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Abstract
A considerable amount of research has focused on how and when the Tibetan plateau
formed in the wake of tectonic convergence between India and Asia. Although far less
enquiry has addressed the controls on river incision into the plateau itself, widely
accepted theory predicts that steep fluvial knick points (river reaches with very
steep gradients) in the eastern Himalayan syntaxis at the southeastern plateau margin
should erode rapidly, driving a wave of incision back into the plateau. Preservation
of the plateau edge thus presents something of a conundrum that may be resolved by
invoking either differential rock uplift matching erosional decay, or other mechanisms
for retarding bedrock river incision in this region where high stream power excludes
the potential for aridity as a simple limit to dissection of the plateau. Here we
report morphologic evidence showing that Quaternary depression of the regional equilibrium
line altitude, where long-term glacier mass gain equals mass loss, was sufficient
to repeatedly form moraine dams on major rivers: such damming substantially impeded
river incision into the southeastern edge of the Tibetan plateau through the coupled
effects of upstream impoundment and interglacial aggradation. Such glacial stabilization
of the resulting highly focused river incision centred on the Tsangpo gorge could
further contribute to initiating and accentuating a locus of rapid exhumation, known
as tectonic anaeurysm.