Subduction-channel model of prism accretion,melange formation,sediment subduction,and subduction erosion at convergent plate margins: 2. Implications and discussion |
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| Authors: | Mark Cloos Ronald L Shreve |
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| Institution: | (1) Department of Geological Sciences and Institute for Geophysics, University of Texas at Austin, 78713-7909 Austin, Texas, USA;(2) Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, 90024-1567 Los Angeles, California, USA |
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| Abstract: | The subduction-channel model predicts five geologically and geophysically distinctive types of convergent plate margin. They differ primarily in the proportions of incoming sediment and upflowing melange offscraped and underplated near the toe of the overriding block. The model predicts complex patterns of uplift or subsidence of the forearc region due to differential underplating or subduction erosion and to initiation or cessation of melange upflow. It shows how subduction speed, sediment input, and pressure gradient along the channel govern the general distribution and magnitude of subduction-zone earthquakes. It can predict the upward trajectories of exotic blueschist or serpentinized peridotite blocks that become entrained in the upwelling melange; and it suggests mechanisms by which much larger tracts of coherent blueschist can be raised to shallower levels. It predicts characteristic structural and metamorphic histories for geologic units accreted during steady-state subduction, showing for example, that offscraped materials undergo subhorizontally-directed compression during final dewatering and accretion, whereas underplated ones undergo large simple shear. It gives the maximum depths reached by upwelled melange, which bear significantly on the metamorphic changes observed in them. It predicts large-scale melange diapirism in certain rare cases, in which the normally adverse pressure gradient is reversed in a short reach of the channel, as at Barbados Island. Finally, it explains why pelagic sediments are seldom found in accretionary complexes and gives insight into the factors controlling what portion of the incoming sediment, and the10Be and other tracers it carries, will reach the region of volcanic-arc magmagenesis. |
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| Keywords: | Subduction zone offscraping underplating dé collement prism dewatering earthquakes blueschists volcanic arc magmas |
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