Chapter 3 Regional context of the geology of the Andaman–Nicobar accretionary ridge

Abstract

Abstract The Andaman–Nicobar accretionary ridge forms the eastern boundary of the Bay of Bengal and is presently being constructed by accretion and underplating of sediments off-scraped from the obliquely colliding Bengal Fan. Net accretion is relatively low ( c. 28%) with the rest subducted mostly into the upper mantle. Although subduction initiated along the margin at c. 95 Ma, large-scale subduction accretion likely accelerated during the Early Miocene by which time wedge top basins had formed. Prior to that time, sediment off-scraped during the Eocene against the ophiolitic backstop was probably derived from the adjacent magmatic arc of Burma. There was also some erosion of continental sources at that time, probably from the Sibumasu Block which forms the western edge of Sundaland. The scale of this accretion was small and potentially interrupted by times of tectonic erosion during the Palaeogene. The influence of continental erosion increased into the Oligocene, potentially accompanied by modest flux from the Indus–Yarlung Suture via the Irrawaddy River. Drainage capture in eastern Tibet in the Early Miocene and opening of the Andaman Sea, probably in the Late Miocene, has removed these source areas to the Andaman Trench.