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The margins bordering the Tibetan Plateau show two end-member morphologies in topographic gradient: steep margins and low-gradient margins. To investigate the formation mechanism of convergent intracontinental plateau margins, we conduct 2D numerical experiments to simulate crustal deformation process across plateau margins. Our numerical experiments demonstrate that heterogeneities in crustal rheological strength control significantly the formation of plateau margins when subjected to crustal convergence. A very steep margin is the result of crustal convergence between plateau with weak lower crust and foreland basin with strong lower crust. By contrast, a low-gradient margin could result from crustal convergence between plateau and foreland with less strength contrast. This finding suggests that the diversity in topographic gradient along the Tibetan Plateau borders reflects heterogeneities in crustal rheological strength across the plateau margins. Steep gradient at the margins indicate large crustal rheological strength contrasts between the weak ductile lower crust of the Tibetan Plateau and its strong surrounding foreland basins, like the Sichuan Basin, the Tarim Basin and the Qaidam Basin. Beneath these steep margins the horizontal flow of the Tibetan ductile lower crust is inhibited and forced to extrude to support escarpments. Low-gradient at the margins indicate less crustal strength variations between the plateau and outer forelands, like at the northeastern and southeastern margins, where they might be outlets for the weak ductile Tibetan lower crust to flow away from the plateau. [ABSTRACT FROM AUTHOR]

The eastern margin of the Tibetan Plateau, the Longmen Shan, has triggered a vigorous debate to explain the uplift and maintaining of its steep and high topography. Several mechanical and kinematic models have been previously developed in order to investigate the tectonic and erosional control on the formation mechanism of the Longmen Shan. But the contribution of crustal shortening has never been taken into account due to the lack of significant surface convergence rate (about 3 mm/a) across the Longmen Shan range. We investigate this question through 2D finite element modelling assuming visco-elasticity rheology. The numerical results are constrained with geological and geophysical observations across the Longmen Shan range. We find that even low rate of crustal shortening contributes significantly to the uplift of the Longmen Shan. This contribution has been greatly underestimated in previous studies. The viscosity contrast between the Tibetan Plateau lower crust and the Yangtze crust crucially controls the uplift of the Longmen Shan due to crustal shortening. The ductile Tibetan lower crust thickens to accommodate most of crust shortening, as the eastward movement of the Tibetan Plateau is obstructed by the mechanical strong and stable Yangtze craton. The uplift rate contrast between the Tibetan Plateau and the Sichuan causes a steep uplift rate gradient to produce and maintain the Longmen Shan steep and high topography in the time scale of several million years. Our results support a possible explanation for the paradox of the high topography and low convergence rate at the Longmen Shan range. [ABSTRACT FROM AUTHOR]