Whole-lithosphere shear during oblique rifting

Brandon M Lutz,Gary J Axen,Jolante W Van Wijk,Fred M Phillips

doi:10.1130/g49603.1

Abstract

Abstract Processes controlling the 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 United States. 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 ~60 km of shear. These deflections underlie an upper-crustal fault zone that accrued ~60 km of dextral slip since ca. 8–7 Ma, when we infer that whole-lithosphere shear began. This dextral offset is less than net dextral offset on the upper-crustal fault zone (~90 km, ca. 13–0 Ma) and total upper-crustal extension (~250 km, ca. 16–0 Ma). We show that, before ca. 8–7 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 collocated with the whole-lithosphere shear zone. Midcrustal strengthening coupled the layered lithosphere vertically and therefore enabled whole-lithosphere dextral shear. Where thick crust exists (as in pre–16 Ma DVR), midcrustal strengthening is probably a necessary condition for whole-lithosphere shear.

Highlights

Continental rupture requires shear zones that cut the entire lithosphere
Where crust is overthickened, the upper crust is commonly decoupled from lower crust and mantle lithosphere (LCML) by a weak middle crust (Burchfiel and Royden, 1985; Block and Royden, 1990), precluding localized whole-lithosphere shear
We describe the development of a whole-lithosphere shear zone that formed during dextraloblique continental extension that overprinted a thrust belt in the Death Valley region (DVR) of the central Basin and Range, United States (Fig. 1)

Summary

INTRODUCTION

Continental rupture requires shear zones that cut the entire lithosphere. where crust is overthickened, the upper crust is commonly decoupled from lower crust and mantle lithosphere (LCML) by a weak middle crust (Burchfiel and Royden, 1985; Block and Royden, 1990), precluding localized whole-lithosphere shear. 7 Ma. Offset features in the upper crust, the Moho, and the LAB define an ∼20–50-km-wide, wholelithosphere shear zone (Fig. 1), in which the entire vertical lithospheric column was sheared dextrally ∼60 km after ca. Upper-crustal kinematics (Lutz, 2021; Animation 1; Fig. S9) predict that, after 8–7 Ma, 57 ± 7 km of dextral slip accumulated across the Furnace Creek (38–48 km) and Stateline fault zones (13–16 km). S5–S8) show generally northwest-increasing Moho depth, with apparent dextral offsets below the Furnace Creek and Stateline fault zones and above a deflection of the LAB depth gradient (described ). Simple cutand-slide reconstruction of the east-northeast– trending LAB depth gradient yielded 56 ± 6 km of lower-lithosphere dextral shear (Fig. S4)

Midcrustal Strengthening and Mechanical Coupling

CONTROLS ON WHOLELITHOSPHERE SHEAR