Exhumation and Uplift of the Shillong Plateau and its Influence on the Eastern Himalayas: new Constraints From Apatite and Zircon (U-Th-[Sm])/He and Apatite Fission Track Analyses

The Shillong plateau is the only raised topography in the foreland of the Himalayas. Located on the trajectory of the Indian Summer Monsoon (ISM), the plateau perturbs the regional distribution of precipitation. As such the Shillong plateau-eastern Himalaya-ISM is a unique system to quantify the couplings between climate, tectonics and erosion. A change in long-term erosion rates along-strike the Bhutan Himalaya was recently attributed to a climatic modulation due to the uplift of the Shillong plateau. To test this interpretation, it is essential to constrain the timing and rate at which the plateau was uplifted and the amount of partitioning of the India-Asia convergence into the plateau. We used apatite and zircon (U-Th-[Sm])/He and apatite fission-track analyses to unravel the thermal histories of thirteen basement samples collected along a N-S transect across the central Shillong plateau. We find that 1) the exhumation of the plateau began at least 9-15 Ma ago, 2) its surface uplift was chronologically decoupled from its exhumation and started after ~3-4 Ma at rates of 0.4-0.53 mm/yr, 3) the long- term horizontal shortening rate accommodated by the plateau is 0.65-2.3 mm/yr, which represents only 10-15 percent of the India-Asia convergence rate. The uplift of the Shillong plateau did not significantly modify the rock uplift rate in the Bhutan Himalaya, which is consistent with the hypothesis of climatic modulation of the Pliocene erosion, tectonic and landscape evolution previously documented along this orogenic front.