The role of low-angle normal faulting in active tectonics of the northern Owens Valley, California

The Owens Valley of eastern California is an extensional graben. The mechanics of extension have traditionally been explained by means of high-angle normal faulting. However, this mechanism appears to be inconsistent with both the accepted tectonic structures of associated basins and with the expected kinematics of regional extension. We have therefore reexamined several lines of evidence that bear on the fault structures bounding the northern Owens Valley. Examination of fault-outcrop geometry indicates that valley-bounding fault planes dip between 26 degrees and similar to 90 degrees. Measurement of numerous fault planes that dip between 25 degrees and 35 degrees demonstrates that low-angle faulting must play an important role in the extensional process. We examined the alluvial fan/drainage basin area ratio of alluvial fans along the west slope of the White Mountains. These vary between similar to 1.00 and 0.05. The larger area ratios are associated with low-angle mountain-front faults, and the smaller ratios are associated with high-angle faults. The Bishop tuff, both in outcrop and in subcrop, shows obvious anticlinal rollovers as the tuff sheet approaches the bounding faults, which may indicate listric faulting geometry. Relocated earthquake hypocenter data define a west-dipping band of seismicity at 4-7 km depth beneath the Owens Valley. Fault-plane solutions for these events permit low-angle westward-directed slip. These observations indicate that the traditional high-angle normal faulting model is inadequate. More plausible alternative structures include low-angle planar normal bounding faults and faulting controlled by either east-dipping or west-dipping master detachment faults.