Differential motion between upper crust and lithospheric mantle in the central Basin and Range

Stretching of the continental crust in the Basin and Range, western USA(1), has more than doubled the surface area of the central province(2). But it is unknown whether stretching affects the entire column of lithosphere down to the convecting mantle, if deep extension occurs offset to the side, or if deeper layers are entirely decoupled from the upper crust(3,4). The central Basin and Range province is unusual, compared with its northern and southern counterparts: extension began later(1); volcanism was far less voluminous(5); and the unique geochemistry of erupted basalts(6-11) suggests a long-preserved mantle source. Here we use seismic data and isostatic calculations to map lithospheric thickness in the central Basin and Range. We identify an isolated root of ancient mantle lithosphere that is similar to 125 km thick, providing geophysical confirmation of a strong, cold mantle previously inferred from geochemistry(6-8). We suggest that the root caused the later onset of extension and prevented the eruption of voluminous volcanism at the surface. We infer that the root initially pulled away from the Colorado Plateau along with the crust, but then was left behind intact during extension across Death Valley to the Sierra Nevada. We conclude that the upper crust is now decoupled from and moving relative to the root.