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

标题：楔形问题：俯冲带的地震各向异性、构造地质和上地幔动力学[ei]: Jeffrey Park和Vadim Levin，耶鲁大学板块构造在很大程度上是由大洋板块在俯冲带的深海海沟的下沉驱动的，这是不同板块之间最常见的收敛边界。地幔“楔子”位于俯冲带的上覆板块和下沉板块之间，这是一条富含橄榄石的地下带，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板块构造热机的主要“产物”——弧火山作用。地幔楔对大陆的增生也有影响，大陆的增生可以通过火山弧的连续堆积而形成。地质学家可以通过深震源地震的位置来追踪俯冲带中板块的下降，但在追踪地幔“楔”的运动方面却不太成功。楔形流动的理论模型有以下几种不同：(1)由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”板块的停滞楔形流动；(3)由三维几何效应引起的海沟平行流动，如下沉的大洋板块的回滚。本项目将利用剪切岩石的弹性各向异性地震指标，探测受其流动变形的地幔楔中的剪切岩石。这些地震指标包括横波双折射，以及从遥远地震的地震记录中P波（纵波）到S波（横波）的部分转换。初步结果表明，俯冲板块顶部可能发育强烈的海沟平行剪切，并且在地幔楔内部存在并可探测到岩石结构的变化。

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.