Geothermochronometric and stratigraphic constraints on the structural and thermal evolution of low-angle normal fault systems : case studies from southwestern Nevada and west-central Arizona
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The structural evolution of low-angle normal faults (detachment faults) has been an extensively debated topic since the initial recognition of these structures throughout the western U.S. Cordillera and their subsequent identification within extensional provinces across the globe. An improved understanding of how detachment faulting occurred at a variety of scales within continental extensional provinces can help refine structural models of how complex detachment fault systems evolved during progressive extensional deformation. This dissertation addresses the evolution of detachment fault systems using a thermochronometric approach that is coupled to hanging wall stratigraphic data in order to evaluate how the thermal history along detachment faults can evolution inform our understanding of the spatial, geometric, and temporal evolution of these fundamental extensional structures. Evaluating the accuracy of thermochronometrically-derived fault slip rates within large magnitude extensional systems has important implications for slip rate interpretations that can be significantly affected by various structural complexities within the footwall and hanging wall. Three new and distinct case studies are presented in order to understand the temporal and spatial development of low-angle normal fault systems and the resulting metamorphic core complexes that have developed within varied extensional settings. Chapter 1 utilizes (U-Th)/He thermochronometry to understand the significance of small-scale (100’s to 1000 m scale) fault blocks within the Bullfrog Hills-Bare Mountain detachment fault system that accommodated transtensional deformation within the southern Walker Lane in southwestern Nevada. The timing of Miocene extensional exhumation was determined in the Bullfrog Hills and Bare Mountain Nevada as well as the effects of several main detachment faults, faults with multiple segments, small scale incisement and excisement detachment faults, and preexisting contractional structures on detachment fault evolution and the interpretation of thermochronometric data from within detachment fault domains. Chapter 2 focused on evaluating the larger scale (km to 10’s of km) structural evolution of progressive detachment fault breakaways that developed along the Buckskin-Rawhide detachment fault system during large-magnitude (~40-50 km) Miocene displacement in the lower Colorado River extensional corridor of west-central Arizona. By coupling geothermochronometry data from within the pre-and synextensional sedimentary record preserved within the Lincoln Ranch hanging-wall basin, this study constrains the timing of a tertiary detachment fault breakaway and provides new insights on the timing of subaerial footwall exposure. Chapter 3 applies a high-density sampling strategy along an ~55 km long, slip-parallel transect within the Harquahala Mountains of west-central Arizona, one of the lesser studied examples of a classic Cordilleran metamorphic core complex in the lower Colorado River extensional corridor. Apatite and zircon (U-Th)/He ages throughout the Eagle Eye detachment fault footwall are combined with geothermochronometry data from sedimentary and basaltic hanging-wall rocks in order to determine the inception and duration of extension, fault displacement magnitude, fault slip rates, fault geometry, and timing of subaerial footwall exposure along the Eagle Eye detachment fault. New results are used to evaluate the structural evolution of the regionally correlative lower Colorado River extensional corridor detachment fault system at the southern extent of the Whipple tilt domain, which has important implications for the coherent behavior of regionally extensive continental detachment fault systems.