WHAT CONTROLS THE GROWTH AND MORPHOLOGY OF THE HIMALAYAN OROGEN?
Collisional orogens are characterized by spatial and temporal superposition of superstructure, infrastructure and orogenic front units that have contrasting metamorphic and deformational histories and styles. This raises questions about how different tectonic styles interact dynamically in space and time. Solving this problem is rendered intricate by strong dynamic coupling between tectonics and climate on million years-timescales.
Structural and metamorphic data and numerical experiments indicate that the mid crust of large, hot collisional orogens deforms according to the channel flow tectonics. Facilitated probably by the very onset of the monsoonal circulation, and increased erosion along the southern slopes of the ancient Himalaya the geologically rapid exhumation of the crustal channel may have promoted formation of a ramp in the incoming Indian crust. Insertion of this colder and stronger Indian crust into the weaker or hotter mid- and lower-crustal orogenic channel underneath southern Tibet may have caused rapid exhumation of rocks from even greater depths. The final steps of exhumation of these rocks and the formation of the landscape in the Himalaya were modulated by synoptic and local climate changes.
Within the active orogenic wedge of the Himalaya, which involves only the molasse-like foreland sediments, our results indicate a distinct west-to-east increase in shortening amount and strain rate, which correlate with plate convergence rates, whereas the wedge morphology correlates inversely with the rainfall amounts. Since the eastward increase in convergence rate would cause higher rate of material accretion and thus a wider wedge, and since the lithology in the Himalayan wedge is similar along the arc, the erodibility and mechanical properties of rocks in the wedge are similar, leaving the erosion, in particular the precipitation rate and the specific stream power as the principal control of the present-day Himalayan wedge morphology.