Australia: The Land Where Time Began

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Island Arc Systems - Morphology

Island arc systems are formed when oceanic lithosphere is subducted beneath other plates of oceanic lithosphere. Island arcs are usually present on the margins of shrinking oceans, such as the Pacific, where the majority of the island arcs are located. The Lesser Antilles (Caribbean) Arc and the South Sandwich (Scotia) Arc, in the western Atlantic, are situated on the eastern margins of small oceanic plates. Transform faults isolate these plates from the general movement that is to the west. Island arc components are usually all convex to the underthrusting ocean, that has been suggested to possibly result from spherical geometry (Frank, 1968). According to Frank's suggestion, if the oceanic lithosphere underthrusts at an angle averaging about 45o, the radius of curvature at the Earth's surface of the island arc is about 2,500 km, which agrees with some, though not all island arc systems (see Source 1). According to Kearey et al., (source 1), spherical geometry probably is the reason for the general convexity of island arc systems, and they suggest deviations are the result of oversimplification of this approach, especially the fact that plate tectonics does not require the conservation of surface area. At the Mariana Arc, with one of the smallest radii of curvature, the underthrusting is almost 90o (Uyeda & Kanamori, 1979).

A flexural bulge about 500 m high is present between 100-200 km from the trench, proceeding from the oceanward side of the system. The trench, the accretionary prism and forearc basin comprise the forearc region. Thrust slices of trench fill (flysch) sediments, as well as probably oceanic crust sediments scraped from the subducting slab at the leading edge of the overriding plate, comprise the accretionary prism. Between the accretionary prism and the island arc is a region of tranquil sedimentation, that is flat-bedded, that is the the forearc basin. An inner magmatic arc surrounded by an outer sedimentary arc form the island arc. In the sedimentary arc, rocks of volcanic origin, that are older than those of the magmatic arc, are overlain by volcaniclastic and coralline sediments. The substrate of volcanic rock is thought to possibly be the result of the initial volcanism associated with the beginning of subduction of the relatively cool oceanic lithosphere. The centre of igneous activity moved backwards to the location that became its 'steady state' position at the magmatic arc, as the cold plate descended further into the asthenosphere. A backarc basin (or marginal basin) behind the island arc is enclosed by the island arc and remnant arc (backarc ridge). Not all backarc basins are formed above an active subduction zone by spreading.

See Source 1 for more detailed information on all aspects of plate tectonics

Sources & Further reading

  1. Kearey, Philip, Klepeis, Keith A. & Vine, Frederick J., 2009, Global Tectonics, 3rd Edition, Wiley-Blackwell.
Author: M. H. Monroe
Last Updated 18/05/2011



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