TECTONICS, VOL. 21, NO. 1, 10.1029/2000TC001246, 2002

1. Introduction

[2] The question of when the Tibetan Plateau attained its current elevation, and how rapidly it did so, is at the forefront of debate over a wide range of first-order problems in continental tectonics. For example, the timing of plateau development places important constraints on the mechanism(s) by which convergence between India and Eurasia has been accommodated [Tapponnier et al., 1982; England and Houseman, 1986], while the rates of uplift of the plateau surface have major implications for the dynamics of plateau formation and the relative significance of deformational processes in the crust and mantle [England and Molnar, 1990]. The development of high elevation in central Asia has been linked to the onset and intensification of the Asian monsoon [Molnar et al., 1993] and perhaps to the drawdown of atmospheric CO₂ via enhanced silicate weathering [Raymo et al., 1988]. Thus the elevation history of the Tibetan Plateau plays a central role in the continuing debate over large-scale linkages between tectonics and global climate change.

[3] Despite the importance of the timing and rate of plateau development, there is little direct evidence for when and how rapidly uplift of the Tibetan plateau occurred [Molnar et al., 1993]. Current estimates for when the plateau attained its current elevation are based on inferences regarding the initiation age of E-W extension in central and southern Tibet [Harrison et al., 1992; Coleman and Hodges, 1995] or of bimodal, potassic volcanism in north central Tibet [Turner et al., 1993, 1996; Chung et al., 1998; Hacker et al., 2000]. Both of these tectonic events are linked to a model for uplift of the plateau surface involving convective removal of a thickened continental lithosphere [Houseman et al., 1981; Molnar et al., 1993; Lenardic and Kaula, 1995]. Although simple geodynamic models linking surface uplift, potential energy increase in the lithosphere, extension, and volcanism are conceptually compelling, a temporal and genetic link between extension, volcanism, and surface uplift remains to be demonstrated. Furthermore, the cause of E-W extension on the high plateau is a matter of active debate; it may be related to a number of processes including strain partitioning during oblique convergence [McCaffrey and Nabelek, 1998], spreading of the Himalayan arc [Seeber and Pecher, 1998], eastward extrusion of Tibet [Armijo et al., 1986], or thickening of crust with a depth-dependent rheology [Royden, 1996].

[4] Rather than envisioning plateau uplift as a spatially and temporally uniform event, a second class of models holds that the plateau grew in lateral extent (primarily to the east) with continuing convergence between India and Eurasia [England and Houseman, 1986, 1988; Royden, 1996]. The deformational history of the Qaidam-Qilian Shan region of Qinghai Province seems to support the inference that shortening and crustal thickening has stepped northward with time [Meyer et al., 1998]. However, this region is characterized by high mountain ranges and intermontane basins, and it may have a tectonic and topographic history independent of the plateau to the south. Although it has been suggested recently that portions of southern Tibet had already attained high elevations prior to the Indo-Asian collision [Murphy et al., 1997], there is virtually no information on how surface uplift of the plateau varied in space and time. To begin to comprehensively address this question, one must first determine when the present margins of the plateau developed.

Figure 1. Regional morphology of the eastern margin of the Tibetan Plateau adjacent to the Sichuan Basin. (a) Index shaded relief image of the Indo-Asian collision zone. Box shows approximate location of the study area. (b) Shaded relief image of the eastern margin. Note the deeply dissected, high-relief escarpment along the margin. Major rivers draining the escarpment are overlain in bold and serve to illustrate the approximate location of the drainage divide. Locations of swath-averaged topographic profiles are shown as rectangular boxes. (c and d) Swath-averaged topography across the eastern margin in the Min Shan and Longmen Shan regions, respectively. Topography was extracted in a 100 km wide, rectangular swath and projected onto a vertical plane located at the midline of the swath. Shown are maximum, mean, and minimum elevations for each swath. Note the remarkably steep topographic front along the Sichuan Basin, and the somewhat more moderate front north of the basin.

[5] In this contribution we investigate the rates and pattern of Cenozoic denudation along the eastern margin of the Tibetan Plateau adjacent to the Sichuan Basin (Figure 1). The topographic margin of the plateau in this region is a 4–5 km high escarpment defined by steep, consequent drainages eroding headward into an unincised plateau surface of low relief. The development of this high topography presents something of an enigma in that shortening of the upper crust during the Cenozoic appears to be rather limited [Burchfiel et al., 1995]. This observation, coupled with the absence of a Cenozoic foredeep in the Sichuan Basin and slow geodetic rates of shortening between the plateau and the Sichuan Basin [e.g., Chen et al., 2000], led Royden et al. [1997] to suggest that the development of the plateau in eastern Tibet was the consequence of thickening and flow within a weak lower crust. Thus, in addition to addressing the regional question of when the eastern plateau developed, estimates of the magnitude and distribution of exhumation across this margin will place first-order constraints on the degree of upper crustal deformation in this region.

Tectonic Setting of the Longmen Shan Region - 2000TC001246

Citation: E. Kirby, P. W. Reiners, M. A. Krol, K. X. Whipple, K. V. Hodges, K. A. Farley, W. Tang, and Z. Chen, Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from ⁴⁰Ar/³⁹Ar and (U-Th)/He thermochronology, Tectonics, 21(1), 10.1029/2000TC001246, 2002.