Deformation of the upper crust in the Kumaon Himalaya analyzed from seismic anisotropy and gravity lineament studies

Abstract Seismic anisotropy of the crust beneath the Kumaon Himalaya region has been investigated by shear wave splitting analysis to unravel deformation processes at the upper crustal depth. The S-wave splitting of 150 local earthquakes recorded by 17 broadband seismological stations reveal a complex pattern of anisotropy in the upper ~20 km of the crust beneath the Kumaon Himalaya. The fast polarization directions are predominantly oriented along NE-SW and NW-SE with significant strength represented by average delay time between fast and slow waves (~0.18 ± 0.03 s) and estimated percentage of anisotropy (2.4%). The anisotropy is found to be maximum at a depth of ~10–15 km. The measurements of fast polarization direction and lineament analysis of Bouguer gravity data in the region indicate that the anisotropy originates due to the combined effect of stress-aligned micro-cracks affected by both regional tectonic stress and local geological features. The fast polarization directions in general show a good correlation with the trend of local lineaments. The lineament trends are observed to be different for different lithological units thereby emphasizing the fact that the deformation in the crust is highly complex. Based on the estimated average crack density (~0.024), it can be envisaged that the upper crust as a whole consists of intact rocks containing individual cracks without large fractures. The lineament and crack density variations suggest that the shallow crust in the Inner Lesser Himalaya is more brittle than the Outer Lesser Himalaya.