The Accretion Process

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When an oceanic plate is subducted under the margin of a continental plate, sediments and other crustal material are "scraped off" and added to the continental margin as the subducted plate disappears below the trench. The oceanic plate functions as a kind of conveyor belt. Island arcs, submarine plateaus, and pieces of the crust large enough to be called microplates are carried to the consuming plate margin. Oceanic crust is too buoyant to be subducted and so is "scraped off" the oceanic plate and attached to the margin of the continent as an

A well-known locality where this process is going on today is the coast of Washington and adjacent British Columbia, where a small remnant of the Farallon Plate (now called the Juan de Fuca Plate) is being subducted. This active plate margin is characterized by volcanoes and earthquakes. A parallel cross section through Vancouver Island shows how sedimentary rocks on the ocean floor are not subducted at the trench, but rather are broken into slices and accreted onto the margin of the continent.

As the subduction continues sedimentary rocks on the downgoing oceanic plate are dragged under the continental margin, but due to buoyancy soon break along a glide horizon (or thrust fault) and become attached to the continental plate rather than the oceanic plate. Thus, in the profile of Vancouver Island (click here or here), at each thrust fault it is the underlying slice that has moved eastwards towards the continent.

Plate Tectonics of Montana (Page 2 of 14)