SUBDUCTION ZONE DYNAMICS AND METAL CYCLING IMPLICATIONS FOR ORE DEPOSITS

Subduction zones are defined by inclined seismic zones where plate convergence is faster than ~3 cm/yr. The entire layer of ocean crust and lithospheric mantle steadily recycle into the asthenosphere. Beneath the forearc block and any accretionary prism, a layer of subducting sediment forms a shear zone, the "subduction channel." Shear zone thickness depends upon convergence speed, sediment supply, plate dip, and density of the hanging wall block. Where convergence speed is about 6 cm/yr and sediment supply is near or exceeds capacity, shear zone thickness is typically about 500 m. Accretionary prisms form where sediment supply is greater than the systems capacity to drag sediment to great depth. Where the incoming sediment supply is less than capacity, all the incoming sediment is deeply subducted. Where the sediment supply is much less than capacity, subduction erosion abrades the front and base of the forearc block and thus truncates the margin. Geophysical studies and drilling indicate substantial sediment subduction at many margins and subduction erosion at some margins. Detection of the short-lived isotope Be-10 in arc magmas at Peru, Central America, the Aleutians, and northeast Japan definitively indicate the entire incoming sediment pile can be dragged to the depths of magmagenesis. Modeling indicates the sediment fluxes that are steadily recycled during subduction are in the range of 1 to 50 square km per m.y. per km length of trench; the higher fluxes where subduction speed is near 10 cm/yr. Sediment recycling volumes are comparable to most estimates of the flux of magma forming volcanic/plutonic arcs. At all margins, the basal, typically siliceous and commonly metal-sulfide-bearing oceanic part of the incoming sediment pile reaches the depths of arc magmagenesis. This is commonly supplemented, and locally dominated, by the overlying terrigenous and/or volcanogenic trench fill.