The Andaman-Nicobar Ridge and the Indo-Burman Range are composed of sediments of the Bengal and Nicobar Fans scraped off the underthrusting Indian plate at the Sunda subduction zone. The Andaman Sea and the eastern part of the central valley of Burma represent a Neogene-Quaternary extensional basin underlain by oceanic crust formed by northwestward rifting of this orogenic ridge away from continental crust of the eastern Burma highlands and the Malay Peninsula. The plate edge is defined by a north-south transform in Burma, by the Sumatran fault system longitudinally bisecting Sumatra, and by a complex system of short spreading rifts and transforms in the central basin of the Andaman Sea. Where sedimentation rates are high over the newly formed oceanic crust, no magnetic a omalies are identifiable; where sedimentation rates are low or negligible, clear magnetic anomalies have permitted dating this phase of opening at approximately 11 m.y. B.P.