Kyle Landry

The ongoing convergence between India and Eurasia since continent-continent collision occurred ~ 55 My ago, formed the Himalayan orogen, the highest mountain range on Earth. Remarkably continuous tectonostratigraphic units and structures along strike characterize its 2000 km-long range front. The topographic uplift of the range induced perturbations of atmospheric circulations patterns sometime between ~20-35 Ma and led to the establishment of the Indian Summer Monsoon (ISM) along the southern flank of the Himalaya. The monsoon is responsible for about 80% of the annual rainfall along the range front and results from the condensation of wet air derived from the Bay of Bengal to the South and travelling northward before being blocked by the Himalayan Mountains. Consequently, strong interactions between tectonic and climatic processes have likely conditioned the exhumational and landscape evolution of the range in the Late Tertiary. Furthermore, the only raised topography outboard the Himalayan range front, the Shillong plateau, is located south of Bhutan on the ISM trajectory and was uplifted in the Pliocene. The Shillong plateau concentrates 30-40% on monsoonal rainfall along its southern slope and consequently central and eastern Bhutan receives about half of the rainfall as the Sikkim Himalaya situated further west. The objective of this study is to quantify Late Tertiary potential changes of exhumation rate in the Trumsing La area (central Bhutan) located in the rain shadow of the Shillong plateau. Four bedrock samples collected along a vertical profile have been dated using (U-Th-Sm)/He thermochronology on apatite crystals (AHe). The effective closure temperature of this thermochronometric system is about 75 ^oC and helps to derive exhumation rates of the upper 1-2 km of the crust. Published apatite fission-track results, a method providing information on the exhumation of deeper crustal levels (closure temperature is 120 ^oC), obtained from the same samples yielded an exhumation rate of 0.6 mm/yr in the Late Miocene. The AHe results yielded an exhumation rate of 1.9mm/yr at the Miocene-Pliocene boundary. Despite the limitations of our experiment, such as the sensitivity of the AHe system to advection of shallow isotherms due to topographic changes, our results help define exhumation rates at the Miocene-Pliocene boundary. Along with numerical data, the major limitations of our experiment have given us information on how to refine our sampling strategy for future experiments so that if this study is repeated, a denser sample interval can be taken, along with a larger amount of grains per sample.

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Pages: 91
Supervisor: Isabelle Coutand