ANATOMY OF A RELEASING STEPOVER BETWEEN THE AIRPORT LAKE FAULT AND THE OWENS VALLEY FAULT THROUGH THE COSO RANGE, CALIFORNIA

Northwest-directed dextral shear in the southwestern Walker Lane is transferred from the Airport Lake fault zone (ALFZ) to the Owens Valley fault (OVF) in a complex releasing stepover across the central and northwestern Coso Range (Monastero et al., 2005). Frankel et al. (2008) described multiple branches of the ALFZ in Indian Wells Valley. Slip on the ALFZ is transferred to the NNE-striking, SE-dipping Coso Wash fault (CWF) in a left-restraining step across the White Hills anticline south of Airport Lake playa (ALP). The CWF (the northern reach of the ALFZ per Roquemore, 1981) bounds the eastern flank of the central Coso Range. As the CWF approaches the southern margin of the Coso geothermal field north of ALP it splays out into a series of traces that dip WNW toward an elongated, NW-trending zone of low P-wave and S-wave velocities in the depth range of ~5–12 km. The base of seismicity shallows above the low velocity zone, suggesting that hot fluids (brines and/or magma) are present below ~5 km (Hauksson and Unruh, 2007). The locus of deformation makes a transtensional left en echelon step across the low velocity zone and the geothermal field to a series of NNE-striking, left-stepping normal-oblique faults in the northwestern Coso Range that bound the western margin of Lower Cactus flat graben. Slip on these normal faults, and perhaps other uncharacterized structures, is transferred to a discontinuous NW-striking dextral fault zone that crosses the NW Coso Range piedmont and joins the 1872 OVF rupture trace. Secondary NNE-striking normal faults and WNW-trending folds deform a Pleistocene wave-cut terrace and pediment associated with a high stand of pluvial Owens Lake. We attribute the complex pattern of NW dextral slip transfer through the Coso Range as reflecting relative youth and immaturity of the SW Walker Lane structures, as well as the presence of the active magmatic system beneath the geothermal field.