新西兰弧后塔拉纳基地区地幔楔各向异性

Anisotropic structure in the back arc region, Taranaki, New Zealand

太平洋板块沿希库兰吉海沟俯冲至新西兰北岛下方300km左右,向南与新西兰南岛发生碰撞,导致北岛发生顺时针旋转。该区域为研究俯冲带地幔楔变形,理解俯冲过程中地幔物质运动及其动力学过程的理想场所。本研究利用近震(深度范围在70~150km)S波分裂计算获得了北岛弧后塔拉纳基地区的地幔楔各向异性特征,结果显示快波方向和延迟时间在空间上存在一定的变化。深度在120km之上的地震对应的快波优势方向为NE—SW,近似平行于海沟走向,反映了地幔楔中平行于海沟方向的地幔流动引起的橄榄岩晶格优势排列;而120km之下地震主要集中在塔拉纳基地区北部,快波方向为NNE—SSW向,且延迟时间随深度增大而增加。由于太平洋板块俯冲到100~150km处的俯冲角度急剧变陡至近似直立,引起深部地幔楔强烈变形。因此,推测弧后北部深度>120km地震的NNE—SSW快波方向可能是受到地幔楔中平行于海沟走向的地幔流动,以及俯冲角度变陡而使深部地幔楔强烈变形的共同作用的结果。弧后北部地幔楔深部的拉张作用更为强烈,是造成各向异性在空间上变化的主要原因。

The Pacific plate is subducting beneath the North Island of New Zealand along the Hikurangi Trench to depths of~300 km,and is colliding with the South Island to the south,causing clockwise rotation of the North Island.Study on deformation of the mantle wedge in the subduction zone is essential to understand the mantle material movement and its dynamic mechanism.In this study,we investigate the anisotropy in the mantle wedge beneath the Taranaki region in the western backarc area of the North Island using the S-wave splitting measurement of local events with depths ranging from 70 km to 150 km.The results show spatial variations in the fast wave direction and delay time.The NE-SW trending dominant fast direction from the events with depths above 120 km is approximately trench-parallel,reflecting the crystallographic preferred orientation of olivine caused by the trench-parallel mantle flow in the mantle wedge.The events below 120 km depth are mainly from north of the Taranaki region.The predominant fast direction of these events is NNE-SSW,which delay times increase with depth.The Pacific slab steepens abruptly to a nearvertical plane at about 100~150 km depth,which could induce stronger shear deformation of upper mantle material in the deep mantle wedge.Therefore,the NNE-SSW trending anisotropy with larger delay times in the deep mantle wedge north of the Taranaki region may be caused by the combination of trench-parallel mantle flow and strong deformation of deep mantle wedge due to steepening of the dipping Pacific slab.The stronger extension in the deep mantle wedge of the northern backarc is the main reason for the spatial variation of anisotropy.