TECTONICS LEADS-CLIMATE CONTROLS: DEFORMATION AND RAINFALL CHANGE IN THE BHUTAN HIMALAYA

We demonstrate that climate influences the tectonic style of the Himalaya in a manner that has not been shown before because the unique set of tectonic, topographic and climatic events and related data allow the quantification of the coupling between tectonics and climate. Precipitation distribution varies in the Bhutan Himalaya, which is the only segment of the orogen where this can be documented and quantified. The change in precipitation distribution was caused by the uplift of Shillong Plateau at the Miocene-Pliocene transition, and by formation of an orographic barrier to the prevailing winds transporting moisture from the Bay of Bengal toward the Himalayan. The effects are condensation and precipitation on plateau's windward side, and a consequent drop in precipitation in the lee. The decrease in rainfall in the Bhutan has led to modification of landscape and decrease in rate of surface erosion. Using apatite fission-track data we document the change in denudation rates between Late Miocene and Pliocene registered in two adjacent areas (one in the lee of the Shillong Plateau and one not). Decrease in surface erosion rates and changes in landscape have in turn caused modifications of tectonic processes in the Himalaya because of the adjustments of lithostatic stresses to changing topographic mass. In addition, we recognise low-relief relict landscapes that can be used to gauge the surface uplift rate as well as the incision rate of the modern landscape and thus separate tectonic- and climate-induced exhumation and surface uplift rates. Finally, this work may provide a field study that can compellingly test the erosionally-controlled channel extrusion model (Beaumont et al. 2001; Grujic et al. 2002).

Beaumont, C., Jamieson, R. A., Nguyen, M. H. & Lee, B., 2001. Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to focused surface denudation. Nature 414, 738-742. Grujic, D., Hollister, L. & Parrish, R., 2002. Himalayan metamorphic sequence as an orogenic channel: insight from Bhutan. Earth Planet. Sci. Lett. 198, 177-191.