Distinguishing tectonic versus climatic forcing on landscape evolution: An example from SE Tibetan Plateau

Abstract : Distinguishing climate from tectonic forcing in shaping the Earth’s surface has been a long-standing issue in the Earth sciences. Great debate exists regarding when and how the SE Tibetan Plateau achieved its current low-relief topography, and both lateral extrusion and lower crust flow have been proposed as the dominant mechanism. Reconstruction of the exhumation history of the SE Tibetan Plateau is key to understanding these formation processes and resolving the significance of different forcing mechanisms. Here we report zircon and apatite (U-Th)/He ages from steep transects across the Lincang granite belt of the SE Tibetan Plateau. Our results reveal a two-stage exhumation history during the Cenozoic with rapid cooling phases in the late Eocene and the middle Miocene. In the late Eocene, the climate was generally dry and there is plenty of evidence for increased extrusion and upper crustal shortening. We suggest tectonic processes are responsible for the first inferred cooling. In contrast, the Asian summer monsoon precipitation increased during the middle Miocene, and we posit the middle Miocene cooling phase records a phase of rapid river incision triggered by the intensified precipitation and associated fault movements. The results are consistent with recent paleo-altimetry work in this region suggesting that the present-day topography of the SE Tibetan Plateau had been largely constructed by the late Eocene. Together, these data suggest that extrusion and/or upper crustal shortening setup the first order topography of the SE Tibetan Plateau, which was then modified by climate-triggered fluvial incision and feedbacks initiated in the middle Miocene.