A first-order feature of the Tibetan Plateau is its triangular geometry on map view with its north-south width increasing from ~500 km in the west to >1500 km in the east. This asymmetry has been explained by either eastward extrusion of Tibet or partitioning of north-south shortening in the Tian Shan and northern Tibet across the strong Tarim block. To quantify the role of lateral extrusion in accommodating intracontinental deformation during the Indo-Asian collision, we must know the timing and magnitude of slip along major Cenozoic strike-slip faults in Asia. This goal may be achieved by examining the termination structures of the strike-slip faults where their motion is transferred to dip-slip structures. In this proposal, we plan to investigate the temporal and spatial evolution of one of such structures: the 220-km long, north-trending Kongur Shan extensional system in the eastern Pamirs. In the lateral-extrusion model, the Kongur Shan extensional 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 the southern 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 northward and its total magnitude of extension decreases from south to north. Alternatively, the oroclinal-bending model suggests that the development of the north-trending Kongur Shan system may have been related to pure bending of the Pamir-Nanga Parbat syntaxis system during Cenozoic northward indentation of the Pamir promontory. This hypothesis not only explains coeval east-west extension in the Pamirs and east-west contraction in the Nanga Parbat region in the western Himalayan orogen, but also predicts specifically that the Kongur Shan extensional system should have lengthened progressively southward and its total magnitude of extension decreases along strike to the south. The distinctive predictions regarding the extensional-magnitude distribution and propagation direction of the Kongur Shan extensional system by the two competing models form the basis of this proposal. We plan to conduct field mapping across the extensional system in four different transects, each is about 20-40 km wide, which will allow us to construct a series of cross sections for estimating the magnitude of extension along individual segment of the system. Field mapping will focus on the question of whether the Kongur Shan extensional system is actually linked with the Karakorum fault. We will integrate field mapping with thermobarometric, 40 Ar/39 Ar thermochronologic, and U-Th-Pb ion-microprobe geochronologic analyses to constrain the P-T-t paths for both the hangingwall and footwall of the extensional system. The results of this study will provide two important constraints on the tectonics of the western 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 and contraction in the western syntaxis system.
