Investigating the Kinematic History of the Kongur Shan Normal Fault System, Western Tibet

青藏高原的一级特征是它在地图上的三角形几何形状，南北宽度从西部的~500公里增加到东部的1500公里。这种不对称性是由西藏向东挤压或田山和藏北南北向缩短分割穿过塔里木强地块造成的。为了量化侧向挤压在印度-亚洲碰撞期间调节陆内变形中的作用，我们必须知道亚洲主要新生代走滑断裂的滑动时间和滑动强度。这一目标可以通过研究走滑断层的末端结构来实现，在那里它们的运动被转移到倾向滑动结构。在这项提议中，我们计划研究其中一个构造的时间和空间演化：位于帕米尔高原东部的220公里长的北向康古尔伸展系统。在侧向挤压模式中，康古尔山伸展体系是长约1000公里的右滑喀喇昆仑断裂的北端，它被认为帮助西藏向东挤压了数百公里。由于假设孔谷山系南端与右滑喀喇昆仑断裂相连，挤压模型要求伸展体系逐渐向北伸展，其总伸展强度由南向北递减。另外，造山斜-弯曲模型表明，康古尔山系向北的发育可能与帕米尔-南加-帕尔巴特组合系统在帕米尔海角新生代向北凹陷期间的纯粹弯曲有关。这一假说不仅解释了喜马拉雅造山带西部帕米尔高原同时代的东西向伸展和南迦帕尔巴特地区的东西收缩，而且还具体预测了孔古尔山伸展体系应逐渐向南伸展，其总伸展强度沿走向向南递减。这两个相互竞争的模型对孔雀山伸展系统的伸展震级分布和传播方向的不同预测构成了这一提议的基础。我们计划在四个不同的横断面上对伸展系统进行野外测绘，每个横断面大约20-40公里宽，这将使我们能够构建一系列横断面，以估计沿系统个别部分的伸展强度。野外测绘将侧重于孔古尔山伸展系统是否真的与喀喇昆仑断层相连的问题。我们将结合现场填图和温度气压、40Ar/39Ar热年代学和U-Th-Pb离子探针地质年代学分析，来约束伸展系统的上盘和下盘的P-T-t路径。这一研究结果将为西喜马拉雅造山带的构造提供两个重要的制约因素：(1)西藏向东挤压的时间和规模；(2)西部构造系统中同时代东西伸缩的启动和发展的动力原因。

A first-order feature of the Tibetan Plateau is its triangular geometry on map view with itsnorth-south width increasing from ~500 km in the west to 1500 km in the east. This asymmetryhas been explained by either eastward extrusion of Tibet or partitioning of north-south shorteningin the Tian Shan and northern Tibet across the strong Tarim block. To quantify the role of lateralextrusion in accommodating intracontinental deformation during the Indo-Asian collision, wemust know the timing and magnitude of slip along major Cenozoic strike-slip faults in Asia. Thisgoal may be achieved by examining the termination structures of the strike-slip faults where theirmotion is transferred to dip-slip structures. In this proposal, we plan to investigate the temporaland spatial evolution of one of such structures: the 220-km long, north-trending Kongur Shanextensional system in the eastern Pamirs. In the lateral-extrusion model, the Kongur Shanextensional system is the northern termination of the ~1000-km long right-slip Karakorum fault,which is thought to have assisted Tibet to extrude hundreds of kilometers eastward. Because thesouthern end of the Kongur Shan system is assumed to link with the right-slip Karakorum fault,the extrusion model requires that the extensional system has progressively lengthened northwardand its total magnitude of extension decreases from south to north. Alternatively, the oroclinal-bendingmodel suggests that the development of the north-trending Kongur Shan system mayhave been related to pure bending of the Pamir-Nanga Parbat syntaxis system during Cenozoicnorthward indentation of the Pamir promontory. This hypothesis not only explains coeval east-westextension in the Pamirs and east-west contraction in the Nanga Parbat region in the westernHimalayan orogen, but also predicts specifically that the Kongur Shan extensional system shouldhave lengthened progressively southward and its total magnitude of extension decreases alongstrike to the south. The distinctive predictions regarding the extensional-magnitude distributionand propagation direction of the Kongur Shan extensional system by the two competing modelsform the basis of this proposal. We plan to conduct field mapping across the extensional systemin four different transects, each is about 20-40 km wide, which will allow us to construct a seriesof cross sections for estimating the magnitude of extension along individual segment of thesystem. Field mapping will focus on the question of whether the Kongur Shan extensionalsystem is actually linked with the Karakorum fault. We will integrate field mapping withthermobarometric, 40 Ar/39 Ar thermochronologic, and U-Th-Pb ion-microprobe geochronologicanalyses to constrain the P-T-t paths for both the hangingwall and footwall of the extensionalsystem. The results of this study will provide two important constraints on the tectonics of thewestern Himalayan orogen: (1) the timing and magnitude of eastward extrusion of Tibet, and (2)the dynamic cause for the initiation and development of coeval east-west extension andcontraction in the western syntaxis system.