INDIAN SUBCONTINENT 285 Impact Structures Lunar and Planetary Institute Contri bution 954 Houston: Lunar and Planetary Institute Grady MM, Hutchison R, McCall GJH, and Rothery RA (eds.) (1998) Meteorites: Flux with Time and Impact Effects Geological Society of London Special Publication 140 London: Geological Society of London Gehrels T (ed.) (1994) Hazards due to Comets and Aster oids Tucson: University of Arizona Press Hodge PW (1994) Meteorite Craters and Impact Structures Cambridge: Cambridge University Press Hoyt WG (1987) The Coon Mountain Controversies Tucson: University of Arizona Press Melosh HJ (1989) Impact Cratering: A Geologic Process Oxford: Oxford University Press Plado J and Pesonen LJ (eds.) (2002) Impacts in Precam brian Shields New York: Springer Verlag Plucker Ehrenbrink BE and Schnitz B (eds.) (2001) Accre tion of Extraterrestrial Matter throughout Geologic Time Norwell, MA: Kluwer Academic Press Spudis P (1993) The Geology of Multi ring Impact Basins Cambridge: Cambridge University Press INDIAN SUBCONTINENT A B Roy, Presidency College, Kolkata, India ß 2005, Elsevier Ltd All Rights Reserved Introduction The Indian Sub-Continent constitutes a distinctive geographic entity: the countries included in the Sub-Continent, including Bangladesh, India, Nepal, and Pakistan, virtually cut off from the rest of Asia by lofty mountain chains Almost half of the SubContinent’s boundary in the north is bordered by the Himalayas and its associated branches: the Sulaiman and Kirthar Ranges to the Hindukush in the northwest, and the Naga Hills and the Arakan Yoma constituting the Indo-Myanmar Arc (Figure 1) The Himalayas extend for over 2500 km, from the Pamir in the west to the Mismi Hills in the east The landscape changes sharply on the southern side of the great mountain ranges, where the high mountain terrain descends down to a huge plain land of unconsolidated sediments, known as the IndoGangetic Alluvial Plain (IGAP) The IGAP, which constitutes the enormous flood plains of the Indus and the Ganga River systems, also includes the narrow basin of the Brahmaputra River in the east and the Thar Desert in the west (including the North Gujarat Plain) South of the IGAP lies the rocky landmass of Peninsular India Except for the Himalayas and associated mountain ranges, the Indian Sub-Continent is traditionally considered as a shield of Precambrian rocks with a younger cover The geophysical data, especially the seismic, Bouguer gravity anomaly, and heat flow patterns, suggest that much of the shield area attained considerable ‘mobility’ during post-Precambrian time In this respect at least, the Indian Shield appears distinctly different from the better-known shield areas like the Canadian or the Ukrainian Shields The history of geological evolution of the Indian Sub-Continent is quite long and complex and, broadly speaking, took place in two stages The first stage covered the entire Precambrian, which was the period of growth and final cratonisation During the second stage of its evolution, the cratonised Indian Shield underwent considerable reconstitution that ultimately produced the present-day geomorphology as well as the tectonic character of the region The following description discusses the regional geology of the Sub-Continent mainly in terms of the history of evolution of the crust, starting from the evolution of the Archaean basement and the formation of Proterozoic basins to the different stages of the Phanerozoic reconstitution Evidence for all of these come from smaller regions, which preserved these stages of crustal development The description of regional geology of the Indian Sub-continent will be divided into the following heads: i Precambrian Indian Crust; ii Geology of the Gondwana basins; iii Between the Jurassic break-up and Himalayan collision iv Geology of the Himalayas; v Quaternary sedimentation and neotectonics Precambrian Indian Crust The Peninsular India lying south of the Indo-Gangetic Alluvial Plain is an old landscape, a considerable part of which is covered by Phanerozoic rocks, the Gondwanas, Deccan Traps, marine Mesozoic-Tertiary formations, and Recent alluvium Tectonically speaking, the peninsula is a shield area that has remained free of any orogenic deformation since the Cambrian The entire region was considered a ‘terra incognita’, even up to about three decades ago Detailed studies in several interdisciplinary fields helped to characterise