Growth and collapse of the Tibetan Plateau: introduction

The Pamir–Tibet–Himalaya orogenic system (Fig. 1) has been growing as a consequence of the collision between the Eurasian and the Indian plates since the Early Eocene (e.g. Hodges 2000). While the time of formation and the magnitude of shortening across the Tibetan Plateau are relatively well constrained, its origin and evolution are disputed, untested, or unknown (e.g. Yin & Harrison 2000). For example, it has been argued that Miocene to Recent geological, geodetic, palaeomagnetic, geomorphological, and geophysical features of the eastern Plateau are best explained by flow of low-viscosity material within the middle–lower crust (e.g. Clark & Royden 2000; Burchfiel et al. 2004). This model contrasts with the classic crustal imbrication–tectonic escape model that calls for thrust–fold belt style thickening and large-scale displacement of lithospheric blocks toward SE Asia. Both models have fundamental implications for crustal rheology, vertical coupling in the lithosphere, and Plateau formation and growth. Advocates of imbrication (e.g. Meyer et al. 1998; Chen et al. 1999; Tapponnier et al. 2001) have emphasized the importance of intracontinental subduction and large-scale thrust-fold belts in accommodating shortening and generating thick crust (Fig. 2). This classic mode of crustal shortening and accompanying lithospheric mantle subduction implies relatively brittle behaviour at substantial depth, with strain concentrated along moderately-dipping, discrete shear zones. Such thrusts/folds may be accompanied by deeply penetrating strike–slip shear zones that accommodate large fractions of collisional convergence by lateral extrusion (e.g. Tapponnier & Molnar 1976; Ratschbacher et al. 1991). In the Pamir–Tibet– Himalaya orogenic system, thickening may have occurred in steps that involved the sequential growth and related thickening/uplift of up to 2000 km sized (along strike) thrust-fold belts. While the crust thickens by folding-imbrication, the mantle decouples from the crust and subducts intra-continentally along pre-existing heterogeneities; in the case of the Pamir–Tibet–Himalaya orogenic system, these likely are the c. east-striking, Palaeozoic–Mesozoic sutures (e.g. Schwab et al. 2004). Accompanying Cenozoic magmatic belts may be related to this subduction. Plateau growth is directed toward weakly constrained margins (i.e. the Pacific subduction zones) and spares lithospheric heterogeneities; that is, the Pamir–Tibet– Himalaya system grows towards the NE and NW into Palaeozoic orogenic belts, like Qinling (NE) and Fergana/Hissar (NW), bypassing the lithospheric heterogeneity of the Tarim basin that involves cratonal crust. Pamir–Tibet–Himalaya orogen formation stepped northward across Tarim, forming the Tien Shan, mostly over the last c. 10 Ma (e.g. Sun et al. 2004). This model describes continental deformation in analogy to plate tectonics, but presumes that highly distributed deformation in the crust veils discrete deeper level mantle subduction zones. Widespread and spectacular upper crustal extension is due to orogenic collapse and lateral extrusion. The crustal flow model appeals to ductile flow of weak lower crust to achieve both Plateau uplift (e.g. Zhao & Morgan 1987) and marginal expansion (e.g. Clark & Royden 2000; Klemperer 2006). To a first order, the geodetically determined velocity field for the Pamir–Tibet–Himalaya orogenic system is similar to results from 3D crustal models that assume a viscous crust deforming above a plate-like mantle (e.g. Shen et al. 2001), suggesting that on length scales of . 100–200 km the behaviour of the Tibetan crust is similar to that of a viscous fluid. A striking outcome of the geodetic work is the observation that within parts of the Plateau margins, crustal thickening and surface uplift have occurred without significant shortening of the upper crust (e.g. Wang et al. 1998). This observation, paired with findings from INDEPTH and other surveys (e.g. Zhao et al. 1993; Nelson et al. 1996; Hacker et al. 2000; Unsworth et al. 2005) that the middle–lower crust beneath most of the Tibetan Plateau is rheologically weak led to models of crustal flow. In these models, material flows from the central Tibet Plateau toward its margins within a weak channel (e.g. Clark & Royden 2000; Klemperer 2006); the driving mechanism is