iPlex Lunch spring-2016

Landscape evolution near the southeastern margin of the Tibetan Plateau: Surface uplift by lower-crustal flow?

Surface uplift on the diffuse southeastern margin of the Tibetan Plateau may result from flow of low-viscosity lower crust, rather than crustal shortening. In the absence of structural indicators, positive evidence for lower-crustal flow is limited. As a result, the incision of river gorges has become important proxy evidence for surface uplift. An ancient, low-relief landscape has previously served as a critical datum for measuring incision. Here, we investigate landscape evolution on the margin of the Plateau using a combination of geochronology and morphology. First, we present terrestrial cosmogenic nuclide (TCN) burial ages and low-temperature thermochronology from near the First Bend of the Yangtze River. We show that the rate of river incision likely declined near the end of the Miocene. This decline is not consistent with existing models of lower crustal flow, which suggest ongoing uplift and incision. This result is consistent with recent stable isotope paleoaltimetry, which indicates that the First Bend has persisted near its present elevation since at least middle Miocene time. Next, we present TCN erosion rates and morphometric data from four study sites extending ~500 km southeast of the Plateau margin. The results are complex, variously indicating river capture, local fault displacement, and regional surface uplift as the dominant drivers of river morphology. Locally, erosion rates and normalized steepness indices vary systematically with the mapped spatial extent of the paleolandscape. However, the landscape response to apparent paleolandscape uplift is remarkably muted in comparison with previous work from higher on the Plateau margin. In conclusion, we emphasize the complexity of landscape evolution southeast of the plateau margin. The mapped paleolandscape may not be a unique feature of regional extent. Detailed examination of landscape elements does not support the argument for lower-crustal flow as the primary driver of surface uplift, at least as currently formulated.