Abstract Rocks exposed across the hundreds of islands that belong to the 800 km long Andaman–Nicobar archipelago provide a condensed window into the active subduction zone that separates the India–Australia plate from the over-riding Burma–Sunda plate. Despite a strategic and seismically active location the Andaman-Nicobar ridge has seen comparatively little research. This Memoir provides the first detailed and comprehensive account of geological mapping and research across the island chain and adjacent ocean basins. Chapters examine models of Cenozoic rifting of the Andaman Sea and the regional tectonic and seismogenic framework. A detailed critical review of the Andaman–Nicobar stratigraphy, supported by new data, includes arc volcanism and a description of Barren Island, India’s only active volcano. Seismic history and hazards and the impacts of the 2004 earthquake and tsunami are also described. The volume ends with an examination of the region’s natural resources and hydrocarbon prospects.

Abstract This chapter examines the history of reconnaissance and geological mapping work on the Andaman and Nicobar islands. To understand early exploration it is necessary to review the driving forces for colonization, including the development of the Andaman Islands as a penal colony for political prisoners. Geological mapping conducted in the colonial era continued after India gained independence in 1947 and expanded in the 1980s to include hydrocarbon and mineral resources. More recent work has placed greater emphasis on supporting field observation data with geochronological, geochemical and petrological analyses.

Abstract The geography and the geomorphology of the Andaman–Nicobar accretionary ridge (islands) is extremely varied, recording a complex interaction between tectonics, climate, eustacy and surface uplift and weathering processes. This chapter outlines the principal geographical features of this diverse group of islands.

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.

Abstract The Andaman Sea evolved from near-pure extension (WNW–ESE) during the Late Palaeogene, to highly oblique extension (NNW–SSE) during the Neogene, to strike-slip-dominated deformation (Late Miocene–Recent). These changes in extension direction and deformation style probably reflect the switch from slab rollback-driven extension to India coupling with Myanmar and driving oblique extension/dextral strike-slip. The East Andaman, Mergui–North Sumatra and Martaban Shelf basins, along with the Alcock and Sewell rises and Central Andaman Basin (CAB), were all involved with this deformation which became increasingly focused on the CAB and the rises with time. Possible revisions to traditional models for the Andaman Sea include: (1) the Alcock and Sewell rises are hyper-extended continental or island arc crust, not Miocene oceanic crust; (2) the East Andaman Basin is predominantly underlain by strongly necked to hyper-extended continental crust, not oceanic crust; or (3) CAB oceanic crust is of Miocene, not Pliocene–Recent age. At present a number of major issues can be addressed but not fully resolved, including: (1) the distribution, timing, volume and origin of magmatism in the basins; (2) the causes and significance of strong crustal reflections imaged on 2D and 3D seismic data; (3) implications for crustal thinning geometries, upper crustal extensional patterns and distribution of igneous intrusions for current models of passive margin development (i.e. volcanic v. non-volcanic margins), and how the back-arc setting modifies these models. Elements of both volcanic and non-volcanic margins are present in the East Andaman Sea, with well-developed necking of continental crust (perhaps due to dry mafic, granulite facies lower crust) and extensive igneous intrusions in the lower and middle crust.

Abstract A wide range of tectonic models exist for the Cretaceous–Cenozoic development of the Sumatra–Andaman–Myanmar region, reflecting outstanding regional issues including: (1) suture zone correlation between Tibet and Myanmar; (2) understanding ophiolitic fragment emplacement; (3) timing of ophiolite emplacement; (4) tectonic setting of ophiolite formation; (5) post-emplacement ophiolite history; (6) number, distribution and accretion timing of different tectonic continental blocks in western SE Asia; (7) how the Andaman–Sumatra subduction zone developed during the Cenozoic, and location and timing of inactive, v. obliquely subducting segments; and (8) considerable variations in regional plate tectonic reconstructions (e.g. latitude of Lhasa Block at the time of collision, amount and direction of block rotation within SE Asia). Following reviews of these issues we propose a relatively simple model whose characteristics are continuity of a single continental mass between Myanmar and Sumatra during the Cenozoic, early Cenozoic ophiolite emplacement as imbricate slices within an accretionary complex and no emplacement of a major overthrusting oceanic slab. Subsequent collisional deformation further dismembered the ophiolites. Approximately 30° clockwise rotation of SE Asia occurred following Asia–India collision, accompanied by transition from a paired Andean-type magmatic belt to regional oblique-slip and strike-slip tectonics. During the Neogene the Andaman sea region became dominantly transtensional, while Myanmar in the Late Neogene became transpressional.

Abstract The Andaman–Nicobar archipelago that forms the western margin of the Andaman Sea is a sediment-dominated accretionary wedge (outer-arc island) associated with a convergent margin tectonic setting. The Andaman accretionary ridge consists of two stratigraphically and structurally distinct terranes, juxtaposed and telescoped into a north–south-trending high-relief fold-thrust belt formed along the obliquely subducting eastern margin of the Indo-Australian oceanic lithospheric plate. The geology and structure of the ridge reflect the complexity of the evolving tectonics and stratigraphy of an accretionary wedge. Pre-Cretaceous meta-sedimentary rocks, Upper Cretaceous ophiolites and Palaeogene–Neogene sedimentary formations indicate rapid, spatial and temporal changes in lithology, sedimentology, sedimentary and tectonic environments, and palaeogeographic setting. This chapter outlines the current geodynamic setting, evidence for the history of accretion and regional geology and introduces the regional stratigraphic framework.

Abstract Andaman–Nicobar Ophiolites (ANO) occur as discontinuous bodies along the eastern margin of the Andaman and Nicobar Islands, the exposed parts of the outer-arc ridge of the present Sunda subduction system. The lithospheric architecture starts with mantle rocks overlain by crustal rocks with a thin transition zone in between. The mantle peridotites and the volcanic rocks exhibit great variability all along the ridge and demonstrate influence of subduction-related magmatism in their origin. Like many Tethyan ophiolites, the ophiolitic rocks of Andaman–Nicobar had their origin in a supra-subduction zone that were juxtaposed tectonically with younger sediments, now exposed on the present outer-arc ridge. The final emplacement of this oceanic lithosphere was unlike typical Tethyan-type ophiolites because, before its final emplacement over the Indo-Burma-Andaman (IBA) microcontinent, the subduction margin was charged with huge sediments from the river delta systems to the north that accreted at the leading age of the overriding plate, similar to some extent to cordilleran-type ophiolites. We propose a two-stage subduction model that displays a sequence of events from birth to resurrection that explains the petrological, geochemical and architectural variations of ANO. Supplementary material: The geochemical tables are available at https://doi.org/10.6084/m9.figshare.c.3634331.v1

Abstract The rocks assigned to the Mithakhari Mélange are composed of conglomerates, gritty and coarse- to fine-grained volcano-lithic to quartzo-feldspathic sandstones, andesitic tuff, siltstone, mudstones, shale, carbonaceous shale and limestones. They occur as coherent and chaotic units as part of the regionally extensive mélange terrane and include olistoliths of pre-ophiolite metasedimentary rocks, ophiolitic ultramafics and basalts, and pelagic-hemipelagic sediments. This chapter describes and assesses the established lithostratigraphic units and examines their palaeontology, geochemistry and geochronology. Focusing on the units with a strong arc signature we consider environments of deposition and palaeogeography. We confirm the existence of active andesite volcanism on the arc massif located east of Andaman arc on the western margin of the Burma–Thai–Malaya peninsula during the Eocene–Oligocene, before the opening of the Andaman Sea in the mid-Miocene, and argue that the Namunagarh Grit, dominated by pyroclast-rich gravity flows, should be treated as a separate unit in studies of Tertiary stratigraphy of Andaman Island. Consideration of the other established units (Hope Town conglomerate and Lipa Shale) suggest these associated rocks types are localized, commonplace and unlikely to be stratigraphically related.

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.

Abstract Interpretation of the origin of Oligocene Flysch exposed in the Andaman–Nicobar Islands has been the subject of debate. Previous work on the provenance of the Andaman Flysch based on samples from South Andaman has indicated major contributions from Myanmar affected by the India–Asia collision, mixed with subordinate detritus from the nascent Himalayas. This study examines the provenance of a larger suite of samples that extend to North and Middle Andaman islands as well as Great Nicobar Island. Rather monotonous petrographic and heavy-mineral assemblages testify to strong diagenetic imprint, leading to a poorly constrained identification of the sediment source. U–Pb zircon ages provide more robust and diagnostic provenance discrimination between the Myanmar Arc and the growing Himalayan range. Combining petrographic and mineralogical data with detrital zircon U–Pb analyses, we find that most of the Andaman Flysch is dominated by a strong continental-crust signal with only a minor contribution from arc material. Statistical analyses of the data show that most of the samples have a provenance similar to Palaeogene Bengal Fan sediments, although the type section on South Andaman has a closer affinity to the provenance of the modern Irrawaddy. Supplementary material: Sample location (Table A1), the complete petrographic (Table A2), heavy mineral (Table A3) and U–Pb zircon-age datasets (Table A4) are all available at https://doi.org/10.6084/m9.figshare.c.3634328.v1

Abstract The geology and stratigraphy of the Archipelago Group are reviewed and new data presented to improve understanding. Major lithofacies are bioclastic limestones, bioturbated mottled marlstones, micritic limestones, trace fossil-bearing mudstones, quartzose sandstones and reworked volcanic material of Mio-Pliocene and Pleistocene age. The succession overlies the Pre-Neogene submarine fan turbidites and tectonic mélanges of ophiolite derivation and Mithakhari rocks, and is overlain by Quaternary deposits of late Pleistocene–Holocene age. The Neogene sequences were deposited in wave- and current-agitated shallow-marine intertidal and relatively deeper-water subtidal, nearshore and offshore shelfal environments. The existing stratigraphic framework is examined and suggestions made for improvement.

Abstract Barren and Narcondam are active and now extinct, respectively, volcanic islands that belong to the inner-arc volcanic belt that extends from Java in the east to Burma in the north. Modern eruptions on Barren Island have been exclusively scoria cone-forming Strombolian-type activity in contrast to historic (1787–32) eruptions that witnessed a switch from scoria cone activity to abundant basaltic lava flows. Prehistoric (late Pleistocene) eruptions were alternating effusive and explosive events. The prehistoric events formed a mafic stratovolcano (island) and the associated large eruptions produced a c. 2.0 km diameter crater at the centre of the stratocone. Barren Island lavas are mainly basalts and basaltic andesites, whereas Narcondam rocks are dacite-andesite-rhyolite and contain evidence of magma mixing of both mantle and crustal material. The types of volcanoes and volcanism associated with Barren and Narcondam have been attributed to differences in tectonic setting and the nature of the basement below the volcanoes. It is likely that continental or transitional crust exists below Narcondam, in contrast to oceanic lithosphere below Barren Island.

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 This chapter examines the history and distribution of seismicity across the Andaman–Nicobar island arc and Andaman Sea. Fundamental differences between the Andaman section of the subduction zone and the Sumatra–Java section to the south help to explain the nature of the deformation. Magnitudes of displacements varied along-strike from uniform dip-slip on the southern (Sumatran) segment to dip-slip and strike-slip components on the Andaman and Nicobar segment. The Andaman section has a more steeply dipping slab and a thicker sediment cover compared to the Sumatra region where coupling with the overlying plate is stronger. Temporal and spatial patterns of seismicity in the Andaman Sea spreading centre are consistent with normal faulting and c. 25 year cycles of dyke injection are considered responsible for the bulk of the spreading.

Abstract The 26 December 2004 earthquake and tsunami had variable impacts on the Andaman and Nicobar islands. While the earthquake rupture caused land-elevation changes (uplift in the west, subsidence in the east and south) and co-seismic shaking damaged buildings and weakened the infrastucture (roads, bridges, jetties), the tsunami caused most devastation. The largely tribal-populated, low-elevation smaller islands in the Nicobars were the most affected with two islands (Katchall and Pilmillow) losing nearly 90% of their population. This chapter documents and presents new evidence of the impacts of the earthquake and tsunami.

Abstract The natural resources of the Andaman–Nicobar Islands and surrounding seas are discussed. Rocks and minerals found on the Andaman Islands include magnetite, pyrite-chalcopyrite, chromite, gold, nickel, sulphur, coal and limestone, but in most cases their concentrations are sub-economic. Lensoid bodies of algal-biohermal limestones of Palaeogene age on Middle Andaman Island are abundant, accessible and of good grade for cement or foundry work. The Neogene limestones are also suitable for this purpose. Basalts and strongly indurated sedimentary rocks, without any overburden, are quarried for construction work. Limited offshore exploration for oil and natural gas shows great promise, but has yet to be developed.