The anatomy and ontogeny of modern intra-oceanic arc systems

Abstract

AbstractIntra-oceanic arc systems (IOASs) represent the oceanic endmember of arc–trench systems and have been the most important sites of juvenile continental crust formation for as long as plate tectonics has operated. IOASs' crustal profiles are wedge-shaped, with crust up to 20–35 km thick; a more useful definition is that IOASs occur as chains of small islands, generally just the tops of the largest volcanoes. A very small fraction of IOASs lie above sea level, but advancing marine technologies allow their most important features to be defined. Modern IOASs subduct old, dense oceanic lithosphere and so tend to be under extension. They consist of four parallel components: trench, forearc, volcanic–magmatic arc, and back-arc, occupying a ≥200 km zone along the leading edge of the overriding plate. These components form as a result of hydrous melting of the mantle and reflect the strongly asymmetric nature of subduction processes. Forearcs preserve infant arc lithosphere whereas magmatism in mature IOASs is concentrated along the volcanic–magmatic front. Mature IOASs often have minor rear-arc volcanism and, because most IOASs are strongly extensional, sea-floor spreading often forms back-arc basins. Sub-IOAS mantle is also asymmetric, with serpentinized harzburgite beneath the forearc, pyroxene-rich low-Vp mantle beneath the magmatic front, and lherzolite–harzburgite beneath back-arc basins. Because most IOASs are far removed from continents, they subduct oceanic lithosphere with thin sediments and have naked forearcs subject to tectonic erosion. IOASs evolve from broad zones of very high degrees of melting and sea-floor spreading during their first 5–10 Ma, with the volcanic–magmatic front retreating to its ultimate position c. 200 km from the trench.