Reconstruction of the Altyn Tagh fault based on U-Pb geochronology: Role of back thrusts, mantle sutures, and heterogeneous crustal strength in forming the Tibetan Plateau

[1] Reconstructing deformation along the northwestern margin of the Tibetan Plateau is critical for evaluating the relative importance of microplate versus continuum models of the Indo-Asian collision. Questions regarding this margin's evolution are as follows: (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? (3) Why does the N-S width of the plateau vary along strike? Ion microprobe U-Pb zircon apparent ages of 17 plutons from NW Tibet, together with regional geologic observations, define a discrete, E-W trending boundary between two tectonic belts that 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 plateau. Shortening appears to have been absorbed in the upper crust by thin-skinned thrusting, in the middle/lower crust by east directed ductile flow and/or subduction, and in the mantle by north dipping subduction. 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 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).