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This study investigates the upper mantle deformation pattern beneath the Indo-Eurasia collision zone utilizing the core-refracted (S(K)KS) phases from 167 earthquakes recorded by 20 broadband seismic stations deployed in the Western Himalaya. The 76 new shear wave splitting measurements reveal that the fast polarization azimuths (FPAs) are mainly oriented in the ENE-WSW direction, with the delay times varying between 0.2 and 1.7 s. The FPAs at most of the stations tend to be orthogonal to the major geological boundaries in the Western Himalaya. The average trend of the FPAs at each station indicates that the seismic anisotropy is primarily caused due to strain-induced deformation in the top ~200 km of the upper mantle as a result of the ongoing Indo-Eurasian collision. A contribution from the mantle flow in the direction of the Indian plate motion is possible. The mantle strain revealed in the present study may be due to a combination of basal shear resulting from plate motion and ductile flow along the collision front due to compression.

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The continuing convergence between India and the Eurasian plate at an average rate of 17 mm/yr (Bilham, 2019) has given rise to the mighty Himalaya mountain range and the Tibetan Plateau. Geologically, from south to north the Himalaya is divided into four major east-west structural tectonic divisions bounded by different faults (Figure 1). From south to north, we denote: (a) the Himalayan Frontal Thrust or the Main Frontal Thrust (MFT), which lies between the sediments of the Indo-Gangetic plains and the Siwalik Himalaya; (b) the Main Boundary Thrust (MBT) that lies between the Siwalik and Lesser Himalayas; (c) the Main Central Thrust (MCT), located between the Lesser and Higher Himalayas, and (d) the South Tibet Detachment (STD) that lies between the Higher and Tethys Himalayas. The Indus Tsangpo Suture Zone (ITSZ) marks the northern boundary of the Tethys Himalaya (Chatterjee et al., 2013;Hebert et al., 2012). Seeber and Armbuster (1981) proposed that the convergence in the Himalaya is accommodated by the Main Himalayan Thrust (MHT), a decollement acting as a root to all three splay faults: the MCT, MBT, and