青藏高原东缘龙门山晚新生代走滑-逆冲作用的地貌标志

GEOMORPHIC EVIDENCE FOR THE LATE CENOZOIC STRIKE-SLIPPING AND THRUSTING IN LONGMEN MOUNTAIN AT THE EASTERN MARGIN OF THE TIBETAN PLATEAU

文章以青藏高原东缘龙门山活动构造的地貌标志为切入点，在汶川-茂汶断裂、北川断裂、彭灌断裂和大邑断裂等主干活动断裂的关键部位，对断错山脊、洪积扇、河流阶地、边坡脊、断层陡坎、河道错断、冲沟侧缘壁位错、拉分盆地、断层偏转、砾石定向带、坡中槽、弃沟和断塞塘等活动构造地貌和断裂带开展了详细的野外地质填图和地貌测量，利用精确的地貌测量数据和测年数据，定量计算了龙门山主干断裂的逆冲速率和走滑速率，结果表明在晚新生代时期龙门山构造带仅具有微弱的构造缩短作用，其中逆冲速率的速度值小于1．1mm／a，走滑速率的速度值小于1．46mm／a，表明走滑分量与逆冲分量的比率介于6：1—1．3：1之问，以右行走滑作用为主。在此基础上，对各主干活动断裂的逆冲速率和走滑速率进行了定量的对比研究，结果表明自北西向南东4条主干断裂的最大逆冲分量滑动速率具有变小的趋势，而走滑分量的滑动速率则具有逐渐变大的趋势，显示了从龙门山的后山带至前山带主干断裂的走滑作用越来越强。由此推测现今的龙门山及其前缘盆地不完全是由于构造缩短作用形成的。而主要是走滑作用和剥蚀卸载作用的产物。另外，根据沉积、构造、盆地充填体的几何形态、地貌、古地磁等标定和对比了龙门山在中生代和新生代的走滑方向，表明龙门山构造带在中生代与新生代之交走滑方向发生了反转，即由中生代时期的左行变为新生代时期的右行。

Geomorphic evidence for active tectonics in the Longmen Mountain at the eastern margin of the Tibetan Plateau has been studied in this paper. We have surveyed some typical geomorphic markers in Wenchuan-Maowen Fault, Beichuan Fault, Pengguan Fault, and Dayi Fault, including terrace offsets, scarps, fault-controlled saddles, dextral shutter ridges, dextral channel offsets, grabens, shatter belts, and pull-apart basins. Electron spin resonance （ESR） ages and thermoluminescence （TL） ages using silty sand taken from below the surface of sediments have been obtained. According to these data, we calculated the rate of thrusting and strike-slipping, and the results indicate that the Cenozoic tectonic shortening at the margin of the Tibetan Plateau is minor with the rate of thrusting smaller than 1. 1 mm/a and the rate of strike-slipping smaller than 1.46mm/a, and the Longmen Mountain is a zone of NNE-directed dextral shear with extensive strike-slip faulting and secondary thrusting with slip-dip ratio of 6 ： 1 - 1.3 ： 1. From NW to SE, the thrust component becomes smaller, but strike-slip component becomes bigger. However, this does not explain the presence of the abrupt topographic escarpment at the margin in the absence of major shortening. We thus infer that some escarpment reliefs are due to the effects of erosional unloading and strike-slipping. Based on comparison of sedimentary evidence, tectonic evidence, geomorphic evidence and paleomagnetism between the Cenozoic and Mesozoic, we believe that the direction of strike-slip faulting has been reversed from SSW-directed sinistral strike-slip in Mesozoic to NNE-directed dextral strike-slip in the Cenozoic.