Crustal Thickness Variations and their Tectonic Implications beneath the Uttarakhand Himalayas and the adjoining Tectonic Segments: Results from 3-D Tomographic Inversion of Local and Regional Earthquake Data

Abstract

We image the high-resolution velocity structures of the crust and uppermost mantle beneath the western part of the Himalayan-Tibetan orogen through tomographic inversion of local and regional earthquake data. We herein reconstruct and present the tomographic image of the variable configuration of the Moho boundary beneath the Himalayan-Tibetan orogen. The thickness of the crust varies between ~40-65 km from south beneath the sub-Himalaya to north beneath the Higher Himalaya. The thickest crust imaged as thick as ~85 km, located ~100 km from ITSZ towards north beneath the southwest Tibet. Our results also report significantly variable geometry of the Moho boundary along the tectonic trend of the Himalayan-Tibetan orogen, which may indicate that the Indian plate subducted/underthrusted beneath the Eurasian plate in a piecewise manner as a consequence of differential convergence rates, counter clockwise rotation of the Indian plate and episodic collision processes. We also image the geometry of the subducting/underthrusting Indian plate beneath the Himalayan-Tibetan orogen. We present the geodynamic model of the two subducted slabs, where the Indian plate subducts/underthrusts towards north and the Tibetan slab subducted southwards beneath the Tibetan plateau. We infer that the Indian plate is torn into pieces differing in its northern limits and angle of subduction/underthrusting. Where its westernmost end subducts/underthrusts below the Eurasian plate with a gentle dip crossing ITSZ and KKMF. On the other hand, towards east the Indian plate subducts/underthrust the Eurasian plate with a relatively greater angle near ITSZ, approximately 250 km distant from HFT.