NEW INSIGHT ON THE OWENS VALLEY FAULT ZONE (EASTERN CALIFORNIA, U.S.A.) FROM DETAILED QUATERNARY FAULT MAP

Owens Valley is a 170 km long late Neogene graben at the boundary between the Sierra Nevada and Basin and Range Provinces that extends southward from Bishop to the Owens Lake-Coso area. It also lies in the narrow Eastern California Shear Zone (active geodetic strain rate ~6-8 mm/yr). In 1872, during a M 7.4-7.6 earthquake, a 116 km segment of the Owens Valley fault zone (OVFZ) ruptured with up to10 m dextral-slip locally.

Field studies and interpretation of 1:12,000 scale, low-sun angle aerial photographs, augment earlier work to show both the extent of the 1872 rupture, and the broader setting of numerous Quaternary faults in the surrounding graben. The low-sun angle aerial photographs cover nearly all the valley floor permitting detailed mapping of the ruptures and relative age classification of scarps. Fault scarps are classified into four age groups based on the age of the youngest deposit offset by the fault and on their aerial photo and field appearance: 1) 1872 rupture, 2) Holocene, 3) Late Pleistocene, and 4) Pleistocene or older. The 1872 rupture, dominantly strike slip, runs mostly in the valley center, while earlier faulting, mainly concentrated in the slopes, displays a strong normal component. In addition, the location of apparently Pleistocene lake shorelines are shown, as some prominent scarps at the SE edge of the lake formerly interpreted as lake highstands are in reality fault-controlled.

The 1872 rupture terminates at late Quaternary basaltic volcanic fields (Big Pine and Coso) at right-steps of the OVFZ. Our map shows that the 1872 scarp terminates south of Dirty Socks Springs near Red Mountain, further south than shown by previous work. Sections of the 1872 rupture with lengths of 34, 11, 10.5, and 10 km show pure transcurrent displacement. Four east-steps, two of which have been documented as restraining bends by recent MSc studies, link the rupture sections. The northern section at the termination of the 1872 rupture shows significantly greater oblique slip. The occurrence of restraining bends on east-steps of a right lateral fault suggests strain partitioning with rotation of small blocks between the range front normal faults and the OVFZ. The pattern of faulting around and within the Owens Lake playa indicates that it is a pull-apart basin, controlled by a right-step of the main NNW right-slip fault zone.