A Geotransect from Dharimanna to Billimora across the Cambay and Narmada-Tapti Rift Basins, India

Abstract

The rift basins of northwestern India are genetically related to the evolution of the west coast of India, which evolved in different stages due to the breakup of Africa, Madagascar, and the Seychelles. A geotransect covering these rift valleys provides significant information regarding their structures and evolution, essential to an understanding of the break-up history of the Indian plate. The Bouguer anomaly map of western India has delineated three sets of linear gravity anomalies representing the Kutch (ENE-WSW), Cambay (NNW-SSE), and Narmada-Tapti (ENE-WSW) basins connected perpendicularly from north to south. A crustal cross section along a geotransect from Dharimanna to Billimora (550 km) along the Cambay and across the Narmada-Tapti basins is presented, combining the known geology and tectonics with the seismic section and the modeling of gravity profiles. The resulting crustal model suggests a shallow Moho at a depth of 31 to 32 km, a high-density layer in the lower crust (3.1 g/cm3) at a depth of 25 to 26 km, and a low-density layer in the upper crust at a depth of 8 to 10 km. These are signatures of extensional regimes, suggesting them to be typical continental rift valleys. High heat flow and occurrences of seismic activities suggest that these rift valleys are still active to a limited extent. The entire Cambay basin is divided into several subbasins by transverse ridges that are fault controlled and may represent mafic, basic, and acidic intrusive bodies as found along its southwestern margin. The maximum thickness of Tertiary sediment varies from 2 to 3 km over the ridges to 5 to 6 km in the depressions. These sediments are underlain by Deccan Traps and Mesozoic (Cretaceous) sediments in certain parts. The Narmada-Tapti section south of the Cambay basin coincides with a gravity low that is a depression, and a major gravity high that extends for ~1000 km from the west coast of India to central India. It presently represents a median uplift in the form of the Satpura high caused by large-scale magmatism in the crust. It is suggested that although the breakup of Africa from India and related magmatism is primarily responsible for the evolution of the Kutch rift basin and the Gulf of Kutch, the latter two events—namely the breakup of Madagascar and the Seychelles-have controlled the evolution of the Cambay and the Narmada-Tapti rift basins from the Gulf of Cambay. These breakups of Indian plate have produced a considerable amount of magmatism in the crust in the form of intrusions and underplating in northwestern India, giving rise to several large-wavelength gravity and magnetic anomalies.