Topographic ooze: Building the eastern margin of Tibet by lower crustal flow

Topography extracted from swath profiles along the northern, southern, and eastern margins of the Tibetan Plateau show two end-member morphologies: st eep, abrupt margins and long-wavelength, low-gradient margins. Because the lack of significant upper crustal shortening across much of the eastern pla teau margin implies that the crustal thickening occurs mainly in the deep c rust, we compare regional topographic gradients surrounding the plateau to model results for flux of a Newtonian fluid through a lower crustal channel of uniform thickness. For an assumed 15-km-thick channel, we estimate a vi scosity for the lower crust of 10(18) Pa.s beneath the low-gradient margins , 10(21) Pa.s beneath the steep margins, and an upperbound of 10(16) Pa.s b eneath the plateau. These results indicate that the large-scale morphology of the eastern plateau reflects fluid flow within the underlying crust; cru stal material flows around the strong crust of the Sichuan and Tarim Basins , creating broad, gentle margins, and "piles up" behind the basins creating narrow, steep margins. These results imply that this portion of the Eurasi an crust was heterogeneous, but largely weak, even prior to construction of the Tibetan Plateau.