Precambrian Rifts and Associated Tectonics of Peninsular India

Abstract

Geophysical data from the Archaean schist belt and granulite terrains of South India are studied. The Bouguer anomaly of the region suggests a thickening of the crust by approximately 6 km in the central part and a NNW-SSE structural trend for the schist belts. The eastern margin of the Chitradurga schist belt depicts sharp gradients in Bouguer anomaly and airborne total magnetic intensity profiles, suggesting deep-seated faults. Airborne magnetic data record high-amplitude magnetic anomalies for the mafic and ultramafic units of the schist belt, like metabasalts and magnetite quartzites with susceptibilities of the order of 15.1 × 10−3 CGS units associated with NW-SE magnetic trends for the schist belts. The granulite terrain is characterized by a large Bouguer ‘low’ acompanied by sharp gradients on either side, suggesting deep faults and thrusts with a thick crust in the center. The airborne magnetic map of this region shows high-amplitude magnetic anomalies over the known charnockite rocks and suggests a predominant NE-SW magnetic trend conforming with the general structural trend of the region. These data have been incorporated to suggest a model in which some form of underplating of the Indian crust has taken place towards the south, creating a compressional regime over the granulites, and farther north an extensional regime in which rifted basins were formed for the greenstones to deposit, as is observed in the present-day back-arc regions of the Himalayas and the Alps. The geophysical signatures of one of the Lower Proterozoic rift basins (Pakhal Basin) are described. Though the Bouguer anomaly of the region primarily reflects the effects of Gondwana (Permian-Jurassic) tectonics in the region, its linear nature, indications of basic intrusions along the marginal faults and the presence of high-density material along the Moho suggest that the Pakhal sediments were deposited in rifted basins which might have influenced an even later orogeny during Gondwana time to form a typical continental graben in the region.