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.