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

Stretching of the continental crust can double its surface area, but it is unknown whether similar amounts of extension occur at depth. Seismic results from the central Basin and Range province, western USA, reveal a thick root of lithospheric mantle that has not been extended and indicates that crustal stretching is decoupled from extension at depth. Stretching of the continental crust in the Basin and Range, western USA1, has more than doubled the surface area of the central province2. 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 crust3,4. The central Basin and Range province is unusual, compared with its northern and southern counterparts: extension began later1; volcanism was far less voluminous5; and the unique geochemistry of erupted basalts6,7,8,9,10,11 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 ∼125 km thick, providing geophysical confirmation of a strong, cold mantle previously inferred from geochemistry6,7,8. 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.