PRESERVATION OF A LOW-RELIEF, REGIONALLY CONTINUOUS EROSION SURFACE IN SOUTHEASTERN TIBET: EVIDENCE FOR THE TRANSIENT CONDITION OF THE SOUTHEASTERN PLATEAU MARGIN

Recent field work and DEM analysis show that remnant, local areas of a low-relief landscape (or "erosion surface") are geographically continuous along the southeastern Tibetan Plateau margin and can be correlated in order to define the maximum envelope of topography of the margin itself. This observation contradicts earlier notions that the low-gradient plateau margin slope is a landscape dominated by dissection and reduction by fluvial incision. Although initial development of the surface is likely diachronous, we propose that a regional, low-relief landscape existed at low elevations prior to uplift and long-wavelength tilt of the southeastern plateau margin. Preservation of perched, relict landscape remnants that reflect slow erosion, low initial elevations and slow uplift rates contrast with the rapidly eroding modern river gorges that incise the surface, indicating that the modern landscape is not in equilibrium. Surface remnants are preserved because incision of the fluvial system has been largely limited to major rivers and principle tributaries, and has not yet progressed throughout the entire fluvial network.

The "transient condition" of the landscape in southeastern Tibet may reflect changes in uplift (baselevel) and/or climate. Evidence that the regional fluvial system is antecedent to margin uplift suggests that initial bedrock river incision into the erosion surface is related to uplift and that the timing of river incision can be used to constrain the initiation of uplift. Results from elevation transects of apatite (U/Th)/He thermochronology indicate that uplift along the highest portion of the southeastern plateau margin began by late Miocene time.

The modern altitude of this relict surface also provides an excellent datum for constraining the broad surface uplift and long-wavelength tilt of the southeastern plateau margin. The absence of major, late Cenozoic-age shortening structures in the upper crust coupled with the long-wavelength uplift pattern imply that the development of the present-day topography of the southeastern plateau margin has largely been accomplished by preferentially thickening the lower crust through pressure-driven flow of weak, lower crustal material evacuated from central Tibet into the eastern foreland.