Accretionary wedge

An accretionary wedge — also called an accretionary prism — is a wedge-shaped mass of sediment and oceanic crust scraped off the downgoing plate at a subduction zone and piled against the overriding plate. It is not a passive accumulation but an actively deforming thrust belt where imbricate faults stack slices of oceanic sediment like cards in a deck, thickening the crust and building topography from material that began as seafloor mud. The wedge grows until its taper reaches a critical angle determined by the balance between accretion rate and basal friction; beyond this angle, material is instead subducted to depth, creating a steady-state recycling boundary. Accretionary wedges are the geological equivalent of a snowplow blade: they determine what gets scraped off the road and what gets driven under.