Bombay gravity high and eruption of Deccan flood basalts (India) from a shallow secondary plume

The western margin of India, especially the area surrounding the gravity high of Bombay, is covered by a thick pile of Deccan flood basalts. The massive eruption is one of the largest continental volcanic events at the Cretaceous/Tertiary biological mass extinction boundary ( ∼ 65 Ma ago). The area is characterized by several hidden tectonic features, and their origin and evolution have remained controversial. In the search for magma source structure, attention was focused on an unusual oval-shaped positive gravity anomaly over the Bombay coastal zone. The gravity analysis indicates the presence of two subsurface structures at depths of 4.5 ± 0.5 km and 18 ± 2 km. These depths correspond to the top of the crustal basaltic layer and an upwarped Moho discontinuity, respectively. The positive gravity anomaly in three-dimensional analysis reveals a possible magma conduit structure and the near absence of the granitic layer. The unusually shallow Moho layer coincides with the region of a broad high heat flow anomaly (average heat flow more than 80 mW/m²). The geophysical signatures from the neighborhood show that the asthenosphere has welled up considerably and is located at a depth of about 40 km. High heat flow and asthenospheric depth estimates suggest melting conditions at very shallow depths. It is suggested that the highly differentiated magma associated with the major eruptive phase of Deccan Traps did not originate at the D′' layer. It is hypothesized that magma was brought to the surface by a major secondary plume at a much shallower depth within the lithosphere and possibly at the crust-mantle boundary, as a consequence of subcrustal erosion and secondary melting caused by the extreme velocity of India during the late Cretaceous and early Tertiary. Substantial melting of the base of the lithosphere due to frictional heating might have resulted in the accumulation of a large quantity of magma below the thinned lithosphere at the pre-existing fracture zone near the continental margin. The presence of a magmatic conduit structure above the Moho, derived from the inversion of gravity data and other tectonic conditions, suggests a possible triggering of the magma reservoir by a large bolide impact ( ∼ 10 km diameter) at the Cretaceous/Tertiary boundary near Bombay.