Orogeny Can Be Very Short
Orogeny can be very short John F. Dewey* Department of Geology, University of California, Davis, CA 95616; and University College, Oxford OX1 4HH, United Kingdom This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on April 29, 1997. Contributed by John F. Dewey, July 12, 2005 In contrast to continent͞continent collision, arc–continent collision margin, common during the early stages of continental collision generates very short-lived orogeny because the buoyancy-driven when, driven by subduction roll-back, arcs bend and wrap impedance of the subduction of continental lithosphere, accom- themselves into remnant oceanic holes (7). Arc͞rifted margin panied by arc͞suprasubduction-zone ophiolite obduction, is re- collision involves the blocking of a subduction zone with buoy- lieved by subduction polarity reversal (flip). This tectonic principle ant, weak, continental crust, followed by subduction polarity is illustrated by the early Ordovician Grampian Orogeny in the reversal͞flip (8, 9). To achieve a flipped polarity, the preflip British and Irish Caledonides, in which a wealth of detailed sedi- subducted lithosphere must be removed by slab break-off to mentologic, heavy mineral, and geochronologic data pin the Orog- allow postflip subduction (10, 11). This process has occurred on eny to a very short Arenig͞Llanvirn event. The Orogeny, from the many active continental margins, including New Guinea during initial subduction of continental margin sediments to the end of the Miocene (9, 12), north Borneo (13), New Caledonia during postflip shortening, lasted Ϸ18 million years (my). The collisional the Eocene (14), the Sierra Nevada during the Jurassic (15), the shortening, prograde-metamorphic phase of the Orogeny lasted 8 Urals during the late Devonian (16, 17), the Appalachian͞ my, extensional collapse and exhumation of midcrustal rocks Caledonian System during the Ordovician (10, 18), and in the lasted 1.5 my, and postflip shortening lasted 4.5 my.
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