LONG AND SHORT-TERM EROSION ON THE EASTERN MARGIN OF THE TIBETAN PLATEAU, A TRANSIENT LANDSCAPE

The eastern margin of the Tibetan Plateau is one of the world's broadest and most dramatic transient landscapes, characterized by a regionally persistent, uplifted, low-relief relict landscape that has been deeply dissected by major rivers and their tributaries. Here, we present results for short and long-term rates of erosion throughout the region derived from cosmogenic radio nuclides and low-temperature thermochronology.

Measured concentrations of 10Be in quartz extracted from stream sediments indicate that short-term (10³–10⁵ yr) basin-wide erosion rates for 35 small catchments (<100 km²) range by two orders of magnitude, from ~0.01-0.03 mm/yr to >1 mm/yr. These erosion rates help us constrain regional patterns of erosion that are crucial to the study of transient river response to Late Cenozoic uplift of the Tibetan Plateau. We also to use this data to: (1) solidify our quantitative understanding of river response to tectonic forcing that can be ported into other landscapes as a refined tool, and (2) provide an important shorter-term complement to the long-term exhumation rate estimates from low-temperature thermochronometers.

Previous thermochronologic studies show that rates of rock cooling on the eastern margin increased dramatically between 9 and 13 million years ago, suggesting that uplift and major river incision began at that time. These data yield estimates of long-term (10⁶–10⁷ yr) exhumation for the Dadu river gorge between 0.25 and 0.5 mm/yr. New bedrock samples collected fall 2003 and 2004 within the Yangtze, Yalong and Dadu river gorges allow us to use (U-Th)/He thermochronology in Apatites and Zircons to further constrain incision and long-term exhumation. Age-elevation relationships from all three gorges show consistent regional long-term exhumation between 0.3 and 0.4 mm/yr. Data from the Dadu transect tightly constrain the initiation of accelerated erosion into a slowly eroding landscape at ~10 my. Data from the Yalong transect, meanwhile, indicate there has been more total dissection than nearby in the Dadu, suggesting that the plateau surface was initially higher and that accelerated exhumation into a slowly eroding landscape started earlier in the Yalong than on the Dadu.