Minimal erosion in central Tibet since the Eocene and implications for plateau development

New apatite (U-Th)/He data from the northern Lhasa and Qiangtang terranes of central Tibet provide new insight into the thermal and tectonic evolution of the Tibetan Plateau. Samples collected away from major late Cenozoic rifts in the northern Lhasa terrane yielded apatite He ages between 54 and 49 Ma. This indicates that less than ~3 km of crust has been removed since ~50 Ma and very slow exhumation rates (~0.05 mm/yr). These findings complement recent K-feldspar 40Ar/39Ar and AFT results from the northern Lhasa terrane by J. Guynn et al. (unpublished data), indicating rapid exhumation during thrust belt development prior to ~50 Ma and minimal exhumation during the Indo-Asian collision. Findings of major Cretaceous-Eocene crustal shortening (>50%) and exhumation, followed by minimal subsequent denudation, raise the possibility of the establishment of a proto-plateau in central Tibet by Eocene time. Whereas the majority of low-temperature theromochronologic results from central Tibet record pre-Indo- Asian collision cooling, we note two exceptions. A granite in the hanging wall of a mapped Tertiary thrust fault north of the Bangong suture in the central Qiangtang terrane yielded an AHe age of 18.0 ± 0.8 Ma. One granite in the northern Lhasa terrane yielded an AFT age of 25 Ma (Guynn et al.). These results may indicate localized thrust belt reactivation during the mid-Tertiary and show the potential for future low- temperature thermochronologic studies to assess geodynamic models of the collision process. Collectively, the exhumation history of central Tibet, away from the influence of late Cenozoic rifts, contrasts sharply with that of the Lhasa region in the southern Lhasa terrane and near the modern margins of the plateau which show prominent signatures of the Indo-Asian collision. Any viable model of plateau development must explain these prominent spatial variations in exhumation history as well as the lack of a corresponding expression in the modern topography.