Seismic structure of the underthrusting Indian crust in Sikkim Himalaya

Abstract

This study presents the first results of the seismic character of the underthrusting Indian crust in the Sikkim Himalaya deduced through an analysis of ∼3600 receiver functions (RFs) abstracted from waveforms registered at 11 broadband stations spanning a 110 km long N-S profile from the foothills to the higher Himalaya. Common conversion point stacks of receiver functions prominently trace the northward dipping geometry of the Indian Moho beneath the Himalaya. Monte Carlo inversion of the azimuthal variations of the RFs at individual stations adopting the nearest neighborhood algorithm approach reveals that the crustal thickness varies from ∼40 km to 61 km from south to north, with a dip varying between 4 and 10 among stations. A Moho doublet prominently seen at a depth of ∼40 km in the higher Himalaya to the north of Main Boundary Thrust has been interpreted in terms of possible (partial) eclogitization of a granulitic Indian lower crust, akin to the finding just north of the study region beneath southern Tibet. A strong layer of anisotropy (∼17%) localized within a low-velocity layer between 20 and 30 km has a NW-SE oriented fast polarization direction counterintuitive to the convergence-parallel and range-perpendicular alignment expected in a convergent setting due to shear processes. Midcrustal transcurrent deformation in Sikkim and Bhutan, evidenced by a conjugate system of strike-slip faulting with NW to NE trending P axis orientations is the most feasible mechanism for causing a near strike parallel oriented fast axis of anisotropy in this segment of Himalaya. Copyright 2010 by the American Geophysical Union.