THE ROLE OF BACKTHRUSTS, MANTLE SUTURES, AND HETEROGENEOUS CRUSTAL STRENGTH IN FORMING THE WESTERN TIBETAN PLATEAU

Reconstructing deformation along the northwestern margin of the Tibetan Plateau is critical for evaluating the relative importance of microplate vs. continuum models of the Indo-Asian collision. Questions regarding the evolution of this margin are (1) what is the total offset along the sinistral Altyn Tagh strike-slip system, (2) how has that offset been absorbed in the western Kunlun Shan, and (3) why does the N-S width of the Tibetan plateau vary so dramatically along strike? I use new U-Pb zircon ages to determine if batholiths in the western and eastern Kunlun Shan belts correlate. Both areas show bimodal populations of ages with a belt of Cambrian to Devonian plutons in the north and a suite of Permian to Jurassic intrusion to the south. These data, in conjunction with regional geologic observations, define a discrete, east-west trending boundary between two plutonic and tectonic belts. This boundary has been offset along the Altyn Tagh system by 475±70 km. Kinematic arguments indicate that this offset cannot be the result of north-directed thrusting in the western Kunlun Shan. Therefore, we propose that south-directed faulting in the Tianshuihai thrust belt both offset the tectonic boundary and produced the asymmetry in the Tibetan plateau. Shortening appears to have been absorbed by thin-skinned thrusting in the upper crust, ductile flow in the lower crust, and N-dipping subduction in the mantle. Factors controlling the formation of the south-directed thrust system appear to be the contrast between the rigid Tarim and the weaker Songpan-Ganzi flysch belt and a north-dipping mantle suture inherited from Late Paleozoic subduction. The evolution of western Tibet leads to a view of continental deformation that integrates elements of the microplate model (e.g., plate-like mantle and crust-mantle decoupling) with aspects of the continuum model (weak crustal flow beneath the plateau).