FORMATION OF A CONTINENTAL WHOLE-LITHOSPHERE SHEAR ZONE

We integrate field observations with geophysical data of the Death Valley region (central Basin and Range, U.S.) to document the formation of a continental whole-lithosphere shear zone. How such shear zones form is debated, but their existence requires that the lithosphere is mechanically coupled from the surface to the base. In our study area, whole-lithosphere shear began around 7-8 Ma. A ~60 km deflection in the depth gradient of the lithosphere-asthenosphere boundary spatially coincides with a deflection of the Moho depth gradient. These deflections underlie an upper-crustal fault zone that accrued ∼60 km of dextral slip since 7-8 Ma. This dextral offset is less than net dextral offset on the upper-crustal fault zone in our study area (∼90 km, ca. 13–0 Ma) and less than total upper-crustal extension (160-170 km, ca. 16–0 Ma). Before 7-8 Ma, a weak middle crustal layer decoupled upper-crustal deformation from deformation in the lower crust and mantle lithosphere. By 7 Ma, the middle crust had strengthened, enabling whole-lithosphere shear. Our model for mid-crustal strengthening includes removal of weak lower crustal material by detachment slip. This study has implications for 3-D geodynamic modeling approaches of oblique lithosphere deformation.