AZIMUTHAL ANISOTROPY OF VELOCITY IN THE MANTLE LID BENEATH THE BASIN AND RANGE PROVINCE

THE Basin and Range province in western North America is one of the largest and best-studied regions of Cenozoic continental extension1,2, yet there is still considerable debate surrounding the geological history of the province, and the cause of the observed widespread rifting. Although recent geological and crustal geophysical observations2 constrain crustal-scale processes within the Basin and Range, more information is needed on the structure and rheology of the mantle part of the lithosphere (the mantle lid), to distinguish between competing geodynamic models1,2. Here we present seismic data showing an azimuthal variation of ∼3.2% in the P-wave velocity in the shallowest mantle beneath the Basin and Range. The direction of maximum seismic velocity coincides with the NW-SE direction of Neogene and present-day tectonic extension, suggesting that penetrative deformation associated with this extension has induced a preferred orientation of the minerals in the mantle lid. Our results thus imply that deformation associated with present-day surface rifting extends at least 30 km into the underlying mantle. © 1990 Nature Publishing Group.