Crustal Seismic anisotropy in Sikkim Himalaya: Implications for deformation

Collision and relentless underthrusting of India beneath Eurasia resulted in large-scale deformation of the Indian lithosphere. Anisotropic parameters serve as a good proxies to decipher deformation in such complex orogenic collision zones. In this study, we present anisotropy characteristics of the crust beneath Sikkim Himalaya using harmonic decomposition of P-to-S converted phases identified in P-wave receiver functions (P-RFs). Analysis of azimuthal variation of these phases enabled parameterizing the crustal anisotropic properties, with depth. Initially, 11,087 high quality P-RFs were computed using waveforms of teleseismic earthquakes having magnitude  ≥ 5.5 and signal to noise ratio  ≥ 2.5 within an epicentral distance range of 30° - 100°, recorded at a network of 38 seismic stations deployed in Sikkim Himalaya and the adjoining foreland basin. Analysis of the first three harmonic degrees (i.e. k= 0, 1 and 2) reveals that the upper crustal anisotropy is oriented WSW-ENE to E-W, coinciding well with the trends of crustal microcracks and fractures. The mid to lower crustal anisotropy aligns predominantly with the dipping decollement layer along which the Indian plate is underthrusting Tibet. An orthogonal reorientation is observed within the extent of the Dhubri-Chungthang Fault Zone authenticating its role in segmenting the orogen. The lower crustal anisotropy is highly perturbed signifying a highly heterogeneous nature of the Moho.  Existence of multiple layers of anisotropy possessing distinct geometries varying with depth could be an indication of a highly complex deformational regime resulting from active crustal shortening.