Dissertations / Theses on the topic 'Structural Nevada'

This dissertation research assesses the behavior of young faults in central Nevada through analyses of landforms associated with these faults. Four large earthquakes have occurred since 1915 in a striking N-S belt in central Nevada; no comparable earthquakes have occurred elsewhere in the Great Basin. The frequency of large-earthquake occurrence, and temporal and spatial patterns and rates of faulting in central Nevada during the Holocene were assessed through geomorphic and geologic studies of young fault scarps. Ages of paleoseismic events were estimated primarily through analyses of fault scarp morphologies and characterization and quantification of soil development associated with alluvial surfaces. Rates of fault scarp degradation were explored through diffusion-based modeling of latest Pleistocene pluvial shoreline scarps. Morphologic scarp age depends strongly on scarp size; modest variations in local climate, particle size, and aspect are less important. Incorporating a factor that depends on scarp size almost always decreases the scatter in scarp age estimates, and is critical if only small scarps exist along a fault zone. An average of ±30% uncertainty about the mean scarp age estimate remains after these analyses. Soil development indices were calibrated using 14 Holocene to latest Pleistocene soil profiles in central Nevada whose maximum ages are constrained. Soil development indices were used to estimate ages of faulted and unfaulted alluvial surfaces along fault scarps. Soils and morphologic fault scarp age estimates for paleoseismic events are generally consistent. Temporal and spatial patterns and rates of faulting during the Holocene were evaluated using age estimates for paleoseismic events. The long-term rate of faulting is about 10 times lower than the historical rate. There were no other N-S belts of faulting during the Holocene, although scarp ages suggest that there may have been other temporal clusters of faulting. There have been spatial clusters of faulting during portions of the Holocene. The extensional deformation rate across central Nevada during the Holocene is about 0.5-0.75 mm/yr. Integrating this rate with fault-slip data from other portions of the northern Great Basin, the Holocene extensional deformation rate is 3.5-6.5 mm/yr, substantially lower than the historical deformation rate.

As a tool in discriminating basic rocks from different tectonic settings, a type of diagram was developed that employs three ratios of trace elements. The diagram separates basic rocks formed in mid-ocean ridge, intra-plate, and volcanic arc settings. It can be used to differentiate basalts from marginal basin, forearc, and arc rift zone settings. A second application of this type of diagram, employing major elements, distinguishes tholeiitic, calcalkaline, and boninitic series volcanic rocks. The southern part of the Foothills terrane, western Sierra Nevada, California, is composed chiefly of Jurassic-Triassic(?) metavolcanic and metasedimentary rocks of lower greenschist grade. Major tectonism affecting the terrane, associated with the Late Jurassic-Early Cretaceous Nevadan orogeny, was sinistral transpression with shearing along the Bear Mountains and Melones fault zones. The line of slip in high shear strain regions is approximated by the modal stretching lineation, which is at a rake of approximately 45° SE to the general shear zone orientation, suggesting sub-equal components of strike slip and dip slip. The sense of shear from kinematic indicators is consistently east side to the northwest. The terrane hosts three types of syngenetic massive sulfide deposits: Cyprus-type Cu deposits, Kuroko-type Zn-Cu-Pb deposits, and Besshi-type Cu-Zn deposits. The Cyprus-type deposits lie at the top of a Triassic(?) tholeiitic - basalt sequence in the lower Penon Blanco Formation. The deposits are part of an ophiolitic sequence that appears to have formed in an open-ocean spreading center environment. Felsic lava facies host the Kuroko-type deposits at the top of the Middle to Late Jurassic upper Gopher Ridge Formation, a dominantly bimodal sequence of meta-rhyolitic lavas and tuffs and meta-basaltic lavas. The tectonic setting appears to have been an arc-rift zone that formed during the transition from arc volcanism forming the lower Gopher Ridge Formation to younger basinal sedimentation forming the Mariposa Formation. The Besshi-type deposits are sediment-hosted in the Late Jurassic Mariposa Formation. They appear to have formed in the median part of a long linear basin between rifted arc segments. The inferred tectonic setting of the sulfide deposits was an early back-arc or interarc basin, which may have been related to transtensional tectonics.

Earthquake ruptures along tectonically active faults nucleate predominantly at depths of 5 to 12km in the crust, so the portions of faults that slip in these events cannot be directly observed. The geometry and composition of seismogenic faults controls the nucleation, propagation and termination of the earthquake rupture process. This study aims to place constraints on the geometry and composition of seismogenic faults by examining ancient faults exhumed from the depths at which earthquakes are observed to nucleate. Faults exposed in the Sierra Nevada, California, show that the internal architecture of earthquake faults is heterogeneous at a variety of scales. Field and microstructural observations are used to describe in detail the architecture of two pseudotachylyte-bearing fault systems in the Granite Pass region of Sequoia and Kings Canyon National Park; the Granite Pass fault (GPF) and associated faults, and the Glacier Lakes fault (GLF) and faults that splay from the GLF. The GPF and sub-parallel faults are 1 to 6.7km long with left-lateral strike-slip displacements up to 80m. The GPF and GPF-parallel faults have architectures that are heterogeneous along strike. They are composed of one to four fault core strands containing cataclasites and ultracataclasites that cross-cut early localized crystal-plastic deformation. Slip surfaces developed at the edges of, within and between fault cores are defined by pseudotachylytes and cataclasites with thicknesses of ~0.01 to 20mm. Fault-related subsidiary structures are developed on either side of fault cores, and comprise damage zones with widths orthogonal to the fault of up to 30m. The GLF and splay faults have architectures that are more homogeneous along strike. These faults are composed of a tabular volume of heavily fractured and altered host rock between approximately planar fault core strands. The fault cores are centimetres wide and contain cataclasites and foliated cataclasites that are cross-cut by pseudotachylytes. Fault-related damage is limited in extent to several metres beyond the bounding fault cores. The GLF contains additional cataclasites, ultracataclasites and pseudotachylytes in a fault core strand within the tabular zone of fractured rock. Thermochronologic analyses of the host rock granodiorite, combined with previously published palaeogeobarometry and apatite fission track data, define the temperature and pressure changes associated with cooling and exhumation of the pluton. The P-T conditions prevalent during the deformation history of the GPF fault system are evaluated by relating recrystallisation mechanisms in quartz to temperature, showing that the early stages of deformation occurred at temperatures of 450 to 600ºC. Dating of pseudotachylytes by the K-Ar isotopic method suggests subsequent brittle deformation took place at temperatures <350ºC and pressures ≤150MPa. A model for the architecture of the GPF architecture therefore has well constrained environmental controls, and should be transferrable to faults with comparable deformation histories. Small faults (cumulative displacements <1m) in the Mount Abbot Quadrangle, 55km north of Granite Pass, have been examined to illustrate the processes associated with the earliest stages of slip in the Sierra Nevada faults. The faults have branched or straight fault traces. Pseudotachylytes in branching faults show that these faults accumulated displacement in high velocity slip events, rather than by quasi-static fault growth. Branching faults without pseudotachylytes contain chlorite breccias interpreted to have formed in response to slip along faults with elevated pore fluid pressure. Straight faults also likely underwent slip events, but contain cataclased chlorite and epidote, suggesting low fluid pressures during slip. The small faults show that fluid-rock interactions are critical to fault geometry, and that lateral structural heterogeneity is established after small finite displacements. Field and thin section observations of exhumed seismogenic faults show that fault architecture and fault rock assemblage are critical to the earthquake rupture process. The heterogeneous composition of slip surfaces in the GPF faults imply that melt lubrication cannot account for all of the dynamic slip weakening as there are no continuous pseudotachylyte generation surfaces through the fault zones. Multiple slip weakening mechanisms must have been active during single rupture events. Slip weakening mechanisms also change at a given point on the fault in response to continued deformation. Splay faults at the GLF termination suggest that structural complexity observed at the terminations of fault surface traces can also be expected at depth. The off-fault damage at the termination of the GLF will change the bulk elastic properties of the host rock and must be accounted for in models of rupture propagation beyond fault terminations, or across geometrical discontinuities. Additionally, aftershock distributions and focal mechanisms may be controlled by the geometry of structures present at fault terminations.

This study focuses on the western Gale Hills located in the western portion of the Lake Mead domain in southern Nevada. The western Gale Hills preserve a record of the Miocene sedimentation and deformation related to the breakup of the hanging walls of the South Virgin-White Hills detachment fault and the Lime Ridge oblique, strike-slip fault of the Lake Mead fault system, the initiation of the right-lateral Las Vegas Valley shear zone in the western Lake Mead domain, and subsequent middle to late Miocene deformation. This study focuses on the lower Horse Spring Formation north of the Las Vegas Valley shear zone. To better understand the stratigraphy and deformation, a detailed geologic map (1:10,000 scale) was produced, data from primary and secondary structures were collected, and ash-fall tuff deposits were dated and correlated through ⁴⁰Ar/³⁹Ar geochronology and tephrachronology. The stratigraphy of the Gale Hills records the initial buttressing of the lower Thumb Member of the Horse Spring Formation onto pre-Tertiary topography. Deposition of the lower Thumb Member records a rapid transgression of the basin margin to the north and northwest across the majority of the Gale Hills. This time was period was then followed by a coarsening up interval and progradation of large alluvial fans in the middle to upper Thumb Member. A transition to a marginal clastic lake in the uppermost Thumb Member then abruptly changed to the Bitter Ridge Limestone algal lake.

Many studies have evaluated the exhumation history of the Gold Butte block in the eastern Lake Mead domain, which forms the footwall of the major South Virgin-White Hills detachment fault and the relationship with the Frenchman Mountain block. This study shows that the Frenchman Mountain block was just south of the Gale Hills during the early to peak stages of detachment faulting from ca. 17-14 Ma. Two new ⁴⁰Ar/³⁹Ar dates (15.35 Ma) from a prominent ash-fall tuff in the Thumb Member in the northern and southern regions of the western Gale Hills is also in the Frenchman Mountain block. In addition, new tephrachronology correlations have tied Proterozoic-clast debris flows in the western Gale Hills to Proterozoic-clast megabreccia deposits in the Frenchman Mountain block, indicating that the two areas were one connected basin during upper Thumb Member time.

This study suggests that the left-lateral Government Wash and Southern Gale Hills faults are reactivated northeast-striking, west-down normal faults that were in the correct orientation to be major Riedel prime shears (R') to the right-lateral Las Vegas Valley shear zone. Map and facies relationships show that the Thumb Member deposits were faulted locally during deposition at ca. 15.5 Ma, with increased fault activity and sedimentation rates throughout the Thumb Member after 15.35 Ma and before ∼14.5 Ma.

An analysis of structures in the western Gale Hills results in a new model of progressive clockwise rotation and faulting along the Las Vegas Valley shear zone that for the first time honors paleomagnetic results and accounts for all major faulting north of the shear zone. The model is primarily based on clockwise vertical-axis block rotation of domains between oblique left-lateral faults that curve progressively toward the Las Vegas Valley shear zone and terminate into major folds or areas of complex deformation. This model predicts that the western Gale Hills began as a north-northeast elongate block that was reduced in length and elongated in an east-west direction from about 20 to 14 km during translation and rotations. Most of this complex faulting occurred from ca. 13.8 to 8 Ma, after deposition of the Bitter Ridge Limestone.

The eastern California shear zone (ECSZ) and Walker Lane belt represent an important inland component of the Pacific-North America plate boundary. Current geodetic data indicate accumulation of transtensional shear at a rate of ~9.2 ± 0.3 mm/yr across the region, more than double the total geologic rate (<3.5 mm/yr) for faults in the northern ECSZ over the late Pleistocene [Bennett et al., 2003, Kirby et al., 2006, Lee et al., 2009, Frankel et al., 2007]. Unraveling the strain puzzle of the Walker Lane is therefore essential to understanding both how deformation is distributed through the lithosphere along this transtensional part of the Pacific-North America plate boundary and how the plate boundary is evolving through time. The observed mismatch between geodetic and geologic slip rates in the central Walker Lane is characteristic of other active tectonic settings, including the nearby Mojave segment of the ECSZ [Oskin et al., 2008] and the Altyn Tagh fault in China [Cowgill, 2007]. In each case, lack of fault slip data spanning multiple temporal and spatial scales hinders interpretation of fault interactions and their implications for lithospheric dynamics. The discrepancy between geodetic and geologic slip rates in the central Walker Lane indicates that if strain rates have remained constant since the late Pleistocene [e.g. Frankel et al., in press], then the "missing" strain is distributed on structures other than the two major dextral faults at this latitude (Death Valley-Fish Lake Valley fault and White Mountains fault). Otherwise the region could presently be experiencing a strain transient similar to that of the nearby Mojave section of the ECSZ [e.g., Oskin et al., 2008], or the rate of strain accumulation could actually increasing over the late Pleistocene [e.g. Reheis and Sawyer, 1997; Hoeft and Frankel, 2010]. The Silver Peak-Lone Mountain extensional complex (SPLM), to which the Clayton Valley faults belong, is the prime candidate to account for the "missing" strain. The down-to-the-northwest orientation of the SPLM faults makes them the most kinematically suitable structures to accommodate the regional pattern of NW-SE dextral shear. We use differential GPS to measure fault offset and terrestrial cosmogenic nuclide (TCN) geochronology to date offset landforms. Using these tools, we measure extension rates that are time-invariant, ranging from 0.1 ± 0.1 to 0.3 ± 0.1 mm/yr for fault dips of 30° and 60°. These rates are not high enough to account for the discrepancy between geologic and geodetic data in the ECSZ-Walker Lane transition zone. Based on geologic mapping and previously published geophysical data [Davis, 1981; Zampirro, 2005], deformation through Clayton Valley appears to be very widely-distributed. The diffuse nature of deformation leads to geologic slip rates that are underestimated due to the effects of off-fault deformation and unrecognized fault strands. Our results from Clayton Valley suggest that the discrepancy between geodetic and geologic strain rates at the latitude of the northern ECSZ is a result of long-term geologic rates that are underestimated. If the true geologic rates could be calculated, they would likely be significantly higher and therefore in closer agreement with geodetic data, as is the case everywhere else in the ECSZ north of the Garlock fault [Frankel et al., 2007a, in press; Kirby et al., 2008; Lee et al., 2009a].

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 structure and regional tectonic setting of an exhumed, 9.3-km long, left-lateral strike-slip fault zone eludicates processes of growth, linkage, and termination for strike-slip fault zones in granitic rocks. The Gemini fault zone is composed of three steeply dipping, southwest-striking, noncoplanar segments that nucleated and grew along preexisting joints. The fault zone has a maximum slip of 131 m and is an example of a segmented, hard-linked fault zone in which geometrical complexities of the faults and compositional variations of protolith and host rock resulted in nonuniform slip orientations, complex interactions at fault segments, and an asymmetric slip-distance profile. Regional structural analysis shows that joints and left-lateral fault zones have accommodated slip within a 4.8-km wide, right-lateral monoclinical kink band with vertical fold axes and northwest-striking axial surfaces. Geometric modeling of the kink band indicates that as little as 1.1 km of right-lateral displacement across the kink band may have produced the observed slip on kilometer-scale faults within the kink band.

I used age structure to examine the role of fire disturbance and climate on the population dynamics of the subalpine forest in the southern Sierra Nevada. I cored trees on ten 0.1 ha plots (3300-3400 m elevation) that varied in species composition, from single-species foxtail pine (Pinus balfouriana) or lodgepole pine (Pinus contorta, var. murrayana), to mixed-species stands of both pines. Crossdating was used to produce accurate dates of tree recruitment and fire events. Age structure varied by plot species composition: lodgepole pine recruitment pattern is pulsed, sometimes forming single-cohort patches in response to fire; foxtail pine plots have a more steady pattern of recruitment; mixed-species plots show an intermediate recruitment pattern. Fire may maintain a species composition mosaic in the subalpine forest. Foxtail pine regeneration may increase in areas opened by fire, although not immediately following fire. Low-intensity fire may spread over areas larger than previously reported under certain conditions in the subalpine zone. In addition, unusually frequent, extreme, and/or extended periods of drought may severely limit subalpine tree regeneration. Growing season frost events and grazing before 1900 may also have affected trees establishing in the subalpine zone.

This study assessed the geologic setting of the Soda Lake geothermal field, which lies in the southern part of the Carson Sink basin of northwestern Nevada within the Basin and Range of the western USA. The Basin and Range is a world-class geothermal province with significant untapped potential, particularly in blind (no surface hot springs or steam vents) geothermal systems. Blind systems probably comprise the majority of geothermal resources in the region, with many lying buried under thick accumulations of sediments in the broad basins that make up >50% of the province. Locating fault-hosted blind geothermal systems in these basins is challenging, and identifying the most prospective parts of these systems is even more demanding. The Soda Lake geothermal field is one of the more deeply buried known systems in this region. This study was undertaken to elucidate the stratigraphic and structural framework of the Soda Lake field, and to determine the probable controls on fluid flow in the production areas. Due to the depth of basin-fill sediments at the Soda Lake field, the structural setting and specific controls on fluid flow are not discernable at the surface. However, the Soda Lake geothermal field has produced electricity for over 30 years, and a wealth of subsurface data has been acquired since the field was first targeted for geothermal exploration in 1972-73. The abundant well data and geophysical surveys in particular provided a foundation for investigation of the geologic setting of the field.

This study was divided into three major parts. In the initial part of the study, a stratigraphic framework was developed for the Soda Lake area from analysis of cuttings, borehole geophysical logs, and radiometric dates of key igneous units. It was validated against exposed stratigraphic sections in the surrounding ranges and interpreted basin-fill sections derived from wells across the Carson Sink basin. Pursuant to this in the second part of the study, a comprehensive 3D geologic model of the Soda Lake field was construct from three inputs: 1) the new stratigraphic framework model, 2) bedding attitude estimates from seismic reflection surveys and borehole logs, and 3) a fault framework derived from both well data and geophysical surveys. The Soda Lake fault framework had been modeled from seismic reflection and borehole data in previous studies. In this study, one of the seismic fault pick sets was enhanced along strike and extended to >2 km depth using well data and forward modeled gravity. This enhanced fault framework served as the initial input to the Soda Lake geologic model. A ‘horizon model’ based on stratigraphic well intercepts and attitude data was then built around the fault framework to generate a 3D geologic block model for the Soda Lake field. In the final phase of this study, the Soda Lake temperature anomaly was modeled in a series of cross-sections extracted from the geologic model. The temperature anomaly was interpreted in context with the geologic model and production data in order to identify the main upwelling and outflow conduits. Key controls on fluid upwelling and probable fluid flow pathways were catalogued based on the spatial relationship between the temperature anomaly and the geologic model of the field area.

There are three major stratigraphic divisions at the Soda Lake geothermal field. The field is situated in and beneath ∼900-1100 m of unconsolidated basin-fill sediments. The basin-fill section is divided into an upper 300-500 m thick, relatively coarse-grained, quartzo-feldspathic unit, and a lower ∼150-300 m thick mud-rich unit. The unconsolidated basin fill is interrupted by a 5.1 Ma trachyandesite body that is up to ∼750 m thick in the central part of the Soda Lake well field. The body consists of a buried vent edifice near one of the main production wells, 50-90 m thick outflow aprons, and a conical root on the west side of the well field that can be traced to the Miocene bedrock contact. About 1 km of Miocene bedrock underlies the basin-fill section. The Miocene bedrock section is dominated by mafic lavas, interbedded with lesser tuff, clastic sedimentary rocks, and minor limestone. No early Miocene or Oligocene strata have been found at the Soda Lake field area. The middle to late Miocene section overlies Triassic-Jurassic metamorphic basement and Jurassic-Cretaceous granite. (Abstract shortened by ProQuest.)

The Ruby-Humboldt Range in Northeastern Nevada exposes the deepest crust in the western portion of the Sevier Hinterland. The product of unique brittle and ductile accommodations, this block of lower crustal rock is a window into the processes of continental thickening and extension. The structure of the northern tip of the Ruby-Humboldt Range core complex is dominated by a large recumbent fold nappe with a southward closeure cored by Paleoproterozoic-Archean gneissic complexes with complex interdigitated field relationships that record polyphase continental metamorphism. Amphibolite-grade metapelitic rocks within the core and Winchell Lake nappe record a wide range of zircon age dates of metamorphic events the oldest of which at ~2.5 Ga is recorded in adjacent orthogneiss as a crystallization age. At least two younger metamorphic events are recorded within this orthogneiss, most significantly at 1.7-1.8 Ga, an event previously unpublished for this region that links it to Wyoming province activity in addition to inherited component of detrital cores up to 3.7 Ga in age that is among the oldest ages reported in Nevada. The youngest overprint of cretaceous metamorphic overgrowth ranges fro 60-90 Ma in age based on zircon rims in the aforementioned units as well as three garnet amphibolites that intrude the core of the nappe and are interpreted to be metabasic bodies.

A geologic map was drafted of the northern Highland Range (1:24,000 scale), rock units defined, and samples of the volcanic units were obtained and analyzed to produce a representative suite of chemical analyses to characterize the range of geochemical variability. The style, relative timing, and orientation of faults and dikes, and the magnitude and variability of stratal tilting was examined to evaluate the structural and magmatic evolution of the northern Highland Range in the context of models for the Colorado River Extensional Corridor and Black Mountains accommodation zone. Methods involved field mapping of the range scale structure and geometry of faulting, structural interpretation, and geochemical analysis of ten representative samples by X-ray spectrometry. Structural data was interpreted with stereonets; geochemical whole rock, and major elemental data was analyzed by comparing elemental oxides; trace elemental data was analyzed by normalizing to chondrite concentrations. The northern Highland Range is a ca. 3,000 m-thick sequence of volcanic and volcaniclastic flows and breccias overlain by regionally extensive tuffs (Mt. Davis and Bridge Spring). Unique mineralogy, geochemistry and lithologic character of some units and volcanic vent facies, as well as the presence of domes and dikes feeding the extrusives argue for local derivation from a dome/stratocone volcanic complex that was mostly restricted to the northern Highland Range.

Outstanding preservation and exposure of Cretaceous tectogenic deposits in the North Muddy Mountains of southern Nevada provide a rare opportunity to examine the influence of frontal structures on provenance and sediment dispersal to the Cordilleran foreland basin. Eastward migration of the Sevier wedge-top depozone into the contiguous foredeep depozone was facilitated by development of the Willow Tank thrust-cored fault-propagation fold. The resulting thrust-cored frontal ridge diverted pre-existing fluvial systems of the Willow Tank Formation and promoted proximal alluvial fan deposition of the Baseline Formation. The Albian Willow Tank Formation represents the earliest foreland basin sediments derived from large, integrated drainage basins of the thrust belt interior. Sandstone point counts reveal an up-section increase in quartz locally derived from erosion from the Jurassic Aztec Sandstone along the frontal anticlinal ridge in the proximal Willow Tank thrust hanging wall. Continued fold growth is documented by growth strata development in and provenance of the overlying White Member of the Baseline Formation. Conglomeratic clast counts and detailed lithofacies analyses in the Cenomanian Red Member of the Baseline Formation record sequential unroofing of the frontal anticline and interaction of adjacent alluvial fan and fluvial depositional environments, respectively. Clast counts of poorly-sorted, massive ungraded conglomerates indicate a reverse clast stratigraphy based on progressively increasing amounts of carbonate framework clasts up-section. This trend is attributed to exposure and erosion of Upper Paleozoic strata in the Willow Tank hanging-wall anticline. Evidence of interfingering depositional environments is shown by up-section trends including: 1) replacement of poorly-sorted and organized coarse-grained conglomerates by well-sorted stratified to normally-graded conglomerates, 2) an overall decrease in grain size, 3) a decrease in soft-sediment deformation and increase in bioturbation, and 4) an increase in lateral bed continuity. This upsection transformation represents a spatial and temporal transition from foldproximal debris flow-dominated fans to an integrated braided stream system that transported detritus from the same source. Facies examination, clast composition and intertonguing relationships between the Red and Overton Conglomerate Members suggest contiguous braided stream networks that tapped separate, distinct source areas. These observations, as well as structural relationships, imply out-of-sequence movement on the Muddy Mountain thrust.

Orbicular and comb layer textures in igneous environments are evidence of an unusual heating and cooling regime in small pockets at the edges of crystallizing magmas. Changes in the composition of a magma spark rapid changes in temperature, which cause the temporary suppression of normal crystal nucleation. As the superheated or supercooled magma returns to equilibrium temperature, crystallization occurs exclusively on pre-existing nucleation surfaces (floating xenoliths or wall rocks), creating orbicular and comb layering textures. Orbs and comb layers collected from two localities in the central Sierra Nevada Batholith were analyzed to determine 1) how they formed and 2) what their formation history reveals about the emplacement histories of their respective host plutons. Geochemical analysis including XRF, U-Pb dating and Sr-Nd and O isotope analysis was used to constrain the characteristics of the orbicular magma. Cathodoluminescence as well as macro and microscale petrography was used to determine the specific growth history of the orbs and comb layers. This study shows that orbs and comb layers from both localities formed due to superheating caused by the influx of water into the orbicular melt. Subsequent cooling was caused by mixing–induced depolymerization and fluid enrichment (Big Meadows Creek) or emplacement into a cooler host rock (Deer Creek). Both locations studied are 2–3 Ma younger than their host plutons, indicating that the processes which form orb and comb layers may cause late melting and magma remobilization in larger plutons.

Home…that space so personal, so distinct, so intrinsic to the human/place relationship that “lies right at the heart of human geography” (Cresswell, 2004, p. 93). Studying the connection people feel toward certain places through concepts of emotion, experience, and attachment to meaning stems outward from the phenomenological and humanist branches of geography (Holt-Jensen, 2009). With every person’s version of home a space unto itself, is it possible for a place so intimate to be studied and defined? My answer is yes. This phenomenological case study investigates the perceptions and emotions of a newly designated conservation neighborhood, the second of its kind in Reno, Nevada. In an area usually looked at as a site for economic development and perhaps initiatives in historic preservation, there is little research undertaken through a cultural geographic lens aiming to understand how different communities in the area view their own home ground in transition and the implications of place creation. This project navigates the allegory of home through the voices and drawn maps of the Wells Avenue Neighborhood Conservation District (WANCD) and is approached through the impressions and attitudes of community groups, merchants, and a patchwork of residents diverse in both their backgrounds and their stories about the place they live. Through the construction of sense of place inside and around the WANCD and with the usage of Geographic Information Systems as a tool for qualitative data collection and comunication, this study investigates how personal experiences and perceptions, community connections and common goals, and specifically-identified areas of personal meaning play into the way in which these different stakeholders experience, participate in, and envision their neighborhood.

Ecological communities are complex, the structure of which is composed of interactions between multiple community characteristics and the abiotic and biotic factors shaping them. Because of this complexity, ecological studies are generally limited in scope and size, often dissecting communities into their component parts to examine them piece by piece. While this might be the most practical method to study communities, this approach often neglects other characteristics that, with their inclusion, would provide a more complete picture of community ecology. The studies described in this dissertation were conducted in an effort to synthesize the complexity that is inherent in ecological plant communities growing on a Mojave Desert bajada. Each study addresses a separate component of community structure, which, taken as a whole, provides a more thorough understanding of arid plant community dynamics. Overall, our results reveal the importance of substrate variables and their role in shaping plant community structure in arid environments. In addition, these investigations provide evidence of the strong role that facilitation plays on this bajada and possibly arid plant communities as a whole. The comprehensive approach described in this dissertation will enable ecologists to gain a more complete understanding of community dynamics and apply this knowledge to various climate change and land management scenarios.

La Sierra Nevada de Santa Marta (SNSM) est peut-être Le massif de la croûte terrestre le plus complexe trouvé dans les Andes du Nord. Sa situation unique comme un massif triangulaire isolé segmenté de la continuité de 7000 km de long Andes montagne comme la dernière position devant les domaines de la plaque des Caraïbes plus jeune, place la SNSM comme une île séparée de toutes les chaînes de montagnes environnantes de la marge continentale. Un relief important caractérise cette montagne atteint l'altitude la plus élevée dans le domaine des Caraïbes entière à 5750 m, et de définir, la SNSM comme le plus grand chaîne de montagnes côtières dans le monde. La SNSM est une caractéristique géologique unique un remarquable que les récifs coralliens Etreintes biodiversité de SES dans les forêts tropicales qui passent d'auge fortement végétalisées la mer des Caraïbes, les forêts de haute nuage, et bruyères, jusqu'à ce que, son magnifique sommet couronné par les glaciers.Par sa position sur la marge nord - ouest de l' Amérique du Sud l'étude de la SNSM donne l'occasion de résoudre des questions importantes sur l'évolution des cycles super-continentales depuis les temps Grenvilliens par l'orogenèse Neoproterozoic Pan-Africain, l'orogenèse Paléozoïque tardif Ouachitan-Appalaches cela a conduit Pangaea assemblée, et Trias Pangaea débâcle suivie par la Rift Atlantique Jurassique centrale et plus récemment par le début de la plaque des Caraïbes accrétion / subduction depuis le Crétacé tardif contre le nord - ouest en Amérique du Sud.Je tenté de démêler l'histoire géologique de la Sierra Nevada de Santa Marta Massif utilisant les plus avancées en géochronologiques, thermochronologiques, géochimiques et isotopiques techniques qui a permis de recueillir une quantité importante de nouvelles données à ajouter à la base de données existante sur la SNSM. Nos résultats comprennent une carte géologique réévaluée 1: 25000, qui comprend la définition de quatre nouvelles unités stratigraphiques, Accompagné de deux sections crustales sur 320 km de longueur ce disséquer le massif, et 8 sections parallèles à l'angle nord - ouest de le ceinture métamorphique de la SNSM. L'ensemble des données géochimiques et isotopiques comprend: i) 17 roches ignées et métamorphiques et six échantillons détritiques datés par ablation laser induite par couplage plasma spectrométrie de masse (LA-ICP-MS) U-Pb zircon géochronologie qui a abouti à 2790 nouvelles dates et in-situ analyse des oligo - élément, ii) 16 roches ignées et métamorphiques qui a donné 31 nouveaux âges thermochronometriques: 12 âgés du traces de fission en zircon 11 âges du traces de fission des apatites et 7 âges (U-Th) / He dans les apatites, iii) Géochimie de la roche entière à partir de 10 échantillons et iv) analyses à la microsonde chimie minérale et cartes x-ray de quatre échantillons a donné aux grenats zonées et péritectiques. Elles ont été données acquises à partir des unités du complex métamorphique nord - ouest du massif SNSM. Avec ces données, nous enquêté i) Les unités sont conformes à la SNSM ce ceintures métamorphiques, de leurs relations chronologiques et stratigraphiques du Précambrien à l'Eocène; ii) Le laps de temps et les conditions P-T du l’événement métamorphique Paléozoïque tardif à Mésozoïque précoce (chapitre 1), iii) Le moment de l'activité ignée, accrétion et l'exhumation des terranes océaniques et continentales au cours du Crétacé tardif à la fin du Miocène. iv) Un mécanisme pour expliquer comment l’exhumation a eu lieu sous un régime collisionnel influencé par un processus climatique à l'érosion et des gradients thermiques élevées (chapitre 2); v) Les processus tardifs de la dénudation et la sédimentation contrôlées par la tectonique dans deux bassins marginaux depuis le Miocène précoce dans le diminution des taux d'érosion et gradients thermiques (chapitre 3)
The Sierra Nevada de Santa Marta (SNSM) is perhaps the most complex crustal massif found in the Northern Andes. Its unique situation as an isolated triangular massif segmented from the continuity of the 7000 km long Andes as the last standing mountain before the domains of the younger Caribbean plate, places the SNSM as an island separated from all surrounding mountain ranges of the continental margin. A prominent relief characterizes this mountain reaching the highest altitude in the entire Caribbean realm at 5750 m, and defines, the SNSM as the highest coastal mountain range in the world. For this reason the SNSM is a unique geological feature that embraces an outstanding biodiversity from its coral reefs in the Caribbean Sea passing trough heavily vegetated tropical rainforests, high cloud forests, and moorlands, until its magnificent summit capped by glaciers.By its position on the northwestern margin of South America the study of the SNSM provides the opportunity to resolve important questions on the evolution of super-continental cycles since Grenvillian times through the Neoproterozoic Pan-African orogeny, the Late Paleozoic Ouachitan-Appalachian orogeny that led to Pangæa assembly, and Triassic Pangæa break-up followed by the Jurassic Central Atlantic Rift and more recently by the start of the Caribbean plate accretion/subduction since the Late Cretaceous against northwestern South America.In this investigation I attempt to unravel the geological history of the Sierra Nevada de Santa Marta Massif using geochronological, thermochronological geochemical and isotopic techniques that allowed to gather a significant amount of new data to add to the existent database on the SNSM.Our results include a reevaluated geological map 1:25000, in which I define 4 new stratigraphic units, accompanied by two crustal-scale cross sections of 320 km length that dissect the massif, and 8 parallel cross sections at the NW corner of the SNSM metamorphic belt. The geochemical and isotopic dataset includes: i) 17 igneous and metamorphic rocks and 6 detrital samples dated by laser-ablation induced-coupled-plasma mass-spectrometry (LA-ICP-MS), U-Pb zircon geochronology that resulted in 2790 new dates and in-situ trace element analyses, ii) 16 igneous and metamorphic rocks that yielded 31 new thermochronometric ages as follows: 12 zircon fission track ages, 11 Apatite fission track ages and 7 (U-Th)/He in apatite ages, iii) Whole rock geochemistry from 10 samples and iv) Microprobe mineral chemistry in spot analyses and x-ray maps from 4 samples that yielded zoned and peritectic garnet. These data were acquired from the units of the northwestern metamorphic suite of the SNSM massif. With these data we investigated i) The units that conform the SNSM metamorphic belts, their chronological and stratigraphic relationships from the Precambrian to the Eocene; ii) The time span and P-T conditions of a Late Paleozoic-Early Mezosoic metamorphic event (Chapter 1), iii) The timing of igneous activity accretion and exhumation of oceanic and continental terranes during the Late Cretaceous to late Miocene. iv) A mechanism for explaining how this exhumation occurred under a collisional regime by a climate influenced process at elevated erosion and thermal gradients (Chapter 2); v) The late processes of denudation and sedimentation controlled by tectonics in two marginal basins since the early Miocene under decreased erosion rates and thermal gradients (Chapter 3)

Fire is a natural and essential component of forests in western North America. Fire maintains biodiversity through the creation of different habitat types, and regular fire rotations reduce the accumulation of woody fuels and thick understory plant densities that give rise to catastrophic fire. The practice of fire exclusion has altered western forests and increased the risk of widespread change under rising temperatures projected for the 21st century. To manage for the reintroduction of fire it is critical that we understand the interactions between fire and forest biota in recently fire-suppressed forests. In Chapter 2, I studied the formation and impact of small-scale fire refugia. Fire refugia are areas within burned forest that experienced relatively little change, and are recognized as important places that offer protection for forest biota (vegetation, wildlife) during and after the fire. Very few studies, however, have examined small-scale fire refugia despite their importance to many organisms (e.g., small mammals, understory plants). In a long-term forest monitoring plot in Yosemite National Park, I mapped all unburned areas ≥ 1 m2 the first year after fire. I found small fire refugia were abundant, somewhat predictable, and fostered increased survival and diversity of nearby plant life. My results suggest that small fire refugia are an important component of burned forests that should be included in management considerations. In Chapter 3, I examined possible fisher habitat in burned areas. Fishers are forest carnivores of high conservation concern due to widespread declines since European settlement and the risk of habitat loss due to fire. An isolated population remains in the Sierra National Forest, where managers are weighing the need to reintroduce fire against possible detrimental impacts to current habitat. My research examined the forest structural characteristics (vegetative cover, heights of different forest layers) surrounding fisher dens. I found suitable thresholds of these structural characteristics in recently burned areas in Yosemite, particularly after low-severity fire. My results suggest that burned areas may offer suitable denning habitat for fishers, though more research is needed to determine if this conclusion holds for all fisher activities (e.g., foraging, resting) and scales of selection.

Le continent nord-américain présente sur sa bordure Pacifique un domaine constitué par l'accrétion du Paléozoïque supérieur au Cénozoïque de fragments lithosphériques : les Cordillères nord-américaines. Ces fragments lithosphériques sont caractérisés par des séquences volcaniques d'arc ou de bassin océanique. Les séquences volcaniques d'arc du Paléozoïque supérieur et du Trias ont longtemps été considérées comme un seul arc. Depuis une dizaine d'années, des équipes françaises ont montré que les séquences d'arc du Paléozoïque et du Trias des Klamath orientales et de Sierra Nevada (Californie du Nord) étaient établies respectivement sur une lithosphère océanique et un bloc continental. Dans le Nevada occidental, les séquences d'arcs du Paléozoïque supérieur affleurent dans deux régions distinctes : Black Rock Desert et Excelsior Mountains. Dans le Black Rock Desert, la séquence permo-triasique de Bilk Creek ressemble en tout point à celle * des Klamath orientales et des Blue Mountains. Il s'agit d'un magmatisme d'arc permo-triasique continu, reposant sur des calcaires permiens inférieurs à affinité téthysienne. Ce magmatisme dériverait d'une source mantellique de type-MORB. Dans les Excelsior Mountains, la formation Black Dyke est constituée de laves et de pyroclastites recouvertes en concordance par des turbidites volcanoclastiques. Cette activité volcanique se produit au Permien inférieur autour de 276 Ma. Ce magmatisme d'arc présente de nombreuses similitudes avec celui de Sierra Nevada. Dans les deux cas, il s'agit d'un magmatisme exclusivement Permien, caractérisé par : (i) des roches volcaniques calco-alcalines; et (ii) de faibles valeurs d'eNd (T=275Ma) comprisent entre -11 et +5,5. Cet arc est séparé du continent nord-américain par un domaine océanique, le bassin de Golconda. En Sierra Nevada, cet arc paléozoïque est établi sur une séquence sédimentaire du Palézoïque inférieur tectonisée. Les faibles valeurs de l'eNd(T) des roches magmatiques de Sierra Nevada et des Excelsior Mountains suggèrent que les magmas dérivent d'une source mantellique, contaminée par une vieille croûte continentale, probablement protérozoïque. Ces magmas subissent au cours de leur remontée et leur stockage dans des chambres des assimilations de matériel crustal lors de leur fractionnement et de leur cristallisation. Le Paléozoïque supérieur volcanique et sédimentaire d'arc de Sierra Nevada et des Excelsior Mountains est plissé, puis recouvert en discordance par des sédiments respectivement du Trias ou du Jurassique. Ces déformations et cette discordance sont liées à la phase orogénique Sonoma, induite par la collision de cet arc avec la marge nord-américaine. Après l'accrétion de cet arc, un magmatisme calco-alcalin à shoshonitique se développe au Trias en bordure du craton. Sa diversité reflète vraisemblablement des variations dans la nature et l'épaisseur de de la croûte qui forme la marge occidentale cratonique américaine et sur laquelle s'édifie cet arc de type andin.

Vers le milieu de l'ere tertiaire, la province des basin and range est passee d'un regime compressif a un severe regime distensif responsable de sa morphologie typique. C'est une des regions les plus etudiees au monde. De tres nombreuses etudes des structures (systemes de failles normales, blocs bascules d'echelle crustale, importantes surfaces de detachement) y ont ete realisees. L'etude des structures distensives vise a clarifier certains aspects de la deformation et de l'evolution de la province. Le travail s'est poursuivi sous la forme de deux approches complementaires. L'une purement qualitative consiste en une observation detaillee des structures et des rapports qu'elles ont entre elles : geometrie des failles, phenomenes de basculements, relations entre les divers types de fracturation, tectonique synsedimentaire, relation avec le volcanisme. . . L'autre est plus quantitative; il s'agit de la reconstitution des paleocontraintes et de l'eavaluation du taux d'extension. La combinaison des resultats qualitatifs et quantitatifs permet d'ameliorer notre comprehension des mecanismes et de l'evolution spatio-temporelle de l'extension. La determination des directions d'extension montre que l'histoire cenozoique de la province des basin and range a ete tres complexe. Chaque phase extensive est caracterisee par l'association de failles normales et decrochantes. Ces dernieres ont ete divisees en deux categories : les decrochements conjugues et les failles de transfert. L'analyse des rapports entre failles normales et decrochantes associee a l'observation detaillee de la tectonique synsedimentaire permet de reconstituer les etapes de la formation des bassins etudies

Thesis (M.S.)--University of Wisconsin--Madison, 1986.
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abstract: The Kinsley Mountain gold deposit of northeastern Nevada, located ~70 km south of Wendover, Nevada, contains seven sediment-hosted, disseminated-gold deposits, in Cambrian limestones and shales. Mining ceased in 1999, with 138,000 ounces of gold mined at an average grade between 1.5-2.0 g/t. Resource estimates vary between 15,000 and 150,000 ounces of gold remaining in several mineralized pods. Although exploration programs have been completed within the study area, the structural history and timing of precious-metal mineralization are still poorly understood. This study aims to better understand the relation between stratigraphy, structural setting, and style of gold mineralization. In order to accomplish these goals, geological mapping at a scale of 1:5,000 was conducted over the property as well as analysis of soil and rock chip samples for multi-element geochemistry. Using cross-cutting relationships, the structural history of Kinsley Mountain has been determined. The deformation can broadly be categorized as an early stage of compressional tectonics including folding, attenuation of the stratigraphy, and thrust faulting. This early stage was followed by a series of extensional deformation events, the youngest of which is an ongoing process. The structural history determined from this study fits well into a regional context and when viewed in conjunction with the mineralization event, can be used to bracket the timing of gold mineralization. The northwest oriented structure responsible for concentrating decalcification, silicification, and mineralization has two generations of cave fill breccias that both pre- and post-date the gold event. The statistical analysis of multi-element geochemistry for rock chip and soil samples has determined that Au is most strongly associated with Te, while weaker correlations exist between Au and Ag, As, Hg, Mo, Sb, Tl, and W. This suite of elements is associated with an intrusion driven system and is atypical of Carlin-type gold systems. From these elemental associations the gold mineralization event is thought to be controlled by the emplacement of a felsic intrusion. The responsible intrusion may be an exposed quartz monzonite to the south of the study area, as suggested by possible zonation of Cu, Pb, and Zn, which decrease in concentration with increasing distance from the outcropping stock. Alternatively, an unexposed intrusion at depth cannot be ruled out as the driver of the mineralizing system.
Dissertation/Thesis
M.S. Geological Sciences 2012

The Weepah Hills Area (Nevada) exposes exhumed metamorphic and plutonic rocks and upper-plate (supradetachment) volcano-sedimentary rocks that have experienced a complex, multi-stage deformational and depositional history. The Weepah Hills metamorphic core complex (WHMCC) is located in a region of the western Cordillera that was affected by both Miocene Basin-and-Range style E-W extension and Mio-Pliocene Walker Lane transcurrent shearing. Mio-Pliocene transcurrent deformation is transferred across a ~175 km releasing bend, known as the Mina Deflection, that kinematically links dextral strike-slip faults of the Death Valley-Fish Lake Valley with the central Walker Lane Belt. Progressive Mio-Pliocene transtension is characterized by core complex detachment faulting and younger high-angle normal faults. Timing of detachment faulting is constrained by both (U-Th)/He thermochronometry of footwall rocks and detailed chronostratigraphy of upper-plate strata to between 9-6 Ma. This age is supported by deformation recorded in the upper-plate strata that is attributed to progressive folding of the detachment associated with corrugation development. Earlier Miocene Basin-and-Range style extension is characterized by N-S trending high-angle normal faults and half-grabens that are strongly overprinted by Mio-Pliocene structures. (U-Th)/He zircon cooling ages from the detachment footwall range from ~12-20 Ma and are attributed to exhumation and unroofing related to E-W Basin-and-Range extension. New detailed sedimentological and geochronologic data show that, in contrast to previous research, the WHMCC upper-plate strata do not form a single supradetachment package, but rather three temporally distinct Miocene stratigraphic packages bounded by angular unconformities. The stratigraphic, structural, and exhumational record preserved in the WHMCC elucidates the timing of deformation and sedimentary basin evolution related to both Basin-and-Range E-W extension and Walker Lane related NW-directed transtension.
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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 Silver Peak Range of west-central Nevada reveals metamorphic tectonites below a low-angle fault deformed in a northwesterly-trending doubly-plunging anticline. The rock units in the region are divided into a lower plate, a Lower Paleozoic upper plate and an Oligocene and younger upper plate. The lower plate assemblage and Lower Paleozoic upper plate rocks share a common structural history, with the exception that peak metamorphic conditions in the lower plate reached lower amphibolite grade, whereas conditions in the upper plate never exceeded lower greenschist facies. Rocks of Oligocene and younger only experienced late-stage brittle deformation which warped the detachment fault into a doubly-plunging anticline. The cooling history of lower plate tectonites and structural evidence from the upper and lower plate rocks indicate an early history associated with Mesozoic thrusting, and a younger history of Miocene extension associated with displacement transfer between the Furnace Creek Fault and Walker Lane Belt.

Thesis (M.S.)--University of Wisconsin--Madison, 2002.
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Mylonitic rocks of the Shoo Fly Complex form a region of epidote-amphibolite grade quartzose and granitoid gneiss, subordinate schist and calcareous rocks, and rare amphibolite in the foothills of the Sierra Nevada range in central California. The Shoo Fly has endured a complicated Phanerozoic structural development involving seven superposed deformations at variable crustal depths. The first four of these (D1-D4) involved tight to isoclinal folding and shearing under medium grade metamorphic conditions. The last three (D5-D7) are marked by open folding and retrograde metamorphism of older fabric elements. The Shoo Fly is in ductile fault contact with east-dipping argillite, chert, and marble of the Calaveras Complex. The Calaveras-Shoo Fly thrust formed during D3 and is a 1-2 km wide syn-metamorphic ductile shear zone. Recognition of D3 overprinting of older Dl+D2 fabrics along the thrust zone indicates that upper plate Shoo Fly rocks record an earlier and more complex structural history than the lower plate Calaveras rocks. Paleozoic gneissic granitoids, an important lithologic component of the Shoo Fly, were intruded as a series of plutons ranging from calc-alkaline gabbro to granitoid (predominate) to syenite. They truncated the early S1 foliation in the Shoo Fly and were folded during regional D2 and D3 events when they were penetratively deformed into augen gneiss, blastomylonite, and ultramylonite. The Sonora dike swarm occurs as an areally extensive (> 1500 km2) subvertical consanguineous suite of andesite, lamprophyre, and basalt dikes that trend east-west across the Calaveras and Shoo Fly Complexes. The metamorphic complexes form the basement to a middle Jurassic calc-alkaline plutonic arc (Jawbone granitoid sequence). A middle Jurassic K-Ar age on the dikes (157-159 m.y.) together with the data of this report indicate that they are petrogenetically related to the Jawbone granitoid sequence and that the dikes probably formed during subduction beneath a continental arc. The dikes provide an important structural marker in the Shoo Fly and Calaveras Complexes. Intrusion of the dike swarm was sensitive to a structural anisotropy in the basement complexes. Since they intruded east-west along a spaced regional schistosity developed during folding of the Calaveras-Shoo Fly thrust, thrusting and subsequent folding were clearly pre-middle Jurassic events. Available geochronologic data sets middle Ordovician to late Devonian intrusive ages for the gneissic granitoids, establishing a pre-late Devonian depositional age for the Shoo Fly. D1 and intrusion of the orthogneiss protoliths may have been precursors of the Late Devonian to Early Mississippian Antler orogeny or, alternatively, may have occurred significantly earlier than the Antler orogeny. Based on cross-cutting relations, D2 formed during the Antler orogeny, D3 and possibly D4 during the Sonoma orogeny, and D5 and D6 during the Nevadan orogeny.

Tectonic signatures such as growth strata, clastic progradation, detrital composition, thickness trends, paleoflow shifts, lithofacies distribution, and vertical stratigraphic stacking patterns provide the basis for a range of tectonic/structural interpretations. Complete understanding of the application and limitations of tectonic signatures is important to maintain consistency and reduce uncertainty of interpretations that use them. This study provides insight into the external controls on two frequently used tectonic signatures in foreland basins: (1) growth strata, and (2) clastic wedge progradation. First, two syntectonic unconformity types are recognized in non-marine, Cenomanian growth strata adjacent to the Sevier thrust-belt in southeastern Nevada, USA. Unconformities with larger angular discordance (>10°, “Traditional Type”) developed when uplift outpaced sediment accumulation. More subtle unconformities with less discordance (2-10°, “Subtle Type”) developed when sediment accumulation nearly kept pace with uplift. Increasing sediment supply with positive net accommodation, allows syntectonic deposits to aggrade above a growing structure, with no change in uplift rate. Hence, sediment supply and regional accommodation impart an important control over growth strata geometries that are often interpreted on the basis of tectonics alone. Identification of unconformity types in growth strata can therefore document additional phases of uplift, particularly for intervals where sediments aggraded above an active structure due to higher sediment supply during regional subsidence, or sea level rise. Second, an anomalous, Campanian clastic wedge is identified in Cordilleran Foreland basin fill, Utah and Colorado. The complex internal architecture, tide-dominated facies and characteristic flat-to-falling shoreline stacking patterns of the wedge reflect rapid progradation of wide (60-80 km), embayed, tide-influenced shorelines; these characteristics distinguish the anomalous wedge from the underlying and overlying clastic wedges in the basin. A high-resolution regional correlation and isopach maps for the anomalous wedge provide evidence that extensive clastic progradation was coeval with both Sevier- and Laramide-style deformation. Stratigraphic relations suggest that development of the anomalous character of Wedge B was due to uplift of a Laramide structure within the foredeep, and possibly enhanced by reduced dynamic subsidence.
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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.

Structural features on megascopic, mesoscopic, and microscopic scales have been examined in a study area located in the central portion of the northern Sierra Nevada (west of Lake Tahoe). Structures observed within the study area are correlated with regional deformational events of the northern Sierra Nevada. Although at least four regionally extensive deformational events have affected the rocks of the northern Sierra Nevada, the effect of only three events are recognized in the study area. Structures associated with post-Ordovician/Silurian and pre-Late Devonian deformation were not identified due to overprinting of later deformational events. There is ample evidence supporting extensive late Paleozoic-early Mesozoic deformation in the study area. Structures from this event formed as the result of compressive deformation along a northwest-striking, east-dipping convergent plate boundary along the western margin of the Sierran province and may be related to the accretion of an exotic terrane, Sonomia. Although a significant component of strike-slip motion may have existed along the plate boundary, the structures appear to be related to the normal component of convergence. The intense, short-lived Nevadan orogeny deformed rocks throughout the study area. This latest Jurassic event is thought to be the result of an arc-continent collision. Nevadan structures, which have an anomalous north to north-northeast trend in the study area, are often indistinguishable from late Paleozoic-early Mesozoic structures due to similarities in style and orientation. Analysis of strain from quartz microfabrics indicates that Nevadan deformation is, at least locally, non-coaxial. The non-coaxial deformation is probably related to left-lateral oblique convergence. The last deformational event that affects rocks in the study area occurred in the Cretaceous. Cretaceous structures have a consistent northwest trend throughout the northern Sierra Nevada. The anomalous trend of Nevadan structures in the study area is most likely related to Cretaceous deformation. In the northern Sierra Nevada, post-Nevadan, dextral oroclinal folding, which occurred prior to or during the Cretaceous deformational event, is the result of right-lateral oblique convergence.

Thesis (M.S.)--Oregon State University, 1990.
Includes plates in pocket. Includes mounted photographs. Typescript (photocopy). Includes bibliographical references (leaves 89-92). Also available on the World Wide Web.

Results from field mapping and analyses of structural and petrochemical data from the southern Sierra Nevada batholith are presented to offer insight into the development of a major intra-arc fault system. The Kern Canyon fault system comprises an early ductile shear zone overprinted in its northern and central segments by a younger, recently active brittle fault. The divergence of these two faults at their middle latitudes poses a complex puzzle with regard to the physical and temporal evolution of deformation in the southern Sierra. Faulting began with ductile thrusting (the Proto-Kern Canyon fault zone) during emplacement of granitic plutons in the central to eastern part of the batholith at ca. 95 Ma. Early thrusting resulted in mismatched levels of pluton emplacement depths across the fault, truncation of significant regional geochemical markers in the batholith, and exhumation of the deepest level of the batholith in its southernmost region. Early ductile thrusting gave way to dextral strike-slip shearing by ca. 90 Ma. The youngest plutons in the batholith, emplaced along the fault between 90 and 80 Ma, are north-south elongate and reflect the dextral transpressional setting into which they were emplaced and deformed. Metamorphic country rocks were also highly sheared along the fault, and paleostress estimates from these deformed rocks suggest stresses along the middle segment of the Proto-Kern Canyon fault were 20–40 MPa, while strain rates were as high as 10-12 s-1 (comparable with other ductile faults). Strain studies and aspect ratios of igneous and metamorphic rocks strung out along the shear zone suggest ductile dextral displacement was 5–15 km. While ductile shearing ceased in the southern part of the batholith by ca. 85 Ma, it continued along the middle and northern segments of the Proto-Kern Canyon fault until ca. 80 Ma, when brittle deformation took over. This chronology suggests that the modern Kern Canyon fault, which shows ample evidence of activity into at least Quaternary time, initiated as a brittle structure in the southwestern part of the batholith, perhaps as early as 85 Ma, and shunted into the ductile shear zone at its middle latitudes ca. 5 Myrs later.