Crustal structure and subsidence history of the Mannar basin through potential field modelling and backstripping analysis: Implications on basin evolution and hydrocarbon exploration

Abstract

The Mannar basin is a hydrocarbon-bearing rift basin located between the southeast Indian and the western Sri Lanka margins. In the present study, an integrated analysis of potential field data and multi-channel seismic profiles along with the biostratigraphic information from the exploratory wells is undertaken to delineate the crustal structure, long-term strength of the lithosphere (Te), and subsidence history of the Mannar basin. The constrained potential field modelling and 2-D flexural backstripping analysis across the Mannar basin revealed the presence of a hyperextended continental crust defined by the Te of 25 km and an elevated Moho geometry than the adjacent continental regions. The tectonic subsidence data derived from the 1-D backstripping of wells revealed that the present-day Mannar basin was developed in two stages of rifting; the first rifting occurred contemporaneous with the separation between the East Indian margin and East Antarctica (130-115 Ma), and the second phase of rifting took place during ~100-65 Ma. Both these rifting events gave rise to minor crustal stretching with stretching factors of β1 = 1.07–1.23 and β2 = 1.09–1.15 respectively. We noticed that the total crustal stretching factor (β) and width of the extended crust increase from north to south along the Mannar basin, indicating the anti-clockwise rotation of Sri Lanka during the rifting. The higher β values in the southern part of the basin might have led to greater accommodation space and more sediment influx. The peak subsidence rates (~0.016–0.037 km/Ma) at ~60 Ma indicate that late Cretaceous intrusive volcanism locally enhances geothermal gradient and hence influence the maturation and hydrocarbon generation of sources.