained.

Seismic studies in the area of the Ruby Mountains metamorphic core complex and adjacent basins of northeast Nevada provide new evidence for Cenozoic tectonic evolution of the Ruby Mountains. Results from interpretation of industry seismic data show that (1) asymmetric basins flanking the Ruby Mountains were created by normal faults beginning in the late Eocene-early Oligocene; (2) the metamorphic core complex detachment fault system was cut by the normal fault system; and (3) total subsidences of Huntington and Lamoille basins, and Ruby basins are ∼4.5 and ∼5.0 km. Analysis of crustal-scale 3-component normal-incidence to wide-angle seismic data shows that (1) the crust along the eastern flank of the Ruby Mountains can be divided into three layers corresponding to the upper, middle and lower crust; (2) upper crustal rocks likely consist of metaquartzite, schist, granite gneiss, and granite-granodiorite with P-wave velocities (Vp) of 5.80-6.25 km/s, S-wave velocities (Vs) of 3.20-3.72 km/s, Poisson's ratios (sigma) of 0.22-0.25, and anisotropy of 0.6-2.5%; (3) possible middle crustal rocks are paragranulite, felsic granulite, felsic amphibolite gneiss, granite-granodiorite, and mica-quartz schist with Vp of 6.35-6.45 km/s, Vs of 3.70-3.75 km/s, and σ of 0.24; (4) lower crustal rocks most likely consist of granulite- rather than amphibolite-facies rocks with Vp of 6.60-6.80 km/s, Vs of 3.85-3.92 km/s, σ of 0.24-0.25, and anisotropy of less than 3%; (4) depth to the Moho varies irregularly between 30.5 and 33.5. Interpretation of these results suggests that (1) Cenozoic extension of the Ruby Mountains and adjacent basins began by late Eocene-early Oligocene; (2) depth to Moho does not reflect local surface relief on the eastern flank of the Ruby Mountains and adjacent basin; (3) fluid-filled fractures and mafic large-scale underplating are unlikely in the lower crust; (4) the present seismic velocities of highly extended core complex crust and normally extended Basin and Range crust are similar; and (5) orientations of fast shear waves near the surface and in the upper crust are parallel to sub-parallel to the regional maximum horizontal compressive stress in the Nevada part of the Basin and Range province.

Structural and geochemical analyses of the Top and Casino deposits, Bald Mountain-Alligator Ridge district, Nevada, were conducted to determine how structures affected gold deposition in Carlin-type deposit s. We also examined how permeability changed over time in a fault that cuts siltstone-dominated sedimentary rocks. The association of gold and related arsenic with faults at the margins of a Jurassic pluton and sedimentary rocks suggests that ore fluids migrated along faults and fracture s. Permeability of the faults changed over time within the Casino deposit, where the ore-controlling fault was a distributed conduit in the early stages of mineralization but a barrier and a localized conduit a t opposite ends of the deposit during later stages. Results indicate that faults may significantly influence patterns of ore deposition and change character over deposit-scale distances, and continued slip along faults may create clay-rich low-permeability faults that are mineralized during early stages of development.

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.

The pre-Tertiary rocks of the Pueblo Mountains are a series of volcanic and volcanogenic rocks intruded by Middle Jurassic and possibly younger plutons. The entire sequence has undergone sub-greenschist to greenschist facies metamorphism. The Pueblo Mountains can be divided into two zones: (1) a northeast-trending, southeast-dipping shear zone in the southeast; and (2) an undeformed zone in the northwest. Three phases of deformation are associated with and restricted to the shear zone, and all show top-to-the-NW sense of shear. $\sp{40}$Ar/$\sp{39}$Ar geochronology for biotite from within the shear zone produces a minimum age for D$\sb1$ of 95 Ma. The Pueblo Mountains shear zone may be related to a similar middle Cretaceous structure in the northern Pine Forest Range, and is also similar to structures developed during and after the poorly understood suturing of the Blue Mountains province to the North American craton.

The Jackson Mountains (JM) are part of the early Mesozoic continental arc in northwest Nevada, which was constructed upon previously accreted Paleozoic basement. The stratigraphy of the Paleozoic basement exposed in the JM has been revised and correlations with nearby age-equivalent rocks in the Pine Forest Range and Bilk Creek Mountains are now more clearly recognized. Upper Triassic strata in the JM (the Carnian-Norian Boulder Creek Beds) herald the onset of Mesozoic arc activity in the region. The Boulder Creek Beds are both overlain and intruded by rocks of the Happy Creek Igneous Complex (HCC). Contact relations and internal features of the HCC indicate that mostly hypabyssal intrusive rocks are now exposed and that the bulk of the supracrustal volcanic succession was eroded prior to deposition of the King Lear Formation (KLF), which unconformably overlies the HCC. The HCC intrudes Norian strata and is cut by plutons that have yielded U-Pb zircon dates of 196-190 Ma and is probably entirely of Early Jurassic age. Igneous rocks associated with the KLF have yielded U-Pb zircon dates that indicate KLF deposition took place in the Early Cretaceous ($\sim$125 Ma). Two phases of Mesozoic deformation are recognized in the JM. The D$\sb1$ phase produced NW trending folds, an axial planar cleavage, and was associated with subgreenschist to amphibolite grade metamorphism. D$\sb1$ structures are found only in rocks older than the HCC and are truncated along intrusive contacts of the HCC. D$\sb2$ deformation produced NE trending folds, an axial planar cleavage, and was associated with very low grade metamorphism. D$\sb2$ affected the HCC and older rocks, but is absent in the KLF. Thus, D$\sb2$ shortening is constrained between late Early Jurassic to Early Cretaceous. D$\sb1$ correlates in style, orientation, and age with deformation in the adjacent Pine Forest Range, but the later D$\sb2$ event is apparently localized in the JM. In the JM, D$\sb2$ fabrics are better developed to the east, towards the back-arc region and may, therefore, have formed during juxtaposition of the arc terrane with the back-arc.

The lower Cretaceous King Lear Formation (KLF) is a gently east-dipping succession of alluvial conglomerates and sandstones that were deposited in a small intra-arc basin. Supracrustal strata of Cretaceous age from within the western U.S. magmatic arc are extremely rare, so the KLF offers an opportunity to obtain paleoenvironmental information about the Cretaceous arc. A new division of the KLF into three members based on clast provenance provides a framework for understanding deposition in the King Lear Basin and thus is essential for paleoenvironmental studies on this portion of the arc. New structural observations and a shallow reflection seismic profile suggest that the KLF was deposited in a half-graben and never experienced compressive deformation. This conclusion means that compressive deformation both in the Jackson Mountains and also in the crustal-scale Luning-Fencemaker Fold and Thrust Belt must have been complete prior to the Early Cretaceous.

The central Basin and Range Province of Nevada and Utah was one of the first areas in which the existence of widespread low-angle normal faults or detachments was first recognized. The magnitude of associated crustal extension is estimated by some to be large, in places increasing original line lengths by as much as a factor of four. However, rock mechanics experiments and seismological data cast doubt on whether these structures slipped at low inclination in the manner generally assumed. In this dissertation, I review the evidence for the presence of detachment faults in the Lake Mead and Beaver Dam Mountains areas and place constraints on the amount of extension that has occurred there since the Miocene. Chapter 1 deals with the source-provenance relationship between Miocene breccias cropping out close to Las Vegas, Nevada and their interpreted source at Gold Butte, currently located 65 km to the east. Geochemical, geochronological and thermochronological data provide support for that long-accepted correlation, though with unexpected mismatches requiring modification of the original hypothesis. In Chapter 2, the same data are used to propose a refinement of the timing of ~1.45 Ga anorogenic magmatism, and the distribution of Proterozoic crustal boundaries. Chapter 3 uses geophysical methods to address the subsurface geometry of faults along the west flank of the Beaver Dam Mountains of southwestern Utah. The data suggest that the range is bounded by steeply inclined normal faults rather than a regional-scale detachment fault. Footwall folding formerly ascribed to Miocene deformation is reinterpreted as an expression of Cretaceous crustal shortening. Fission track data presented in Chapter 4 are consistent with mid-Miocene exhumation adjacent to high-angle normal faults. They also reveal a protracted history dating back to the Pennsylvanian-Permian time, with implications for the interpretation of other basement-cored uplifts in the region. A key finding of this dissertation is that the magnitude of crustal extension in this region has been overestimated. The pre-extensional width was increased by a factor of two across Lake Mead, through a combination of high-angle normal faulting and strike-slip deformation. Data from the transect across the Beaver Dam Mountains suggest substantially less extension, with the difference accommodated for the most part by displacement on the intervening Las Vegas Valley Shear Zone. The Colorado Plateau-Basin and Range transition zone may be a long-lived tectonic boundary where this assumption may be especially ill-suited.

Many geologic studies have inferred that the California continental margin in the vicinity of the western Mojave Desert was tectonically disrupted after emplacement of the Cretaceous Cordilleran batholith and prior to Neogene displacements on the San Andreas fault system. The causes of this regional deformation, however, are poorly understood. Located along the northern margin of this disrupted region at the southern end of the comparatively little deformed Sierra Nevada batholith, the eastern Tehachapi Mountains are ideally situated to study the possible mechanisms of this disruption. In view of this, the geology and structure of the eastern Tehachapi Mountains were investigated using geologic field mapping at scales of 1:6,000 through 1:24,000, detailed petrographic studies, and structural and kinematic analysis of deformation fabrics and structures in the field and in the lab. The study area is divided by a generally N trending shallowly SE dipping ductile-cataclastic fault zone called the Blackburn Canyon fault into the eastern Tehachapi gneiss complex in the footwall and the Oak Creek Pass complex in the hangingwall. The eastern Tehachapi gneiss complex is composed of two different sequences of metasedimentary rocks that have been intruded by three generations of plutonic rocks. The Brite Valley group metasedimentary rocks consist largely of pelites and graphitic quartzite with subordinate marble. The Antelope Canyon group metasedimentary rocks consist of a lower section composed mostly of thinly laminated dirty quartzite overlain by an upper section of marble. The earliest intrusive rocks in the area (group I orthogneisses) are lithologically diverse and include granite augen gneiss, garnetiferous hornblende diorite gneiss, and hornblende biotite quartz diorite gneiss. Both groups of paragneiss and the group I orthogneisses are intruded by group II plutons of the Tehachapi Intrusive Complex. The Tehachapi Intrusive Complex is composed of comagmatic gabbro, quartz diorite, and tonalite and it is inferred to be continuous with the large ~100 Ma Bear Valley Springs tonalite pluton exposed to the west. The group III intrusives are small bodies and thin sheets of leucocratic biotite granite which intrude all of the other lithologies. The rocks in the gneiss complex have had a complex deformational history. The metasedimentary rocks are folded into map-scale N to NW trending SW vergent isoclinal F1 folds. Later (?) intrusion of the group I orthogneisses was accompanied (?) and followed by amphibolite facies metamorphism and the localized formation of NE trending shallow plunging open to tight F2 folds. During (?) and after intrusion of the ~100 (?) Ma Tehachapi Intrusive Complex the gneiss complex was metamorphosed at amphibolite facies and deformed by map-scale open to tight NW trending SW vergent F3 folds. After much of the F3 folding the basement rocks in the Tehachapi Valley area appear to have been folded into a regional dextral-sense convex-west F4 oroclinal fold. In the later stages of F4 folding part of the southwest limb of the Tehachapi Valley orocline is inferred to have been transposed into a NW trending shallow NE dipping noncoaxial ductile shear zone called the eastern Tehachapi shear zone. The shear zone has a structural thickness of ~1 km, top to the S-SW shear sense, and most shearing appears to have occurred during greenschist facies retrograde metamorphism. The shear zone appears to continue to the north across Tehachapi Valley where it is inferred to merge with the steeply E dipping dextral-slip proto-Kern Canyon fault. Motion on the shear zone is inferred to have ended at about the time when the Late Cretaceous (?) group III leucogranites intruded. Following shear zone activity rocks in the gneiss complex locally were folded in gentle NE trending subhorizontal F5 folds. Late top to the NE shearing in the upper structural levels of the gneiss complex suggests that a normal fault may be concealed beneath the alluvium of Tehachapi Valley. The lithologies and deformation history of the Oak Creek Pass complex are very different from the eastern Tehachapi gneiss complex. The Oak Creek Pass complex is composed mostly of granodioritic plutonic rocks (group IV intrusives) which commonly are cataclastically deformed and metamorphosed at greenschist and lower grade. Arkosic sandstones and conglomerates of the Late Cretaceous (?)-Eocene (?) Witnet Formation locally are unconformable above the granodiorite. Emplacement of the Oak Creek Pass complex above the eastern Tehachapi gneiss complex along the Blackburn Canyon fault took place after most of the activity along the eastern Tehachapi shear zone. Shear sense along the Blackburn Canyon fault is top to the S or SE. The Oak Creek Pass complex is divided into a number of structural plates by low-angle (?) ductile-cataclastic fault zones one of which is the NE trending Mendiburu Canyon fault. Synclinal F6 folding of the Witnet Formation and NW vergent overthrusting of the Witnet Formation by granitic rocks along the Mendiburu Canyon fault are interpreted to postdate motion along the Blackburn Canyon fault. Deformation of the Witnet Formation is inferred to be pre-Miocene in age based on correlation with a similar deformation across Tehachapi Valley. The Brite Valley group metasedimentary rocks are suggested to correlate with the western facies of the Triassic-Jurassic age Kings sequence and the Antelope Canyon group rocks may correlate with the eastern facies of the Kings sequence or possibly with Late Proterozoic-Cambrian age rocks of the miogeocline. Juxtaposition of the two groups of metasedimentary rocks may have been along a cryptic structure that was active prior to intrusion of some of the group I plutons which are inferred to be mid-Cretaceous in age. Formation of the NE trending F2 folds between ~117 Ma and ~100 Ma is suggested to have resulted from the local reorientation of the regional stress field in the vicinity of a weak strike-slip (?) fault such that the direction of maximum compressive stress during the deformation was oriented subparralel to the trend of the Sierra Nevada batholith. The F3 folds, F4 folds, and the eastern Tehachapi shear zone are interpreted to have formed more or less sequentially during a protracted period of contractional deformation in the middle to lower crust of the southern Sierra Nevada batholith from ~100 Ma to ~80 Ma. Top to the S-SW motion along the shear zone may reflect the and underthrusting of Rand schist beneath the batholith at lower structural levels during low-angle Laramide subduction. The Blackburn Canyon fault and a number of other previously identified low-angle faults in the southern Sierra Nevada region are suggested to be extensional faults along which part of the southern Sierra Nevada batholith was unroofed. The source region for the out of place Oak Creek Pass complex and other inferred allochthonous rocks is suggested to be the area in the Sierra Nevada east of the proto-Kern Canyon fault and south of South Fork Valley. Exposures of Witnet Formation may be the remnants of a synextensional sedimentary deposit that accumulated in a supradetachment basin. This inferred extensional exhumation of the southeastern Sierra Nevada may have begun as early as ~85-90 Ma and ended at ~80 Ma or later based on data from previous studies in the region. Thus, contractional deformation in the middle crust of the southern Sierra Nevada region may have been coeval with upper crustal extensional deformation in Late Cretaceous time. Correlation of the Cretaceous structural histories of the eastern Tehachapi gneiss complex and the northern Salinian block in the Coast Ranges of central California supports previous suggestions that the two areas may have evolved in close proximity to one another. The relative westward offset of the Salinian block from the Sierra Nevada prior to the Neogene may in part be the result of Late Cretaceous-early Cenozoic (?) westward extrusion of wedges of middle to lower crust bounded by thrust faults below and E dipping extensional faults above in a manner analogous to recent models for deformation in the Himalayas. The upper plate rocks of the Blackburn Canyon fault appear to be rotated about 90° clockwise relative to their inferred source region and the F4 folds in the Tehachapi area appear to have dextral vergence. The vergence of the folding and the sense of rotation both are consistent with Late Cretaceous dextral-oblique convergence indicated by plate motion models and with the presence of Late Cretaceous synbatholithic dextral transpressional and strike-slip shear zones in the Sierra Nevada to the north.

Geologic mapping provides structural and stratigraphic observations which lead to new insights into the magnitude, timing, and rate of Cenozoic extensional tectonism in the Death Valley region of the Basin and Range province in the western United States. Detailed mapping of the Grapevine Mountains, in northeastern Death Valley, yields new information on the structural evolution of the Titus Canyon anticline, a west-vergent fold of the Cordilleran thrust belt. The Grapevine Mountains contain the longest exposure of west-vergent folding in the Death Valley region, and detailed mapping supports previous interpretation of this structure as a piece of a single, laterally continuous fold, whose extensionally dismembered fragments form a key marker in reconstructions of Basin and Range extension. Such an interpretation suggests >100 km of west-north-west translation of the Grapevine Mountains away from the Sheep Range in late Cenozoic time. Correlation and re-interpretation of Cenozoic sedimentary and volcanic strata between the Sheep Range and the Grapevine Mountains indicate that this extension occurred on two separate extensional systems, the Sheep Range detachment system, and the Northeastern Death Valley detachment system. The former was active from 16-14 Ma, while the latter was active from 12.5-8 Ma. In contrast, stratigraphic and sedimentological data from the Eagle Mountain Formation suggests that, although extension across the central Death Valley region accommodated a similar magnitude of extension as the northern Death Valley region, ~100 km, extension across this region occurred post-11 Ma, and largely between 8-6 Ma. New geodetic and paleoseismic data are also presented from the eastern Basin and Range. These data indicate that slow (~4 mm/yr), long term (100s ka) strain accumulation is accommodated, geologically, by short (1000s yr) periods of fast (>1cm/yr) strain release, suggesting that the appearance of diffuse deformation across the eastern Basin and Range is likely due to time-averaging of many temporally discrete high-strain release earthquake clusters. These observations together suggest that the diffuse nature of intra-continental extension in the Basin and Range province may be the result of the summation of many spatially and temporally distinct extensional events, which, when active, progress at very high rates

Field mapping, petrography, U/Pb zircon geochronology, and Rb/Sr geo-chemistry on the crystalline rocks of the southernmost Sierra Nevada and Tehachapi Mountains north of the Garlock fault have 1) generated a structural, geo-chemical, and geochronological framework; 2) demonstrated a continuation of Sierran plutonic and metasedimentary rocks into the Tehachapi Mountains; 3) indicated that the region, in particular the gneiss complex of the Tehachapi Mountains, represents the deepest exposed levels of the Sierra Nevada batholith; 4) placed constraints on possible mixing models between upper mantle and meta-sedimentary components to generate the observed geochemical signatures of the rocks; and 5) resolved a major mid-Cretaceous deformation event.

The main crystalline rocks of the study area are the rocks of the Bear Valley Springs intrusive suite and the gneiss complex of the Tehachapi Mountains. The Bear Valley Springs suite is a mid-Cretaceous tonalite batholith complex with coeval gabbroic intrusives. The gneiss complex of the Tehachapi Mountains consists dominantly of early-Cretaceous orthogneiss, with subordinate paragneiss and local domains having granulite affinities. The orthogneisses are dominantly tonalitic in composition, with significant layers and domains of granodioritic to granitic and lesser dioritic to gabbroic gneiss. Quartz-rich metasedimentary rocks and marble constitute the main framework assemblage into which the plutonic rocks were emplaced. Field relations demonstrate assimilation of metasedimentary material into the orthogneisses and magma mixing between mafic, tonalitic, and anatectic granitic material derived from the metasediments.

Crystalline rocks of the region, with the exception of metasedimentary framework rocks, fall into a narrow age range of 90-120 Ma, and exhibit three main age suites. Most samples have zircon populations with systematics indicative of igneous crystallization, with signs of zircon inheritance or entrainment in the vicinity of metamorphic septa. Strongly discordant samples are relatively rare, and include the granodiorite of Claraville (concordia intercepts of 90/1900 Ma), the paragneiss of Comanche Point (108/1450), and a quartzite in the Kings sequence metasedimentary framework rocks (1700 Ma upper intercept).

The rocks in the first age suite (gneiss complex of the Tehachapi Mountains and augen gneiss of Tweedy Creek) exhibit a greater degree of deformation, especially under moderate to high grade conditions. Major deformational fabrics are expressed as gneissic banding, mylonitization, recrystallization, boudinaging, and transposition of internal contacts. Internally and externally concordant zircon systematics of the orthogneisses in this suite indicate igneous crystallization between 110-120 Ma. Discordant zircon systematics suggest entrainment of minor amounts of mid-Proterozoic zircon and/or open system lead loss in response to the 100 Ma magmatic culmination (Bear Valley Springs event).

The second suite, 100±2 Ma Bear Valley Springs intrusive suite (tonalite of Mount Adelaide, tonalite of Bear Valley Springs, hypersthene tonalite of Bison Peak, and metagabbro of Tunis Creek) contains igneous rocks which locally cross-cut the older suite. These rocks have a late-stage deformational fabric shown primarily in the tonalites as pervasive foliation and faint gneissic banding. The zircon systematics of this suite are internally and externally concordant, indicating igneous crystallization ages, with only local evidence of entrainment of mid-Proterozoic zircon. The deformation of the suite was synplutonic, with later phases within the suite lacking significant deformational fabrics. The major deformational fabrics exhibited in the Tehachapi and Bear Valley Springs suites may be the result of the intrusion of the tonalite batholith into the lower crust, and/or the result of intra-arc shearing that was preferentially concentrated in various intrusive bodies.

The third suite, late deformational intrusive rocks, consists of units which cross-cut deformational features in both the older suites. These youngest rocks are themselves slightly to nondeformed. The members in the suite have ages of 90 Ma (granodiorite of Claraville), 93 Ma (tonalite stock at Tweedy Creek), and 94 Ma (pegmatite dike at Comanche Point).

Field mapping and petrography have shown a southward continuation of Sierran plutonic and metasedimentary framework rocks to the region of Tejon Creek. The plutons show a constant age spread and overall composition throughout the region, with a greater degree of solidus to hot sub-solidus deformation exhibited southward. The metamorphic septa have a higher grade, and are more strongly deformed southwards, becoming migmatitic. The southern margin of the tonalite of Bear Valley Springs consists of a gradational contact with the hypersthene tonalite of Bison Peak, which is believed to represent the floor or conduit phase of the batholith. Along its southwestern margin, the tonalite of Bear Valley Springs grades into the gneiss complex of the Tehachapi Mountains through a region of tonalitic gneiss that appears to be derived through the mixing of tonalitic magmas and migmatitic melts produced from paragneiss components in the gneiss complex. Paleomagnetic and structural restoration of the southwestern margin of the tonalite indicates that it may represent the uptilted floor of the batholith that originally spread out over its gneissic substrate.

The crystalline rocks of the southernmost Sierra Nevada represent the deepest exposed levels of the Sierra Nevada batholith. Saleeby and others (1986a) indicate a continual increase in depth of exposure from the central to southern part of the batholith. Elan (1985) shows metamorphic conditions of 3.0 kb and 700°C in the south-central Sierras, while Sharry (1981b) has suggested that parts of the gneiss complex have a deep-seated (8 kb) origin with rapid late-Cretaceous uplift. Granulitic nodules of similar character to parts of the gneiss complex have been described by Domenick and others (1983) as originating from a similar depth beneath the central Sierra. Gneissic granitoids have numerous lenses of mafic to ultramafic cumulates showing igneous crystallization under granulite facies conditions. The domains of "granulite" in the gneiss complex of the Tehachapi Mountains are believed to be hot, relatively dry zones in a crystallizing and deforming batholithic complex. Magmatic epidote-bearing tonalites and late stage sub-solidus autometamorphic garnet growth are further indicators of a deep (≥6 kb) level of origin for the region.

The "granulites" (metagabbro of Tunis Creek and hypersthene tonalite of Bison Peak) are interpreted to be of an igneous origin. Evidence for this interpretation consists of: relict olivine grains and cumulate textures; foliation believed to be the result of igneous flow; zoned plagioclase necessitating the presence of a magma; tonalites that contain epidote that is interpreted to be of magmatic origin; δ¹⁸O and Rb/Sr isotopic values in the igneous range; abundance of retro-grade but paucity of prograde mineral reactions; gradational contacts between plutonic units; and observed intrusive contacts. Pyroxene within the "granulites" is believed to be of a pyrogenic origin. The rocks typically have a retrograde assemblage that consists of olivine → orthopyroxene and pyroxene → amphibole. The mineral assemblages all point to a downward P-T path.

Simple two-component mixing models have been constructed for samples from the southernmost Sierra Nevada, and involve incorporation of partial to complete melts of metasedimentary material into "primitive" upper mantle orogenic mafic magmas prior to crystallization. The two possible end-members are the quartzite-paragneiss of Comanche Point and the hypersthene tonalite of Bison Peak-metagabbro of Tunis Creek. Initial ⁸⁷Sr/⁸⁶Sr correlates directly with δ¹⁸O, and generally correlates inversely with Sr content for most of the samples. Simple isotopic mixing models indicate incorporation of up to 33% metasedimentary material in the granitic rocks, and up to 15% in the tonalites, with younger and more easterly samples requiring a larger metasedimentary component. The non-correlation of Sr_o with Sr content for some of the Pastoria Creek samples indicates an oceanic-affinity source with little interaction with continental crustal material. A number of samples appear to require a third, probable lower continental crustal and/or oceanic crustal-upper mantle component that may have a Paleozoic age.

Based on Rb/Sr and K/Ar age systematics, the region was uplifted in a regional cooling event at ~85 Ma perhaps as part of regional thrusting event(s) in southern California. The crystalline rocks were subsequently exposed and unconformably overlapped by Eocene marine sediments. Paleomagnetic data suggest about 45-60° of clockwise rotation between 80 and 16 Ma for the southern end of the Sierras, possibly as the result of the thrusting event responsible for the regional uplift.

Saleeby and others (1986c) have suggested that the lower crust beneath the Sierra Nevada batholith is comprised in part by granulitic and mafic intrusive rocks. Experimental studies by Christensen and Fountain (1975) also suggest the presence of granulites in the lower continental crust. The interpretation that the study area represents the deepest exposed level of the southernmost Sierra Nevada batholith leads to the implication that granulitic-affinity rocks comprise the lower part of the continental crust. Therefore, this study provides some degree of confirmation to the aforementioned hypotheses.

Indiana University-Purdue University Indianapolis (IUPUI)
The Antler and Sonoma orogenies occurred along the southwest-trending passive Pacific margin of North America during the Paleozoic concluding with the accretion of the McCloud Arc. A southeast-trending sinistral transform fault truncated the continental margin in the Permian, becoming a locus for initiation of an east-dipping subduction zone creating the Sierran magmatic arc. Constrained in age between two early Triassic tuff layers, the volcanic clasts in the breccia of Frog Lakes represent one of the earliest records of mafic magmatism in the eastern Sierra Nevada. Tholeiitic rock clasts found in the breccia of Frog Lakes in the Saddlebag Lake pendant in the east central Sierra Nevada range in composition from 48% to 63% SiO2. Boninites produced by early volcanism of subduction initiation by spontaneous nucleation at the Izu-Bonin-Mariana arc are more depleted in trace element concentrations than the clasts while andesites from the northern volcanic zone of the Andes produced on crust 50 km thick have similar levels of enrichment and provide a better geochemical modern analogue. Textural analysis of the breccia of Frog Lakes suggest a subaqueous environment of deposition from a mature magmatic arc built on continental crust > 50 km thick during the Triassic. The monzodiorites of Saddlebag and Odell Lakes are temporal intrusive equivalents of the breccia of Frog Lakes and zircon geochemistry indicates a magmatic arc petrogenesis.