Figure 8. Map showing the individual splitting measurements obtained at different stations. The length of each line is proportional to the delay time between the fast and slow S  waves. Arrow represents the direction of the Indian Plate motion as defined by the NNR-NUVEL-1A plate model. (From Kumar V.P., et al., 2022)
  Another study on shear wave splitting (Kumar N., et al., 2021) reveals that both stress and structure-induced anisotropy prevail in the Kumaun Himalaya. The anisotropy directions are mainly NE-SW, N-E and NW-SE, in agreement with the observed gravity lineaments (Hajra et al., 2022b).

3.2.3 Shillong Plateau

Mohanty and Singh (2022) investigated the shear wave splitting using SKS, SKKS, PKS phases for Shillong Plateau and have found that the deformation patterns beneath the northern and central Shillong Plateau are dominated by the asthenospheric forces controlling the absolute plate motion (APM) of the Indian plate in a no net rotation frame in a distinctive NE direction. Also, they have reported that at the southern proximity of the Shillong Plateau, the deformation pattern seems to be aligned parallel to the major regional geological structures. The coherent lithospheric deformation along with transpressional tectonics act as the major source of anisotropy at this southern end.

3.2.4 Rajasthan Craton

Shear wave splitting parameters are obtained at four broadband seismic stations in Rajasthan using core-refracted phases (Mandal, 2019c). The delay time was found to vary from 0.3 to 2.4 s and clustered around 1.6 and 1.7 s. The FPAs are found to vary from 8\(^{\circ}\) to 175\(^{\circ}\). However, most of them are along the NE direction, parallel to the absolute plate motion (APM) direction of the Indian plate in the no-net-rotation frame. The basal drag could be the primary cause for the observed APM parallel anisotropy beneath the Rajasthan craton. It is inferred that the coherent lithospheric fabrics in the Rajasthan craton were formed during the Archaean and survived subsequent Paleoproterozoic tectonic events.

3.2.5 Kachchh rift zone

Shear wave splitting observations beneath twelve broadband seismic stations in the Kachchh rift zone (KRZ), Gujarat, are estimated using the core-refracted phases (Singh and Mandal, 2020; Mandal, 2021). In total, 443 new measurements are obtained. The mean value of FPAs varies from 49.41\(^{\circ}\) to 103.78\(^{\circ}\) at the stations, with an average value of 76.91\(^{\circ}\). Similarly, the delay times are found to cluster around 1.39 to 2.34 s at the stations. Most of the FPAs are clustered around 60\(^{\circ}\)and 80\(^{\circ}\), suggesting an ENE-WSW direction, near parallel to the E-W trending Kachchh rift axis. It is suggested that the upper mantle anisotropy beneath the KRZ is parallel to the Kachchh rift axis. It is generally observed that rift zones are generally characterized by large delay times, which also suggests significant anisotropic contribution from the asthenospheric flow-induced anisotropy. Thus, the KRZ region is associated with a thick anisotropic layer, with mantle anisotropy parallel to the Kachchh rift axis direction caused by both frozen lithospheric anisotropy and asthenospheric flow-induced anisotropy.

3.2.6 Western Ghats

The mantle deformation pattern beneath the Western Ghats, India, is investigated using shear wave splitting of core-refracted phases at 17 broadband seismic stations (Sribin et al., 2021). In total, 193 measurements are obtained, comprising 52 splitting and 141 null measurements. The delay times are found to vary from 0.3 to 2.8 s, and the FPAs from N6\(^{\circ}\) to N177\(^{\circ}\). The dominant direction is found to be NE, parallel to the APM direction of the Indian plate in a no-net-rotation frame. The dominant cause for the observed anisotropy is shear at the base of the lithosphere. The E-W orientation at stations close to the western coast, especially in the northern part of the Western Ghats can be associated with the lithospheric stretching along the west coast, associated with rifting process. Also, the coast parallel FPAs oriented along N-S and NNW-SSE direction with delay times varying from 0.6 to 1.2 s at stations away from the coast could be associated with the edge flow due to the transition from a thick to a thin lithosphere.

3.2.7 Profile across the Dharwar Craton and the Cuddapah Basin

Upper mantle anisotropy is investigated along a west-to-east profile having 38 broadband seismic stations covering mid-Archaean Western Dharwar Craton (WDC), late-Archaean Eastern Dharwar Craton (EDC), and Proterozoic Cuddapah Basin (CB) (Saikia et al., 2019). The orientation of FPAs varies from -50\(^{\circ}\) to 5\(^{\circ}\) in the WDC, -40\(^{\circ}\) to 30\(^{\circ}\) in the EDC and -5\(^{\circ}\) to 85\(^{\circ}\) in CB and further east. The delay times vary from 0.4 to 2.0 s, with the average being 1 s. In the WDC, the orientation of FPAs is found to align along the strike of shear zones and faults. This suggests frozen-in anisotropy in the lithosphere, possibly established during the lithospheric evolution in mid-late Archaean. In the EDC, the orientation of the FPAs deviates from the APM direction, suggesting anisotropy frozen-in from the episodes of late Archaean to Proterozoic period. The splitting trend beneath the CB and Eastern Ghats (EG) follows the strike of the rift along with plate motion direction, indicating that the anisotropy is influenced by a combination of frozen anisotropy due to continental rifting along the eastern margin of the Indian plate and active asthenospheric flow.

3.2.8 NW-SE Profile across the DVP and the EDC

The upper mantle anisotropy is investigated beneath the DVP and the EDC at fifteen broadband seismic stations located along a NW-SE profile (Sivaram et al., 2022). In total, 71 measurements are obtained by performing shear wave splitting analysis of core-refracted phases. The orientation of FPAs suggests variation in splitting parameters along the profile. In the DVP, the orientation of FPAs is along NE-SW with the delay times varying from 0.5 to 1.2s. In the EDC and EGMB (Eastern Ghat Mobile belt), the FPAs are along NW-SE in EDC and NE-SW in EGMB, and the delay times vary from 1 to 1.4 s. The non-APM orientation in EDC suggests frozen-in anisotropy in a thick lithosphere, associated with the late Archaean to Proterozoic events or the last major episode of tectonic and magmatic activity during 2.6 Ga. In the DVP, the deformation seems to represent the predominant APM-trending asthenospheric anisotropy beneath a thinned lithosphere. Possibly, the upper mantle is influenced by shear interactions from the geologically recent \(\sim\) 65 Ma Deccan plume event. In the EGMB, the FPAs are sub-parallel to the APM direction, which suggests imprints of rifting.

3.2.9 Eastern Ghat Mobile Belt (EGMB)

Jana et al. (2019) carried out shear wave splitting analysis using core refracted PKS, SKS, SKKS phases to capture the collisional signature preserved beneath the EGMB. This is the first geophysical evidence deciphering the episodic collisional history of EGMB. The fast axis directions have shown absolute plate motion as the dominant cause. Though in Rengali province and Mahanadi rift zone, splitting shows signatures of previous collisions. Jana et al. (2021) evaluated the seismic anisotropic signatures and mantle deformation patterns using Reference Station Technique for the EGMB and its surroundings. This study found new evidence of frozen anisotropic signature demarcating eastern Phulbani domain from western Phulbani domain. The dominant effect of absolute plate motion has been observed beneath the Bastar craton. The distinct nature of Chilka lake from its surroundings suggests Chilka lake to be a separate block.

3.2.10 Synthesis of upper mantle anisotropy beneath India

Around \(\sim\) 2500 published individual shear wave splitting measurements from more than 350 broadband seismic stations are synthesized to present the mantle deformation scenario beneath India (Roy et al., 2021a). On a continental scale, the delay times are found to cluster around 0.8 s, with the FPAs predominantly along the APM direction. This is attributed to basal shear due to the interaction between the lithosphere and asthenosphere. A significant deviation from the APM is observed from south to north. The deviation from APM is categorized into four sub-regions, namely northeastern (NE), north, central, and south India (Fig.9). For the NE and north India, it is attributed to the Indo-Eurasian collision tectonics. For NE India, it is found to be parallel to the strike of the orogens, suggesting coherent deformation in the upper mantle. For central India, the deviation is attributed to frozen anisotropy associated with widespread magmatism in the 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, WDC and the northern part of Southern Granulite Terrain (SGT). This probably reflects the lithospheric evolution process in the mid-to-late-Archaean, 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.