Research Article Crustal Density and Susceptibility Structure beneath Achankovil Shear Zone, India M. Prasad, 1,2 C. P. Dubey , 1 K. B. Joshi , 1 and V. M. Tiwari 3 1 Solid Earth Research Group, National Centre for Earth Science Studies, 695011, Thiruvananthapuram, India 2 Department of Marine Geology and Geophysics, Cochin University of Science and Technology (CUSAT), 682022, Kochi, India 3 CSIR-National Geophysical Research Institute, 500007, Hyderabad, India Correspondence should be addressed to C. P. Dubey; p.dubey48@gmail.com Received 7 April 2021; Accepted 4 August 2021; Published 4 October 2021 Academic Editor: Sayandeep Banerjee Copyright © 2021 M. Prasad et al. Exclusive Licensee GeoScienceWorld. Distributed under a Creative Commons Attribution License (CC BY 4.0). The Southern Granulite Terrain (SGT) is a large tract of exposed Archean continental crust, divided into the Madurai Block (MB), Trivandrum Block (TB), and Nagercoil Block (NB). These crustal domains are linked with the NW-SE trending Achankovil Shear Zone (AKSZ). We combine gravity and magnetic data with previously published ground observations and geochronological data to re-evaluate the crustal architecture, evolution of the AKSZ, and possible extension of AKSZ into Madagascar. Analyses indicate that the long wavelength trends of the magnetic anomalies originate at ~20 km depth of different SGT blocks. These observations are corroborated with the gravity as well as computed gravity gradient anomalies. The presence of khondalite outcrops in Trivandrum Block implies that high magnetization crust is the main source of positive magnetic anomalies. Such magnetic anomalies advocate that SGT preserves the remanent of Archean crustal blocks in South India, a part due to variation in thermal and geochemical processes. The AKSZ, TB, and MB exhibit contrasting magnetic crustal signatures. The joint modeling results reveal a three-layered crustal configuration with varying Moho ranging from 41 to 34 km in NE to SW, respectively. It is also noted that AKSZ is a narrow and deep structure near to the Western Ghats Escarpment while it is wide and shallow in the far-east, which implies that the evolution of the Western Ghats is a late geological event. 1. Introduction The crustal structures of shear zones in the southern part of India have attracted wide attention owing to their vital role in reconstructing the Precambrian evolution of Southern Granulite Terrain (SGT). The SGT is by far the largest exposed Precambrian deep continental crust consisting of multiple deformed Archean and Neoproterozoic high- grade metamorphic and magmatic rocks which are dissected by several shear/suture zones [1, 2]. The crustal structure and nature of large scale geological or tectonic features in southern part of SGT are not fully understood. The key to unraveling the evolution of South India, and its role in the amalgamation of Gondwana supercontinent, lies in the exposed lower crustal section of the SGT which is associated with several shear and suture zones [2–5]. The Achankovil Shear Zone (AKSZ) forms the southern tip of the SGT and separates the Madurai Block (MB) to the north and the Trivandrum Block (TB) to the south. The AKSZ plays a crucial role in Gondwana correlations and global tectonics. It provides an opportunity to understand the tectonic and geodynamic evolution of Western Ghats Escarpment (WGE), SGT and its associated lithological blocks, viz., Trivandrum and Madurai Blocks. It is also believed that the final amalgamation of Gondwanaland took place during the Pan-African, and Madagascar is placed against India in the Gondwana assembly. Such assemblage discrepancies in the SGT and their connection to Madagas- car for the Gondwana reconstruction is of immense interest as the crustal architecture are still debated in terms of phys- ical parameters (density and magnetization) and geological history. Most of the geophysical studies in SGT involve a single transect of Vattalkundu-Kanyakumari profile [6–11] except for velocity analysis with limited seismic data from two sta- tions by [12]. The seismic tomography and reflection studies GeoScienceWorld Lithosphere Volume 2021, Article ID 6017801, 17 pages https://doi.org/10.2113/2021/6017801 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2021/6017801/5421904/6017801.pdf by guest on 06 October 2021