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Sedimentary Basins related to Volcanic Arcs M08353 Basin Analysis Reading - start with: • Reading, H.G.: Sedimentary Environments – 2nd edition. Tectonics & Sedimentation chapter by Mitchell & Reading – 3rd edition. Volcaniclastics chapter by Orton, p. 549- Volcanic arcs may develop... • within oceanic lithosphere, where ocean floor subducts beneath ocean floor, and an island arc results, e.g. Lesser Antilles arc • or at the edge of a continent, where oceanic lithosphere subducts beneath continental lithosphere, and a continental margin magmatic arc forms, e.g. Andes Basins related to volcanic arcs • fore-arc • back-arc • intra-arc • All may be either submarine or subaerial, or may have marine & subaerial parts • Much sediment is supplied from active arc. Fore-arc basins • Lie in the arc-trench gap, between volcanic arc and submarine trench • range from small basins on trench slope to large basins (50 to 100 km wide, and > 500 km long) with thick fills (several km) • Basins tend to become wider and shallower with time, partly because of accretion at trenches fore-arc basins Sediment sources: • volcanic arc • outer arc • longitudinally from a continent Tectonic style varies: • compressional • extensional • strike-slip Back-arc basins • • • • lie behind the magmatic arc often the site of extension & thinning of crust may overlie either ocean or continental crust oceanic back-arc basins are eventually subducted and destroyed, or preserved in thrust complexes related to ocean closure. • back-arc basins on continental crust - more varied facies, because of terrigenous input; higher preservation potential. Intra-arc basins • Sedimentary basins within magmatic arcs, between volcanoes, or between older and younger belts of the arc • Some are fault-bounded and subside rapidly. Faulting due to extension within arc, or flexure of lithosphere due to weight of volcano. • With time, position of the arc migrates, and basins may change between intra-arc, backarc and fore-arc. Sediment supply and transport • Sediment supply varies according to volcano behaviour, governed by magma viscosity and gas content. • In deep water, explosive activity is suppressed by hydrostatic pressure. • More silicic magmas in more evolved arcs therefore greater explosive activity, more supply of pyroclastic sediment. Sediment transport and deposition is controlled by: • topography - both subaerial and submarine • volcanic processes, especially eruption column height, direction of pyroclastic flows • sediment transport systems - e.g. rivers, prevailing winds