Collaborative Proposal: Active Faulting Within the Eastern Tibetan Margin and Western Sichuan Basin, P. R. China

西藏东缘是世界上最神秘的高原边缘之一。海拔从四川盆地内不到500米上升到6.5公里以上，距离约30到50公里。这比喜马拉雅前锋更陡峭，喜马拉雅前锋的起源应归因于印度次大陆在西藏之下的活跃冲断。然而，与喜马拉雅形成鲜明对比的是，西藏东部边缘显示出活动冲断的微不足道的证据。此外，边缘附近缺乏明显的沉积盆地和大地缩短作用，进一步证明了活动逆冲作用的重要性。新的野外调查表明，尽管活动逆冲作用确实很少，但有充分的证据表明存在明显的活动走滑断裂。初步野外勘察发现三条或可能四条走向北东的重要右旋走滑断层和两条走向南北向的主要左旋走滑断层。目前的研究重点是确定这些断层的速率和详细几何形状，每一条断层都是限制该区域相对地壳运动的计划尝试的关键组成部分。这些观察结果能够约束各种假设，从而使我们能够了解这部分青藏高原边缘的演化。早期的调查也揭示了四川盆地内存在重大活动断裂的可能性。这一点特别令人担忧，因为可疑断层可能代表着巨大的强烈地面运动，而四川盆地大约有6000万人。断层滑动的速度是通过仔细的采样策略来确定的，通过宇宙成因的放射性核素分析来估计被断层抵消的地貌的年龄。活动断裂的样式和模式是通过图像分析、地形分析和以前在侦察现场工作中确定的区域的详细野外测绘的组合来确定的。滑移率和断层几何形状的组合产生了一套区别于竞争模型的运动学。

The eastern margin of Tibet is one of the most enigmatic plateau margins in the world. Elevations rise from less than 500 m within the Sichuan Basin to more than 6.5 km over a distance of about 30 to 50 km. This is steeper than the Himalayan front, which owes its origin to the active thrusting of the Indian subcontinent under Tibet. In stark contrast to the Himalaya, however, the eastern margin of Tibet shows insignificant evidence of active thrusting. Moreover, the lack of a significant sedimentary basin adjacent to the margin and the absence of geodetic shortening further testify to the insignificance of active thrusting.New field investigations indicate that while active thrusting is indeed scarce, there is ample evidence for significant active strike-slip faulting. Initial reconnaissance field investigation reveals three and possibly four significant right-lateral strike-slip faults that trend NE, and two major left-lateral strike-slip faults that trend ~N-S. Research is now focused on determining the rates and detailed geometry of these faults, each of which are critical components in planned attempts at constraining relative crustal motions in the region. These observations are able to constrain the various proposed hypotheses and therefore enable us to understand the evolution of this part of the Tibetan margin.Early investigation has also revealed the likelihood of significant active faulting within the Sichuan Basin. This is of particular concern, because the suspect faults may represent significant strong ground motions and the Sichuan Basin is home to about 60 million people.Rates of fault slip are being determined by a careful sampling strategy whereby the ages of landforms that have been offset by faulting are being estimated by cosmogenic radionuclide analyses. The style and pattern of active faulting is being determined by a combination of imagery analysis, topographic analyses, and detailed field mapping in regions previously identified during reconnaissance field work. The combination of slip rates and fault geometry yields a set of kinematics that discriminate among the competing models.