High times on the Tibetan Plateau: Paleoelevation of the Thakkhola graben, Nepal

Timing of surface uplift of the Tibetan Plateau has received much attention because of proposed links between high topography and change in regional and global climate, southern Asian paleoecology, and ocean chemistry (Quade et al., 1989; Edmond, 1992; Prell and Kutzbach, 1992; Raymo and Ruddiman, 1992; Molnar et al., 1993; Derry and France-Lanord, 1996). However, the actual vintage of high elevation of the Tibetan Plateau is undocumented. A Pliocene-Quaternary age of uplift has been inferred on the basis of fossil plants from Tibet, the nearest modern relatives of which grow at low elevations (Xu, 1981; Li, 1995). A suggested late Miocene age of uplift (Pan and Kidd, 1992; Coleman and Hodges, 1995; Harrison et al., 1995) assumes that east-west extensional faults and basins of late Miocene age in the southern part of the Tibetan Plateau developed in response to gravitational collapse of a thickened, high plateau (Molnar and Tapponnier, 1978; Armijo et al., 1986; Harrison et al., 1992; Molnar et al., 1993). In this view, basins are assumed to be symptomatic of high elevation, and their ages then give the timing of the uplift. However, McCaffrey and Nabelek (1998) and Seeber and Pêcher (1998) challenged the link between high elevation and east-west extension, pointing out that kinematic models predict extension parallel to the arcuate Himalayan belt and do not require high elevation to explain such extension. Previous attempts to understand the elevation history in the southern Tibetan Plateau lacked information from basin deposits. We present oxygen isotopic data from carbonate rocks in the Thakkhola graben as a proxy of relative change in paleoelevation of the southern Tibetan Plateau. Age constraints are provided by new magnetostratigraphic and carbon isotopic data. The Thakkhola graben is located in northcentral Nepal between the South Tibetan detachment system and the Indus suture zone (Fig. 1A). The South Tibetan detachment system is a lowangle, north-dipping, normal fault system, located just north of the highest part of the Himalaya. The Indus suture zone is the boundary between Indian and Asian crust. Mesozoic Tethyan series rocks are exposed in the eastern footwall of Thakkhola graben and Paleozoic Tethyan series rocks are in the western footwall (Colchen et al., 1986) (Fig. 1B). The basin contains two depositional sequences, separated by an angular unconformity: the Tetang Formation, to 230 m thick; and the overlying Thakkhola Formation, to 720 m thick (Fort et al., 1982; Garzione et al., 1999) (Fig. 1B). The Tetang Formation crops out in the