EVOLVING CONCEPTS OF INTRA-OCEANIC ARC SYSTEMS: LESSONS FROM THE WESTERN PACIFIC

The intraoceanic arc-trench system (IOAS) is one of the most important concepts to come from Pacific Rim geoscientific studies. IOAS is the oceanic convergent margin endmember and are mostly found today in the Western Pacific, although fossil IOAS's are important parts of many orogens. IOAS is mostly submerged - with only the tops of the tallest volcanoes rising above sealevel as “island arcs”- for two reasons: 1) they are built on oceanic lithosphere; and 2) because they mostly subduct old, dense oceanic lithosphere, IOAS's tend to be under extension. Advancing marine geoscientific technologies command to the evolving IOAS concept continues to evolve, with contributions from many geoscientists. IOAS's consist of 4 parallel components that reflect the asymmetry of subduction: trench, forearc, volcanic/magmatic arc, and backarc, occupying 200 km+ along the leading edge of the overriding plate. IOAS's have wedge-shaped crustal profiles up to 20-35 km thick. IOAS magmas form when convecting asthenosphere decompresses and melts, following fluxing by fluids and melts from the subducted slab. IOAS's evolve from very high degrees of melting and seafloor spreading during the first 5-10 m.y. after a new subduction zone forms. This early evolution can be studied in IOAS forearcs, where this infant arc lithosphere is preserved and where most ophiolites form. Because most IOAS's are far-removed from continents they have low sedimentation rates, so naked forearcs that are vulnerable to tectonic erosion. Mature IOAS magmatism is concentrated along a stable volcanic/magmatic front, ~150-200 km from the trench. Low-P fractionation results in eruption of mostly evolved arc basalts. High heat flux beneath IOAS magmatic arc melts middle and lower crust to generate felsic melts, which may explosively erupt or form tonalitic middle crust. IOAS crustal processing also yields lower crustal mafic cumulates and residues, which may sink through a chemically transparent Moho back into the mantle. 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.