大地构造——Gregory A.DevisDD- 6-8 Subduction-IIIa.pdf

Gregory Davis Dizhi Daxue 6/8/2011 The deepest parts of the ocean are the ocean trenches next to continents and arcs. They receive sediment eroded from the nearby land masses AND they receive sediment deposited out in the oceans and carried into the trenches on the tops of the subducting plates. WHAT HAPPENS TO THIS SEDIMENT? ACCRETIONARY WEDGES Accretionary Wedges .au/…/Report2/hydro. htm www.phys.ocean.dal.ca/…/lect12/lect12.htm l ACCRETIONARY WEDGE (PRISM) An oversimplified view of an accretionary prism or wedge (both terms are used). Prisms typically grow by the addition of sediment at their near-trench fronts, but they can also grow from below by “underplating” of subducted materials (sediments, sea-floor volcanoes, etc.) Accretionary Wedges .au/…/Report2/hydro. htm www.phys.ocean.dal.ca/…/lect12/lect12.htm l This geometry of a Chilean subduction zone, South America, is closer to reality, but still has a vertical exaggeration .. The “backstop” (in some wedges it dips seaward) True scale geometries of various accretionary wedges (and the Himalayan thrust belt) The best example of what happens to sediments when they enter an oceanic trench is the Nankai trough (trench) south of Shikoku Island, Japan Subduction of the Philippine plate in the Nankai Trough of southwestern Japan The accretionary wedge (prism) inland from the Nankai Trough continues onshore at Shikoku island. This figure shows only that part of the huge wedge that lies between the Trough and the island. The V.E. is about 2x (V:H). Most of this wedge is Pliocene and younger in age ( 6 Ma)! At a convergent