Disparate deformation of the crust and upper mantle beneath the Doda-Kisthwar region, NW Himalaya

Abstract

This study attempts to characterize the nature of seismicity, anisotropy and coherency of deformation in the crust and upper mantle beneath the Doda- Kisthwar region (DKR), Jammu, located in the Kashmir seismic gap of NW Himalaya. Location of earthquakes using local waveforms, and shear wave splitting in local and teleseismic waveforms recorded at 5 temporary stations, are performed for this purpose. The relocation of 890 earthquakes reveals two clusters along the NE and SW part (referred to as Kisthwar and Doda cluster respectively), which is attributed to the under-thrusting of the Indian plate and uplifting of Kisthwar Window respectively.The shear wave splitting analysis yielded a total of 47 local S and 11 XK(K)S splitting measurements. The corresponding Fast Polarization Azimuths (FPAs) show two distinct spatial patterns. In the first pattern, the FPAs are oriented along the NW direction, are perpendicular to the maximum horizontal stress, and are parallel to the structural trend as a result of oblique compression in the NW Himalaya. In the second pattern, the FPAs are oriented along the ENE-WSW direction and the corresponding delay times are found to be larger than those in the DKR. This could be because of localized deformation; the strike of fluid-filled fractures in the fault zone is also along the ENE-WSW direction, parallel to the course of the Chenab river. Thus, for both the trends, structure-induced anisotropy seems to be the most possible cause for the observed anisotropy. Further, the splitting parameters show a slight variation with depth for the respective orientation patterns.The delay times of the XK(K)S phases vary from 0.3 to 1.3 s, and cluster around 0.5 s, with the majority of FPAs, oriented along the ENE-WSW direction. This can be reconciled in terms of a combined effect of absolute plate motion (APM) related strain and finite strain induced by compression. The inconsistency in the upper crust and upper mantle anisotropy does not favour a coherently distributed lithospheric deformation in this segment of the NW Himalaya. The source of anisotropy in the upper crust is complex compared to that in the upper mantle.