1,721,405 research outputs found
The 26 December 2004 earthquake and tsunami
No full textThe 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
Mithakhari deposits
No full textThe 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
Seismicity of the Andaman–Nicobar Islands and Andaman Sea
No full textThis 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
Geological framework of the Andaman–Nicobar Islands
No full textThe 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
The archipelago group: current understanding
No full textThe 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
Provenance of Oligocene Andaman sandstones (Andaman–Nicobar Islands): Ganga–Brahmaputra or Irrawaddy derived?
No full textInterpretation 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
Submarine fan deposits: petrography and geochemistry of the Andaman Flysch
No full textThe 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
Introduction to the geography and geomorphology of the Andaman–Nicobar Islands
Full text linkThe 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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