Wedge Issues: Seismic Anisotropy, Structural Geology and Upper-Mantle Dynamics in Subduction Zones

Yale University, New Haven CT

Investigators

Abstract

Park Jeffrey J. EAR-0106867 Abstract for proposal EAR0106867 (PH # 33x) Title: Wedge Issues: Seismic Anisotropy, Structural Geology and Upper-Mantle Dynamics in Subduction Zones PI's: Jeffrey Park and Vadim Levin, Yale University Plate tectonics is driven largely by the sinking of oceanic plates at the deep-ocean trenches of subduction zones, the most common type of convergent boundary between distinct plates. The mantle "wedge" lies between the overriding and the sinking plates at a subduction zone, a buried strip of olivine-rich rock typically 100-200-km thick and 100-400-km wide. The chemistry and dynamics of the mantle wedge determine arc volcanism, a major "product" of the plate-tectonics heat engine. The mantle wedge also influences the accretion of continents, which can grow by the successive accumulation of volcanic arcs. Geologists can track the descending plate in a subduction zone by the locations of deep-focus earthquakes, but have had less success tracking the motion of the mantle "wedge." Theoretical models for wedge flow differ between (1) a 2-D "corner flow" driven by shear-coupling to the descending plate, (2) a stagnant wedge that is already transforming to a stable "continental" plate, (3) trench-parallel flow induced by 3-D geometric effects, such as the rollback of the sinking oceanic plate. This project will detect sheared rock in the mantle wedge, deformed by its flow, by using seismic indicators of elastic anisotropy in the sheared rock. These seismic indicators include shear-wave birefringence, and the partial conversion of P waves (compressional) to S waves (shear) in seismograms from distant earthquakes. Preliminary results suggest that strong trench-parallel shear can develop at the top of the subducting plate, and that changes in rock fabric exist and are detectable within the mantle wedge itself.