Discerning crustal deformation patterns beneath Sikkim Himalaya

Sikkim Himalaya is an actively deforming part of the Himalayan orogen which formed as a result of an impactful continental-continental collision. Recent studies have characterized the region by bimodal seismicity resulting from a dynamic multi-fault system against the backdrop of spatially varying geological and geophysical features. In this study, we attempt to map the distribution of crustal inhomogeneity beneath Sikkim Himalaya using peak delay time (T_pd) analysis of the S-wave envelope. Quantitative estimations at 6 Hz central frequency shows predominant path dependence (B > 0.5) suggesting strong multiple forward scattering due to presence of inhomogeneities. Further, we produced 3-D peak delay perturbation (ΔLog(T_pd)) map to investigate the depth distribution of inhomogeneities. Spatial variation maps at six distinct depths reveal high ΔLog(T_pd) between 0-15 km in the south, southwestern Sikkim, and eastern Nepal region which elucidate the presence of a highly heterogeneous decollement surface along which the Indian plate is underthrusting beneath the Tibetan plateau. On the contrary, the shallow crust of northern Sikkim portrays negative ΔLog(T_pd) which evidences an undeformed medium, corroborated with the lack of seismicity at shallower depths. Investigation of the depth section along the southeast-northwest direction reveals a zone of highly deformed crust across MHT with prevalent micro-seismic activity. The said zone coincides with low S-wave velocity and low coda attenuation parameter which transpires to the presence of numerous inhomogeneities with the possible presence of fluid as well. We also observe deformation in the foothills of Himalaya possibly induced due to the upliftment of the Shillong plateau.