Probing Mantle Deformation beneath the Southern Granulite Terrain Using Seismic Attenuation Anisotropy

The Southern Granulite Terrain (SGT) in peninsular India is a high-grade metamorphic region formed by intricate Precambrian tectonic processes, serves as a natural laboratory for examining the seismic properties of solid continental lithosphere. Attenuation anisotropy shows how seismic energy loss changes with direction, giving extra information beyond just how fast seismic waves move through rock. It is particularly good at showing processes like grain-boundary relaxation, dislocation creep, and fluid assisted deformation. We have measured the shear-wave splitting parameters (, ) and attenuation anisotropy (, ) for the SKS phases recorded at 13 stations spread over the SGT using the second eigenvalue minimisation method and the instantaneous frequency matching technique, respectively. The attenuation anisotropy parameters for each station, obtained through a weighted-stacking process, vary from 0.1s to 0.85s for differential attenuation () with an average of ~0.36s and -82° to 88° for fast polarisation direction (), with the apparent fast wave () attenuating more, indicating the presence of fluid-filled fractures. Removing the attenuation effects, the station-averaged delay time () lies between 0.73s and 1.27s, with an average of ~0.99s, and fast polarisation direction () lies between -87° and 58°. We further analysed the backazimuthal dependence of the splitting parameters. The melt inclusions and the anisotropic layers beneath each station are characterised using the squirt flow model. The fractures are striking at an angle between ~49° and ~306°, and dipping at an angle between ~36° and 50°. The anisotropic layer thickness varies from 33 km to 115 km beneath the stations. Variations in attenuation anisotropy across major shear zones, like the Palghat–Cauvery and Achankovil sutures, offer important information about reactivated shear deformation, fossil lithospheric fabrics, and potential asthenospheric contributions in the SGT. This information helps to clarify the tectonothermal evolution of this ancient crustal block.