Imprints of diverse mantle deformational episodes in the Cauvery Suture Zone, South India

Abstract

This study attempts to understand the upper mantle deformation pattern beneath the northern part of Southern Granulite Terrain (SGT), located to the south of Archean Dharwar craton in south India. The study region comprises the tectonic blocks – Nilgiri, Biligiri Rangan, Shevaroy and Madras blocks – separated from the northern part of Madurai block by the Cauvery Suture Zone (CSZ), associated with subduction–accretion–collision tectonics. Shear wave splitting observed on high quality XK(K)S (SKS, SKKS, PKS) waveforms from 119 teleseismic events recorded by 19 broadband seismic stations, mostly within the CSZ and the northern part of Madurai block, is utilized in this study. The 122 splitting measurements obtained, show an average delay time of 0.8s between the fast and slow waves. Strong lateral variations in the fast polarization azimuths (FPA) are observed, with the splitting parameters at the majority of stations showing a dependence on back azimuth. Importantly, excluding the Madras block, a poor correlation between the FPA and the present day absolute plate motion (APM) direction is observed. This dominance of non-APM strain reflects complex anisotropy preserved in the lithosphere. By plotting the fast polarization trends based on the ray piercing points at a depth of 150km, three distinct splitting patterns associated with the (i) Biligiri Rangan block and the eastern part of the Nilgiri block, (ii) eastern part of the CSZ and the northern Madurai block (NMB), and (iii) Madras block and its vicinity within the CSZ, are observed. The strong lateral heterogeneity in the deformation of the continental lithosphere might be either related with the shear zones that are associated with frozen magmatic intrusions, two episodes of ocean closure and the collision that followed, plate motion related strain or a combination of these. Similar deformation patterns in the northern part of Madurai block might be related to remnants of subducted oceanic lithosphere.