HR: 11:35h
AN: T42A-06 INVITED    [Abstracts]
TI: Anatomy of Intra-Oceanic Arc Systems
AU: * Stern, R J
EM: rjstern@utdallas.edu
AF: Geosciences, U TX Dallas, Box 830688, Richardson, TX 75083-0688, United States
AB: Intra-oceanic arc systems (IOAS) are ultimately embedded in orogenic belts and added to the continental crust. Reconstructing fossil IOASs in collision zones requires understanding the salient features of a typical IOAS. IOASs have the relative dimensions of tagliatelle (flat) pasta: much wider (~250 km) than thick (10-30 km), much longer (1000's of km) than wide. IOASs begin to form when subduction begins, either spontaneously (SNSZ) or by forced convergence (INSZ). For SNSZ, IOASs start as broad zones of seafloor spreading associated with subsidence of the adjacent lithosphere, whereas INSZ IOASs are built on trapped crust. IOAS magmatism manifests the evolution of its subduction zone and indirectly the breadth of the subducted ocean. Two stages in SNSZ IOAS magmato-tectonic evolution exist: infancy and maturity. Infancy lasts 5-10 Ma and results in broad zones of seafloor spreading of tholeiite/boninite; this becomes forearc for the mature IOAS and is emplaced as ophiolite during collision (subduction zone failure). Arc maturity begins with true subduction, as the subducted slab reaches depths ~130 km, focusing magmatism to begin building the magmatic arc ~200km away from the trench and allowing the forearc to cool and hydrate. Mature magmatic arcs mostly yield low-K tholeiitic and medium-K calc-alkaline magmas. Magmatic focusing begins crustal thickening beneath the magmatic arc, at ~500m/Ma for the Izu-Bonin-Mariana IOAS. No systematic compositional evolution to more LIL-enriched primitive magmas occurs once IOAS maturity is reached, except when upper plate stress regime (BAB formation, strike- slip faulting) or the nature of subducted material (more/different sediments, young oceanic crust) changes. Thickening is accompanied by processing of crust beneath the magmatic arc, with progressive differentiation into upper volcanic, middle tonalitic, and lower mafic layers, producing an increasingly effective density filter for magma ascent. Crustal layer formation involves anatexis of amphibolite and mafic melt fractionation to form nests of felsic plutons, accompanied by drip-delamination of pyroxene-rich residues and cumulates back into the mantle. Active IOASs thus have mass transfer in both directions across the crust-mantle boundary beneath the magmatic arc, leading to small P-wave velocity differences between gabbroic lower crust and pyroxenitic upper mantle. Forearcs, in contrast, are underlain by serpentinized harzburgite. Intra-oceanic arc systems are rarely associated with accretionary prisms; because most are far-removed from continental sources of sediment, they subduct oceanic lithosphere with thin sediments and have naked forearcs subjected to tectonic erosion. These aspects of IOASs should be revealed in accreted ancient arcs: 1) Ancient IOAs should be large, both wide and thick; and 2) Ancient IOASs should be asymmetric. Scraps of IOASs could be smaller slivers of crust, brought into place by strike-slip faulting, but a true accreted arc should be as obvious to a geologist as a beached whale is to a beachcomber.
DE: 3042 Ophiolites (8140)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 5475 Tectonics (8149)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting