板塊下方地幔震波非均向性與其動力學之意含: 科科斯與興都庫什隱沒帶

Abstract

上部地幔中的非均向性礦物會因外在應力而有其傾向的排列位態(Lattice preferred orientation, LPO),其中非均向性礦物之快軸排列方向在地震學觀 測上多假設為平行地幔流場的方向,此論文是利用 S 波反應近地震震源震波非均向性(source-side seismic anisotropy)來推測其 LPO 之位態,並探討隱沒板塊下方或周遭地幔的流場與其動力學上的意義。論文第一部份為探討興都庫什(Hundu Kush)隱沒帶地幔的震波非均向性,我們的觀測結果顯示此區域的震波非均向性,其快方向以隱沒的興都庫什板塊為中心呈現環形的排列。此環形分布情況看似是由隱沒板塊回滾(rollback)擾動周遭上部的地幔所造成。但經由合成波模型顯示,此環形排列所顯示為視方向(Aparent direction)而不是非均向性礦物之實際位態(orientation),而是徑向震波非均向性(radial anisotropy)與近垂直拖曳的地幔流場的綜合。實際 LPO 仍為平行向地幔深部隱沒的垂直方向。而此現象與逕向非均向相關,也暗示低含水量的 A 型橄欖石或是 AG 型橄欖石此地的地幔主要組成。論文第二部份是探討科科斯(Cocos)隱沒板塊下方地幔的震波非均向性。過去對於此區域近地震震源震波非均向性的研究指出,地幔中 LPO 多依照絕對板塊運動模型所描述的隱沒方向,而我們的量測結果顯示其快方向比隱沒方向順時鐘旋轉約 50 度。近一步的分析震波的振動極化方向顯示我們用來探查非均向性之震波極化方向與 APM 平行,此觀測結果對平行 APM 排列之 LPO 極不敏感,並指示板塊下方地幔存在另一層與 APM 非平行之 LPO。經由逆推發現,板塊下方地幔如果包含一層淺部與APM 平行之 LPO,以及另一層 N70E 方向排列之LPO,即 可以解釋觀測資料上快方向的順時鐘轉向。此模型可作為支持回滾(rollback)的隱沒板塊下方地幔同時在極流(poloidal flow)與環流(toroidal flow)的證據。

In this thesis, we investigate how the subducting slab shapes the mantle flow structure by exploring the seismic anisotropy which caused by the stress induced alignment of the lattice preferred orientation of anisotropic mineral in the upper mantle. We retrieve the anisotropy in the ambient mantle of the subducting slab, the source-side anisotropy, by analyzing shear wave splitting (SWS) of the direct S phase after correcting the receiver-side anisotropy with the SKS splitting parameters. First, we present the source-side SWS from Hindu Kush intraslab events to sample the surrounding mantle and the observed fast directions exhibit a circular pattern around the slab. We propose that the observed pattern is produced by the sub-vertical shear flow entrained by the steep descent of the slab and the ongoing breakoff. This scenario requires the existence of A-type or AG-type olivine fabrics with strong orthorhombic anisotropy in mid- to lower upper mantle. This interpretation circumvents the debate on the cause of trench-parallel anisotropy in some oceanic subduction zones where slab entrainment and rollback may coexist, and supports the notion that orthorhombic anisotropy of olivine may play an important role in shaping mantle anisotropy. Then we present the source-side anisotropy in the subslab mantle of the Cocos subduction zone, our results show that fast directions predominantly align at N85oE, or 50o clockwise from the direction of the absolute plate motion (~35o), and analysis of the fast directions and the initial polarization of the shear wave leads us to model the layered structure in the subslab mantle. Our model shows that the upper and lower layers are characterized by a fast direction of 38o and 76o, respectively. The upper layer of the structure immediately entrained by the slab is descending normally and the lower layer is oblique in subduction, and the layered structure which may present the coexist of the entrained flow and the toroidal flow in the subslab mantle.