Seismic structure of the crust and the upper mantle beneath the Himalayas: Evidence for eclogitization of lower crustal rocks in the Indian Plate

Abstract

Variations in the seismic velocity structure of the Himalayan collision zone include significant differences between its north and south portions, with transitions in physical properties across the Greater Himalaya. We combined P- and S-wave traveltimes from a temporary broadband seismic network in eastern Nepal and southern Tibet with arrival times at the permanent station network of the Department of Mines and Geology of Nepal to determine the seismic velocity structure across the Himalaya, using local earthquake tomography and traveltimes of regional earthquakes. The P-to-S velocity ratio (Vp/Vs) structure marks the difference between the Indian Plate and the overlying materials, with the Vp/Vs ratios being high for the former and low for the latter. We also found a significant increase in the uppermost mantle seismic velocities from south to north, reaching P-wave velocities (Vp) over 8.4 km/s north of the Greater Himalaya. These high Vp values do not seem to be the result of biases due to anisotropy in the upper mantle beneath the Greater and Tethyan Himalayas. Instead, we suggest that rocks in the lower crust of the underthrusting Indian Plate undergo metamorphism to eclogite as they plunge to greater depth beneath the mountain range, explaining the high seismic velocities. Copyright 2008 by the American Geophysical Union.