Morphotectonic signatures and revised timing of opening of the Andaman Backarc Basin, Northeast Indian Ocean

Abstract

The Andaman Backarc Basin is believed to have been formed by the present episode of seafloor spreading in the Andaman Sea, separating the Alcock Rise in the north (Burmese Plate) and the Sewell Rise in the south (Sunda Plate). The geophysical investigations carried out so far in this basin provided detailed information on the morphotectonic signatures of its central sector, however, such understanding of the eastern sector is still awaited. Those studies also inferred continual seafloor spreading in the Andaman Backarc Basin starting from 4.0 Ma, however, some of the subsequent studies, based on deep water sedimentation, postulated an episodic seafloor spreading responsible for the formation of the Andaman Backarc Basin. To address these problems, we revisited the morphotectonic signatures and evolution of the Andaman Backarc Basin using a new set of multibeam bathymetry and magnetic data from the central and eastern sectors of the Andaman Backarc Basin, complemented by the existing geophysical data from the central sector. Our updated bathymetry map of the newly surveyed northernmost segment (referred as Segment D), is characterized by the presence of a well-defined rift valley, with an along-axis gentle dip from the east to the west. The segment D is also characterized by the absence of significant magnetic anomalies, as in the case of segment C, corroborating the concept of replacement of layer 2A by a sill-sediment-metasedimentary layer, in a sedimented spreading centre. Further, our revisit to the magnetic anomaly identifications suggests that the magnetic anomalies over the segment B of the Andaman Backarc Basin can reasonably be explained in terms of continual seafloor spreading from chron C3An.1ny (5.894 Ma) till the present (0.0 Ma). Based on these identifications of magnetic anomalies and the plate tectonic reconstructions, we revise the age of opening of the Andaman Backarc Basin from 4.0 Ma to ~5.9 Ma.