甘肃疏勒河冲积扇发育特征及其对构造活动的响应

甘肃疏勒河冲积扇是面积达2400km²的一个巨型冲积扇,位于青藏高原北缘阿尔金断裂带东端。区域构造上该扇发育在阿尔金主断裂、三危山断裂和南截山断裂组成的阿尔金断裂带左旋走滑构造域。根据野外调查获得的沉积层序、地貌形态和构造变形等资料,该冲积扇可划分为老冲积扇和新冲积扇。老冲积扇扇顶紧靠阿尔金主断裂,由已显著构造变形的早更新统玉门砾岩组成;新冲积扇是在老冲积扇基础上发育的,扇顶已向下游移动到大坝附近,距阿尔金主断裂4km,主要由中更新统酒泉砾石和晚更新统戈壁砾石组成。疏勒河冲积扇发育对区域构造活动的响应主要表现为:1)在第四纪早期,阿尔金断裂的左旋走滑兼逆冲运动,导致了由玉门砾岩组成的疏勒河老冲积扇扇头被左旋错动约8km,同时扇体,尤其扇头明显褶皱隆升,扇顶向山外移动;2)此后随着阿尔金断裂继续作左旋走滑兼逆冲运动,老冲积扇扇头又被左旋错动了约2km,总共左旋位移了约10km,并且扇顶下移了约4km,形成新的冲积扇;3)在冲积扇内从早更新世至晚更新世沉积中心不断向下游移动,呈现向下游超覆沉积的特征,这也一定程度上反映了祁连山隆升、向外扩展和侵蚀作用显著

RESPONSE OF THE DEVELOPMENT OF THE SHULE RIVER ALLUVIAL FAN TO TECTONIC ACTIVITY

The Shule River alluvial fan, is a very large alluvial fan with an area of about 2 400km ², located in the eastern part of the Altun Tagh Fault zone at the northern margin of the Qinghai Xizang Plateau. The fan has developed in a left lateral strike slip domain setting, provided by the Sanwei Shan Fault and Dongbatu Shan Fault, which are associated with the general Altun Tagh Fault structures. On the basis of the alluvial stratigraphy, geomorphology and the details of the tectonic deformations, the Shule River Fan can be divided into an older and a younger fan. The older fan, with the fan head located close to a main fault line of the Altun Tagh fault, is composed of strongly deformed Yumen Conglomerate——the deformation has taken place since the Early Pleistocene. The younger fan, with the fan head at Daba, a distance of about 4km from the main fault line, overlies the older fan. It is mainly composed of the Middle and Late Pleistocene Jiuquan Gravel and Gobi Gravel formations. From the field evidence it is to conclude that the response and evolution of the Shule River Fan to the tectonic events that have characterized this region during the Quaternary took the following form: (1) the Yumen Conglomerates in the head zone of the older fan were folded and displaced some 8km to the west. This was in response to the major left lateral strike slip movement, accompanied by a minor thrust component, that took place along the main fault of the Altun Tagh structural zone in the Early Pleistocene; (2) with the continuing movement of the main fault (similar to that which occurred in the Early Pleistocene), a younger fan developed, with fan head displaced by some 4km towards the lower fan reaches through uplift and erosion of the Yumen Conglomerates at the head of the older fan. Associated with this, the older fan head was displaced an additional 2km more toward the west——resulting in a total left lateral strike slip movement of about 10km since the end of the Early Pleistocene; and (3) the general depocenter of the system has been displaced toward the north, with the associated sediments now completely covering the area of the fan. The general results indicate that the Qilian Frontal Range has been uplifting and expanding in association with the left lateral strike slip movement and thrusting of the Altun Tagh main fault during the development of the Shule River Fan.