The Southern Sierra Nevada Drip and the Mantle Wind Direction Beneath the Southwestern United States

Abstract

The Miocene-Pliocene convective removal of the southern Sierra Nevada batholithic root and its sinking through the upper mantle provides a natural experiment to estimate the direction and velocity of mantle flow beneath the southwestern United States. Xenolith data, volcanism patterns, and geologic evidence are used to constrain the location and timing of the initial detachment. Seismic tomography images of the upper mantle are used to determine the displacement of the downwelling "tail" by the background mantle flow. The "mantle wind" direction based on this analysis is estimated to be SSW, in contradiction to another recent estimate based on a combination of geodetic data and shear-wave splitting measurements. The conflicting conclusions can be reconciled if the splitting measurements are dominated by anisotropy in the shallow mantle imposed by lithospheric motion. The SSW direction is approximately consistent with global models of asthenospheric counterflow directions, and therefore supports the idea that self-driving plate forces can explain the absolute motion of the North American plate.