3.2 Upper Mantle Deformation
3.2.1 Ladakh - Karakoram zone (LKZ)
Seismic anisotropy of the upper mantle beneath the eastern LKZ and northwest Himalaya has been investigated based on splitting in SKS waveforms recorded at 28 broadband seismic stations (Fig.6). In the frontal part of the Himalaya, the Fast Polarization Directions (FPDs) are mostly parallel or sub-parallel to the strike of the Himalayan orogeny suggesting deformation in the shallow lithospheric mantle under compression owing to the India-Asia collision. On the other hand, FPDs observed in the Lesser, Higher, and Tethyan Himalaya largely follow the NE-oriented absolute plate motion (APM) of the Indian plate which can be attributed to basal shear as the Indian plate moves above the asthenospheric mantle, with a minor contribution from shallow lithospheric deformation. A complex anisotropy pattern is observed in the Indus Suture Zone. The FPDs near the Karakoram Fault Zone are parallel or sub-parallel to its strike. The study suggests that KF extends up to the lithospheric mantle accommodating the India-Asia collision and facilitating extrusion in the Tibetan Plateau (Paul A., et al., 2021).
3.2.2 Western Himalaya
Upper mantle anisotropy beneath the western Himalaya is investigated using data from 62 broadband seismic stations (Biswal et al., 2020; Kumar V.P., et al., 2022). Of these 62 stations, around 18 stations are located along a linear profile in the Kumaun-Garhwal Himalaya traversing the Himalaya from south to north. The study region mostly comprises of Himachal (Biswal et al., 2020) and Kumaun-Garhwal (Kumar V.P., et al., 2022) Himalaya. The FPAs are mainly oriented along the ENE-WSW and \(\sim\) E-W directions, respectively, and a few in the NE direction (see Fig.4 of Kumar, V.P., et al., 2022). Also, very few observations in the Kumaun-Garhwal Himalaya are oriented along the \(\sim\) SE-NW direction. The delay times mostly vary from 0.2 and 1.7 s, mostly between \(\sim\) 0.4 to \(\sim\) 1.0 s. In the Kumaun-Garhwal Himalaya, the average delay time gradually decreases from 0.9 to 0.7 s in the sub- and lesser Himalaya to 0.6 s in the MCT zone and the higher Himalaya. There is not much variation in the orientation of FPAs from south to north in the Himachal and Kumaun-Garhwal Himalaya.
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.