拉萨地体东南缘地壳深部结构

Deep crustal structure of the southeastern Lhasa Terrane

拉萨地体南缘作为印度欧亚大陆碰撞的最前缘是研究碰撞带地壳接触关系的最佳区域。拉萨地体东南部在新生代陆陆碰撞前还经历了中生代特提斯大洋岩石圈的俯冲演化阶段,多期次构造演化导致地壳结构复杂,致使不同分辨率地球物理方法观测地壳深部结构不清晰,宽频带地震观测和深地震反射数据对于俯冲的印度地壳前缘位置的解释并不一致。本研究在前人布设的测线周围平行布设了一条短周期密集台阵测线。通过本次研究的高分辨远震P波接收函数计算结果发现,前人通过宽频带数据所界定持续向北延伸的印度下地壳实际并不连续,印度地壳仅存在于雅鲁藏布江缝合带下方,这一认识与深地震反射结果一致。拉萨地体的中上地壳存在对应南北向挤压形成的逆冲构造。宽频带数据观测结果中看到的下地壳榴辉岩化区域位于雅鲁藏布江缝合带以北,主要位于中拉萨以及北拉萨南部。

The southern Lhasa Terrane is the foremost area to investigate plate boundary interactions in the India Eurasia collision zone.Prior to Cenozoic continental collision,the southeastern Lhasa Terrane underwent multi-stage tectonic evolution under Mesozoic subduction of the Tethyan oceanic lithosphere,which resulted in a complex regional crustal structure that renders inconsistent observations of its deep structure using different geophysical methods with different resolving power.Presently,broadband seismic observation and deep seismic reflection data disagree on the location of the subduction front at the Indian plate.To gain further insight,a short-period dense array survey line was laid parallel to the previous survey line.By using high-resolution P-wave receiver functions,we discovered that the previously defined northward extension of the Indian lower crust based on broadband data is actually not continuous.In fact,the Indian crust exists only beneath the Yarlung Zangbo suture zone,which is consistent with deep seismic reflection results.The middle and upper crust of the Lhasa Terrane develops a thrust fault under south-north compression,while lower crustal eclogitization observed in the broadband seismic data is mainly located in the north of the Yarlung Zangbo suture zone in the middle Lhasa Terrane and southern part of the North Lhasa Terrane.