The role of crustal strength variations in shaping orogenic plateaus, with application to Tibet

[1] The Tibetan Plateau is the type example of a large orogenic plateau formed as a result of continent-continent collision. The morphology of the plateau and its margins suggests that preexisting variations in crustal strength have influenced the growth of the plateau. We have developed a three-dimensional numerical model of deformation in a viscous crust in order to investigate the effects of lateral heterogeneities on plateau growth. The model includes a two layer crust and allows for lateral variation of viscosity in both the upper and lower layers. Model results indicate that crustal strength variations have a dramatic effect on the morphology and dynamics of a developing plateau. A region of strong crust is characterized by a very steep plateau margin that propagates extremely slowly, does not accommodate significant shortening strain, and is subparallel to local upper and lower crustal velocities. A weak crustal region develops a gently sloping margin; uplift propagates rapidly across the weak zone, and crustal material within the plateau is diverted toward the low-strength region. With a relatively simple distribution of strength variations, corresponding to strong Tarim and Sichuan Basin crust and a weak southeastern corner, our model produces a plateau with many similarities to the Tibetan Plateau, including the overall morphology, rotation around the eastern syntaxis, and E–W extension. Analysis of model results suggests that E–W extension of the central plateau may be related to the rapid eastward flow of crustal material into a weak zone in the SE corner without significant change in plateau elevation.

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