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This study attempts to interrogate the upper mantle deformation pattern beneath the Kumaon-Garhwal region, located in the western Himalaya, using shear wave splitting (SWS) analysis of core-refracted (XK(K)S) phases recorded at 53 broad-band stations. The fast polarization azimuths (FPAs) revealed by 338 well constrained measurements are dominantly clustered around ENE–WSW, with a few along the NE and E–W directions. The delay times vary from 0.2 to 1.4 s, with an average of 0.6 s that is smaller than that for the Indian shield (∼0.8 s), central and eastern Himalayas. The northern part of the lesser Himalaya shows a slightly smaller delay time compared to the southern part, which is attributed to the weakening of azimuthal anisotropy caused by the dipping of the Indian lithosphere. In order to understand the crustal contribution, its anisotropy is measured by analysing the splitting of Ps conversions from the Moho (Pms), akin to that of the XK(K)S phases. However, reliable results for crustal anisotropy could be obtained only at 10 stations. The average delay time due to crustal anisotropy is 0.47 s, with a variation from 0.2 to 0.9 s. Although the dominant period of Pms is smaller than that of SK(K)S, crustal anisotropy contributing to splitting of the latter phases cannot be ruled out. The orientation of FPAs obtained from Pms phases is found to be parallel or sub-parallel to those from XK(K)S phases, suggesting a similar deformation mechanism in the mid- to lower-crust and upper mantle. On the basis of FPAs derived from XK(K)S measurements, the Kumaon-Garhwal Himalaya (KGH) region can be divided into four subregions. In the western and eastern parts, the FPAs are mostly aligned along NE and ENE–WSW, and NE, respectively. In the central and south-eastern parts, their orientation is along ENE–WSW and NW, respectively. The strong ENE–WSW orientation in the central part could result from a slightly variable anisotropy in the crust to the upper part of the lithosphere or basal topography causing deflection of mantle flow. Also, the NW orientation in the south-eastern part of KGH is associated with a shallow source within the lithosphere. Application of the spatial coherency technique to single-layered anisotropic parameters results in a depth of 220–240 km, implying that the dominant source of anisotropy could lie in the upper mantle. [ABSTRACT FROM AUTHOR]

[Display omitted] • A large spatial variability in crustal deformation is found beneath the western and eastern parts of the Arunachal Himalaya. • Small delay times and large scattering in the fast axis of anisotropy characterize the western part. • Crustal anisotropy beneath the western part is due to a combination of structure- and stress-induced mechanisms. • Correlation of local S and XK(K)S measurements in the central part suggests coherent deformation. • Combined delay times from the central and eastern parts show a correlation with depth. Shear wave anisotropy of the crust beneath the Arunachal Himalaya is investigated by analysing the splitting of steeply incident (i c ≤ 45°) direct S-phases of 396 local earthquakes, recorded at 32 stations. The 76 well constrained splitting measurements reveal that the delay times between fast and slow waves range from 0.02 to 0.30 s, clustered around 0.07 s. A large variation in the orientation of fast polarization directions (FPDs) is found. The smallest delay times (mostly < 0.10 s) and large variation in the orientation of FPDs are found at stations located in the western part of the Arunachal Himalaya. This is likely associated with heterogeneities in the lithological properties and a combined effect of stress-induced and structure-induced anisotropy. For a small crustal block in the central part of Arunachal, the FPDs are parallel to the strike of the Himalayan arc and are associated with structure-induced anisotropy. In this region, the FPDs representing the upper mantle are also found to be parallel to the strike of the Himalayan arc, suggesting a coherent deformation. In the eastern part, a variation in the FPDs is observed from north to south, being parallel to the strike of the Siang river in the former and mostly parallel to the average maximum horizontal stress in the latter. The delay times show a correlation with depth when the observations in the central and eastern parts of the Arunachal Himalaya are combined. [ABSTRACT FROM AUTHOR]

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. • Analysed the local S and XK(K)S phases to examine the coherency of deformation in the crust and upper mantle • The fast axis of anisotropy in the upper crust does not correlate with the regional stress pattern. • The anisotropy in the upper mantle is due to the combined effect of basal shear and extension induced by collision. • Lack of coherent deformation in the crust and upper mantle suggests a decoupled nature. [ABSTRACT FROM AUTHOR]