GEOMORPHIC EVIDENCE FOR ACTIVE SLIP ALONG A LOW-ANGLE NORMAL FAULT: PANAMINT VALLEY, CALIFORNIA

The appreciation of the significance of low-angle normal faults during extensional intracontinental deformation stands among the many contributions of B.C. Burchfiel and G.S. Davis to the field of tectonics. In the southwestern Basin and Range province, large magnitude extension during Miocene – Pliocene time was accommodated along a regionally extensive low-angle detachments. Whether these faults remain active today and, if so, whether they rupture during large earthquakes, are questions central to understanding the geodynamics of distributed lithospheric deformation and associated seismic hazard. Geologic and geophysical constraints on Pliocene – Recent dextral transtension along the Panamint Valley fault system present one of the most compelling cases for opening of the modern physiographic valley as a pull-apart (Burchfiel and Stewart, 1966) bounded by a low-angle detachment (Burchfiel et al., 1987; MIT and Biehler, 1987). Here, we reconstruct the Pleistocene – Holocene slip history of this fault system combining analysis of high-resolution topography with a detailed chronology of alluvial fan and lacustrine deposits. The range-front fault system is coincident with a low-angle (15-20°), curviplanar detachment fault that is linked to strike-slip faults at its southern and northern ends. Isochron burial dating (²⁶Al/¹⁰Be) of fanglomerate deposits juxtaposed with footwall breccia across a clay-rich gouge zone requires significant displacement along the detachment during the past ~800 ka. Moreover, the degree of soil development in younger alluvial deposits in direct fault contact with the footwall block suggests that low-angle normal fault remained locally active until as recently as ~80-100 ka. Finally, the geometry of Holocene fault scarps and kinematics inferred from displaced geomorphic markers are consistent with slip on a low-angle (~15-20°) fault system beneath the valley. Radiocarbon ages of juniper logs from debris-flow deposits bracket the youngest scarp-forming event to 300-600 ka, consistent with results from a paleoseismic trench ~50 km to the south. Thus, we conclude that ongoing dextral shear along the margin of the Basin and Range continues to be accommodated during co-seismic slip along detachment faults in the shallow crust beneath Panamint Valley.