Pace and Tempo of Deformation in Eastern California Over the Past 3 Ma: Implications for the Geodynamic Evolution of Right-Lateral Shear along the Western Margin of the Basin and Range

Along the Eastern California shear zone (ECSZ), dextral shear of 9-12mm/yr exceeds geologic slip rates by a factor of two. Whether this difference reflects transient rates of deformation or a true acceleration in tectonic loading remains a outstanding question. Below, we summarize recent work along the Owens Valley (OVFZ) and Panamint Valley (PVFZ) fault systems and integrate these data into a regional framework for the evolution of faults in the ECSZ over the past ~3Ma.

We focus on three aspects of fault evolution: timing of initiation of right-lateral slip, acceleration of slip during the past ~1Ma, and spatial variations in displacement along individual structures over the past ~100ka. First, structural and stratigraphic observations indicate that right-lateral, oblique slip did not initiate along the OVFZ until ~2Ma and along the PVFZ until ~3Ma. This timing coincides with initiation of strike-slip faults in the northern Walker Lane (Henry et al., 2007), and marks a regionally-significant reorganization of the ECSZ. Second, geomorphic analysis of the footwall ranges along the PVFZ reveal a transient wave of incision that reflects a sustained increase in fault slip between ~0.7–1.4Ma. Finally, estimates of slip rate reveal coordinated decreases in rate toward fault tips in Owens and Fish Lake Valleys.

Collectively, these observations suggest that fault systems in the ECSZ are responding to a system-wide change in the locus and rate of deformation during the latest Pliocene. Subsequent acceleration in fault slip is likely associated with continued maturation of fault systems. These data force us to consider the possibility that differences between geodetic velocities and geologic slip rate may not reflect transients, but rather may be a manifestation of ongoing strain localization. We speculate that this reorganization could be driven by relatively recent thermal weakening of the lower crust consequent to removal of Sierran mantle lithosphere.