Evidence of Seismic Anisotropy Beneath the Western Margin of Eastern Dharwar Craton, India

Abstract

To evaluate seismic anisotropy beneath ten broadband seismic stations in the Western margin of Eastern Dharwar Craton (EDC) (near Hyderabad region of India), we perform the shear wave splitting analysis using core refracted phases (such as SKS, SKKS) of teleseismic events. Seismic anisotropy is quantified by measuring the shear wave splitting parameters: the direction of the fast-polarized wave (Φ) and the delay time (δt) between the two components. These parameters indicate the orientation in which seismic waves travel fastest due to the material’s anisotropic properties and the strength of the anisotropy, respectively. We estimate the Φ and δt using the Rotational Correlation and Minimum Energy methods. In the upper mantle, minerals like olivine tend to align along the direction of maximum shear, as reflected in the orientation of Φ. Our results across all stations show that in a NNR NUVEL-1A, no-net- rotation reference frame, the estimated Φ and δt range from (54)° to (82)° and 0.42 to 0.90 s, respectively. The average (Φ) orientation is N(68 ± 4)⁰E, which is sub-parallel N(25 ± 4)⁰E to the absolute plate motion (APM) and the average (δt) is (0.53 ± 0.002)s. Our analysis shows discrepancy of shear wave splitting of SKS/SKKS phases which makes us believe that the source of seismic anisotropy beneath this region possibly lies in the lower mantle, as also observed in other similar studies. The observation of small (δt) lead to an interpretation of the weak anisotropy, however, due to lack of sufficient SWS data to support two—or more layers, we are not able to give a depth constraint, but the anisotropy layer may be located n the lower mantle. This observed lower mantle anisotropy may be driven by paleo-lithospheric plastic deformation in the deeper mantle (with anomalous D” structure). While under favourable temperature–pressure conditions, another possibility also exists for the phase transformation from lower mantle minerals like perovskite (Pv) to post-perovskite phase (pPv), resulting in the lattice preferred orientation (LPO) of these minerals. This observation of well detected seismic anisotropy is in contrast to many earlier researchers that had characterized this region as having either null or insignificant upper mantle seismic anisotropy. Due to sparsity of our splitting data, we are unable to constrain the depth of lower mantle anisotropy, however, this study provides valuable inputs for the studies on geodynamic evolution along the western margin of EDC.