Gravity study along the Kavali-Udipi deep seismic sounding profile in the indian peninsular shield: Some inferences about the origin of anorthosites and the eastern ghats orogeny

Abstract

The crustal section along a 600 km long profile across the Indian peninsular shield between Kavali on the east coast and Udipi on the west coast, running approximately along 14°N latitude, can be represented by a generalised three-layer crustal model in accordance with the interpretation of gravity data along this profile based on the structural picture obtained from deep seismic soundings (DSS) (Kaila et al., 1979). These three crustal layers are: an upper layer of density 2.70 g/cm 3 corresponding to an average P-wave velocity ( V p ) of 6.2 km/sec, an intermediate layer of density 2.75 g/cm 3 ( V p = 6.5 km/sec), and a deeper layer of density 2.85 g/cm 3 ( V p = 6.8 km/sec). Crust of varied thicknesses and comprising the above three layers, sits in general on upper mantle rocks of 3.3 g/cm 3 corresponding to V p = 8.2 km/sec. The regional gravity anomaly pattern along the profile can convincingly be explained as being due to the combined effect of intracrustal structural features and the configuration of the Moho boundary as developed through tectonic activities along a number of faults detected by DSS. In the eastern portion of the profile, corresponding to the Cuddapah basin, there exists an additional deep crustal layer of density 3.05 g/cm 3 ( V p = 7.2 km/sec), and an emplacement of highdensity material at shallow depths along weak zones on the eastern margin of the Cuddapah basin. This magmatic high-density mass at shallow depth may extend as a linear subsurface feature along the entire Eastern Ghats belt as evidenced by the continuity of the similar gravity anomaly pattern and may be responsible for the Eastern Ghats orogeny and uplift. This feature may be responsible for the plutonic activity to which the origin of anorthosite rocks, found along the Eastern Ghats, can be associated. In the western part of the Cuddapah basin the basaltic layer has thickened considerably with its top reaching a shallow depth of about 8 km. This thickening of the basaltic layer is considered responsible for the generation of numerous dykes and sills in this region.