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Collision between an undeformable indenter and a viscous region generates isostatically compensated topography by solid-state flow. We model this process numerically, using a finite element scheme. The slope, amplitude and symmetry of the topographic signal depend on the indenter size and the Argand number of the viscous region, a dimensionless ratio of gravitational body forces to viscous forces. When applied to convergent continental settings, these scaling rules provide estimates of the position of an indenter at depth and the mechanical properties of the viscous region, especially effective viscosity. In Tibet, forward modelling suggests that some elevated, low relief topography within the northern plateau may be attributed to lower crustal flow, stimulated by a crustal indenter, possibly Indian lithosphere. The best-fit model constrains the northernmost limit of this indenter to 33.7°N and the maximum effective viscosity of Eurasian middle and lower crust to Pa s. [ABSTRACT FROM AUTHOR]

A viscous region with deformable boundaries is used to model simultaneous crustal flow and lithospheric coupling in northern Tibet. This model suggests that (1) the deformation of northern Tibet is different from that of southern Tibet because of structural differences between the regions; (2) the viscosity contrast between the crust and the mantle lithosphere is relatively small beneath northern Tibet; and (3) crustal flow is compatible with crust-mantle coupling under these conditions. [ABSTRACT FROM AUTHOR]

Cosmic ray exposure ages for formation of perched alluvial terraces and for abandonment of an alluvial/debris-flow fan on opposite sides of the Tangtse Valley (the outflow at the northwest end of Lake Panggong, which is in the Karakorum Range of Western Tibet) provide evidence of a humid period at ∼11.5 to ∼7 ka. This is consistent with other regional records and supports a controversial chronology for the sedimentary record from Lake Panggong. Fan abandonment appears to have occurred at ∼11.5 ka as the climate presumably became more humid in response to initiation of enhanced monsoonal circulation, consistent with previously reported onset of humid conditions in a sedimentary record from the easternmost basin of the lake. In contrast, the terraces did not form until about 7 ka with downcutting of the transverse valley by overflow from Lake Panggong. This lag can be explained in light of the bathymetry of Lake Panggong; the modern lake consists of five basins separated by shallow sills, and outflow through the Tangtse Valley could not occur until the water level was substantially above its present level. The easternmost basin receives the inflow of the major rivers feeding the lake, making its chemistry highly sensitive to changes in precipitation. However, sustained wet conditions are required to fill the basins to the west to the sill depth necessary for overflow through the Tangtse Valley and resultant downcutting and terrace formation. [Copyright &y& Elsevier]