Mountain Building, Erosion, and the Seismic Cycle in the Nepal Himalaya

The Himalaya is the most impressive example on earth of an active collisional orogen. It combines rapid crustal shortening and thickening, intense denudation driven by the monsoon climate, and frequent very large earthquakes along an incomparably long and high mountain arc. It has therefore been the focus of a variety of investigations that have addressed various aspects of mountain building on various timescales. Geological and geophysical studies give some idea of the structure of the range and physical properties at depth. The long-term geological history of the range, over say several millions to a few tens of millions of years, has been documented by structural, thermobarometric, and thermochronological studies. Morphotectonic investigations have revealed its evolution over several thousands or tens of thousands of years; and geodetic measurements and seismological monitoring have revealed the pattern of strain and stress built-up over several years. This chapter is an attempt to show that the results of these investigations can be assembled into a simple and coherent picture of the structure and evolution of the range. The author also intends to illustrate the interplay between these various processes operating at different timescales. One important example of processes that interact via feedback mechanisms is particularly clear in the Himalaya: the thermal structure of the range, which is a result of the long-term crustal deformation and pattern of exhumation, governs, through its influence on rheology, the pattern of deformation as well as the seismic behavior of the range-bounding thrust fault. In this chapter the key role played by surface processes is emphasized. These processes have carved morphologic features that can be used to deduce vertical displacements. They also have generated the molasse deposits that have filled the subsiding foreland basin, providing a record of mountain building. In addition, they have participated actively in the evolution of the ADVANCES IN GEOPHYSICS, VOL. 46