HR: 0800h
AN: S41C-1003    [Abstracts]
TI: Unusual Upper and Lower Mantle Discontinuities Detected Under the Western Pacific
AU: * Bagley, B
EM: bbagley@tamu.edu
AF: Texas A&M University, Department of Geology & Geophysics, College Station, TX 77843 United States
AU: Fletcher, J
EM: jkathlee@u.washington.edu
AF: University of Washington, Department of Physics, Box 351560, Seattle, WA 98195 United States
AU: Revenaugh, J
EM: justinr@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, 310 Pillsbury Dr. S.E., Minneapolis, MN 55455 United States
AB: Studies of upper mantle reflectivity using multiple ScS reverberations have demonstrated distinct reflector ensembles beneath subduction complexes and ocean basins. The former usually features a well developed G discontinuity (LID-LVZ transition) and, on occasion, small impedance increases and decreases between depths of 300 km to the 410-km discontinuity. The latter also features G, albeit usually at shallower depths, and only very rare features above the transition zone although recent work with SS and PP precursors has uncovered occasional L discontinuities (impedance increases between 200 and 300 km depth) beneath ocean basins. In this study, we examine multiple ScS reverberations from earthquakes in the Marianas and Izu-Bonin subduction complexes recorded on the island of Hawaii and in Japan. The paths defined by the earthquake-receiver pairs divide cleanly into old ocean basin and subduction-dominated end members. Results for the former are marked by a strong G discontinuity at ~80 km depth, 410-km, 520-km and 660-km discontinuities, and a weak reflector in D". In addition, there is a strong impedance increase observed at 300 km depth. These data sample a broad swath of the western Pacific, suggesting the widespread occurrence of this feature which we dub X following the lexicon of Revenaugh and Jordan [1991]. We believe it signals the presence of stishovite in eclogitic mid-upper mantle. The subduction-dominated path also shows clear transition zone discontinuities, but contains two additional reflectors of note: an impedance decrease approximately 80 km above the 410-km discontinuity and an impedance increase at 850 km depth. The former may be evidence of melt ponding on top of the olivine-to-wadsleyite transition, as previously detected on paths sampling the back-arc basins to the west. The deeper feature corresponds roughly with high velocity bodies observed in local tomography and lies in sub-slab mantle. If due to paleosubduction, it implies westward drift of the subduction zone or large-scale folding of the slab in the lower mantle. Revenaugh and Jordan, Mantle layering from ScS reverberations 2: the transition zone, J. Geophys. Res., 96, 19763-19780, 1991.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: Fall Meeting 2005