Toroidal Mantle Flow Induced by Slab Subduction and Rollback Beneath the Eastern Himalayan Syntaxis and Adjacent Areas

A total of 431 well-defined and 632 null shear-wave splitting measurements obtained from 115 broadband seismic stations located in the eastern Himalayan syntaxis and adjacent areas is largely inconsistent with predicted fast orientations by absolute plate motion models. Spatial coherency analysis of the splitting parameters suggests that the observed azimuthal anisotropy is mostly located in the upper asthenosphere or the transitional layer between the lithosphere and asthenosphere, and the disagreement between the fast orientations and regional tectonic fabrics suggests an insignificant lithospheric contribution to the observed anisotropy. The observations may be attributed to flow systems that are driven by the westward rollback of the Indian slab beneath the Indo-Burma block and are modulated by a previously revealed gap between the northward and eastward subducting slabs of the Indian plate.