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.
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The Andaman–Nicobar Accretionary Ridge: Geology, Tectonics and Hazards
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.