A MECHANISM FOR SYN-COLLISIONAL ROCK EXHUMATION AND ASSOCIATED NORMAL FAULTING - RESULTS FROM PHYSICAL MODELING

Properly scaled physical modelling of the subduction of continental lithosphere is performed with a three-layer lithospheric model. The model includes a strong upper crust, a weak ductile lower crust, and a strong mantle part. The lithosphere is underlain by a low-viscosity asthenosphere. Subduction is produced by a piston (push force) and the pull force from the mantle lithospheric layer which is slightly denser than the asthenosphere. The results of the modelling are tested on the example of the Himalayas. In the experiments all lithospheric layers subduct into the mantle to a depth corresponding to 200-300 km in nature until the upper crustal layer fails in front of the subduction zone, forming a first major thrust fault (the MCT in our interpretation of the Himalayas). Underthrusting along this fault increases relief, which upon reaching a height corresponding to several kilometres in nature, is removed mechanically by a blade (the blade is moved horizontally back and forth throughout the experiment). This 'erosional' unloading mechanism causes the previously subducted segment of the buoyant upper crust to slide upward over the weak lower crust at a rate comparable to the subduction rate. The rise of the crustal slice accelerates displacement along the crustal thrust, while producing a normal sense motion and exhumation of the material from great depth (up to 40-50 km depending on the model. parameters) along the upper surface of the slice. The rapid uplift of the crust then ceases, and the orogenesis occurs further due to continuing underthrusting along the first major thrust. At a certain stage the crust fails again, producing a new major crustal thrust (the MBT in the Himalayas).