WHOLE-LITHOSPHERE SHEAR DURING OBLIQUE RIFTING

Processes controlling formation of continental whole-lithosphere shear zones are debated, but their existence requires that the lithosphere is mechanically coupled from base to top. We document the formation of a dextral, whole-lithosphere shear zone in the Death Valley Region (DVR), southwest USA. Dextral deflections of depth gradients in the lithosphere-asthenosphere boundary and Moho are stacked vertically, defining a 20-50 km-wide, lower lithospheric shear zone with 55 ± 15 km shear. These underlie a dextral fault zone that accrued 52 ± 9 km of slip since ca. 8-6 Ma, when we infer that whole-lithosphere shear began. This dextral offset is less than net dextral offset on the upper-crustal fault zone (82 ± 8 km, ca. 13-0 Ma) and total upper-crustal extension (~250 km, ca. 16-0 Ma). We conclude that, before 8-6 Ma, weak middle crust decoupled upper crustal deformation from deformation in the lower crust and mantle lithosphere. Between 16 and 7 Ma, detachment slip thinned, uplifted, cooled, and thus strengthened, the middle crust, which is exposed in metamorphic core complexes co-located with the dextral shear zone. Thus mid-crustal strengthening enabled whole-lithosphere dextral shear. Where thick crust exists (as in pre-16 Ma DVR), mid-crustal strengthening is probably a necessary condition for whole-lithosphere shear.