印度板块下地壳北向俯冲与榴辉岩化的地震学证据:接收函数成像结果

印度地壳与岩石圈地幔的俯冲前缘和俯冲形态，对认识高原构造变形、隆升机制有重要意义.本文基于青藏高原西缘分布的流动宽频带地震台站（TW-80测线和Y2台网）记录的远震波形数据，通过接收函数H-κ网格搜索与CCP叠加方法，对研究区地壳结构进行成像.结果显示：（1）研究区西侧北西—南东向剖面（剖面1，2），狮泉河逆冲断裂带以南，深度67~80 km范围内均观测到连续的Moho界面；40~55 km范围内存在另一组横向上可连续追踪的界面，其形态与之下Moho面横向变化趋势近乎平行；（2）研究区东侧剖面3下方，Moho面从南端喀喇昆仑断裂带下方向北逐渐加深，在雅鲁藏布江缝合带附近增至大约67 km，进入拉萨块体至台站WT20和WT03下方至最深75~80 km，然后向北有所抬升.基于成像结果和岩石学研究成果推测藏南块体下方，自西向东均存在俯冲印度板块下地壳的榴辉岩化现象，可以用来指示印度板块地壳尺度的俯冲前缘，其在青藏高原西部（约80°E）位于班公湖—怒江缝合带附近，向东逐步递减至拉萨块体中部.

Eclogitization of Indian lower crust during its northward subduction revealed by receiver function depth migration images

The receiver functions calculated at the broadband seismic stations of TW-80 and Y2 array are used to reveal the main subduction features of the Indian plate beneath western Tibet. The H-κ grid-searching results and CCP depth migration image show that the observed Moho structure at depth 67~80 km is laterally traceable from south to north on the west profiles in the study area, while there is another discontinuity interface at depth 40~55 km almost parallel to the Moho up to the station SQAT nearby the Shiquanhe thrust fault. This feature also can be seen on the CCP migration image on the east profile of the study area. Based on the evidence derived from this and previous petrology studies, we infer that the Indian lower crust front beneath western Tibet is located at Bangonghu-Nujiang suture, while it gradually reduces southward to the internal Lhasa block beneath central and east Tibet.