HR: 1340h
AN: MR23B-0070    [Abstracts]
TI: A Step in the D'' Shear Velocity Discontinuity Beneath the Cocos Plate Imaged by Kirchhoff Migration
AU: * Hutko, A
EM: ahutko@pmc.ucsc.edu
AF: University of California Santa Cruz, Earth Sciences, Santa Cruz, CA 95064 United States
AU: Lay, T
EM: tlay@pmc.ucsc.edu
AF: University of California Santa Cruz, Earth Sciences, Santa Cruz, CA 95064 United States
AU: Garnero, E
EM: garnero@asu.edu
AF: Arizona State University, Dept. of Geological Sciences, Tempe, AZ 85287-1404 United States
AU: Revenaugh, J
EM: justinr@umn.edu
AF: University of Minnesota Twin Cities, Geology and Geophysics, Minneapolis, MN 55455 United States
AB: We use 270 horizontally-polarized S waves from 15 deep earthquakes under South America recorded at broadband stations in western North America to image shear-velocity structure in the deep mantle beneath the Cocos Plate. We use a Kirchhoff migration approach, assuming isotropic scattering from a three-dimensional grid of possible scattering nodes in the lowermost mantle. Several 3D mantle tomography models are used to correct for first-order travel-time perturbations due to volumetric heterogeneity, and waveforms are migrated with respect to either S or ScS arrivals. We observe an East-West striking abrupt 50-150 km change in the depth of the D'' shear velocity discontinuity near 6°N. This feature is apparent in migrations for a 1D reference model and in migrations that use different 3D aspherical models to account for volumetric velocity effects. Our results do not contain significant topography elsewhere on the boundary, and are compatible with a relatively flat D'' discontinuity on either side of the step. The vertical step is constrained to occur over less than 100 km laterally. The step may be due to strong temperature and or chemical gradients, both of which require an active dynamical process to sustain such a steep feature. One dynamical process that can account for the step is folding and piling of a cold slab that has reached the core-mantle boundary, as observed in numerical and experimental models, resulting in a 100 km elevation of the post-perovskite phase boundary due to a 700K lateral temperature reduction in the folded slab. We also detect localized low velocities along the boundary of the imaged D'' discontinuity, which may involve upwellings caused by the slab laterally displacing a thin hot thermal boundary layer. Preliminary efforts to migrate broadband and short period P wave data also reveal complicated D'' structure in this region, however these results are much lower resolution and will be explored in greater detail.
DE: 7203 Body waves
DE: 7207 Core (1212, 1213, 8124)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8121 Dynamics: convection currents, and mantle plumes
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005