INVESTIGATING MOUNTAIN BUILDING PROCESSES: A REVIEW OF METHODS AND OUTSTANDING ISSUES

A variety of techniques (geodesy, geomorphology, thermochronology, paleomagnetism and structural geology) are now available to investigate the mechanics of crustal deformation. They generally differ in the timespan they typically cover but also in the very nature of the physical quantities that are measured. As a result, their analysis requires some physical models to insure that the data are interpreted in view of a physically plausible and internally consistent framework. In the presentation, I will describe what we have learned from applying these techniques to some active orogens, with some focus on the Nepal Himalaya. I will discuss some models of mountain building in the Himalaya over the last 15Ma which was derived from the inversion of the available data covering that time period. This particular example demonstrates the benefit, or rather the necessity, of using physical modeling to analyze the various types of dataset which have been collected. Based on this particular example I will then discuss some outstanding issues regarding fault mechanics and the development of fold-and-thrust belts and orogens: What is the fraction of the geodetic strain that is elastic and which is released on active faults? What is the partition between seismic and aseismic deformation? What is the influence of surface processes on crustal deformation? How important is the interplay between deformation and the thermal evolution of the crust? What is the best rheological model for the continental crust?