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

Abstract

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).

Crustal–lithospheric structure and continental extrusion of Tibet

“…These offsets and slip rates are similar to the c. 425 km proposed by Cowgill et al (2003) for early Palaeozoic piercing points. Initiation of slip along the Altyn Tagh Fault is thought to have been late Oligocene-earliest Miocene (c. 23 Ma) from strata in the Xorkol Basin bounding the Altyn Tagh Fault at 928E (Yue et al 2001(Yue et al , 2003.…”

“…The Altyn Tagh Fault may have geological offsets of as much as c. 400 km (Ritts & Biffi 2000;Yue et al 2001Yue et al , 2003Cowgill et al 2003) and Cenozoic average slip rates of 12-16 mm a À1 . Quaternary and active slip rates measured from InSAR and GPS are between 5 and 15 mm a À1 , but with a decrease towards the east (Wright et al 2004;Elliott et al 2008;Jolivet et al 2008) and are not high enough to support large-scale continental extrusion.…”

“…The offsets of .400 km suggested by Lacassin et al (2004a) and Valli et al (2007Valli et al ( , 2008 we regard as invalid correlation of offset geological markers (see Lacassin et al 2004b, discussions). Evidence for larger offsets (c. 400 km) along the Altyn Tagh Fault, however, does appear to be more robust (Yue et al 2003Zhang et al 2001;Cowgill et al 2003). Leloup et al (2001) and Replumaz & Tapponnier (2003) illustrating the continental extrusion model.…”

“…Cowgill et al 2003;Yue et al 2003); (2) Quaternary estimates derived from alluvial terrace ages and geomorphological offsets (e.g. Meriaux et al 2005;Gold et al 2009); (3) modern decadal geodetic measurements of interseismic strain from GPS and InSAR (e.g.…”

Localization of shear along a lithospheric strength discontinuity: Application of a continuous deformation model to the boundary between Tibet and the Tarim Basin

“…Reconstruction of the tectonic evolution of this region indicates that lateral strength variations may play a significant role in the distribution of continental deformation. Cowgill et al [2003] relate southward thrusting in the western Kunlun in part to the strength contrast between the relatively rigid Tarim basin and the weaker Songpan-Ganzi flysch terrain. Paleozoic sutures and faults [e.g., Sobel and Arnaud, 1999] may contribute additional weakness to the Tibetan lithosphere.…”