Evidence for plate motion related strain in the Indian shield from shear wave splitting measurements

Abstract

The predominantly Precambrian Indian lithospheric plate experienced a long history of deformation ever since its breakup from Gondwanaland and its passage over four hot spots. The nature of mantle deformation of this fast moving plate has hitherto remained elusive in view of the paucity of measurements of shear wave anisotropy. In this paper we present the first results of 182 measurements of azimuthal ansiotropy obtained by analyzing the SKS and SKKS waveforms from 85 earthquakes recorded at 35 broadband seismic stations located on various geological units of the Indian shield. Results show that the delay times (δt) between the fast and slow axes of anisotropy are close to 1s, which is the global average for continental shield regions. The fast axis azimuths do not reveal coherent patterns that suggest dominance of asthenospheric flow where a basally flat Indian lithosphere is dragged over an inactive sublithospheric mantle. The northern part of the Indian shield where the lithosphere is thick shows a strong correlation between plate motion and azimuths of fast polarization, due to the effect of the basal cratonic keel plowing through the sublithospheric mantle. Most of the fast axes azimuths can be explained by the absolute plate motion (APM) related strain, considering the basal topography of the lithosphere. The consistent NS orientations along the west coast may be due to edge flow associated with a transition from a thicker to a thinner lithosphere. The Southern Granulite Terrain and the Cuddapah basin shows evidences for fossilized anisotropy, particularly in regions close to continental shear zones. Copyright 2008 by the American Geophysical Union.