Why Study Arc Sytems?
Plate tectonics explains how earth's lithospheric plates are created, move, interact, and are destroyed. Seafloor spreading at mid-ocean ridges and subduction at convergent margins are necessary and complementary manifestations of the plate tectonic process. Subduction zones profoundly perturb the thermal and chemical structure of earth's upper mantle. Subducted lithosphere cools and stirs the upper mantle, and perfuses it with elements that are normally sequestered in the crust and ocean. Arc systems include all crustal and lithospheric elements that owe their existence to the subduction process. This includes forearc regions, magmatic arcs, actively rifting back-arc basins, and extinct elements such as remnant arcs and back-arc basins.
We do not know for how much of earth's history subduction has been going on, but thick crust of mafic to intermediate composition is produced at convergent margins, and this is where continental crust forms. Our understanding of how convergent margins evolve implicates several other first-order scientific and societal questions. Among the scientific questions are the roles of subducted water, sediments, and crust in forming magmas, the origin of ophiolites and exotic terranes, and the role of subduction as a driving force for plate tectonics. Aspects of convergent margins that are of immediate societal interest include seismic and volcanic hazards in the circum-Pacific ' Ring of Fire' and mineral and hydrocarbon resources. There is no question that a better understanding of convergent margins is important to a wide range of scientific and societal interests.
Why Study Intra-oceanic convergent margins (like the IBM Arc System)?
There are two great classes of convergent margins; those built on continental crust (Andean-type) and those built on oceanic crust (intra-oceanic). Although both are due to subduction, studies of the two types have proceeded independently. Andean-type arcs have been the domain of 'on-land' geoscientists and much of what we know about convergent margins comes from these well-exposed and well-studied systems. We know much less about intra-oceanic convergent margins because they are mostly submerged, but these promise to yield insights into fundamental subduction zone process that are unobtainable by studying Andean-type arcs. Intra-oceanic convergent margins are built on oceanic crust,where original crustal thicknesses are known. With suitable geophysical studies, we can easily track crustal growth rates and processes. Intra-oceanic convergent margins are built on refractory oceanic crust, thus obviating the question of the role of crustal contamination on the evolution of arc magmas, a question looming over all petrologic interpretations for Andean-type arcs. Because intra-oceanic convergent margins are far-removed from terrestrial sediment sources, forearc architecture is largely exposed, and we can study the composition and structure of the earliest subduction-related rocks, and use this to understand how a subduction zone forms. This same lack of sediments allows us to directly sample and more readily interpret subduction-related fluids venting through the forearc, and it also means that back-arc rifting can be more easily studied. Because back-arc basin basalts reflect a hybrid melting process that is intermediate between melting due to adiabatic decompression (like that producing basalt beneath the mid-ocean ridges) and melting due to hydrous fluxing like that forming arc magmas, studying back-arc basin basalts offers insights into how both arc and MORB magmas form. The formation of hydrothermal ore deposits in arcs and back-arc basins has obvious implications for scientific and economic endeavors .