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Abstract:
We explore the pattern of seismic anisotropy beneath a ~300 km transect in the Kumaon Himalaya, an active, orogenic region formed by the collision between the India-Eurasian plates, and carrying world-wide attention for its mysterious geometry and seismic potential. Applying the shear wave splitting analysis of the teleseismic core-refracted phases (SKS, SKKS, PKS and PKKS) on a linear profile of 32 broadband seismograph stations, we uncover the signatures of enigmatic strain histories and mantle dynamics. The measured shear wave splitting parameters i.e. Φ (Fast Polarisation Direction, FPD) and δt (Delay Time, DT) carry extraordinary information of the anisotropy formed through the complex mantle-deformation. We observe a strong dependence of the splitting data on the back-azimuth directions, which fits fairly well with a two-layer anisotropy model, with the lower layer having a low dipping angle at most of the stations. We present a geodynamic model depicting the preserved and the current situation of the compression-forces at the India-Asia collision zone. Incidentally, the mean Φ are primarily sub-parallel to the North East (NE)-oriented absolute plate motions (APM), and nearly orthogonal to the major geological boundaries in the Higher Himalaya. In the Lesser Himalaya, the mean Φ are nearly orthogonal to the APM and are sub-parallel to the surface expressions of major geological features. Similarly, we observe a wide variation in the mean δt. Our detailed analysis indicates a predominantly asthenospheric source of anisotropy, and a possible slab-tearing for this segment of the Kumaun Himalaya.
