Precambrian Tectonics of the South Indian Shield Inferred from Geophysical Data

Abstract

Geophysical parameters such as MAGSAT, LANDSAT, gravity, deep seismic sounding, seismicity, and heat flow data have identified cratonic and mobile units in the Precambrian continental crust of the South Indian shield. LANDSAT imagery and gravity field data suggest that the shield is divided into four geophysical domains from north to south, namely: (1) the Deccan trap-covered gneissic basement and the gneiss terrain around Hyderabad, (2) the Dharwar craton, subdivided into its eastern and western blocks, (3) the granulite terrains of Kerala, Karnataka, and Tamil Nadu, and (4) the Eastern Ghats granulite belt. The smooth transition in the gravity field from the Dharwar greenstone terrain to the Deccan trap-covered area to the north indicates the northward continuity of the granite-gneiss basement. However, the trap-covered region lacks any significant short-wavelength anomalies of the type associated with the Dharwar greenstone belts, indicating the probable absence of any such large-scale high-density belts underneath the Deccan traps. The Chitradurga schist belt, a 450 km-long linear feature of Archean-Early Proterozoic age, is technically one of the most important features of the shield. Geophysical studies suggest that the eastern margin of the Chitradurga belt (CDB) is the site of crustal shortening during Late Archean-Early Proterozoic times, a result of E-W compressional forces operating at the time of the evolution of the Early Proterozoic mobile belt (EPMB) in the eastern block of the Dharwar craton. The CDB probably represents a paleo-rift on the Archean crust where early tensional forces created a rift opening followed by later closure under compressional forces. In its northern part, the southern granulite terrain is geophysically continuous with the Dharwar craton, but in more southern portions it is segmented by fault-bounded blocks of different degrees of uplift. Gravity and seismic crustal investigations suggest that the Eastern Ghats belt (EGB) is a continental collision zone produced by some kind of plate convergence. Suggestions that the Eastern Ghats trends are geophysically continuous into the southern granulite terrain are disproven by the existence of an elongate gravity high in the southern part of the cratonal eastern block, which truncates gravity-map trends.