Active and passive seismic experiments show that the southern Sierra, despite standing 1.8 to 2.8 kilometers above its surroundings, is underlain by crust of similar seismic thickness, about 30 to 40 kilometers. Thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range, but little subcrustal lithosphere is present beneath the eastern High Sierra and adjacent Basin and Range. These and other data imply the crust of both the High Sierra and Basin and Range thinned by a factor of 2 since 20 million years ago, at odds with purported late Cenozoic regional uplift of some 2 kilometers.

We display the velocity model in several cross sections a, nd maps of Moho depth and average crustal velocity. The measured velocities in the upper and mid crust of the Sierra Nevada batholith are in good agreement with laboratory measurements on Sierra Nevada tonalites after corrections for density a.nd temperature. Peridotite xenoliths from the eastern Sierra Nevada suggest strong upper mantle anisotropy, which could explain some of the velocity heterogeneity in the Sierra Nevada mantle. By the time Creta, ceous subduction-related magmatism ceased, the Sierra Nevada arc must have had a thick marie lower crust; yet a principal result of our work is that today the batholith has a crust of mainly felsic composition throughout.A subcrustal layer with velocities below normal P• velocities (<7.6 kin/s) may indicate the presence of lower crustal material in eclogite facies.

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Abstract. New seismic wide-angle data from the eastern Aleutian Islands show a. mafic composition and a 30-kin-thick island-arc crust. Traveltimes of P and S refracted arrivals and prominent crustal and mantle reflectors observed to offsets of over 300 km were used to derive velocity models for the eastern Aleutian Arc between the islands of Atka and Unimak using a three-dimensional finite difference modeling and tomography code. We interpret the highest crustal P wave velocities

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