Peneplain formation in southern Tibet predates India-Asia collision and plateau uplift

Hetzel, R., Dunkl, I., Haider, V., Strobl, M., von Eynatten, H., Ding, L., Frei, D.

Abstract

The uplift history of Tibet is crucial for understanding the geodynamic and paleoclimatologic evolution of Asia, however, it remains controversial whether Tibet attained its high elevation before or after India collided with Asia ~50 m.y. ago. Here we use thermochronologic and cosmogenic nuclide data from a large bedrock peneplain in southern Tibet to shed light on the timing of the uplift. The studied peneplain, which was carved into Cretaceous granitoids and Jurassic metasediments, is located in the northern Lhasa block at an altitude of ~5300 m. Thermal modeling based on (U-Th)/He ages of apatite and zircon, and apatite fission track data indicates cooling and exhumation of the granitoids between ~70 and ~55 Ma followed by a rapid decline in exhumation rate from ~300 m/Ma to ~10 m/Ma between ~55 and ~48 Ma. Since then the peneplain has been a rather stable geomorphic feature, as indicated by low local and catchment-wide erosion rates of 6–11 m/Ma and 11–16 m/Ma, respectively, which were derived from cosmogenic 10Be concentrations in bedrock, grus, and stream sediment. The prolonged phase of erosion and planation that ended ~50 Ma ago removed 3–6 km of rock from the peneplain region, which was likely accomplished by laterally migrating rivers. The lack of equivalent sediments in the northern Lhasa block and the presence of a regional unconformity in the southern Lhasa block indicate that the rivers delivered this material to the ocean. This implies that erosion and peneplanation proceeded at low elevation until India’s collision with Asia induced crustal thickening, surface uplift, and long-term preservation of the peneplain.