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