Browsing by Author "Law, Richard D."

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    Advances in Subduction Zone Processes
    Gorce, Jennifer Shannon (Virginia Tech, 2018-06-29)
    Subduction zones are an important recycling center at which material from the exterior of the Earth is transported to Earth's interior. The processes that occur along subduction zones have important implications for elemental cycles, geodynamics, and material mass transport. The cold, dense subducting lithosphere experiences prograde metamorphism as it transitions from blueschist to eclogite facies resulting in the breakdown of volatile-bearing minerals and producing anhydrous minerals and a free fluid phase. Previous works attempting to understand the evolution of subducted lithologies have provided a firm foundation in which to apply field work, computational thermodynamic modeling, and geochronological techniques in order to better constraint the Pressure-Temperature-time (P-T-t) paths and dehydration of subducted lithologies. This dissertation; 1.) Explores novel approaches to modeling and predicting fluid/rock interactions during deep (>60km) subduction, and 2.) Questions what the calculated P-T-t path from eclogite lithologies reveals about early exhumation of subducted terrains. The second chapter focuses on how externally-derived hydrous fluids can decarbonate subducted basalt, liberate carbon and transfer it to the overlying mantle wedge, where it can be incorporated into melt that forms volcanic arcs. Here, the thermodynamic response to the infiltration of external fluids assuming open system, pervasive fluid flow, is quantified. It was determined that while hotter subduction zones have more favorable P-T conditions in which to facilitate decarbonation than colder subduction, the extent of decarbonation is largely dependent on the availability of fluid from the dehydration of underlying serpentine. The third chapter constrains the P-T-t paths of subducted lithologies from Syros, Greece using a combination of thermodynamic modeling, 147Sm/144Nd garnet geochronology, and quartz-in-garnet geobarometry. This provides insight into early exhumation of subducted lithologies, and allows for the exploration of assumptions made in thermodynamic modeling and in quartz-in-garnet geobarometry. Results suggest that garnet grew over a 4.31my period from 45.71±0.98Ma to 41.4±1.7Ma, during initial exhumation from maximum subducted depths. Calculated exhumation rates are a relatively rapid, 0.4-1.7 cm/yr. Because field relationships on Syros suggest the width of the subduction channel along the slab/mantle interface is not adequate to facilitate buoyancy-driven ascension of metabasic blocks, initiation of southward retreat of the Hellenic Subduction Zone and subsequent slab rollback is proposed to have played an important role in the exhumation of subducted lithologies. The final chapter investigates the compositional controls on the P-T conditions at which dehydration due to the breakdown of hydrous minerals occur during subduction (blueschist/eclogite boundary), and the implications they have on the rheology, seismicity, and densification of the down going slab. Total Alkali Silica (TAS) diagrams reveal that eclogites are more alkali rich, implying that initial alteration of the seafloor controls the mineral evolution of subducted basalt in many cases.
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    Applications of Roll-Along Electrical Resistivity Surveying in Conjunction with Other Geophysical Methods for Engineering and Environmental Site Characterization
    Sayer, Suzanne (Virginia Tech, 1996-04-29)
    Roll-along electrical resistivity surveying was used with seismic refraction, magnetometer and gravity surveying in geophysical characterization of sites with a specific environmental or engineering problem. Three examples are presented where resistivity surveying provided vital constraints on acquisition and interpretation of other data in chaotic terrane. A commercially resistivity meter was used with prototype equipment designed, assembled, and tested at Virginia Tech. The equipment included a multiconductor cable consisting of interchangeable segments and a circuit allowing selection of numerous electrode configurations. The Sinking Creek Landfill, a 10-acre site, was used for disposal of municipal waste in the early 1970’s. Roll-along resistivity proved to be the most useful geophysical tool in ascertain its internal structure. Wenner configuration resistivity data, sensitive to both conductive leachate and ferrous metals, showed trenches within the landfill displayed in profile. Magnetic field measurements revealed anomalies over some trenches suggesting a method for discriminating between ferrous metal and leachate. Results of a resistivity survey can help planners of a refraction survey avoid low velocity “blind” layers. The Mid County Landfill borrow area is a 26 acre site situated within the Max Meadows Breccia and used for cover material for an adjacent landfill, The engineering problems were to measure the volume of rippable material, but travel time data were somewhat ambiguous. The refraction data interpreted using a) conventional 3-layer analysis b) horizontal 3-layer analysis of single shots, and c)continuous velocity gradient analysis of single end shots were compared with auger refusal depth. The single end horizontal analysis matched auger refusal depths most closely. Roll-along resistivity pseudo-sections made along the refraction lines proved to be effective for qualitatively imaging pinnacles and megaclasts. Excavation of fill material from a 75 acre river terrace in Pembroke exposed an antiform cut by high angle, near surface faults. Geophysical characterization was undertaken to determine the thickness of the alluvial deposit, and the relationship of the faults with structures in the underlying bedrock. Seismic refraction showed the terrace was as much as 134 feet thick. Resistivity pseudosections revealed a resistivity anomaly associated with the graben could be detected for a horizontal distance of several hundred feet. A gravity gradient paralleling the resistivity anomaly extends the feature more than 1000 ft from the exposed structure. Tenuous evidence of a bedrock escarpment beneath the near surface structure is found in a combination of seismic refraction, gravity, and electrical resistivity data. Roll-along resistivity has proved to be key to geophysical interpretation of these three areas. Images displayed on pseudosections reveal lateral inhomogeneity more clearly than could be discerned from seismic, gravity and magnetic data. Roll-along resistivity data can provide information for efficient siting of additional geophysical studies.
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    Constraining Metamorphic and Tectonic Evolution in Convergent Terranes: How Trace Elements and Mineral Inclusions Shape Mechanical and Reconstructive Models
    Ashley, Kyle T. (Virginia Tech, 2015-06-01)
    Conventional thermobarometry in metamorphic systems has been primarily limited to constraining peak temperature (or pressure) along a generalized P-T loop. This is largely attributed to the assumption that mineral assemblages and chemistries achieve a state closest to equilibrium with the maximum thermal (and therefore energetic) input at these peak conditions. However, this traditional approach is limited in providing much information about the evolution of a metamorphic terrane, which is modified by tectonic (kinematic) forces, fluid and component mobility, and heating duration. The ubiquity of quartz in the continental crust has driven much interest in using the phase for thermobarometric purposes. In this dissertation, I discss the application of elastic theory in reconstructing conditions of inclusion encapsulation through inclusion pressure estimation with Raman spectroscopy. In some instances, overpressuring of quartz inclusions in garnet give evidence for high-pressure formation conditions. When analyses are collected from garnet core to rim, pressure paths along garnet growth can be inferred (if temperature can be reasonably estimated). In high-T, low-P terranes, quartz may become dilated if the inclusion adheres to the host. If a quartz inclusion is sufficiently stretched, transformation to a low-density polymorph may occur. Trace element uptake, particularly Ti, have been characterized in quartz and understood to be the result of a temperature- (and to a lesser extent, pressure-) sensitive substitution for Si4+. However, the application of the Ti-in-quartz thermobarometer in quartz mylonites has led to mixed results due to the low-Ti resetting that occurs with dynamic recrystallization. We applied defect energy simulations and took a global assessment of deformed quartz trace element chemistries to infer that sweeping grain boundaries provide short pathways that allows localized re-equilibration with a Ti-undersaturated medium, resulting in Ti removal from the quartz lattice. In addition, thermodynamic pseudosection modeling has provided a method to assess Ti activity as a dynamic parameter – one that evolves as the phase stability changes through prograde and retrograde metamorphic reactions. With this understanding, better growth-composition models can be derived to infer complex pressure-temperature-time-deformation (P-T-t-D) histories of metamorphic rocks. These techniques and results are coupled with conventional thermobarometry techniques to provide a more comprehensive picture of the conditions experienced by a rock through the evolution, from burial to exhumation to the Earth's surface. The thermal evolution is used to provide conceptual thermal-kinematic models to explain tectonic evolution and heat advection in the continental lithosphere in ancient mountain belts.
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    The Determination of Lithospheric Rheology and Long-Term Interplate Coupling in Japan: Finite Element Modeling
    Huang, Shaosong (Virginia Tech, 1996-09-26)
    Northeast Japan experienced an approximately constant, compressional deformation during the last 5 million years resulting from the steady subduction of the Pacific plate. Because the direction of the maximum compression axis is approximately perpendicular to the strike of the island arc, 2-D finite-element modeling can be used to examine the deformation over time of the island-arc lithosphere. The model geometry is based on geophysical and geological data, and each model run requires an assumed rheology and interplate coupling. Novel to our modeling is the ability to include erosion/deposition loading and the creation of strike-slip faults, based on a dynamically-applied fracture criterion. The criterion for acceptability is how well a model matches observed present-day topography, gravity, and seismicity patterns. Results given below are for models that satisfy this criterion. The long-term effective elastic thickness is 10 km in the inner arc, increasing to about 50 km near the trench. The effective elastic thickness in the inner arc is therefore much smaller than the about 30 km short-term elastic thickness estimated from seismological data. The viscosity of the lower crust is on the order of 1022 Pa s or less. The strength of interplate coupling off Sanriku is about two to four times greater than off Miyagi, and there is about twice as strong a coupling at greater depths. The relative strength of coupling correlates well with the observed interplate seismicity. Hence the inferred weaker coupling off Miyagi indicates a lack of seismogenic potential -- a low probability for large earthquakes in that region, not just a long return cycle. The same modeling procedure was also applied to southwest Japan. The viscosity of the lower crust is not more than 1021 Pa s, and the elas tic thickness is about 10 km. The calculated strength of interplate coupling for southwest Japan is about 1.5 times greater than for the off-Sanriku region in northeast Japan, which correlates well with the fact that there have been great (M>8) earthquakes in the Nankai Trough region, but none that large in the off-Sanriku region.
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    Emplacement of the Santa Rita Flat pluton and kinematic analysis of cross cutting shear zones, eastern California
    Vines, John Ashley (Virginia Tech, 1999-12-14)
    This study documents the deformation history of the Santa Rita Flat pluton, eastern California, from the time of emplacement to post-emplacement transpressional shearing, and consists of manuscripts that make up three chapters. The first chapter addresses the emplacement of the Santa Rita Flat pluton using anisotropy of magnetic susceptibility (AMS). The second chapter describes the kinematic analysis of cross-cutting shear zones within the western margin of the pluton. The third chapter is an informal paper on the U/Pb dating of two sheared felsic dikes from the pluton. AMS of the Santa Rita Flat pluton indicates that the paramagnetic and ferromagnetic minerals define a foliation which is arched into an antiformal structure in the central to southern parts of the pluton. The northern part of the pluton displays an east-west striking magnetic foliation which lacks a fold-like geometry. Previously published field mapping and petrologic surveys of the pluton and surrounding wall rocks indicate that the southern margin and northern part of the Santa Rita Flat pluton represents the roof and core of the pluton, respectively. Integration of our analysis of the internal structure of the pluton with previously published work on the regional structure of the surrounding metasedimentary wall rocks, suggests that the pluton may have initially been intruded as a sill-like or "saddle reef" structure along a stratigraphically controlled mechanical discontinuity in the hinge zone of an enveloping regional-scale synform. Subsequent vertical inflation of this sill resulted in local upward doming of the overlying pluton roof and formation of the antiformal structure now observed at the current erosion level in the central-southern part of the pluton and overlying locally preserved roof rocks. No corresponding fold structure is indicated by AMS analysis in the northern part of the pluton, which is exposed at a deeper level, and represents a section closer to the pluton core. Emplacement of the Santa Rita Flat pluton at 164 Ma overlaps in time with regional deformation at ~185 - ~148 Ma (Middle - Late Jurassic) recognized in the southern Inyo Mountains. Northwest trending folds are pervasive along the western flank of the Inyo and White Mountains, and may have accommodated strains at the lateral tips of thrust faults which crop out in the southern Inyo Mountains. We speculate that space for initial emplacement of the Santa Rita Flat pluton may have been produced by layer-parallel slip and hinge-zone dilation, accompanied by axis-parallel slip during formation of a regional scale thrust-related synform. The Santa Rita shear system (SRSS) is composed of a series of discrete NW-SE striking steeply dipping shear zones that cut and plastically deform granitic rocks of the Santa Rita Flat pluton. The shear zones exhibit a domainal distribution of gently and steeply plunging stretching lineations, and are located at planar mechanical discontinuities between the granite and a series of felsic/mafic dikes which intrude the pluton. Mylonitized dikes within the shear zones contain syntectonic mineral assemblages not observed in dikes outside the shear zones, indicating that the dikes were intruded prior to shear zone development. Correlation with geometrically similar shear zones in the Sierra Nevada batholith to the west, suggests that the SRSS probably nucleated from a regional stress field in Cretaceous times (~90-78 Ma). Strain is heterogeneous within the shear zones, with local development of protomylonite, mylonite, ultramylonite and phyllonite. Strain heterogeneity within the granite is attributed to fluid infiltration and chemical reaction and alteration of feldspar to fine-grained mica. These deformation-induced mineral changes would have resulted in progressive mechanical weakening over time of rocks within the SRSS. The phyllonites occur predominantly within steeply lineated shear zones and contain mylonitized foliation-parallel quartz veins. The pattern of c-axis preferred orientation in these quartz veins indicates that deformation within the shear zones occurred under plane strain conditions. Locally, quartz veins also cut the foliation planes, reflecting high pore fluid pressures during evolution of the SRSS. These cross-cutting quartz veins are also plastically deformed, and their c-axis patterns indicate weak constrictional strains. The orientation of the shear zones, together with their strain paths, are used to develop a transpressional kinematic model for development of the SRSS within a progressively rotating stress field.
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    Evaluation of Coupled Erosional Processes and Landscape Evolution in the Teton Range, Wyoming
    Tranel, Lisa Marie (Virginia Tech, 2010-06-17)
    The evolution of mountain landscapes is controlled by complex interactions between large-scale tectonic, surficial and climate conditions. Dominant processes are attributed to creating characteristic features of the landscape, but topographic features are the cumulative result of coupled surficial processes, each locally effective in a different climate or elevation regime. The focus of erosion by glacial, fluvial, or mass wasting processes is highly sensitive to small changes in boundary conditions, therefore spatial and temporal variability can be high when observed over short time scales. This work evaluated methods for dissecting the history of complex alpine landscapes to understand the role of individual processes influenced by changing climate and underlying bedrock. It also investigated how individual and combined mechanisms of surficial processes influenced the evolution of topography in the Teton Range in Wyoming. Detrital apatite (U-Th)/He thermochronology and cosmogenic radionuclide erosion rates were applied to determine spatial and temporal variability of erosion in the central catchments of the range. Spatial variability existed between the glacial and fluvial systems, indicating that sediment erosion and deposition by these processes was controlled by short-term variability in climate conditions. Effective glacial incision also controlled other processes, specifically enhancing rock fall activity and inhibiting fluvial incision. Short-term erosion rates were highly variable and were controlled by stochastic processes, particularly hillslope failures in response to slope oversteepening due to glacial incision and orientation and spacing of bedrock fractures. Erosion rates averaged over 10 ky time scales were comparable to long-term exhumation rates measured in the Teton Range. The similarity of spatial erosion patterns to predicted uniform erosion and the balance between intermediate and long-term erosion rates suggests the landscape of the Teton Range is approaching steady-state, but frequent stochastic processes, short-term erosional variability and coupled processes maintain rugged topographic relief.
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    Evolution of Deformation Along Restraining Bends Based on Case Studies of Different Scale and Complexity
    Cochran, William Joseph (Virginia Tech, 2018-06-25)
    Globally, deformation along obliquely converging plate margins produce a wide variety of complex fault patterns, including crustal pop-ups, fault duplex structures, restraining bends, and flower structures. Depending on the plate velocity, plate obliquity, crustal rheology, length-scale, and climate, the evolution of faulting into translational and vertical strain can range in complexity and fault slip partitioning (i.e. vertical vs. horizontal strain). In this dissertation I studied two restraining bends to understand how these factors influence patterns of deformation along two major plate boundaries: The North American-Caribbean and the North AmericanPacific plate boundaries. First, I estimate the exhumation and cooling history along the Blue Mountains restraining bend in Jamaica using multiple thermochronometers. Three phases of cooling have occurred within Jamaica: 1) initial rock crystallization and rapid emplacement of plutons from 75-68 Ma, 2) slow cooling from 68-20 Ma, and 3) two-stage exhumation from 20 Ma – Present. During the most recent phase of Jamaica’s cooling history, two stages of exhumation have been identified at 0.2 mm/yr (20 – 5 Ma) and ~1 mm/yr (5 Ma – Present). Given the plate velocity to exhumation rate ratio during the most recent phase, we suggest that the climate of Jamaica increases the erosivity of the Blue Mountain suite, whereby the Blue Mountains may be in an erosional stead-state. Second, I studied the long-term evolution of a restraining bend at San Gorgonio Pass in southern California by relating fault kinematics within the uplifted San Bernardino Mountains to the nearby Eastern California shear zone. Using highresolution topography (i.e. UAV and lidar surveys), I studied the plausibility of faulting along two potentially nascent faults within the San Bernardino Mountains, namely the Lone Valley and Lake Peak faults. We found that while both faults display evidence for Quaternary faulting, deciphering true fault slip rates was challenging due to the erosive nature of the mountainous landscape. Coupled with evidence of Quaternary faulting along other faults within the San Bernardino Mountains, we suggest a western migration of the Eastern California shear zone.
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    Facies, Sequence Framework, and Evolution of Rudist Buildups, Shu'aiba Formation, Saudi Arabia
    Al-Ghamdi, Nasser Mohammad (Virginia Tech, 2006-05-19)
    The Cretaceous (Early Aptian) Shu'aiba Formation, Shaybah field, Saudi Arabia, is 60 km long by 12 km wide and 150 m thick, and is a giant carbonate reservoir. It formed on a regional carbonate ramp bordering an intrashelf basin. The succession consists of a composite sequence of seven high frequency sequences. Sequences 1 and 2 formed a deeper open platform of Palorbitolina-Lithocodium wackestone, with maximum flooding marked by planktic foram mudstone. Sequence 2 built relief over northern and southern blocks, separated by an intraplatform depression. They form the composite sequence TST. The remaining sequences developed a platform rimmed by rudist rudstone backed by rudist floatstone back-bank and lagoonal fine skeletal peloidal packstone; slope facies are fine skeletal fragmented packstone. Aggradational sequences 3 to 5 make up the composite sequence early highstand. Progradational sequences 6 and 7 are the composite sequence late highstand marking the deterioration of the Offneria rudist barrier and deposition of widespread lagoonal deposits, where accommodation may have been created by syn-depositional growth faulting that moved the northern block down. Shu'aiba deposition on the platform was terminated by long-term sea-level fall and karsting. The succession is dominated by approximately 400 k.y., 4th order sequences and 100 k.y. parasequences driven by long term eccentricity and short term eccentricity respectively, similar to the Pacific guyots of this age. This suggests that early Cretaceous climate may have been cooler and had small ice sheets and was not an ice-free greenhouse world.
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    Fault Behavior and Kinematic Evolution of the Eastern California Shear Zone
    Garvue, Max Martin (Virginia Tech, 2024-10-07)
    The geomorphic expression, sedimentation, and near-field deformation of a fault system may be characterized to obtain an understanding of its kinematic evolution and potential seismic hazards. The dynamics and deformation history of the Eastern California shear zone (ECSZ), a wide and complex network of right-lateral strike-slip faults, is not well understood, despite hosting three large (>Mw 7.0) earthquake ruptures in recent decades. The low-net slip faults of the ECSZ (each with <10 km) offer a unique opportunity to assess strain distribution in a developing, kinematically immature strike-slip system. To do so, I conducted field-based investigations of these faults within the Mojave Block of the ECSZ. First, I investigated the morphology, structure, and controls of restraining bend growth along the numerous faults of the ECSZ via field mapping and numerical deformational modeling. I found that the ECSZ restraining bends are small (kilometer-scale), exhibit high-angle, doubly fault-bound geometries with positive flower structures, and have self-similar morphologies characterized by a "whaleback" longitudinal profile and an arrowhead shape in map view. Gradual changes in form with increasing restraining bend size suggest a common growth mechanism influenced more by the kinematics of local fault geometries than by the fault's obliquity to plate motion. Modeling results indicate that concentrated shear strain at single transpressional bends facilitates the development of new secondary faults with cumulative strain as a mechanism to accommodate horizontal shortening via uplift between the faults. The ECSZ restraining bends contribute minimally to regional contractional strain due to their small size, steep fault angles, and shallow crustal penetration (< 5 km), which also suggests that they are unlikely to obstruct large earthquake ruptures. Second, I conducted a spatiotemporal slip rate analysis of the Calico fault with new mapping and geochronology of offset alluvial fans from North Hidalgo Mountain. From this work I obtain several findings. 1) The slip rate along North Hidalgo Mountain ranges from 1.5-2.1 mm/yr in the Holocene and 0.8-2.0 mm/yr in the late Pleistocene. 2) The similarity in slip rates between North Hidalgo Mountain and the Rodman Mountains suggests that this 38 km stretch is a kinematically coherent fault segment with a relatively steady slip rate of 1.7 +0.4/-0.3 mm/yr over the past 60 ka. Faster rates reported from Newberry Springs suggest either a significant increase in slip rate from the Rodman Mountains to Newberry Springs or temporal variations in slip rate. 3) The new rates support previous work which showed the central section of the Calico fault has the highest slip rate in the Mojave Block. However, it does not resolve the discrepancy between ECSZ geodetic and geologic slip rates, implying that transient changes in slip rate, or the contribution of off-fault deformation or other structures may be required. Additionally, the lack of geological slip rate data might contribute to this discrepancy if significant spatial and temporal variations exist on other ECSZ faults.
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    Field based study of thrust faults in the Appalachian Valley and Ridge Province Newport, Virginia
    Overby, Kyle Eugene (Virginia Tech, 2016-03-24)
    This study focuses on a series of thrust sheets exposed in the Appalachian Valley and Ridge Province Blacksburg-Pembroke area in southwest Virginia. Structures in the hanging wall of the Saltville thrust (Saltville thrust sheet) and the footwall of the Saltville thrust (Narrows thrust sheet) are examined. The first part of this study involves the construction of a series of thrust transport-parallel 1:24,000 scale geologic cross sections to constrain the subsurface geometry of fault and fold structures within the Saltville and Narrows thrust sheets. The second part of the study involves an outcrop-scale study of geologic structures exposed along a series road cuts in the footwall of the Saltville thrust and the geometric and relative timing relationships between folding, cleavage formation and thrust faulting. The cross sections show a series of interconnected splay faults branching off of the Saltville thrust and cutting both its hanging wall and footwall. Angle of dip and magnitude of dip-slip displacement on thrust and splay faults progressively decrease from hinterland to foreland within this fault system that is referred to as the Spruce Run Mountain-Newport (SRMN) fault system. Bedding within this fault system essentially forms a structural transition zone between the Saltville and Narrows thrust sheets, defining a km-scale fractured synform-antiform fold structure that has many structural attributes usually associated with fault propagation folding. In the road cut outcrops, early meter-scale faults are folded by later foreland-(NW) vergent folds. Although cleavage defines convergent cleavage fans about these folds, subtle obliquities between folds and cleavage indicate that folding post-dates early layer-parallel shortening and associated foreland-vergent thrusting.
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    Flow Properties of Moine Thrust Zone Mylonites in Northern Assynt, NW Scotland
    Roth, Benjamin Louis (Virginia Tech, 2010-12-07)
    Quartz-rich mylonites present along the Moine Thrust Zone are well suited for the application of various analytical techniques designed for investigating the flow processes by which rock deformation occurred. These analytical techniques were applied to a suite of samples from the footwall and hangingwall of the Moine thrust exposed along the Allt Pol aâ Mhadaich stream located in the northern part of the Assynt window. Vorticity analyses were performed to determine the relative contributions of pure and simple shear deformation within the penetratively deforming thrust sheets. Integration of vorticity data with 3D strain analyses demonstrated that sub-vertical shortening perpendicular to the flow plane, accompanied by thrust transport parallel extension, occurred during mylonitization, and was driven by emplacement of the overlying Moine nappe. Quartz c-axis fabrics in the mylonites are characterized by well-defined asymmetric Type-1 cross girdles in which internal and external skeletal asymmetries are indicative of a top-to-the-WNW shear sense, compatible with regional thrusting. These c-axis fabrics were also used to estimate deformation temperatures. Differential flow stresses associated with mylonitization were estimated from the grain size of dynamically recrystallized quartz. Deformation temperature and flow stress data were then incorporated into a dislocation creep flow law for quartz to estimate strain rates. Finally, along strike variation in these flow properties at the base of the Moine nappe to the north and south of the APM section were investigated and results from the APM section compared with previously published studies of mylonites exposed in eastern Assynt that occupy similar structural positions.
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    Fluid inclusions as a monitor of progressive grain-scale deformation during cooling of the Papoose Flat pluton, eastern California
    Brauer, Nancy A. (Virginia Tech, 1997-12-18)
    Analyses of fluid inclusions and microstructures within the Papoose Flat pluton were used to investigate the chemistry and temperatures of fluids circulating with the pluton during cooling. Based on previous microstructural analyses, the interior of this late Cretaceous granitic to granodioritic pluton has been divided into three domains: i) a central core characterized by magmatic microstructures, ii) a middle domain of high temperature (>500°C) solid-state deformation, and iii) an outermost domain characterized by relatively low temperature (<5000°C) solid-state deformation. According to previously published anisotropy of magnetic susceptibility analyses and pluton cooling models, plastic flow occurred in both the outer part of the pluton and within its aureole rocks while the core of the pluton was still molten. Solid-state deformation is proposed to have stopped when the pluton interior cooled through its solidus less than 100,000 years after magma emplacement. Microstructural analysis of samples from all three domains confirmed the transition from magmatic flow in the core of the pluton to solid-state deformation at the pluton margin. However, weakly developed solid-state microstructures overprint the dominant magmatic microstructures in samples from the core domain. The existence of solid-state microstructures in all three domains indicates that deformation continued during and after crystallization of the interior of the pluton. Two phase, low salinity (< 26 wt% NaCl equivalent), liquid-rich aqueous fluid inclusions predominate within both quartz and feldspar grains in all samples. Throughout the pluton, the majority of fluid inclusions are hosted by deformed grains. Feldspar-hosted primary inclusions are associated with sericitic alteration. Inclusions were also observed in feldspar as secondary or pseudosecondary inclusions along fractures. Inclusions in quartz are frequently found near lobate grain boundaries or near triple junctions; linear pseudosecondary inclusion assemblages are commonly truncated against lobate boundaries between adjacent quartz grains, indicating that discrete microcracking events occurred during plastic deformation. Homogenization temperatures overlap for all three microstructural domains. Coexisting andalusite and cordierite in the contact aureole, and the intersection of the Mus + Qtz dehydration reaction with the granite solidus, indicate trapping pressures between 3.8 and 4.2 kb. Ninety-eight percent of the calculated fluid inclusion trapping temperatures at 3.8 - 4.2 kb are below the granite solidus of 650°C. Seventy-six percent of the trapping temperature data fall within the more restricted range of 350-500°C; i.e. at temperatures which are lower than the commonly cited brittle-ductile transition temperatures for feldspar at natural strain rates, but above those for quartz. No correlation could be established between trapping temperatures and either host mineral or microstructural domain within the pluton. The similar, relatively low trapping temperatures indicate that the majority of inclusions preserved in all three domains were trapped during the late low strain magnitude stages of solid-state deformation. The most common fluid inclusion trapping temperatures (400-500°C) in all three microstructural domains are similar to the deformation temperatures indicated by microstructures and crystal fabrics in the outer part of the pluton; these trapping temperatures are obviously lower than temperatures associated with contemporaneous solid state and magmatic flow in the pluton interior. The similar trapping temperatures within the pluton core and margin must indicate that the inclusion-trapping event migrated from the margin to the core of the pluton as it cooled, because fluid inclusions would rapidly equilibrate to a density appropriate for the PT conditions of their host minerals.
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    Geologic evolution of the Archean Buhwa Greenstone Belt and surrounding granite-gneiss terrane, southcentral Zimbabwe
    Fedo, Christopher M. (Virginia Tech, 1994-09-05)
    The Archean (~3.0 Ga) Buhwa Greenstone Belt, and surrounding granite-gneiss terrane, is the least understood major greenstone belt in the Archean Zimbabwe Craton, despite occupying a critical position between an early Archean continental nucleus and the Limpopo Belt. The cover succession in the Buhwa Greenstone Belt, which was probably deposited on the margin of this nucleus, is divisible into shelfal and basinal facies associations separated by a transitional facies association. The shelfal association consists mostly of quartzarenite and shale, but also contains a thick succession of iron-formation. Geochemical characteristics of the shales indicate that the source terrane consisted of several lithologies including tonalite, mafic-ultramafic volcanic rocks, and granite that underwent intense chemical weathering. Basinal deposits consist dominantly of greenstones, with less abundant chert and ironformation. The cover succession, which was deposited on a stable shelf transitional to deep water, has no stratigraphic equivalents elsewhere on the Archean Zimbabwe Craton. However, time and lithologic correlatives in the central zone of the Limpopo The Archean (-3.0 Ga) Buhwa Greenstone Belt, and surrounding granite-gneiss terrane, is the least understood major greenstone belt in the Archean Zimbabwe Craton, despite occupying a critical position between an early Archean continental nucleus and the Limpopo Belt. The cover succession in the Buhwa Greenstone Belt, which was probably deposited on the margin of this nucleus, is divisible into shelfal and basinal facies associations separated by a transitional facies association. The shelfal association consists mostly of quartzarenite and shale, but also contains a thick succession of iron-formation. Geochemical characteristics of the shales indicate that the source terrane consisted of several lithologies including tonalite, mafic-ultramafic volcanic rocks, and granite that underwent intense chemical weathering. Basinal deposits consist dominantly of greenstones, with less abundant chert and ironformation. The cover succession, which was deposited on a stable shelf transitional to deep water, has no stratigraphic equivalents elsewhere on the Archean Zimbabwe Craton. However, time and lithologic correlatives in the central zone of the Limpopo ~2.9 Ga in southern Africa. At ~2.9 Ga, the northern margin of the greenstone belt experienced kilometerscale, oblique-slip dextral shearing. This shear zone and the surrounding margins of the greenstone belt were later intruded by the ~2.9 Ga Chipinda batholith, which ranges from granitic to tonalitic in composition. A number of events occurred during the time period spanning 2.9-2.5 Ga and current geochronology cannot separate their order; some are known to be coeval. Crustal shortening to the northwest, which resulted in map-scale folding of the cover succession (and surrounding batholith) and greenschist-facies metamorphism, occurred along a set of discrete high-angle reverse-sense shear zones in response to uplift the Northern Marginal Zone of the Limpopo Belt over the Zimbabwe Craton. Two suites of potassic granites were intruded into the area near the end of reverse shearing. Analysis of a conjugate fault pair that is developed within one of the potassic granite suites, yields a principal compressive stress consistent with continued northwest-directed crustal shortening. The region was stabilized by ~2.5 Ga, with intrusion of the Great Dyke of Zimbabwe. It is possible that the last events to affect the area, which include sinistral shearing, transecting cleavage development, and northwest-striking open folding, took place during the 2.9-2.5 Ga time intervaL These structures post-date regional folding and metamorphism, but because of limited magnitude and extent, do not show obvious cross-cutting relationships with other rocks or structures. A tenable alternative is that these late structures formed at ~2.0 Ga. an age that is proving to be of great significance in the evolution of the Limpopo Belt and along parts of the southern margin of the Zimbabwe Craton.
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    Geology and mineralization in the vicinity of the Morning Star precious-metal deposit of the Ivanpah Mountains, San Bernardino County, California
    Sheets, Ronald Wynn (Virginia Tech, 1996)
    Disseminated precious-metal mineralization occurs at the Morning Star deposit, in the upper-plate of the Morning Star thrust which represents intermediate age (105 and 90 Ma) faulting along the Mesozoic foreland fold and thrust belt (MFFTB). Deformation along the Morning Star thrust consists of both ductile and brittle events, and is different than deformation on other structures in the MFFTB in the northeastern Mojave Desert. The different style and episodic nature of deformation, together with the spatial relationship to the Teutonia batholith, accounts for mineralization on the Morning Star thrust and the absence of mineralization on other thrusts in the belt. Two stages of mineralization have been identified in the Morning Star deposit. Six distinct types of electrum have been identified. Electrum occurs as free grains, fracture fillings, or as inclusions in minerals during primary mineralization, and as rims around early electrum or as intergrowths with covellite and acanthite during secondary mineralization. Textures and compositions of the electrum and Ag+Au-sulfides indicate Au and Ag remobilization was isochemical in the lower portions of the deposit, while remobilization at upper levels of the deposit resulted in gold enrichment. Hydrothermal fluids (H₂O-CO₂-6 wt.% NaCl), that were driven by Late Cretaceous igneous activity, scavenged metals from the host rocks to form the Morning Star mineralization. Early mineralization is syn-tectonic, but the bulk of the mineralization is post-tectonics. The bulk of the main stage mineralization was precipitated into open spaces at temperatures between 280° and 330°C by a combination of reduction and increased acidity of the fluid due to wall rock reactions. Ore grades have subsequently been increased by Au and Ag remobilization.
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    Geology and Tectonic Significance of the Late Precambrian Eastern Blue Ridge Cover Sequence in Central Virginia
    Wang, Ping (Virginia Tech, 1991)
    The Late Precambrian cover sequence in the Blue Ridge of central Virginia includes rocks of the Moneta Formation and the overlying Lynchburg Group. The Moneta Formation comprises arnphibolites, felsites and biotite gneisses that unconformably overlie the Grenville basement. The Lynchburg Group in central Virginia is divided into three formations. Lynchburg I is made up of massive to thick bedded coarse-grained feldspathic arenites and conglomerates, which are interpreted as slope-apron deposits. Lynchburg IT contains mainly medium to fine grained feldspathic arenites and graphitic schist (black shales) with subordinate conglomeratic rocks. These are believed to be channelized submarine fan turbidites formed in an anoxic environment. Lynchburg ill consists of fine to medium grained feldspathic quartz arenites and a minor amount of conglomeratic rocks, which are considered to be channelized submarine turbidites with a more open marine environment and wider shelf. Three metamorphic facies and two deformation events are recognized in the cover sequence of the study area. The current tectonic models tend to view most of the mafic-ultramafic rocks and the host sedimentary rocks of the Lynchburg as ophiolitic melange, thus creating a suture, of Precambrian to Ordovician age. Detailed field mapping shows that the Lynchburg Group does not have the characteristics of melange and the mafic-ultramafic rocks in it do not resemble ophiolite. Rather, the cover sequence is related to the Late Precambrian Iapetan rifting event. Some tectonomagmatic discriminant diagrams have been used to support the current tectonic model and they are considered one of the most important arguments for ophiolites. These diagrams were tested by plotting samples from Jurassic rift basalts-diabases of eastern North America (ENA). The ENA samples, as well as the post Grenville mafic rocks in the Blue Ridge, tend to plot outside the within-plate field. It is clear that geochemical data alone may give a wrong tectonic classification, and that a knowledge of field relations is of paramount importance for interpretation.
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    Geometry, kinematics and age of the northern half of the White Mountain shear zone, eastern California and Nevada
    Sullivan, Walter Andrew (Virginia Tech, 2003-06-06)
    The White Mountain shear zone (WMSZ) is a zone of intense penetrative deformation that lies along the western front of the northern White-Inyo Range in eastern-most California and western-most Nevada. The northern half of the WMSZ is characterized by a NNE to NNW-striking steeply dipping foliation and associated shallowly plunging NNE to NW-trending stretching lineations. S-C fabrics observed in outcrop, microstructural shear sense indicators and kilometer-scale foliation geometry all indicate dextral movement. Localized discrete zones of coeval steeply plunging stretching lineations are present in the northern half of the WMSZ. Microstructural data from these domains indicate a high component of pure shear within a separate coeval kinematic framework and hence a transpressional history. The WMSZ appears to be tectonically related to both the Sierra Crest shear system to the west and the Santa Rita shear system to the south. Correlation between the WMSZ and the Santa Rita shear system indicates that Late Cretaceous dextral transpression may extend up to ~120 km along the western front of the White-Inyo Range. Cross-cutting relationships with Late Cretaceous plutons bracket the age of the WMSZ at between 72-92 Ma. A lack of annealing recrystallization in deformed quartz and the presence of high temperature crystallographic fabrics near the margins of the ca. 72 Ma Boundary Peak pluton indicate significant strain accumulation within the WMSZ subsequent to emplacement of the Boundary Peak pluton. These observations extend the duration of Late Cretaceous dextral transpression in eastern California to at least as recent as 72 Ma.
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    A geophysical characterization of New River terrace deposits in Giles County, Virginia
    Cyrnak, Jennifer Sue (Virginia Tech, 1996-06-05)
    A series of geologically recent faults was discovered within a folded succession of stratified alluvial sediments, commonly referred to as New River terrace deposits, near the town of Pembroke in Giles County, southwest Virginia. Geological and geophysical investigations were utilized to characterize the terrace deposits and investigate the nature of the observed faults. Geologic mapping of the underlying carbonate bedrock provided orientation measurements of fault, bedding, and joint planes; however, none of the features mapped within the bedrock could be directly correlated with the observed faults. The results of geophysical studies suggest significant variations in lithology within the alluvial sediments. Seismic velocities (P-wave) obtained from reversed seismic refraction profiles range from 900-1700 meters/second, while apparent electrical resistivity values vary from 300-2000 ohm-meters. A 75 meter wide, east-west trending low resistivity zone extending across the center of the study area is juxtaposed against an extremely high resistivity zone which is present to the north. The results of seismic reflection and electrical resistivity data analysis are interpreted to suggest that several small and large scale extensional faults may exist throughout the sediments, possibly reaching depths of up to 30 meters. An isopach map of preserved terrace thickness indicates that the faults lie within an east-west trending zone of maximum terrace thickness (35-40 meters thick), which corresponds to a topographic rise in land surface. A structure contour map of the bedrock surface reveals an area of lowest bedrock elevation beneath this zone, implying inverted topography. Results indicate that the evolution of the terrace might be related to ongoing karst processes within the bedrock.
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    Heterogeneous internal fabric of the Mount Barcroft pluton, White Mountains, of eastern California: an anisotropy of magnetic susceptibility study
    Michlesen, Karen Joyce (Virginia Tech, 2004-02-02)
    Anisotropy of magnetic susceptibility (AMS) have been used with great success for determining the internal structure and fabrics of Jurassic and Cretaceous plutons of felsic-intermediate compositions in the White-Inyo Range of eastern California. However, application of the AMS techniques to the Mount Barcroft pluton, located in the northern White Mountains, has yielded anomalous scalar and directional AMS data indicative of unprecedented heterogeneity on the meter-kilometer scale. The 165 Ma Mount Barcroft pluton is primarily of granodiorite composition and was intruded into the Barcroft Structural Break, a northeast striking, steeply dipping structure that juxtaposes Mesozoic metavolcanic rocks to the north against Proterozoic-Paleozoic metasedimentary rocks to the south. Two oriented hand samples (A and B) were collected at each of 78 sites distributed on a 1 kilometer grid pattern across the 5 by 15 kilometer Mount Barcroft pluton and oriented cores were prepared from these hand samples for AMS analysis. Microstructure identification of single thin sections prepared for each sample site yielded primarily magmatic with minor solid-state structures. A highly heterogeneous distribution of scalar parameters (Km, P%, F%, L%, T) was documented both between sample sites and between the A and B cores at individual sites. The heterogeneity may be the result of complex mineral assemblages and the interaction between different magnetic mineral species ranging from single domain to pseudo-single domain to multidomain magnetite. More problematic are the directional parameters between A and B cores in orientation and fabric type (e.g. prolate and oblate susceptibility ellipsoids) occur which cannot be readily explained by a complex mineral assemblage. Different fabric types in A and B cores at individual sample sites could be the result of discrete, temporally unrelated, magma pulses of variable composition and viscosity. Heterogeneity of scalar and directional AMS parameters in the Mount Barcroft pluton, and its contrast with the homogeneous AMS signatures within similar age plutons to the south, may provide evidence for a previously unrecognized magma source beneath the northern White Mountains.
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    How Hot, How Deep, How Long: Constraints on the Tectono-Metamorphic Evolution of Granulite Terranes
    Guevara, Victor Emmanuel (Virginia Tech, 2017-06-05)
    Granulites are the dense, strong metamorphic rocks that are produced during high- (HT) to ultrahigh-temperature metamorphism (UHT) and partial melting of Earth's crust. Granulites are ubiquitous in exhumed Archean cratons and are thought to comprise much of Earth's stable lower crust. Understanding the mechanisms responsible for crustal heating in Archean terranes is thus paramount to understanding the stabilisation of early continental crust, and whether such mechanisms resemble modern tectonic processes. It is therefore important to quantify the pressure–temperature–time (P–T–t) paths of Archean granulites, as such paths can be diagnostic of heating mechanism. This dissertation explores: 1) novel approaches to reconstructing the P–T–t paths of granulites, and 2) what the deciphered P–T–t paths of rocks from two Archean granulite terranes reveal about Archean crustal heating. The first chapter shows how petrologic modelling at multiple scales from a texturally heterogeneous granulite can provide "snapshots" of the P–T path, which would be difficult to reconstruct otherwise. The remaining chapters are focused on reconstructing the P–T–t paths of two Archean granulite terranes: the Beartooth Mountains, and the Pikwitonei granulite domain (PGD). The second and third chapters present evidence for cryptic HT metamorphism of the Beartooth granulites at ~2.7 Ga characterized by rapid (< 1 Ma) exhumation at HT and fast cooling (~10-100 C/Ma) in the middle crust. This suggests advective/conductive heating over short length-scales. In the fourth chapter, thermobarometric data suggest the western PGD experienced UHT decompression followed by cooling in the lower crust. High-precision zircon and monazite dates reveal apparently episodic crystallization over at least ~24 Ma. This episodicity could reflect multiple thermal cycles or the control of local reactions on zircon/monazite crystallization during cooling. High-spatial resolution petrochronology provides temporal constraints on prograde metamorphism. These data suggest metamorphism in the PGD was driven by a long-lived heat source over large length-scales near the base of the lithosphere. Disparities in the timescales, length-scales, and the depth and amount of heating between the terranes may suggest different crustal heating mechanisms in each, and that the late Archean Earth may have been tectonically diverse.
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    Hydrogeology and Simulated Water Budget of the Rio Cobre and Rio Minho-Milk River Basins, Jamaica, West Indies
    Wishart, DeBonne Natalie (Virginia Tech, 2000-09-20)
    An investigation was undertaken to better understand the hydrogeologic framework of the Rio Cobre and Rio Minho-Milk river basins, Jamaica, West Indies. A quasi three-dimensional finite-difference groundwater flow model was used to conceptualize flow conditions and establish a hydrogeologic budget of the region. The Rio Cobre and Rio Minho-Milk river basins lie on the Clarendon Block, an area with a complex geologic history. The geologic history includes: 1) the intrusion of calc-alkaline granites, 2) morphotectonic sedimentation, 3) three episodes of deformation by transpressional and transcurrent tectonics, 4) the deposition of a highly permeable, Tertiary carbonate platform, and 5) the development of near surface karst oriented with the major NNW-SSE fault trend in the basins. Since deposition, compression, faulting, and solution have modified the distribution and thickness of carbonate rocks impacting the ground-water flow of the region. The most notable features are the older NNW-SSE trend dip-slip faults and the younger E-W trend strike-slip faults, notably the South Coast Fault (SCF) formed during the Laramide Orogeny. The White Limestone aquifer is the principal aquifer of the Rio Cobre and Rio Minho-Milk river basins in the parishes of St. Catherine, Clarendon, and partly in Manchester. It is characterized by intercalated sequences of permeable rubbly and micritic carbonate rocks. The age of the rocks range from Late Cretaceous (Maastrichtian) to Recent. The permeability of the South Coast fault and the high hydraulic conductivity value associated with the Tertiary carbonate platform (480 m/d) in the Lower Rio Minho-Milk River basin control the gradient of the potentiometric surface and ground water flow in that region The agreement between the measured and the simulated hydraulic heads obtained for this steady-state model suggests that the values assigned to the hydraulic properties that characterize the ground-water flow of the White Limestone aquifer are reasonable. Recharge to the area occurs as net recharge in addition to upland subsurface inflow across the general head boundary in the northern part of the study area. Comparisons of calculated and observed values of head indicate that simulated groundwater flow field generally agree with field conditions. Several simplifying assumptions were made for the conceptualization and simulation of flow in the basins: 1) during the 1998 water year, ground-water in the basins was considered at steady-state, 2) pumping does not significantly affect the level of hydraulic heads; therefore pumping wells are not simulated, 3) Net recharge from precipitation varies spatially, 4) karstification and aquifer heterogeneity impact on the distribution of hydraulic conductivity, 5) Darcy's law is applicable to flow through the fractures and solutions openings in a karst region, 6) flow in the White Limestone aquifer occurs in the uppermost 650 m and vertical flow is assumed to be controlled by intervening units, 7) evaporation was not explicitly simulated in the model. Recharge rates were considered as "net recharge," and 8) submarine discharge occurs from the aquifer along the coast where aquifers are hydraulically connected to the sea. Ground-water flow in the basins was conceptualized as a quasi three-dimensional flow system in which two model layers were used. The model boundaries selected to represent natural hydrologic boundaries include (1) a no-flow along the western and eastern boundaries, (2) a constant head boundary along the freshwater/saltwater interface; (3) a general head boundary along the northern boundary; and (4) a horizontal-flow barrier boundary along the South Coast Fault; and (5) river leakage boundaries along major rivers draining the coastal basins. The simulated region is an area of 2,550 square kilometers, two-thirds of which is hilly and the remainder, irrigated plains with small swamps draining the area. The model consists of over 337,500 cells and employed a regular grid spacing of 200m x 160m. The model was designed and calibrated to steady-state conditions from data observed/estimated during water year 1998. The Water Resources Authority of Jamaica (WRAJ) will use the results of the modeling study as a predictive tool for long-term management and monitoring of water resources in the region. The model was calibrated using a manual trial-and-error adjustment of parameters. Hydraulic conductivity values in both model layers, hydraulic conductivity at the general-head boundary, and streambed conductance were adjusted during successive simulations until computed head values approximated field conditions. The computed potentiometric surface is an adequate or reasonable match on a regional scale, with the general horizontal hydraulic gradient oriented with the main fault trend NNW-SSE in both basins. Sensitivity tests of the calibrated model were conducted on net recharge, hydraulic conductivity, hydraulic conductivity assigned along the general-head boundary, and streambed vertical conductance to determine if differences between simulated and observed values were similar to the range of uncertainty in the values of input data and boundary conditions. Based on the results obtained from the sensitivity analysis, it is apparent that the model is extremely sensitive to changes in horizontal hydraulic conductivity and recharge in the form of precipitation. The model is least sensitive to streambed vertical hydraulic conductivity.