HR: 0800h
AN: T31C-1321    [Abstracts]
TI: Role of Tectonics in the Tomography and Anisotropy of the Southern California Uppermost Mantle
AU: * Davis, P M
EM: pdavis@ess.ucla.edu
AF: UCLA, Earth and Space Sciences UCLA, Los Angeles, CA 90095 United States
AB: SKS splitting measurements in southern California have been interpreted (a) normal to the direction of most compressive stress (b) related shearing of the asthenosphere as the lithosphere moves over the mantle and (c) shearing along the San Andreas fault. No single mechanism appears to explain all the data. Splitting results from the LARSE II seismic array that crossed the San Andreas reveal it has minimal influence splitting. Surface waves show that azimuthal anisotropy may extend to depths of 200 km, that is, well into the asthenosphere. The recognition of the high velocity body under the San Gabriel Mountains seen in P-wave tomography has led to the suggestion that the mantle lithosphere has delaminated and is sinking in a sheet like structure that has developed a twist with depth. We compare the three dimensional distribution of the tomographic anomaly with that expected if delaminaton began at the time of the transition of the margin from transtension to transpression and the formation of the San Gabriel Mts. We assume the North American and Pacific plates are moving over a relatively stagnant mantle into which the mantle lithosphere sinks. We examine whether the finite shear strains that are necessarily associated with the delamination process might explain the distribution of SKS splitting by orienting olivine crystals in a three-dimensional flow. Such alignments may also be important in interpreting the tomography, that hitherto has been assumed to be due to isotropic changes in the velocity structure.
DE: 8123 Dynamics, seismotectonics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting