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

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 131 = 1.07-1.23 and 132 = 1.09-1.15 respectively. We noticed that the total crustal stretching factor (13) 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 13 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.

Automated summary

The study revealed a hyperextended continental crust in the Mannar basin, with two stages of rifting occurring between 130-115 Ma and -100-65 Ma, resulting in minor crustal stretching. The southern part of the basin experienced higher crustal stretching, potentially leading to increased accommodation space and sediment influx. Late Cretaceous intrusive volcanism in the area influenced geothermal gradient and hydrocarbon generation.