Tectonics and history of the Andaman Sea region

The Andaman Sea is an active backarc basin lying above and behind the Sunda subduction zone where convergence between the overriding Southeast Asian plate and the subducting Australian plate is highly oblique. The effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right-lateral fault system. The late Paleocene collision of Greater India and Asia with approximately normal convergence started clockwise rotation and bending of the northern and western Sunda Arc. The initial sliver fault, which probably started in the Eocene, extended through the outer arc ridge offshore from Sumatra, through the present region of the Andaman Sea into the Sagaing Fault. With more oblique convergence due to the rotation, the rate of strike-slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike-slip motion. These basins in sequence are the Mergui Basin starting at ∼32 Ma, the conjoined Alcock and Sewell Rises starting at ∼23 Ma, East Basin separating the rises from the foot of the continental slope starting at ∼15 Ma; and finally at ∼4 Ma, the present plate edge was formed, Alcock and Sewell Rises were separated by formation of the Central Andaman Basin, and the faulting moved onshore from the Mentawai Fault to the Sumatra Fault System bisecting Sumatra.