ACCRETIONARY COMPLEX

An accumulation of oceanic sediments scraped from the surface of a plate undergoing subduction at a destructive plate margin, and accreted to the edge of the over-riding plate (see plate tectonics). It is also known as an accretionary prism, accretionary wedge or subduction complex. The geological structure is dominated by thrust faults, allowing accretionary complexes to be recognised in the geological record. Fragments of oceanic crust may be scraped off the subducting plate and preserved as ophiolite complexes. The accreted material may partly fill the oceanic trench and may protrude above sea level to form a non-volcanic island arc close to the trench (sometimes known as an outer-arc or outer-arc ridge). Modern examples include Barbados in the West Indies, the continental slope region of East Japan and the Andaman and Nicobar Islands on the Indian Ocean side of the Sunda Arc.