Eastern Himalayan syntaxis formation controlled by slab flattening and crustal shear localization

Plate corners in collisional orogens are often characterized by sharply curved orogenic zones, that is, orogenic syntaxis zones, where tectonic mountain building and surface processes vigorously interact. The formation and evolution of syntaxis zones, which are particularly well expressed in the eastern and western terminations of the Himalayan orogen, remain debated. Here, based on 3D thermomechanical surface process numerical experiments, we propose that the formation of the Eastern Himalayan syntaxis is predominantly driven by flattening of the Indian slab and shear localization in the weak Eurasian crust. Our models indicate that a northward migrating syntaxis initially forms during collision with India when the middle to lower crust in the Eurasian plate is rheologically weak beneath a proto-plateau. This syntaxis becomes a barrier for the laterally expanding plateau. The corner of the collision zone is dismembered by the localization of crustal-scale strike-slip fault zones driven by slab flattening and differential plate advance. The middle to lower crust locally flows within a narrow channel from the plateau towards the east and south along the syntaxis. These links between slab dynamics and crustal shear localization drive the structural and topographic evolution of orogenic syntaxis.