Unusual Upper and Lower Mantle Discontinuities Detected Under the Western Pacific

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.