KINEMATICS OF THE CERRO COSO FAULT AND ITS INTERSECTION WITH THE GARLOCK FAULT, SOUTHERN INDIAN WELLS VALLEY, CA

The Cerro Coso fault (CCF) is a NW-SE oriented, dextral strike-slip fault in southern Indian Wells Valley, California, that projects SE from the Sierra Nevada Frontal Fault to the Garlock fault. Markers mapped in the NW half of the study area show offsets of ~1m on the youngest streams to ~1km on a Pleistocene alluvial fan. Offsets indicate a minor component of S-side-up dip motion. In the SE portion of the study area in the Radamaker Hills, a Cretaceous granite contact has been offset by ~7.5km. Southeastward, the CCF splits into at least two separate splays, both of which are exposed along Searles Station Road. The eastern splay contains both subvertical and subhorizontal zones of fault gouge, which intersect and are mutually offsetting. The subvertical faults are more prominent, and have gouge zones up to 0.75m thick. The western splay is the contact of granite hills with a depression filled with ponded volcanic ash.

The CCF loses definition southward and trends into a set of E-W bedrock and alluvial hills as it projects to the Garlock. Thrust faults placing basement rocks over alluvial deposits are exposed at multiple locations along the E-W hills. To the east, along Red Mountain Road, caliche layers within the alluvium have been folded and faulted. Stepped terraces in the hills indicate that there may be 2 to 3 thrust faults. A ~1km long ground penetrating radar line was run from the Garlock fault across these hills and confirms the presence of several S-dipping thrust faults.

We interpret these faults to accommodate some dextral slip on the CCF. Because the hills rise only a few 100m above the valley, it seems unlikely that the ~7.5km of offset CCF has been taken up by thrusting adjacent to the Garlock. Instead, we interpret that only ~1km of offset, shown by the Pleistocene alluvial fan in the NW portion of the study area, is taken up by thrusting. The remaining ~6.5km may be explained by 1) having motion on the Garlock fault iteratively remove fault slices related to CCF motion, or 2) having slip occur prior to initiation of the Garlock. Lack of modern seismicity along the CCF suggests that the CCF is either slipping aseismically or has a relatively low slip rate. Due to its proximity to the Little Lake/Airport Lake Fault Zone, it is probable that the Cerro Coso fault plays only a minor role in accommodating strain along the eastern California shear zone.