Ingeology,accretion is a process by which material is added to atectonic plate at asubduction zone, frequently on the edge of existing continentallandmasses. The added material may be sediment,volcanic arcs,seamounts,oceanic crust or other igneous features.[1][2][3]
Accretion involves the addition of material to atectonic plate viasubduction, the process by which one plate is forced under the other when two plates collide.[3] The plate which is being forced down, the subducted plate, is pushed against the upper, over-riding plate. Sediment on the ocean floor of the subducting plate is often scraped off as the plate descends. This accumulated material is called anaccretionary wedge (or accretionary prism), which is pushed against and attaches to the upper plate.[3] In addition to accumulated ocean sediments, volcanic island arcs or seamounts present on the subducting plate may be amalgamated onto existingcontinental crust on the upper plate, increasing the continental landmass.[3]
Continental crust differs significantly from oceanic crust. Oceanic crust is primarily composed ofbasaltic rocks, which have a higherdensity than the rocks making up the majority of continental crust.[4] Island arcs and other volcanic rocks are also lower in density than the oceanic crust, and are therefore not easily subducted along with the oceanic crust that surrounds them. Instead, these less-dense bits of crust will collide with existing continental crust on the upper plate once the oceanic crust separating them is completely subducted.[3] Most continents are composed of multiple accreted "terranes", pieces of low-density continental crust with different origins. For example,North America is made up of multiple accreted terranes that collided with theLaurentian proto-continent, such as theProterozoicYavapai,Mazatzal, andGrenville Province Terranes.[5] Accreted terranes along modern subduction plate boundaries include theNankai accretionary complex of Japan (subduction of thePhilippine Plate beneath theEurasian Plate), theBarbados Ridge in the Caribbean (subduction of theCaribbean Plate beneath theNorth American Plate), and theMediterranean Ridge (subduction of theAfrican Plate beneath the Eurasian Plate). These examples of accreted terranes also all include accretionary wedges.[6][7][8]
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