New constraints on the formation and evolution of the Andaman Sea, a sedimented back arc spreading center in the South East Asia, from seismic reflection studies.

Abstract:

The Andaman Sea is an enigmatic feature in the Indian Ocean region. To the west, it is bounded by a near arc parallel Andaman subduction system and to the east by the Malaya Peninsula. It hosts volcanic provinces like Alcock and Sewell Rises and the Andaman Sea Spreading Center (ASSC) that connects the sliver strike-slip Sagaing Fault in the north with the Andaman Nicobar and Great Sumatra Faults in the south. The actual spreading center follows a succession of basins, starting by the spreading of the Mergui basin in the south-east, 32 Ma ago, that shifted to the actual position of the spreading closer to the subduction trench. Several hypotheses have been proposed for the formation of the Andaman Sea basins: (a) Pull-apart basin along the Sagaing-Sumatra fault system, driven by the collision-extrusion mechanism and/or by the slip-partitioning induced by the oblique subduction, (b) Back-arc spreading due to the subduction. There is a debate about the orientation of the present spreading/extension between the North-South motion along the strike-slip faults and the NW-SE opening of the ASSC. We have access to 7000 km of high-resolution deep seismic reflection data, and high-resolution bathymetry data, which we combine with relocated earthquake data to shed light upon the formation and evolution of the Andaman Sea Basin. The central basin contains up to 4 km thick sediments. The crustal thickness is about 5-8 km in the central basin and increases to 13-15 km beneath the Alcock and Sewell Rises, which are devoid of sediments. Here we show how both the collision and the subduction play a role in the position and orientation of the extension in the Andaman Sea Basin, and how they influence the accretion at the spreading center.