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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]

We attempt to synthesize the mantle deformation patterns beneath India by assembling ∼2500 published shear wave splitting measurements from more than 350 broadband seismic stations. On a continental scale, the delay times are clustered around 0.8s, with the fast polarization azimuths (FPAs) predominantly oriented parallel to the absolute plate motion (APM) direction. This can be attributed to basal shear due to the interaction between the lithosphere and asthenosphere. In addition, the FPAs reveal a significant deviation from the APM from south to north. This deviation is analysed in four sub-regions namely northeast (NE), north, central and south India. In the NE and northern India, the deviation is attributed to the Indo-Eurasian collision tectonics. In NE India, the deviation from APM is found to be parallel to the strike of the orogens, suggesting coherent deformation in the upper mantle. In central India, the deviation from APM is attributed to frozen anisotropy associated with widespread magmatism in the Deccan Volcanic Province (DVP), paleo rifting and collisional events in the eastern Indian shield. The deviation is stronger in southern India than in central or northern India, primarily in the DVP, Western Dharwar Craton and northern part of Southern Granulite Terrain (SGT). This probably reflects the lithospheric evolution process in the mid-to-late-Archean, continental rifting in the western and eastern margins, ocean closure and subduction in the northern part of SGT. Back azimuthal variation in the splitting measurements in southern India suggests layered anisotropy and/or variation among different blocks. [ABSTRACT FROM AUTHOR]

This study presents new results of shear wave splitting at 56 broadband stations installed in the Eastern Himalaya, Burmese Arc, and adjoining regions. The fast polarization directions (FPDs) within the Himalaya, Burmese Arc and the foredeep are parallel to the strike of the orogens, suggesting a coherently deformed lithospheric mantle under compression. The FPDs follow the Main Boundary Thrust and the Main Central Thrust in the central segment of the Arunachal Himalaya. Smaller delay times (∼0.7 s) can be best explained by multiple layers of anisotropy, where combined effects of the anisotropic fabric of a coherently deformed lithosphere of the Arunachal Himalaya and plate motion related strain are playing a role. Null measurements in the Siang window may be due to the Indian plate interaction with Eurasia and Burma plates, causing different layers of anisotropy. Another possibility is the coincidence of source polarization direction with the fast axis. In the Bengal Basin, the Nulls could be due to annihilation of frozen anisotropy in the downwelling Indian lithosphere and the APM‐related strain, or a highly heterogeneous mantle affected by the Kereguelen plume magmatism, at ∼116 Ma. Key Points: New results of shear wave splitting from the Eastern HimalayaCoherently deformed lithospheric mantle under compressionVariable nature of anisotropy along the Himalaya [ABSTRACT FROM AUTHOR]