Abstract 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 anomalies 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.

Abstract Sumatra is an active (Andean) continental margin that would be linked by land to SE Asia if sea level fell by as little as 50 m. Present-day tectonic processes are controlled by three major fault systems, the most obvious of which is the subduction thrust which crops out in the Sunda Trench. The trench curves very little in the 800 km between Enggano and Nias, i.e. off central Sumatra (Fig. 2.1), but is markedly convex towards the Indian Ocean both further north and further south. Water depths of more than 6000 m are reached in the south but the maximum in the north may be less than 5000 m. The difference is usually, and convincingly, attributed to the presence on the Indian Ocean plate of the Nicobar Fan, consisting of sediments, derived ultimately from erosion of the Himalayas, which increase steadily in thickness towards the north (e.g. Hamilton 1979). Continuing subduction is attested by a Wadati-Benioff Zone (WBZ) that extends to depths of the order of 200 km (e.g. Newcomb & McCann 1987 ) and by volcanic activity in the Barisan mountains, the peaks of which generally lie within a few tens of kilometres of the coast. The change, of more than 45Ð, in the trend of the trench between 96ÐE and 97ÐE (the ‘Nias Elbow‘) may have been initiated by subduction of the 2 km high Investigator Ridge (Investigator Fracture Zone), which trends approximately north-south at about 98ÐE. Sieh & Natawidjaja (2000) defined a “Central Domain” of mainland Sumatra between

Abstract The Andaman–Nicobar subduction system is the northwestern segment of the Sunda subduction system, where the Indian Plate subducts beneath the Sunda Plate in a nearly arc-parallel direction. The entire segment ruptured during the 2004 great Andaman–Sumatra earthquake ( M w =9.3). Using recently acquired high-resolution seismic reflection data, we characterize the shallow structure of the whole Andaman–Nicobar subduction system from west to east, starting from the nature of the subducting plate in the Bay of Bengal to back-arc spreading in the Andaman Sea. We find that the Ninety-East Ridge is overlain by thick continental margin sediments beneath the recent Bengal Fan sediments. The boundary between these two sedimentary units defines the plate interface. We observe evidence of re-activation of fracture zones on the subducting plate beneath the forearc, influencing the morphology of the upper plate. The forearc region, which includes the accretionary wedge, the forearc high and the forearc basin, is exceptionally wide (250 km). We observe an unusually large bathymetric depression within the forearc high. The forearc high is bounded in the east by a normal fault, whereas the forearc basin contains an active backthrust. The forearc basin is floored by the continental crust of Malayan Peninsula origin. The active sliver strike-slip fault lies in a deep basin, created during the rifting of the forearc continental crust and the Malayan Peninsula. The sliver fault connects with the Great Sumatra Fault in the south and with the Sagaing Fault in the north, via the Andaman Sea spreading centre and a large transform fault in the Andaman Sea.

Abstract The Andaman Flysch of Oligocene age comprises basinal-scale deposits of marine turbidites from an axially fed submarine fan. It is intermittently exposed across the entire chain of the Andaman–Nicobar Islands from the northernmost tip of North Andaman to the southernmost part of Great Nicobar. This chapter reviews the field and petrographic attributes, heavy mineral assemblages, major and trace elements, and isotope geochemistry of the most accessible outcrops on the main Andaman islands.