Late Cenozoic Tectonic Evolution of the Northern Himalaya, China

After the India-Asia collision and the following thickening, the northern Himalaya began experiencing extensional tectonics since Eocene-Oligocene transition. Both the two-mica granite caused by thickening melting in the northern Himalayan gneiss domes and the leucogranite intruding along the STDS give the same U-Pb age of about 35Ma. This indicates a tectonic transition from thickening to extension along the northern Himalaya at this time, and the STDS might begin to be active since then, which is much earlier than the results of current studies. The thrusting and thickening before 35Ma caused the partial melting in middle-lower crust, which formed the weak root of the thickened crust or the so-called “channel flow”. This “channel flow” triggered the collapse of the orogen and the onset of the extensional tectonics of the STDS. The melts intruded along the STDS, forming the leucogranite with the older ages of around 35Ma. The STDS’s extension and the resultant thinning led to the further partial melting of the crust, which caused the larger-scale magmatism related to the emplacement of leucogranite in Miocene. The diapirism of these leucogranites formed the northern Himalayan gneiss domes which exposed the greater Himalayan crystalline complex and the shear zones of the STDS in northern Himalaya. A change in tectonic regime happened in northern Himalaya at about 13Ma, when the N-S extension of the STDS ceased and gave the way to E-W extension of the N-trending rifts. This change possibly represent the start of the eastward channel flowing, or a change of the convergent velocity between India and Asia.