LARGE LARAMIDE DEXTRAL SHEAR ACROSS OWENS VALLEY, EASTERN CALIFORNIA, AND EXTENSIONAL UNROOFING OF THE SOUTHERN SIERRA NEVADA

Recognition of 65 km of post-83 Ma dextral shear across Owens Valley (Kylander-Clark et al., 2003, GSA Cord. Sect. Abs.), and possibly twice that amount since 148 Ma (Glazner et al., 2003, GSA Cord. Sect. Abs.), raises the question of how this magnitude of right slip fits into a regional framework. Historic seismicity and geodesy indicate ongoing right slip across the Owens Valley fault zone, but this right slip probably only recently supplanted predominantly normal-sense Cenozoic faulting because the Owens Valley fault zone does not offset the Garlock fault at their near-orthogonal intersection. The modern Owens Valley fault therefore appears to reactivate an older dextral shear zone such that up to 90% of the 65 km of shear probably accumulated in latest Cretaceous-early Tertiary (Laramide) time. We propose that the Laramide Owens Valley shear zone made an extensional stepover into the southern Sierra Nevada and that this contributed to local unroofing of deep elements of the Cretaceous arc.

After restoration of 65 km of late Cenozoic left slip across the Garlock fault, the Owens Valley shear zone projects southeastward into the east-central Mojave Desert. It is difficult to accommodate 65-130 km of post-148 Ma dextral shear there because the shear zone would intersect an apparently continuous belt of middle to late Jurassic (~170-148 Ma) igneous rocks and fold-thrust structures. Continuing the shear zone in the western Mojave, where it might link to the Mojave-Snow Lake fault zone, is more feasible but requires a rightward, i.e., extensional, stepover. Such a stepover may be found in the southern Sierra Nevada, which differs from the rest of the range in exposing deeper (15-30 km) Cretaceous arc rocks. These rocks apparently were unroofed by Late Cretaceous-early Tertiary normal faults, which Wood and Saleeby (1998, Intl. Geol. Rev.) interpreted to be caused by gravitational collapse. Reconnaissance near Forester Pass revealed post-83 Ma normal faults, previously mapped as strike-slip faults, that are suitably oriented and situated to transfer dextral shear from Owens Valley into the southern Sierra Nevada. An extensional stepover of a Laramide dextral shear system into the southern Sierra Nevada would imply that transtension played a significant role in the localized greater unroofing of arc rocks in that area.