GEOMORPHIC EVIDENCE FOR CO-SEISMIC SLIP ON AN ACTIVE LOW-ANGLE NORMAL FAULT: PANAMINT VALLEY, CALIFORNIA

The feasibility of co-seismic displacement along low-angle normal fault systems remains an outstanding problem in structural geology and tectonics. In the southwestern Basin and Range, large magnitude extension during Miocene – Pliocene time was accommodated along a system of low-angle detachment faults. Whether these faults remain active today and, if so, whether they accumulate and release elastic strain remains uncertain. Here we examine the relationship between Late Pleistocene – Holocene fault scarps and a low-angle detachment system along ~60km Panamint Valley fault zone. We combine analysis of LiDAR-based topography with a detailed chronology provided by a calibrated soil chronosequence to characterize the recent paleoseismic history of the fault system.

The range-front fault system along the eastern side of Panamint Valley is a low-angle (15-30°) curviplanar detachment fault that is linked to strike-slip faults at its southern and northern ends. A recent paleoseismic investigation of the southern fault revealed evidence for 3-4 surface ruptures during the Late Holocene (~4-5 ka; McAuliffe et al., 2013), the most recent of which (MRE) occurred ~330-485 cal yr BP. Scarps related to the MRE can be traced for at least ~35 km northward along the range front to Happy Canyon, where a young debris flow is displaced across a ~ 3 m scarp. Radiocarbon dating of detrital wood in the deposit constrains the age of this event to < 600 cal yr BP and confirms. Analysis of displacement of latest Holocene alluvium from LiDAR suggests that the MRE involved slip of ~2-3 m along the length of the rupture, consistent with an moment magnitude between 6.5 - 7. The geometry of the rupture mimics range-scale variations in strike of the curviplanar detachment fault, suggesting that scarps merge with the detachment at depth. Moreover, rupture kinematics inferred from displaced geomorphic markers implies dextral 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). Thus, we conclude that the most recent rupture along the Panamint Valley fault system likely activated the low-angle normal fault beneath the valley. Displacement of older alluvium suggests at least 2-3 previous ruptures have occurred during the Late Holocene.