Extensional collapse of the Himalayan orogen in the Late Miocene.

The collision of the Indian and Eurasian plates ~ 55 Ma formed the Himalaya, one of the youngest mountain belts. This convergence led to two significant metamorphic stages: M1, which occurs under high pressure and low temperature in a thick crust, and M2, resulting from crustal thinning in a high-temperature, low-pressure environment, evolved the gneissic domes. This study provides the first apatite fission track (AFT) and zircon fission track (ZFT) thermochronological record from one of such gneissic domes in the NW Himalaya viz., the Gianbul Dome (GD). The dome is bounded by two extensional shear zones, namely the South Tibetan Detachment System (STDS) dipping towards NE and the Khanjar Shear Zone (KSZ) dipping towards SW.  The AFT cooling ages range from 14.2 ± 1.2 to 5.7 ± 1.1 Ma, and ZFT ages range from 22.8 ± 2.2 to 14.6 ± 0.9 Ma. The ZFT ages remain almost constant across the dome, suggesting thermal relaxation during this period, while the AFT ages are young towards the extensional shear zones of KSZ and STDS. The fission-track data, in combination with the published Ar-Ar and (U-Th)/He cooling ages, has been modeled using a thermo-kinematic inverse and forward model to analyze the processes that led to the exhumation of the dome. Various scenarios like river incision, lithology, deformation along faults like Main Himalayan Thrust, Main Central Thrust, STDS, glacier control, and erosion control over exhumation have been tested. Our results suggest that the extension of normal fault is the primary mechanism for the exhumation of the GD. The extension happened in two phases: (a) during the initial normal sense movement along the STDS when the reverse sense of shear was switched to the usual sense of shear during the early Miocene, and (b) during the Late Miocene. The initial phase of extension is a well-recognized phenomenon in the Himalayan orogen that has been explained through models like channel flow or ductile wedge extrusion. However, the first report of extensional activity along the STDS during the Late Miocene allows us to test whether it is a local phenomenon or a regional event that happened in the brittle stage. Thus, we compiled all the published geochronological and thermochronological data of all the prevailing gneissic domes in the Himalayas from west to east and ran the 3D thermokinematic model to assess the exhumation path of the rocks and brittle stage deformation history. Our results suggest that two phases of extension happened in the entire arc of the Himalayan orogen. The first phase facilitated the southwest migration of ductile materials of rocks from mid-crustal depths accompanying the extension because of gravitation, favoring the channel flow concept. The second phase of extensional collapse happened during ~7-3 Ma ago in the brittle stage. We hypothesize that a drop in gravitational potential energy led to the reactivation of extensional faults along the Himalayan arc. Thus, we propose that extensional collapse in the collisional mountain belts is a cyclic phenomenon that happens to attain a stable, steady state of the orogens.