缅甸弧中部俯冲带下方地幔间断面起伏形态研究

Study on the topography of mantle discontinuities beneath the central Burma arc subduction zone

新生代以来,印度板块和欧亚板块发生碰撞形成了喜马拉雅造山带和青藏高原,印度板片在喜马拉雅东构造结处缅甸弧俯冲带进入深部地幔.开展缅甸弧俯冲带下方地幔间断面的研究有助于认识印度大陆岩石圈的碰撞-俯冲过程及其对上地幔结构的影响.本文选用了发生于缅甸弧地区的3个中源地震事件,获取了欧洲和美国阿拉斯加地区多个密集地震台网/台阵记录的高质量宽频带波形,利用N次根倾斜叠加方法(N=1,4)提取了缅甸弧中部俯冲带下方地幔间断面处产生的次生SdP转换震相.通过研究我们发现:(1)410-km间断面的深度范围为392~407 km,平均深度为400 km;相比于IASP91模型,410-km间断面的平均抬升幅度为10 km.我们推测印度板片在缅甸弧中部俯冲带下方已经穿过410-km间断面并进入到地幔转换带,410-km间断面的抬升与冷俯冲板片的影响有关;(2)X间断面深度在289~314 km之间,平均深度为306 km,推测该间断面可能与大陆地壳深俯冲背景下形成的超高压变质岩中柯石英(coesite)到斯石英(stishovite)的相变有关.

The collision between the Indian and Eurasian plates since the Cenozoic causes the formation of the Himalayan orogenic belt and Tibetan Plateau,and the Indian plate enters into the deep mantle beneath the Burma arc subduction zone in eastern Himalayan syntaxis.Study on mantle discontinuities is helpful to understand the collision-subduction process of the Indian continental lithosphere and its influence on the upper mantle structures.In this study,we select three intermediate-depth earthquakes occurred in the Burma arc,and retrieve broadband seismograms based on several dense networks/arrays in Europe and Alaska of America.Using the N-th root slant stack method(N=1,4),we extract SdP phases converted at mantle discontinuities below the sources.We find that the depths of the 410-km discontinuity appear at 392~407 km with an average of 400 km and an average uplift of 10 km compared with the IASP91 model.We suggest that the Indian slab has penetrated across the 410-km discontinuity and entered into the mantle transition zone,and the uplifts of the 410-km discontinuity are likely induced by the cold subducted slab.The depths of X-discontinuity in our study area appear at 289~314 km with an average of 306 km.We infer that the X-discontinuity may relate to the coesite to stishovite transformation in the ultrahigh pressure metamorphic rocks under the deep subduction of the continental crust.