Gravity signatures, derived crustal structure and tectonics of Achankovil Shear Zone, southern India

The kinematics of the crustal-scale Achankovil Shear Zone in the Southern Granulite Terrane, India is often debated. This shear zone is considered to mark the terrane boundary between Madurai Granulite Block towards the north and Kerala Khondalite Belt towards the south. Closely spaced 4500 new gravity measurements in the region reveal the detail crustal fabric across the Achankovil Shear Zone. A subdued Bouguer gravity anomaly along a N–S profile that trends approximately orthogonal to the Achankovil Shear Zone signifies that this part of the crust is devoid of any crustal-scale density discontinuity. Furthermore, horizontal-gradient, analytical signal and second vertical derivative analyses of the gravity data suggest that the density inhomogeneity across the Achankovil Shear Zone is relatively shallow in origin thereby refuting it as a terrane boundary. A prolonged zone of positive gravity gradient, as much as 125 km wide towards south, is due to the effect of the continental margin. 2 1/2D gravity modelling along the profile, constrained from seismic results, reveals a three layer crustal configuration with the depth to Moho varying from 41 km beneath the Vattalkundu to about 34 km beneath the Kanyakumari and then attaining to about 32 km beneath the continental shelf region. The 22 km thick quasi-continental crust in the adjoining Indian Ocean indicates a transitional crust in the Gulf of Mannar region. Analysis of new gravity data thus supports the idea that the Achankovil Shear Zone is an intracratonic litho-tectonic feature and the two provinces across it are related by a continual progression in single metamorphic terrain rather than an ancient geo-suture.