Upper-mantle anisotropy beneath the south Indian Shield: Influenced by ancient and recent Earth processes

Abstract

We obtained shear-wave splitting parameters from core-refracted phases like SKS, SKKS, and PKS at 75 digital broadband seismic stations almost uniformly spread over the south Indian Shield representing varied geological terrains, including the western Dharwar craton, the eastern Dharwar craton, the Southern granulite terrain, and the continental margins along the west and east coasts. A majority of the stations over the Dharwar craton show delay times of 1-1.6 s, indicative of a 150-200-km-thick anisotropic layer correlating with the lithospheric root, while segments like the Pan-African Southern granulite terrain have delay times of 0.5-0.7 s, suggesting an internally deformed and thin anisotropic layer, possibly due to recent plate-tectonic and geodynamic processes. The average direction of anisotropy is generally ~N30°E, correlating with the present-day plate motion, with local deviations where the direction of anisotropy correlates with the orientation of the regional shear zones. Stations close to the continental margin show large time delays (up to 2 s) with the fast axis parallel to the rift axis. Further, we infer a layered anisotropic lithosphere in the south Indian Shield as revealed in 90° periodicity of the two anisotropy parameters (fast direction and delay time). The upper lithosphere represents the depleted Archean mantle, while the lower lithosphere could be transformed to more fertile mantle due to subsequent deformation. This study suggests that the observed anisotropy over the south Indian Shield is the result of complex interplay between the architecture of an Archean craton and its subsequent deformation in different geological domains due to deep Earth processes.