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Timescales and mechanisms of crustal thickening and post-orogenic extension in the North American Cordillera
AuthorLevy, Drew Alexander
AdvisorZuza, Andrew V
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Plate boundary forces are transmitted far into the interior of continents creating wide orogenic belts that modify large continental regions and extends geological hazards far from plate margins. Understanding the mechanisms that accommodate intracontinental deformation requires knowledge of the timescales over which fault systems develop in relation to other components of the orogenic system. The Basin and Range province is wide zone of distributed crustal extension that developed within the hinterland of the North American Cordillera. Extensional exhumation of the hinterland crust exposes the contractional structures that accommodated crustal shortening and thickening within the Cordillera. Evaluating the timing, magnitude and kinematics of hinterland thrust belts improves our understanding of the mechanisms of crustal thickening during Mesozoic orogenesis, and the effect on the Cenozoic tectonics of western North America. Furthermore, factors controlling the initiation of extension and the significance of metamorphic core complexes in the evolution of the Basin and Range province remain debated. To understand the factors controlling the timing and localization of initial Basin and Range extension, and the influence of Mesozoic orogenesis, this dissertation presents an integrated study of initial subduction initiation and crustal shortening along the incipient convergent North American plate boundary and the Jurassic to Miocene structure, metamorphism and magmatism within the Sevier hinterland of northeast Nevada and northwest Utah, USA.We investigated the driving mechanism of the Permian Last Chance thrust system in southwest Laurentia to understand the transition from transform margin to subduction zone prior to the development of the Mesozoic Cordilleran arc. Here we present the results of new geological mapping, detrital zircon U-Pb geochronology, and a synthesis of regional tectonics to inform a kinematic model of the Last Chance thrust system and outline the Permian-Triassic tectonic evolution of the plate boundary during induced subduction initiation. The evolution of this transpressional system and subduction zone is recorded by development of the Last Chance thrust system of the Death Valley region. Geological mapping in the Last Chance Range, northern Death Valley National Park, and the Inyo Mountains reveals the east-directed Last Chance thrust system was constructed by repetitive out-of-sequence deformation consistent with transpressional strain. The Last Chance thrust system accommodated a minimum of >75 km (60%) shortening across these thrust faults, based on cross-section restorations guided by regional stratigraphic relationships and restoration of subsequent Mesozoic deformation. Large-magnitude shortening along the CCt accommodated a significant component of the convergent plate motion as the Panthalassan crust was thrust below the continental margin before initial slab sinking. Development of the Last Chance thrusts system accommodated initial thickening of the Cordilleran hinterland. Early to Late(?) Jurassic crustal shortening progressed eastward in the retroarc region, which likely played a key role in the cumulative thickening of the hinterland region. Understanding the timing and magnitude of Jurassic shortening in modern northeast Nevada is key to resolve the crustal structure of the Cordillera and the influence on subsequent extensional tectonics. The Elko extensional domain of northeast Nevada encompasses the set of highly extended ranges from the Ruby Mountains in the west to the Grouse Creek Mountains in the east. The kinematic development of the Pilot-Toano ranges, within the eastern Elko extensional domain, was reconstructed and 40Ar/39Ar thermochronology and zircon U-Pb geochronology were applied to constrain the timing of faulting. Retrodeformation of ~30 km of extension along the Pilot Peak detachment fault and later high-angle normal faults yields the structure of the Middle-Late Jurassic thrust belt of the Elko Orogeny. Northeast striking thrust faults accommodated ~12 km of shortening and <5 km of burial of the Neoproterozoic-lower Cambrian section in the lower plate. Initial extensional faulting 20-15 Ma was accommodated by the Pilot Peak detachment fault, which initiated at high angle and rooted into and reactivated the regional Late Jurassic thrust décollement. Imbrication of the upper plate resulted in east-tilted fault blocks and a west-migrating extensional front that propagated westward to the Ruby Mountains-East Humboldt Range (REH) metamorphic core complex. Despite exposing the same Neoproterozoic-Cambrian protolith as the REH metamorphic core complex, the Pilot-Toano ranges lack extensive Oligocene intrusions and high-strain mylonitic fabrics. The kinematics and conditions of the REH mylonitic shear zone were evaluated using field study, microstructural analysis and thermobarometry to understand the mechanisms controlling its development. Field observations indicates the 0.75-1 km-thick mylonite zone accommodated >80% attenuation of the originally 3.5 km-thick Neoproterozoic-Ordovician stratigraphic section. Microstructural analysis of REH mylonites indicates general shear strain at strain rates of 10-13 to 10-12 s-1 at temperatures of 400–600℃. Cross-cutting intrusions bracket mylonitic shear between 29 and 17 Ma, but thermochronology records cooling below the nominal temperature limit of quartz crystal plasticity (350 ℃) by 23 Ma. The mylonite zone is thus ~10 Myr older than extensional exhumation of the range after 16 Ma. To explain these data, we argue 40-29 Ma magmatism and remobilization of Late Cretaceous leucogranitic melts enhanced mid-crustal buoyancy. Reduced viscosity due to thermal, melt and fluid-related weakening facilitated diapiric flow accommodated by attenuation of the mylonitic shear zone. This model is consistent with regional field relationships indicating minimal upper crustal extension prior to the Middle Miocene. The key finding of this research is the protracted episode of Eocene-Oligocene melting within the Ruby Mountains-East Humboldt Range (REH) following initial ca. 39 Ma magmatism controlled the Oligocene development of the metamorphic core complex due to buoyancy driven crustal flow. Thus, the REH mylonite zone is not a result of a large-displacement low-angle normal fault. Pre-existing structures of the Late Jurassic thrust belt controlled the geometry of initial Early to Middle Miocene extensional faulting regionally, however the presence of high-strain mylonitic fabrics focused large-magnitude extension during Middle Miocene unroofing the REH metamorphic core complex. The Basin and Range metamorphic core complexes may thus be expressions of focused Eocene-Oligocene heating rather than records of initial collapse within the Sevier hinterland.