Seismic anisotropy and mantle deformation beneath Eastern Ghats Mobile Belt using direct-S waves

Abstract

Seismic anisotropy observations provide a potential tool to constrain the signatures of various continental-scale deformations that have shaped the lithospheric structure of various mobile belts and ancient cratons across the world. They shed light on various aspects of evolutionary tectonics in any region. The current study primarily aims at examining the direct S-wave derived seismic anisotropy beneath the Eastern Ghats Mobile Belt (EGMB) and the adjacent Archean Singhbhum and Bastar Cratons. The granulite Terrane of EGMB represents the complex deformational history and is a crucial link in the reconstruction of Rodinia and Gondwana supercontinents. The Terrane underwent collision and rifting in Mesoproterozoic, followed by thermal overprint of the mid-Neoproterozoic to early Phanerozoic orogeny modifying its lithospheric structure. We have implemented the Reference Station Technique and obtained 854 well-constrained individual splitting measurements. The observations are based on 185 earthquake events (with Mw⩾5.5, and epicentral distance ranging 30° to 90°) recorded at 27 seismic stations deployed in the study area. The large splitting delay times (1.2–2.3 s) suggest that the lithospheric mantle is highly anisotropic. The NNE-SSW oriented Fast Polarization Directions (FPDs) observed at the Singhbhum Craton can be devoted to the evolutionary tectonic regime of the craton. The strained minerals at the deeper depths, result in FPD patterns sub-parallel to the Kerajung Fault Zone (~70°) in the Angul Domain. The FPDs are predominantly oriented along the Absolute Plate Motion (APM) direction of the Indian plate (~39°) for the Bastar Craton. However, the varying FPD patterns in the Eastern Ghats Boundary Shear Zone (Khariar Domain) signify the influence of the lithospheric deformation along with APM. The imprints of the continental building orogenies, that have modified the lithospheric structure of the study area over the Mesoproterozoic, mid-Neoproterozoic, and early Phanerozoic periods, are preserved as frozen anisotropic signatures. Our observations highlight these signatures. The study also investigates seismic anisotropic patterns in the hitherto uncharted regions close to Chilka Lake using the core-mantle refracted (SKS, SKKS, and PKS) and direct S phases. The markedly distinct E-W (average ~95°) oriented fast wave azimuths, against the ~47° average FPD of the adjacent western Phulbani Domain, distinguishes it as a separate block amongst the collage of domains forming the Eastern Ghats Province.