Mantle deformation in Sikkim and adjoining Himalaya: Evidences for a complex flow pattern

New measurements of azimuthal anisotropy within the Himalayan collision zone and southern Tibet, obtained by applying the SKS-splitting technique to waveforms from 31 broadband seismic stations, reveal significant variations in the strength and orientation of the fast axes. Stations in the Sikkim Himalaya, show strong anisotropy south of Main Central Thrust (MCT), with delay times ∼1 s. The fast polarization directions are nearly parallel to the strike of the Himalaya mountain chain and vary consistently along the profile, from 105° in the south to 130° in the north. However, in the adjacent Nepal Himalaya, a predominance of plate motion related strain is observed, at least up to the MCT. The western portion of southern Tibet, south of the Indus–Tsangpo suture zone, reveals characteristics of E–W oriented anisotropy, contrary to null anisotropy found in the eastern part. The complex flow pattern revealed in the present study may be due a combination of basal shear resulting from plate motion and ductile flow along the collision front due to compression.