Nature of the crust below the Southern Granulite Terrain (SGT) of Peninsular India across the Bavali shear zone based on analysis of gravity data

Abstract

Abstract The Bavali shear zone is a major Proterozoic structure in the Southern Granulite Terrain (SGT) of Peninsular India. A detailed gravity survey has been carried out in order to delineate the crustal structure and tectonics of the shear zone and adjacent areas. The Bouguer anomaly map of the region shows a number of features: (i) an E-W gravity gradient zone with an average gradient of 3.0 mGal/km in the west and 0.4 mGal/km in the east across the shear; (ii) a broad gravity low with a relief of 20 mGal in the Wynad plateau SE of the shear; (iii) a series of gravity lows in the southern part of the shear between Kannur and Vayittiri; (iv) a broad gravity high of 30–40 mGal encircling the charnockite province in the NW part of the shear, and; (v) a sharp localised gravity high closure of 40–50 mGal towards coast at Nileswaram. Spatially filtered gravity anomaly maps clearly exhibit contrasting gravity signatures on either side of the shear zone with relatively higher gravity values within the northern block. Crustal thickness estimates, in general, vary from 36 to 41 km with a thinning of 2 to 3 km towards the north below the shear zone. Using two-dimensional crustal models determined from gravity anomaly profiles across the Bavali shear zone, it is interpreted that the shear zone extends downward close to a plane separating blocks of different crustal densities. The shear zone dips steeply towards the south and its downward extension meets with a slight Moho rise suggesting that the Bavali shear zone is a crustal-scale structure. Eastwards below the Wynad plateau, however, the shear zone becomes less prominent suggesting local variations in its geometry. The models also point out the presence of a thick high-density body (2.98 g/cc) in the lower part of the crust below the northern charnockite province, which narrows towards the Wynad plateau region in the east. The presence of such a high-density body could be due to factors such as evolution together with the protoliths of overlying charnockitic rocks, a dense lower crustal residual fraction during anorthosite genesis, or emplacement during Pan-African/later anorogenic events. The models also indicate that the Wynad plateau region is characterised by a relatively homogeneous crust.