LATE QUATERNARY – RECENT SLIP ON A LOW-ANGLE NORMAL FAULT: INFERENCES FROM ALLUVIAL DEPOSITS ALONG THE PANAMINT VALLEY FAULT SYSTEM

Although the significance of normal-sense displacement on low-angle (<30°) fault systems has been recognized in both extensional and collisional orogens worldwide, relatively few have been shown to be presently or recently active. Part of the difficulty of recognizing recent slip in the geologic record lies in the nature of hanging-wall deposits; high-angle faults typically displace young, unconsolidated alluvial deposits near or at the range front. Although this is commonly interpreted as evidence that low-angle faults observed within the range have been abandoned in favor of newly-formed structures, the exact nature of the intersection between both fault systems is often uncertain. Here we examine the relationship between Late Pleistocene – Holocene fault scarps and a low-angle detachment system along ~60km of the central and southern Panamint Valley fault zone. We develop a preliminary chronology of Pleistocene – Holocene alluvial fans and lacustrine deposits utilizing both radiocarbon and in situ cosmogenic isotopes. Several lines of evidence indicate that slip on these young fault scarps is a direct consequence of displacement on the master detachment. First, the geometry of fault scarps within alluvium mimics range-scale variations in strike on the curviplanar, low-angle detachment fault, suggesting that scarps merge with the detachment at depth. Second, the kinematics of recent faulting inferred from displaced geomorphic markers suggests oblique-normal slip, consistent with the long-term slip vector inferred from piercing lines across the Hunter Mountain fault (e.g., Burchfiel et al., 1987). Third, alluvium of late Pleistocene age is juxtaposed against bedrock across a ~25-30° fault surface characterized by well-developed fault gouge. Finally, direct observations of 3-5 meter fault scarps at the intersection of the low-angle range-front fault with the valley floor indicate recent (probably seismogenic) slip on the detachment system itself. We conclude that the neotectonics of the Panamint Valley fault zone reflect active slip above a low-angle detachment fault.