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

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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.