喜马拉雅造山带西部拉昂错蛇绿岩带区域地壳深部结构及其可能构造归属研究

前人研究表明喜马拉雅造山带西部出露的拉昂错蛇绿混杂岩为新特提斯洋壳岩石圈的一部分,代表了新特提斯洋的关闭及其随后大洋岩石圈物质的仰冲.鉴于拉昂错蛇绿岩的构造演化历史尚不明确,前期对于拉昂错蛇绿岩带构造归属的研究主要基于岩石学研究和地表地质调查等,缺少精细的深部地壳结构进行运动学指示,因此证明拉昂错蛇绿混杂岩体的构造归属并非易事.本次研究中,我们对前期获得的一条南北向延伸穿过雅鲁藏布江缝合带和喜马拉雅造山带西部拉昂错蛇绿岩体的112 km长的深反射地震剖面进行了构造解释.高分辨率的深反射地震剖面清晰地显示了喜马拉雅山脉西部造山带内发育良好的地壳双冲构造几何结构,该地壳尺度双冲构造将印度俯冲地壳物质从底部运移到上部.同时,地震剖面还显示拉昂错蛇绿岩体和雅鲁藏布江蛇绿岩体在上地壳深处呈倾向相反但底部相通的结构构造.结合前人的岩石学/地球化学/地表地质研究成果,我们认为拉昂错蛇绿岩体为雅鲁藏布江缝合带蛇绿岩体的一部分.印度俯冲前缘的双冲构造折返将深部物质带到地表过程的同时,还将部分雅鲁藏布江蛇绿混杂岩携带至南侧距主缝合带位置大约20 km的拉昂错蛇绿岩区域.

Seismic evidence for tectonic affinity of the Yungbwa ophiolitic complex,Western Tibet

The Yungbwa ophiolitic complex in the western Himalayas represents segment of the Neo-Tethyan oceanic lithosphere and its subsequent obduction onto the continental margin. Given the unclear stages of evolution, elucidating the affinity of this complex is not a simple task. To address this issue, models were proposed mainly from conventional observations, such as petrological studies and surficial geology investigations. In this case, additional constraints on crustal-scale geometry derived from deep seismic reflection imaging were required. In this study, we presented a 112 km long deep seismic reflection profile that cuts through both the Yarlung-Zangbo suture zone and the Yungbwa ophiolite in the western Tibetan Plateau. A high-resolution seismic image clearly shows the well-developed, duplexing crustal geometry up to the surface within the western Himalayan orogen. The seismic image also shows the opposite dipping but connected structures at depths between the Yungbwa ophiolite and the Yarlung-Zangbo ophiolites at a upper-crustal scale. Coupled with previous studies, we suggest that the Yungbwa ophiolites were emplaced from a deep segment of the original Yarlung-Zangbo suture zone to the surface allochthonous ophiolites. This process occurred as an accommodation to the top-to-the-south crustal-scale duplexing of the subducted Indian crust from the lower plate to the upper plate during subduction reorganization.