Estimation of source localization and central depth of anisotropy beneath an active seismic region plays an important role in understanding the deformation mechanism and current configuration of tectonics. Though shear wave splitting measurements are capable of deciphering the mantle dynamics and deformation patterns, they are constrained from analysing the actual depth of anisotropy or heterogeneity, which may have large influences in interpreting the geodynamics of a particular region. The present study fills this gap of estimating the source localization and depth of seismic anisotropy beneath the Shillong Plateau and Himalayan foredeep region through a well‐established spatial coherency analysis of splitting parameters based upon the Fresnel zone concept. The model suggests a central depth of heterogeneity at around 100 km beneath the Shillong Plateau mass, coinciding with the lithosphere‐asthenosphere boundary and strengthens our implication that the asthenospheric drag at the base of the Indian lithosphere is responsible for absolute plate motion of India, is the major source of anisotropy and controls the deformation patterns of the Shillong mass. In a similar manner, the spatial coherency analysis predicts a central depth of anisotropy at around 150 km beneath the Himalayan foredeep region. The asthenospheric force corresponding to this particular depth is suggested as the major source of anisotropy for this region.

<p>SKKS phase being an unique one among other core refracted phases like PKS, SKS etc. is capable of imaging the anisotropic contribution from lower mantle as per its raypath is considered. Its unique property of reflection at the core-mantle boundary enables it to carry forward the lower mantle contribution in case of seismic anisotropy is concerned. The lower mantle as a whole is assumed to be isotropic except the lowermost 200-300km (D’’ layer) which pertain a distinct diversity in the raypath of SKKS phases beyond 130<sup>o</sup> epicentral distance and thereby manifest the possible influence of lower mantle in the deformation pattern of any region. The present study of SKKS splitting analysis comprising an epicentral range of 140<sup>o</sup>-180<sup>o</sup> is primarily intended to complement the existing shear wave splitting dataset associated with north east India as well as to understand the effect of lower mantle on the splitting parameters (fast polarization direction (FPD, ϕ) and delay time (δt)). The motive of the study can be further extended to decipher the implication of such narrow epicentral range on splitting analysis. The analysis suggests that, beneath sub-Himalaya, the Indo-Eurasia collision derived lithospheric force along major thrust faults is the prime source behind the deformation, while Assam foredeep is somewhat influenced by the seismogenic Kopili Fault. There exist a striking difference in anisotropic directions between northern and southern fringe of Shillong plateau where deformations are governed by the absolute plate motion (APM) of Indian plate driven asthenospheric flow and seismically active Dauki and Dapsi faults respectively. Such disparity in splitting attributes can be inferred as the interplay of constricted back-azimuthal distribution and lean range of epicentral distance of seismic events, though the probability of lower mantle involvement cannot be ignored completely.</p>

<p>For the understanding of deformational mechanism and geodynamics of a tectonic set up, the source localization and central depth of anisotropy plays a vital role. Though mantle dynamics and deformation patterns can be understood from studying the shear wave splitting mechanism, the true interpretation of under earth mechanism governing the geodynamics remains little biased and unrealistic without the  proper justification and identification of the source localization and depth of anisotropy. Our present study is focused on the possible central depth determination and source localization of anisotropy beneath the Sikkim Himalayan region based upon the well-established spatial coherency method of Splitting parameters, an improved and dynamic principle of grid search analysis based on the Fresnel zone concept. The principle is based upon the maximum coherency relation between the splitting parameters suggested by a minimization in the variation factor as a function of true depth of the anisotropy. Sikkim Himalaya, sandwiched between the central Nepal Himalaya and the eastern Bhutan Himalaya, demarcates the distinct change in the width of the Himalayan foreland basin and the Main Himalayan Thrust (MHT), which is a part of the active deforming eastern Himalayan fold axis and thrust belt. The Spatial coherency analysis of splitting parameters suggests the central depth of heterogeneity at around 130 km beneath this Sikkim Himalayan region as a consequence of the deformation patterns governed by the complex lithospheric mass at this particular depth.</p><p> </p><p>KEYWORDS</p><p>Spatial coherency, Shear wave splitting, Sikkim Himalaya, lithosphere.</p>