ACTIVE FAULTING ALONG THE SOUTHWESTERN INYO MOUNTAIN FAULT ZONE (SWIM), CALIFORNIA

The Southwest Inyo Mountain (SWIM) fault zone is located along the southwest front of the Inyo Mountains, California within the Eastern California Shear Zone (ECSZ). Characterized by active strands of NNW-striking normal, dextral and oblique slip faults, the SWIM fault zone cuts a variety of rock units including highly deformed Paleozoic bedrock, Pliocene basalt lavas, and Quaternary alluvial fan surfaces. Alluvial surfaces, from oldest to youngest, are Qf1, Qf2a, Qf2b, Qf3a, and Qf3b to Qf4, and are distinguished on the basis of relative elevation, inset geometry and surface morphology. Characteristic geomorphic features indicative of active faulting include shutter ridges, offset alluvial drainages, fault scarps in bedrock and alluvial fan deposits, and ponded late Quaternary sediments.

A ~ 4 km wide zone of subparallel, WSW-dipping normal faults is primarily exposed in the southern portion of the study area along the western flank of Malpais Plateau. The basalt lavas have been vertically offset ~750 m across the fault zone and, if the same age as those on the Darwin Plateau (~4.3-7.1 Ma; Schweig et al., 1989), indicate a vertical slip rate of ~0.1-0.2 mm/yr. If we assume a fault dip of 60°, then the horizontal extension rate was ~0.1 mm/yr. A linear fault trace, ponded sediments and apparently beheaded basalt flows suggest a component of dextral slip along the westernmost normal fault strand. Small normal offsets (~2-3 m) observed in late Quaternary alluvial fan surfaces to the west suggest that normal faulting continues today.

The zone of normal faults continues northwestward into a zone of right-oblique, E-dipping faults that offset late Quaternary alluvial fan surfaces. The oldest surface, Qf1, exhibits vertical offset ranging from ~80 m to ~23 m. Younger Qf2a surfaces exhibit minimum vertical offsets of ~8-10 m and Qf2b surfaces show a minimum of ~4 m vertical offset. The youngest surfaces, Qf3 and Qf4, are not faulted. Measurements of offset channels indicate maximum dextral offsets ranging from ~650 to ~220 m since incision of Qf1 surfaces.

Our studies indicate a transition from dominantly EW-extension during the Pliocene to a combination of EW-extension and NW-dextral shear during the Quaternary. This temporal pattern of deformation kinematics is the same as has been documented throughout this part of the ECSZ.