Sedimentary units in the Inyo Mountains of eastern California can be traced into the aureole of the Papoose Flat pluton where, due to intense crystal-plastic deformation, they are thinned to less than 10% of their original stratigraphic thickness. This deformation is constrained by textural relationships to have occurred during contact metamorphism. The strain within the dominantly S>L tectonites is partitioned by lithology. The deformation is in bulk, noncoaxial, but examination of individual lithologic units reveals large components of either pure shear or simple shear deformation. C-axis fabric analysis of quartz tectonites within veins in the gneissic border of the pluton, and quartzites within the pluton aureole, indicate that quartz tectonites deformed under approximately simple shear deformation, and suggest that the pluton/wall rock contact is the zone of highest strain. C-axis fabrics measured from the quartz tectonites are distinctly different between the veins and the quartzites. Veins display a higher maximum concentration of c-axes per uniform distribution and fabrics are characterized by single girdles. Quartzites display decreasing maximum concentrations of c-axes with increasing distance from the pluton/wall rock contact and fabrics are characterized by asymmetric cross girdles. Structural and field relationships indicate that; 1) pure shear deformation was dominant within the deformed border of the pluton, 2) flattening (chocolate tablet boudinage) occurred locally at the pluton/wall rock contact, 3) dominantly pure shear deformation produced the porphyroblast-matrix relationships within the aureole schists and, 4) pure shear plane strain (symmetrical boudinage) occurred locally within the aureole marbles. The direction of maximum extension within the boudinaged marbles is parallel to a stretching lineation which is well developed throughout the deformed western margin of the pluton, and is associated with the simple shear deformation observed in the quartz tectonites. Both pure shear and simple shear deformation are believed to be occurring concurrently with intrusion. Porphyroblast-matrix relationships within the aureole schists indicate that andalusite porphyroblasts have not rotated with respect to compositional layering. In sections viewed parallel to lineation and perpendicular to foliation, a constant angular relationship is observed between planar porphyroblast inclusion trails and matrix foliation traced around the western margin of the pluton. Unwrapping of the matrix foliation, from concordancy with the pluton margin into a planar reference frame, "unspins" the planar inclusion trails into a parallel and constant geographic orientation. This relationship, along with inclusion trail geometry, suggests that initial metamorphism was static, and that penetrative deformation did not occur until late in the growth history of andalusite. Deformation is recorded in the rims of andalusite porphyroblasts as a curvature of inclusion trails into parallelism with the matrix foliation. Inclusion trails begin curving at the same point in porphyroblast rims where inclusions coarsen in grain size and decrease in abundance, therefore a second ‘stage’ of metamorphism is believed to be synchronous with deformation. The strong component of pure shear deformation (plane strain and flattening) observed within various lithologic units, the variation in strain around the pluton/wall rock contact, and the synchroneity of the second stage of metamorphism with deformation suggest that the Papoose Flat pluton was forcefully emplaced. The deformation is believed to be a result of a complex path of forceful inflation, where inflation is not symmetrical as a balloon, but is heterogeneously developed and possibly directed more in the orientation of the lineation. A regional deformational event may, or may not, have occurred synchronously with forceful emplacement. / Master of Science

The Striped Rock pluton of the late-Proterozoic Crossnore Plutonic-Volcanic suite is located beneath the Fries Thrust zone in the Blue Ridge province of southwest Virginia. The multiphase granite pluton has been affected by episodes of brittle and crystal plastic deformation at both the microscopic and mesoscopic scales. Brittle deformation preceded and postdated crystal plastic deformation. The pluton is cut by pervasive centimeter-scale cataclasite zones and ductile shear zones that vary in width from a few millimeters to several hundred meters. The majority of mylonite zones in the pluton strike east and northeast and are inclined moderately southeast. Cataclasite zones strike northeast and northwest. Deformation is most intense along the southern contact with the Cranberry gneiss where both pluton and country rock are deformed into a northeast-striking zone of mylonitic augen gneiss. The intensity of deformation decreases northwestward. Southeastdirected normal fault displacement is common to east and northeast-trending shear zones. A minor group of northwest-oriented shear zones dip moderately southwest and northeast and show sinistral, strike-slip displacement. Quartz-, chlorite- and stilpnomelane-filled cracks and veins with northeast and northwest trend uniformly overprint mylonite and cataclasite zones of all scales. Microstructure changes progressively with increasing strain. Feldspar grains are cut by at least two generations of mineralized, dilatant microcracks. Minerals precipitated in the early set of microcracks have undergone extensive crystal plastic deformation. Late-stage microcracks are filled with completely undeformed minerals. The spatial distribution of normal fault mylonite zones is geometrically consistent with generation during 1) late-Proterozoic extension, 2) Mesozoic extension, 3) rigid-body rotation during Paleozoic thrusting, or 4) "gravitational collapse" during Paleozoic thrusting. Field and microstructural evidence favor (4). The exact timing of deformation is not, however, well-constrained. / Master of Science

Proterozoic rocks in the Needle Mountains include ca. 1750 Ma amphibolite-grade, metavolcanic and metaplutonic gneisses and ca. 1690 Ma granitoids that comprise the basement to the siliciclastic Uncompahgre Group. The mafic and felsic gneisses underwent synkinematic metamorphism and two phases of isoclinal folding and foliation development during D_B, prior to emplacement of the ca. 1690 Ma plutons. D_BC deformation caused folding of D_B fabrics in the gneisses, development of a subvertical, east-striking foliation in the granitoids, and generation of a macroscopic sigmoidal foliation pattern throughout the area prior to 1430 Ma. D_BC structures in the basement are correlated with macroscopic structures in the Uncompahgre Group, which was deformed into an east-trending cuspate synclinorium during this event. Gently plunging mineral lineations and asymmetric kinematic indicators in the basement record a component of dextral strike-slip shearing in domains of east-striking foliation and sinistral shearing in areas of northeast-striking foliation. A model for D_BC involving the development of conjugate strike-slip shear zones in response to north-northwest shortening is most consistent with the kinematic and fabric orientation data. A zone of phyllite, derived largely from basement, occurs everywhere along the basement-cover contact. Kinematic indicators along and near the contact record upward movement of the cover relative to the basement on each side of the synclinorium and imply that the cover rocks are parautochthonous. Stratigraphic facing of the cover rocks away from the basement supports the interpretation of this contact as an unconformity at the base of the Uncompahgre Group. Alteration of the basement rocks along this contact involved hydration and the loss of CaO, MgO, SiO₂, and Na₂O. The phyllite zone is interpreted as a metamorphosed and deformed regolith that localized out-of-synform movement while the basement and its parautochthonous cover were folded together during D_BC. Rocks in the Needle Mountains comprise part of the Colorado Province, one of several terranes that were possibly accreted to the Archean Wyoming Craton during the Proterozoic. Age constraints on the timing of deformation indicate that D_B and D_BC are representative of two regionally extensive deformational episodes. Pre-1700 Ma deformation is attributed to the assembly of volcanogenic terranes and their accretion to the Wyoming Craton along the Cheyenne Belt. Post-1700 Ma deformation resulted from regional north-northwest crustal shortening induced by tectonic interactions along the southern margin of the Colorado Province. These results support the hypothesis that terrane accretion was important in the Proterozoic crustal evolution of southwestern North America. / Ph. D.

The Early Ordovician, Upper Knox Group consists of meter-scale shallowing-upward cycles that were deposited on a low-sloping ramp. Cycles formed in response to short term (<100 k.y.) eustatic sea-level fluctuations and typically have well developed tidal flat caps. Cycles are bundled into five transgressive-regressive sequences which correspond to third order (1-10 m.y.) sea-level fluctuations defined by Fischer plots. The Upper Knox Group is 90% dolomite of which greater than 75% predates Middle Ordovician, Knox Unconformity development. Early dolomitization occurred penecontemporaneously with tidal flat progradation during fifth-order (up to 100 k.y.) sea-level falls as indicated by: abundant dolomite in cycles with well-developed tidal flat caps and scarce dolomite in cycles with no or thin laminite caps; decrease in dolomite abundance with distance below tidal flat caps; dolomitized cycles decrease basinward; and dolomite clasts veneer cycle tops and the Knox Unconformity surface. Third-order sea-level fluctuations also strongly controlled early dolomitization as indicated by Fischer plots; limestone, subtidal-dominated cycles correspond to third-order sea level rises and completely dolomitized, peritidal-dominated cycles correspond to third-order sea level falls. "Early" dolomite was metastable and its geochemical composition was modified during initial stabilization by marine brines during progradation of each cycle, and by mixed fresh/marine waters of the Knox aquifer associated with unconformity development. Much "early" dolomite however, remained metastable into the deep burial environment where it was replaced and overgrown by burial fluids as suggested by: covariant trends between crystal size, mole % CaCO₃, Sr²⁺, Mn²⁺ and δ¹⁸O; similar regional trends defined by stable isotope values of "early" dolomites and burial dolomites; and water-rock modeling of trace element and stable isotopic trends. Trace element and stable isotope compositions of least-altered "early" dolomite however, record a memory of a precursor evaporative dolomite. Cathodoluminescent dolomite stratigraphy defines five generations of burial dolomite that can be correlated over 100,000 km². Burial dolomites postdate a regional dissolution event attributed to migration of organic acid-rich fluids through the Knox carbonates. Regional dolomitization occurred coeval with Late Paleozoic deformation and was closely associated with MVT mineralization and hydrocarbon migration. The δ¹⁸O values and trace element contents of burial dolomites in conjunction with fluid inclusions, suggest that burial fluids were warm (135 to 200°C), saline (13 to 22 wt. % NaCl equiv.), ¹⁸O-enriched (+2 to +9 % SMOW) fluids with geochemical compositions similar to present day basinal brines. Mn²⁺ and Fe²⁺ contents of the dolomites suggest a redox control over Mn and Fe fluid chemistry, and in conjunction with regional δ¹³C trends, likely record precipitation from organic acid-rich fluids. Regional trace element and δ¹⁸O trends record a basinal fluid source and regional northwestward flow. Stable isotope values of burial dolomites and fluid inclusions from dolomites and associated minerals, define a prograderetrograde sequence that formed during basinwide, gravity-driven fluid flow which developed in response to Late Paleozoic thrusting and uplift. / Ph. D.

No description available.

No description available.

No description available.

No description available.

The northeastern Rincon Mountains record a superposed history of low-angle normal-slip shear strain. Moderate- to low-angle faults, mapped previously as Laramide thrust faults, are recognized as normal faults of Tertiary age. Two faults are predominant: a younger-overolder ductile fault forms the base of a metasedimentary carapace, a ductile shear zone (decollement zone) of southwest vergent slip, and an older-over-younger (locally younger-over-older) fault named herein as the San Pedro basal detachment fault forms a brittle shear surface of west-southwest slip. The decollement zone is characterized by passive-slip folding, flexural-flow folding, boudinage, stretched pebbles, and low-angle ductile normal faults. Structural analysis reveals southwest- ergent simple shear strain with a component of superimposed pure shear strain (vertical flattening). The San Pedro basal etachment fault underlies a faulted, distended allochthon. The internal structure of the allochthon is characterized by an imbricate shingling of tilted fault blocks against west-dipping normal faults, suggesting emplacement from the east by an extensional and/or gravitional mechanism. Detachment faulting involved Late Oligocene sedimentary rocks yet cuts ∼26 m.y. old dikes. Mid-Miocene (?) faults form north-trending fault blocks which have rotated rocks of the metamorphic basement and the allochthon eastward. High-angle normal faults of the Basin and Range disturbance form an eastern fault margin across which the northeastern Rincon Mountains have been uplifted. The deformation recorded in the northeastern Rincon Mountains is interpreted to reflect mid-Tertiary crustal extension. Early structural elements define a ductile shear zone which is either truncated or overprinted by a subsequently thinner zone of brittle shear. The shear zone descends stratigraphically westward across the Rincon Mountains. Reconstructions of its mid-Tertiary configuration show the shear zone to be a surface of normal-slip. Displacement near the surface is by brittle shear, but is progressively replaced by ductile shear down-dip. Evolution of the surface superimposes the region of brittle shear against the region of ductile shear. Late Cenozoic block faulting has segmented, tilted, and exhumed the surface.

No description available.