Abstract:

By using two seismic profiles along 30°N and 31°N in western Sichuan, China deployed by Institute of Geology, China Earthquake Administration, Virtual Deep Seismic Sounding (VDSS) method was applied to study the crustal structure in Sichuan-Yunnan region. The results show distinct Moho depth from VDSS at Sichuan-yunnan Rhombic Block, Songpan-Garze block and Yangzte block. 1) The Moho depth from VDSS in Sichuan basin is about 40 km, 2) Moho depth from VDSS beneath Sichuan-yunnan Rhombic Block is about 45-50 km, 3) Moho depth from VDSS beneath Songpan-Garze block is about 30-40 km. The Moho depth from VDSS in Sichuan basin is consistent with that predicted from Airy isostasy, while the Moho depth from VDSS at Chuan-Dian Fragment and Songpan-Garze block is clearly shallower than the Moho depth from previous receiver function studies, and those predicted from the Airy isostasy as well. These results may suggest that the crustal structure beneath Sichuan basin is relatively simple while there are complex crustal structure beneath Sichuan-yunnan Rhombic Block and Songpan-Garze block. In addition, the Moho depth from VDSS are clearly reduced at Xianshuihe fault and Anninghe fault, which might reflect mantle upwelling along the fault zones. The results provide new constrains to understand the deformation mechanism at southeastern margin of Tibetan Plateau.

Key words: Sichuan-Yunnan region, virutal deep seismic sounding, Moho, crustal structure

摘要：

基于中国地震局地质研究所在中国四川西部布设的流动地震观测台阵数据, 用近年发展起来的虚拟地表震源反射测深方法研究川滇地区的地壳结构。结果表明, 川滇地块、松潘?甘孜地块和杨子地块 3 个地块虚拟地表震源反射测深的莫霍面深度存在明显差异: 1) 四川盆地为 40 km左右; 2) 川滇地块为 45~50 km; 3) 松潘?甘孜地块为 30~40 km。四川盆地虚拟地表震源反射测深的莫霍面深度与艾里重力均衡模型所预测的结果基本上一致, 而川滇地块和松潘?甘孜地块虚拟地表震源反射测深的莫霍面深度明显小于前人得到的接收函数莫霍面深度和艾里重力均衡模型预测的结果。可能与四川盆地地壳结构简单, 而川滇地块及松潘?甘孜地块地壳结构复杂有关。同时, 结果显示, 在鲜水河断裂和安宁河断裂处虚拟地表震源反射测深的莫霍面深度明显变浅, 可能与这些深大断裂处地幔物质的上涌有关。研究结果可为认识青藏高原东南缘的构造变形模式提供新的约束。

关键词: 川滇地区, 虚拟地表震源反射测深法, 莫霍面, 地壳结构