Crustal structure and nature of emplacement of the 85°E Ridge in the Mahanadi offshore based on constrained potential field modeling: Implications for intraplate plume emplaced volcanism

Abstract

An integrated interpretation of multi-channel seismic reflection, gravity and magnetic datasets belonging to northern most part of the 85°E Ridge in the Mahanadi offshore is carried out to study the crustal structure and mode of its emplacement. The basement structure map of the ridge reveals that it is 130–150km wide and is composed of an eastern high which appears as a continuous, broad and smooth topographyand the western high characterized by several steep isolated highs. The seismic velocities reported for the first time over the ridge indicate several sedimentary sequences ranging in velocities between 1.6 and 4.0km/s above the acoustic basement top. The salient aspects of the sedimentary velocities are; a low velocity layer (2.6–3.2km/s) within the Cretaceous sequence in the intervening depressions encompassing the flank region, and a regionally widespread higher velocity layer (3.5–3.8km/s) belonging to the Eocene–Oligocene section overlying the ridge. A layer having a velocity of 4.2–4.7km/s probably made of volcanoclastic rocks is observed immediately below the acoustic basement. The sediment isopach maps presented here for three major horizons are used to compute the 3-D sediment gravity effect to obtain a crustal Bouguer anomaly map of the region. Detailed analysis of the gravity and magnetic anomaly maps clearly demonstrates the continuity of ridge up to the Mahanadi coast at Chilka Lake. Seismically constrained gravity and magnetic models indicate that the ridge is composed of volcanic material that was emplaced on continental crust in the shelf-slope areas and over the oceanic crust in the deep offshore areas. The modeled crustal structure below the ridge further indicates volcanic emplacement of the ridge on a relatively younger lithosphere. We propose two alternative models for the emplacement of the ridge.