青藏高原东南缘地壳结构与变形机制研究进展

Review on the crustal structures and deformations in the southeastern margin of the Tibetan Plateau

新生代青藏高原的隆升改变了整个亚洲的构造格局,对气候、环境均产生了重要的影响,但高原的隆升扩展机制众说纷纭.青藏高原东南缘作为扩展前缘,其构造演化对了解整个高原的扩展机制具有重要的意义.本文总结了近年来对青藏高原东南缘地壳结构研究的最新进展,特别是2011年中国地震科学探测台阵计划开展以来,利用密集地震台阵取得的新成果,探讨了青藏高原东南缘地壳的结构与变形机制.这些研究发现青藏高原的地壳由高原向外围减薄,但在高原边界断裂附近存在地壳厚度突变带;下地壳中存在两个独立的低速异常,一个位于松潘—甘孜块体下方,被高原的边界断裂所围限,另一个位于小江断裂带下方,呈NE-SW向展布.我们认为青藏高原东南缘下地壳物质被边界(丽江—小金河)断裂所围限,并没有继续向边缘流出,但是地壳挤出产生的应力作用继续向东南方向传递,造成了小江断裂带附近的地壳变形.

The uplift of the Tibetan Plateau in the Cenozoic has changed the tectonics of the whole Asia and affected the climate and environment significantly.However,the mechanism of the plateau uplift and expansion is still in debate.The southeastern margin of the Tibetan Plateau,as the one of the expansion frontiers,provides the key to understand the tectonic evolution of the plateau.In this study,we review the progress of the researches on the crustal structures in the southeastern plateau margin,especially those achieved after the ChinArray project since 2011.The latest results show that the thickness of the crust descends from the plateau to its southeastern margin with an abrupt change under the plateau boundary.Two independent low-velocity zones are revealed in the lower crust:one under the Songpan-Ganzi terrane which is part of the high plateau,and another orientating NE-SW under the Xiaojian fault zone.We propose that the lower crustal materials extruded from the high plateau are blocked around the plateau margin(i.e.,Lijiang-Xiaojinhe fault)by a strong crustal block in the western margin of the Yangtze block.However,the stress is further transferred to the Xiaojiang fault zone and induced the crustal deformations there.