Geologic mapping and zircon (U-Th)/He thermochronology of the Avawatz Mountains, California; Implications for the eastern terminus of the Garlock fault
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Authors
Johns, Wes M.
Issue Date
2023
Type
Thesis
Language
Keywords
Avawatz Mountains , Garlock fault , Geology , Southern Death Valley , Structural Geology , Tectonics
Alternative Title
Abstract
The Avawatz Mountains are located at the eastern tip of the left-slip Garlock fault where the fault intersects the NW-striking right-slip Southern Death Valley fault (SDVF). The Avawatz Mountains were constructed via late Cenozoic contractional deformation involving active thrust and/or oblique-slip faults. Contrasting structural models suggest that the Avawatz Mountains formed as either a fault-termination thrust belt at the eastern end of the Garlock fault, orwithin a transpressional restraining bend along the SDVF. Here we present detailed geologic mapping, structural analysis, and low-temperature thermochronology data to test these proposed structural and evolutionary models and to resolve the timing and kinematics of deformation. Field observations show numerous subvertical west-striking strike-slip faults in the western portion of the range and a prominent west-dipping reverse fault along the eastern rangefront. Zircon (U-Th)/He thermochronology (ZHe) from a ~1 km vertical transect in the hanging wall of the eastern rangefront fault yields cooling ages ranging from 79 ± 11 Ma in the structurally highest samples to 12.3 ± 3.1 Ma in the structurally lowest samples. Across the vertical sampling transect, most samples yield average cooling ages that cluster around ca. 15 Ma. This age distribution is consistent with exhumation initiating in the middle Miocene (ca. 20-15 Ma), synchronous with the initiation of slip on the Garlock fault. Vertical exhumation rates calculated from ZHe data are < 1 mm/yr, which corresponds to a horizontal shortening rate of ~1 mm/yr based on the observed dip of the primary west-dipping reverse fault structure. Our observations suggest a complex fault and exhumation history that may reflect the transfer of left-lateral slip on the Garlock fault to an east-directed termination thrust system that was later overprinted by slip on the SDVF and the development of a restraining bend.