Anisotropic elastic velocities of some upper mantle xenoliths underlying the Sierra Nevada Batholith

Abstract

Seismic properties were determined for two Iherzolite xenoliths found in a basalt pipe cutting the Sierra Nevada batholith. These xenoliths were derived from the upper mantle under the Sierra Nevada. We assume that the fabrics and ultramafic mineralogy of these xenoliths are representative of this upper mantle. The P velocities in a single direction for each of the two samples were determined as a function of pressure and temperature to 9 kbar and 260°C. Physical and modal properties and fabric data for the principal minerals in the samples were used to calculate compressional velocities. Measured velocities agree with the calculated ‘Voigt’ velocities to within 2.5% but disagree with the ‘Reuss’ velocities by 10% and 13%. If one assumes no alteration products, the calculated velocities show an anisotropy of 0.5–0.6 km/s at upper mantle pressure-temperature conditions under the Sierra Nevada. The calculated maximum and minimum Vp at 50 km and 500°C are 8.4±0.1 and 7.85±0.12 km/s. The seismic refraction value of 7.9 km/s is nearly parallel to the long axis of the batholith, which is roughly in the same direction as the minimum Vp in the ocean off California. These results are consistent with the existence of large-scale upper mantle anisotropy under the Sierra Nevada as a consequence of subduction of oceanic lithosphere. The assumption of an isotropic upper mantle having the ultramafic mineralogy of these xenoliths does not agree with calculated temperatures for the upper mantle under the Sierra Nevada.