Dynamic Processes in Extensional and Compressional Settings - Mountain Building: From Earthquakes to Geological Deformation

This chapter describes the processes at work in the development of collisional orogens, with some focus on the Nepal Himalaya. It is shown that the range's topography, structure, exhumation history, and metamorphic organization can be understood in the light of active tectonic processes. Current deformation across the central Nepal Himalaya is indeed well documented. Geodetic and geomorphologic investigations indicate that the orogenic wedge is thrust up at about 20mmyr^-1 along a major intracrustal detachment fault, the Main Himalayan Thrust fault (MHT), with little internal shortening of the wedge itself. Slip along the MHT is aseismic at depth deeper than about 15-20km, probably because of a transition from stick-slip to stable sliding as temperature gets higher than about 250-350°C. At shallower depth, motion along the MHT results primarily from recurrent large magnitude earthquakes. The convergence rate across the Himalaya has not changed much over the last ∼15My, but the geological structure of the range indicates that the MHT has not been a stationary feature, however. The MHT must have migrated into the footwall allowing for a transfer of mass from the underthrusting Indian basement to the overthrusting Himalayan wedge, with the development of a duplex system at midcrustal depth. As a result, the orogenic wedge has grown primarily by underplating rather than by frontal accretion. This model explains well available thermometric data, in particular the so-called inverted thermal gradient, and thermochronological data spanning the last ∼15My.