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Toward a dynamic concept of the subduction channel at erosive convergent margins with implications for interplate material transfer

Abstract:

Convergent plate boundaries accommodate intraplate displacement within a ∼100-1000 m thick shear zone. Marine geophysicists typically define this zone, the subduction channel (SC), as the sedimentary layer between the downgoing oceanic crust and the base of the upper plate. Geologists and modelers, instead, perceive the SC as a specific type of shear zone. The original theory of SCs was developed when the net accretion of marine sediments to the forearc was thought to typify a convergent margin. While erosive margins were briefly mentioned, their mechanics were not discussed in any detail. We now realize that subduction erosion is taking place at roughly half of the modern subduction margins. Here we review and revise the theory of erosive SCs (1) to unify this concept across disciplines, focusing on the meaning of the channel's boundaries; (2) to redefine the portions of the forearc included in the SC concept; and (3) to better idealize this dynamic system where material supply to the channel, fluid content, and the heterogeneity of deformation all influence the SC's upper and lower boundaries. Migration of the channel boundaries controls the downdip variation of tectonic mechanisms that shape the margin. Within the shallow, <15 km deep part of the SC, a gradual change of physical properties defines three zones; zone 1 of rapid fluid dewatering, zone 2 of overpressure, and zone 3 with metamorphic fluid release. A SC is a dynamic feature with along-strike and downdip variations caused by changes in channel material, in trapped fluid contents, and in interplate boundary geometry. Copyright 2012 by the American Geophysical Union.