)
3.2.11 Pn anisotropy of the Indian shield
High-resolution P-wave velocity and anisotropy structure of the hitherto
elusive uppermost mantle beneath the Indian shield and its surrounding
regions are obtained using 19,500 regional Pn phases from 172 broadband
seismic stations (Illa et al., 2021a). The effect of continental
rifting, collision and orogeny is reflected in the Pn velocity image.
The cratons in the Indian shield have uppermost mantle velocities
ranging from 8.02 to 8.42±0.05 km/s. Prominent highs and lows are
observed in the shield region related to mantle deformation episodes, as
the Indian plate has experienced major tectonic activity during and
after the breakup from the Gondwanaland. The Pn and SKS anisotropic fast
axis directions are consistent, except for the Indian shield, revealing
that the Indian cratons are distinct with an altered uppermost mantle
preserving the remnant anisotropy. The FPAs are consistent in the
collision environments in the west, Himalaya, and Burmese arc region.
3.2.12 Anisotropy of NW DVP using Surface waves
The anisotropic and isotropic variations within the crust and upper
mantle beneath the NW DVP are investigated using surface waves (Sharma
J., et al., 2021). Results reveal different intra-crustal layers, lid,
and a low-velocity zone (LVZ). The LVZ comprises a uniform
asthenospheric low-velocity layer (LVL) of average VSV 4.44 km/s and VSH 4.47 km/s, and another LVL with an
average VSV 4.45 km/s and VSH 4.41 km/s.
A negative radial anisotropy is observed in the LVZ, indicating the
dominance of vertical flow. This could be related to partial melts,
volatile materials and/or a thermal anomaly.