HR: 1340h
AN: T23A-0531    [Abstracts]
TI: Seismic Anisotropy beneath Pacific Ocean Lithosphere Subducted at Kamchatka
AU: * Russo, R M
EM: russo@geology.ufl.edu
AF: Dept. of Geological Sciences University of Florida, P.O. Box 112120 241 Williamson Hall, Gainesville, FL 32611
AU: Lees, J M
EM: jonathan_lees@unc.edu
AF: Dept. of Geological Sciences University of North Carolina, Chapel Hill, NC 27599, Chapel Hill, NC 27599
AB: We present new measurements of source-side shear-wave splitting from the Kamchatka portion of the Pacific plate subduction zone. The study region includes the abrupt lateral termination of subducted Pacific lithosphere at the Bering fault, termed the slab edge. Source-side splitting results from shear-waves emanating from slab earthquakes crossing an anisotropic upper-mantle region lying strictly beneath the subducting Pacific plate. Observations of split core shear phases (SKS, SKKS) at a suite of distant stations allows calibration of upper mantle anisotropy directly below the stations, and then correction of splitting of teleseismic S phases from Kamchatka earthquakes. Thus, we can isolate splitting from the sub-slab regions sampled by these S waves. Our results show that the fast axis of anisotropy beneath the Pacific slab is strongly trench parallel (i.e., NE); splitting delay times range from 1-4 s, indicating long (100-450 km) anisotropictravel paths through the sub-slab asthenosphere. Fast-axis measurements for several events are consistently trench-normal in regions of possible slab modification by seamount subduction. Shear-wave splitting observations made using core shear phases recorded at stations deployed on Kamchatka (Side Edge Kamchatka Slab experiment) are also strongly trench-parallel in the region south of the slab edge, but become trench-normal near the slab edge itself. Both sets of splitting measurements indicate northeastward upper mantle flow parallel to the trench beneath the Pacific plate. The primary upper mantle anisotropy observed in the study region lies beneath the Pacific slab and not in the subduction zone's upper mantle wedge, and that the slab base guides flow until the slab edge is reached, whereupon asthenosphere flows around the slab edge. The width of the trench-parallel flow region beneath subducted Pacific slab is not more than 200 km, indicating that the asthenospheric channel and underlying viscous mesosphere are both subducted in the upper mantle beneath the slab.
DE: 7218 Lithosphere (1236)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 8162 Rheology: mantle (8033)
SC: Tectonophysics [T]
MN: Fall Meeting 2005