Tectonics of the northern Himalaya since the India–Asia collision

Abstract

The India–Asia collision resulted in the construction of the vast Himalayan orogen. The northern Himalayan domain began to experience extensional tectonics since Eocene–Oligocene transition (EOT) when the Himalayan orogeny was still in progress. Major structures generated by the extension in the northern Himalaya include the south Tibet detachment system (STDS), the north Himalayan gneiss domes (NHGD), and the north–south trending rifts (NSTR). The earliest emplacement of syn-deformational leucogranite at ~36–32Ma along the STDS marks the initial transformation from thickening to thinning in the northern Himalayan domain at EOT. The thickening before EOT caused crustal partial melting, which formed the weak root of the thickened orogen or the so-called “channel flow”. This “channel flow” triggered the collapse of the orogen, extrusion of the greater Himalayan crystalline complex (GHC), and the onset of extensional tectonics of the STDS. The similarities in tectono-thermal history and geochemistry of rocks between the STDS and NHGD suggest that the formation of the NHGD has a direct relationship to the activity of the STDS. The extension of STDS and the resultant thinning caused further partial melting of the crust, leading to the larger-scale emplacement of leucogranite during Miocene (27–13Ma). Diapirism of these plutons shaped the domes in NHGD, exposing the GHC and the shear zones of the STDS in the northern Himalaya. In Gyirong, another tectonic transform from extension to shortening occurred after ~18.7Ma. In Dinggye region, the STDS was offset by the NSTR, culminating the deformation at ~13Ma when the NSTR began to be active. This indicates another tectonic transform in the northern Himalaya at ~13Ma, when the N–S extension of the STDS ceased and gave way to E–W extension of the NSTR, marking the end of the peak granitic magmatism in northern Himalaya. Multiple episodes of deformation in the major structures, such as the STDS, MCT and NSTR, and superposition of different structures indicate a multiphase orogenic process in the Himalayas, in which mountain building and collapse occurred alternately, with the formation of NSTR and conjugate shear zones in Tibet. This tectonic scenario was possibly controlled by the India–Asia convergent rate. We propose that the extension was an integral part of the orogenic process rather than a simple marker of the culmination of the orogeny.