Eastern Himalayan syntaxis formation controlled by slab flattening and transient crustal channel flow

Plate corners in collisional orogens are often characterised by sharply curving orogenic syntaxis zones, where plate tectonic mountain building processes and surface processes interact vigorously. The formation and evolution of these zones, which are particularly well expressed in the two margins of the Tibetan-Himalayan orogen, remain debatable and partly enigmatic. Here, based on high-resolution 3D thermomechanical-surface process numerical experiments, we propose a new unifying model for the Eastern Himalayan syntaxis formation that is predominantly driven by slab flattening and transient mid-lower crustal channel flow. Our models indicate that the northward migrating syntaxis forms when the middle to lower crust in the Eurasian plate are rheologically weak in the continental collision zone, e.g. weakened in a pre-existing proto-plateau. As soon as the syntaxis has formed, it becomes a barrier for the laterally expanding plateau. Therefore, the middle to lower crust extrudes from the plateau toward the East and South around the corner. This crustal flow occurs in a narrow channel and is enhanced by differential advance of the Indian indenter. Our models also suggest that both crustal outflow and syntaxis structure formation are controlled by intense flattening of the Indian slab, which is an overlooked process governing recent structural and topography evolution of the Himalayan orogen. The discovered links between syntaxis formation, crustal flow and slab flattening suggest that corners of collision zones may in fact determine and govern the evolution of the entire orogenic system.