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

Park Jeffrey J. P. -0106867提案摘要EAR 0106867（PH #33 x）标题：楔形问题：俯冲带中的地震各向异性、构造地质学和上地幔动力学PI's：Jeffrey Park和Vadim Levin，耶鲁大学板块构造主要是由俯冲带深海海沟处的海洋板块下沉驱动的，这是不同板块之间最常见的会聚边界类型。地幔“楔”位于俯冲带的上覆板块和下沉板块之间，俯冲带是一个富含橄榄石的岩石的埋藏带，通常厚100-200公里，宽100-400公里。地幔楔的化学和动力学决定了弧火山作用，这是板块构造热引擎的主要“产物”。地幔楔也影响大陆的增生，大陆可以通过火山弧的连续积累而增长。地质学家可以通过深源地震的位置来追踪俯冲带中板块的下降，但追踪地幔楔的运动却不太成功。楔形流的理论模型在以下几种情况下是不同的：（1）由与下降板块的剪切耦合驱动的二维“角流”，（2）已经转变为稳定的“大陆”板块的停滞楔形，（3）由三维几何效应（如下沉的海洋板块的回滚）引起的沟槽平行流。该项目将利用剪切岩石中弹性各向异性的地震指标来探测地幔楔中因其流动而变形的剪切岩石。这些地震指标包括剪切波双折射，以及来自远距离地震的地震图中P波（压缩）向S波（剪切）的部分转换。初步结果表明，强烈的海沟平行剪切可以在俯冲板块的顶部发展，岩石组构的变化存在，并在地幔楔本身是可检测的。

Park Jeffrey J.EAR-0106867Abstract for proposal EAR0106867 (PH # 33x)Title: Wedge Issues: Seismic Anisotropy, Structural Geology and Upper-Mantle Dynamics in Subduction ZonesPI's: Jeffrey Park and Vadim Levin, Yale UniversityPlate 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.