Seismic Anisotropy beneath the Eastern Himalayas and Adjacent Areas

Abstract:

Seismic anisotropy beneath the eastern Himalayan orogenic belts and the adjacent Lhasa block and the northern extreme of the Indian continent has been investigated by many studies using splitting of P-to-S converted phases from the core-mantle boundary. Significant discrepancies exist in the measurements and in the geodynamic models proposed by previous studies to explain the shear-wave splitting (SWS) results. Here we report results from a comprehensive analysis of XKS (including PKS, SKKS, and SKS) data recorded by 112 stations in the area from 83 to 93 degree East, and 26 to 31 degree North. A total of 236 pairs of resulting splitting parameters show systematic spatial variations. In the western one-third of the study area (approximately west of 86 degree East), the fast orientations are mostly E-W which is parallel to the strike of the orogenic belts, while stations east of this longitude demonstrate mostly NNE-SSW fast orientations which are similar to those observed on the Indian continent. Based on the spatial variations revealed by this and previous studies, we propose that seismic anisotropy beneath the study area is the result of two contrasting mechanisms. The first is the simple shear between the lithosphere of the subducting Indian slab and the asthenosphere, which leads to NNE-SSW oriented anisotropy; and the second is lower crustal flow which induces mostly E-W oriented anisotropy. Because the fast orientations in the two layers are approximately orthogonal to each other, the observed fast orientation at a given area reflects that of the layer with stronger anisotropy, and the observed splitting time is the difference in splitting time between the two layers. Under this model, the western part of the study area is dominated by lower crustal flow, while in the eastern part, simple shear between the slab and the asthenosphere produces stronger anisotropy than crustal flow.