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 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 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 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 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.

The Andaman Islands are part of the Andaman-Nicobar Ridge, an accretionary complex that forms part of the outer-arc ridge of the Sunda subduction zone. The Tertiary rocks exposed on the Andaman Islands preserve a record of the tectonic evolution of the surrounding region, including the evolution and closure of the Tethys Ocean. Some of the Paleogene sediments on Andaman may represent an offscraped part of the early Bengal Fan. Through field and petrographic observations, and use of a number of isotopic tracers, new age and provenance constraints are placed on the key Paleogene formations exposed on South Andaman. A paucity of biostratigraphic data poorly define sediment depositional ages. Constraints on timing of deposition obtained by dating detrital minerals for the Mithakhari Group indicate sedimentation after 60 Ma, possibly younger than 40 Ma. A better constraint is obtained for the Andaman Flysch Formation, which was deposited between 30 and 20 Ma, based on Ar-Ar ages of the youngest detrital muscovites at ca. 30 Ma and thermal history modeling of apatite fission-track and U-Th/He data. The latter record sediment burial and inversion (uplift) at ca. 20 Ma. In terms of sediment sources the Mithakhari Group shows a predominantly arc-derived composition, with a very subordinate contribution from the continental margin to the east of the arc. The Oligocene Andaman Flysch at Corbyn's Cove is dominated by recycled orogenic sources, but it also contains a subordinate arc-derived contribution. It is likely that the sources of the Andaman Flysch included rocks from Myanmar affected by India-Asia collision. Any contribution of material from the nascent Himalayas must have been minor. Nd isotope data discount any major input from cratonic Greater India sources.

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 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