Late Cenozoic uplift of southeastern Tibet

Abstract

Recent field work and DEM analysis show that remnant, local areas of a low-relief land scape (or "erosion surface") are geographically continuous across the southeastern Tibetan Plateau margin and can be correlated in order to define the maximum envelope of topogra phy of the margin itself. This observation contradicts earlier notions that the low-gradient plateau margin slope (i.e. the maximum elevation of the margin) is a product of landscape dissection and reduction by fluvial incision due to the presence of major rivers which drain this portion of the plateau and plateau margins. Although initial development of the erosion surface is likely diachronous, we propose that a continuous low-relief landscape existed at low elevations prior to uplift and long-wavelength tilt of the southeastern plateau margin. The modern altitude of the erosion surface provides an excellent datum for constraining the total amount of surface uplift of the southeastern plateau margin. The long-wavelength tilt of the surface across the plateau margin without major disruption mirrors the low-gradient decrease in crustal thickness across the plateau margin, which suggests that crustal thickening has occurred in a distributed manner. Because large-magnitude compressional structures of late Cenozoic age are lacking, we propose that crustal thickening beneath the southeastern plateau margin has largely been accomplished by preferential thickening the lower crust. Perched, relict landscape remnants that reflect slow erosion, low initial elevations and slow uplift rates contrast sharply with the rapidly eroding modern river gorges that incise the surface, indicating that the modern landscape is not in equilibrium. Surface remnants are preserved because incision of the fluvial system has been largely limited to major rivers and principle tributaries, and has not yet progressed throughout the entire fluvial network.

(cont.) This "transient condition" of the landscape in southeastern Tibet reflects the initiation of rapid bedrock incision into a developing plateau margin, and the altitude of the remnant erosion surface can also be used as a datum by which to measure the total amount of erosion since the beginning of plateau uplift. 2.1 Introduction The continent-continent collision between India and Eurasia is largely responsible for creating the Tibetan Plateau, the most extensive region of elevated topography on Earth [Figure 2.1]. The development of such an anomalously high landmass has been of interest to scientists in a broad range of disciplines ranging from lithospheric dynamics to the inter action between tectonics, climate and surface processes. Studies of the Tibetan Plateau have raised several first-order questions such as: 1) how is plate convergence accommodated in the continents and what are the relative contributions of continental subduction, uniform or differential shortening in the upper and lower crust, and lateral extrusion of rigid lithospheric blocks?; 2) how do spatial (or temporal) variations in crust and mantle rheology partition deformation throughout theorogen?; 3) does the convective removal of the mantle lithosphere contribute to surface uplift and high plateau elevation? ...