Collaborative research RUI: Timing of slip along the Sierra Nevada frontal fault zone, California: A thermochronologic study

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The eastern flank of the southern Sierra Nevada, California, forms an impressive topographic escarpment with some of the highest elevation peaks in the conterminous United States. The escarpment along Owens Valley is structurally controlled by the active Sierra Nevada frontal fault zone. This research project focuses this zone where Pacific-North America plate interaction processes intersect with the extending Basin and Range. Thus, the eastern margin of the Sierra Nevada has been of long-standing interest to the tectonics community and the timing of faulting and uplift of the region remains under debate. This research project focused on the southern Sierra Nevada frontal fault zone will shed light on the long-term structural evolution of the normal faults bounding the impressive Sierra Nevada mountain front and address several fundamental questions: When did the range front form? Is the Sierra Nevada frontal fault zone the product of a single or two distinct episodes of late Cenozoic normal fault slip? And did the southern Sierra Nevada lose or gain elevation during these episodes of normal slip? The project advances desired societal outcomes by: (a) training the next generation of scientists via involvement of undergraduates, including minority students, and graduate students in all aspects of the research; (b) developing a diverse, globally competitive STEM workforce through graduate and undergraduate student training; (c) fostering international collaboration and (d) enhancing scientific literacy by developing a field-based outreach program in the Sierra Nevada for undergraduate freshman and transfer STEP students (Science Talent Expansion Program, which is centered on scientific training of traditionally underrepresented students).This research project will test two primary hypotheses: (1) the southern Sierra Nevada frontal fault zone records two episodes of late Cenozoic normal fault slip and (2) the southern Sierra Nevada either lost or gained elevation during the first episode of normal slip and gained elevation during the second episode of normal slip. Integration of new field studies (geologic mapping, structural, and kinematic) and low-temperature thermochronology (apatite fission track, (U-Th)/He, and 4He/3He), with published geologic data, in forward and inverse thermokinematic-landscape evolution modeling will be used to test these hypotheses. Examination of data on the timing of late Cenozoic normal slip and the surface uplift histories in light of model results will allow assessment of the validity of different geodynamic processes (e.g. development of a slab window, increase in relative rate of Pacific-North America plate motion, a flexural-isostatic model, and replacement of foundering dense lithosphere with asthenosphere) postulated as driving both faulting along the Sierra Nevada frontal fault zone and changes in mean surface elevation of the Sierra Nevada. This research is motivated by the long-standing disagreement whether the Sierra Nevada records uplift in the late Mesozoic followed by no change or a decrease in elevation throughout the Cenozoic, or uplift in the late Mesozoic followed by a decrease in elevation during the middle Cenozoic and then a second pulse of uplift in the late Cenozoic. Data acquired during this research - normal fault slip kinematics, slip histories, and slip rates along the Sierra Nevada frontal fault zone - will be combined with published results on erosion rates, thermobarometry, and other geologic constraints into forward and inverse thermokinematic-landscape evolution models to characterize the normal fault slip and late Cenozoic uplift histories of the southern Sierra Nevada, and to assess the soundness of proposed geodynamic processes driving normal fault slip and uplift. This research is the first to apply low-temperature thermochronometry studies directly to the entire Sierra Nevada frontal fault zone escarpment and to use this combination of investigative tools to characterize the late Cenozoic slip history along the Sierra Nevada frontal fault zone and uplift history of the Sierra Nevada.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.