LAUSR

Abstract Continental collision between the Indian and Eurasian plates in the past ~55 ± 5 Ma has led to the development of the Tibetan Plateau, where nearly doubled crustal thickness indicates intense lithospheric thickening during the collision. On the other hand, geological evidence indicates significant lateral extrusion of Tibetan materials around the edges of the plateau, especially around the Eastern Himalaya Syntaxis. Two end-member models, “pure shear thickening” and “tectonic escape”, emphasize respectively the vertical and lateral deformation of Tibetan lithosphere during the collision. However, their competing roles in accommodating the Tibetan continental collision remain debated. Here, we developed a series of 3-D high-resolution numerical models to systematically study strain partitioning between crustal thickening and lateral extrusion during the collision. The model focuses on eastern Tibetan Plateau, where most of the escaping tectonics occurred. The model results indicate that the amount of crustal thickening is generally several times higher than that of lateral extrusion in accommodating the lithosphere shortening. The partitioning between crustal thickening and lateral extrusion depends on the rheological contrast between the Tibetan and neighboring blocks, whereas the relic suture zones within the Tibetan lithosphere do not control the strain partitioning but affect the morphology. The southward motion of the Indochina block, resulting from extension caused by trench retreating, has major impacts on the development of large-scale lateral extrusion tectonics. The vertical variation of lithospheric rheology leads to faster motion of the crust than the mantle lithosphere in the extruding block, hence mechanical decoupling between the extruding crust and lithospheric mantle.