Abstract

Rapid surface uplift or subsidence and voluminous magmatic activity have often been ascribed to regional-scale downwelling of lithospheric mantle. However, because lithospheric drips—sinking plumes of cold and dense lithosphere—are relatively small and transient features, direct evidence of their existence has been difficult to obtain. Moreover, the significant vertical mantle flow that should be associated with such structures has not been detected. Here we integrate seismic anisotropy data with tomographic images to identify and describe a lithospheric drip beneath the Great Basin region of the western United States. The feature is characterized by a localized cylindrical core of cooler material with fast seismic velocities and mantle flow that rapidly shifts from horizontal to vertical. Our numerical experiments suggest that the drip can be generated by gravitational instability resulting from a density anomaly of as little as 1|[percnt]| and a localized temperature increase of 10|[percnt]|. The drip tilts to the northeast—opposite to the motion of the North American plate in the hotspot reference frame—and thereby indicates northeast-directed regional mantle flow.