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Recent Marine Geological Research in the Mariana and Izu-Bonin Island Arcs! Pacific Science (1990), vol. 44, no. 2: 95-114 © 1990 by University of Hawaii Press. All rights reserved REVIEW ARTICLE Recent Marine Geological Research in the Mariana and Izu-Bonin Island Arcs! PATRICIA FRYER 2 ABSTRACT: Over the last decade models of the structure, composition, and evolutionary processes of the Mariana and lzu-Bonin island arc systems of the western Pacific have been changed profoundly by an interdisciplinary and multi­ national approach to studying the details ofthese regions. The standard marine geological studies have been superseded by detailed sea-floor mapping, sea-floor observations using submersibles, and deep-ocean drilling efforts. The increased level of effort is the result of discoveries of new geological phenomena in these regions and the desire to approach the study of ancient convergent margin terrains exposed on land in the light ofthe resultant insights. The new discoveries include active eruption ofserpentine muds forming large volcano-like seamounts in the Mariana forearc, recent protrusion ofserpentinite debris flows from fault­ dissected horsts of metamorphosed supra-subduction zone mantle also in the Mariana forearc, similar ancient seamounts in the Izu-Bonin forearc region , evidence of recent forearc rifting, petrogenetically complex arc volcanoes situated within the backarc basin setting in both the Mariana and lzu-Bonin systems, the occurrence in close proximity to one another ofmagmas generated from different sources (the deep arc source and the shallower backarc basin basalt source) in neovolcanic zones ofthe incipient rifting portions ofthe Mariana and Izu-Bonin backarc rifts, and a variety ofunique hydrothermal systems and fauna associated both with the arc volcanoes and with the active backarc spreading centers. THEACTIVE DEEP OCEAN trenches of the west­ cesses of subduction in an environment unin­ ern Pacific mark the zones of collision and fluenced by preexisting continental crust. In subduction between the Pacific lithospheric these relatively uncomplicated island arc sys­ plate and the Asian continent. South ofJapan tems it is simpler to decipher the components this zone of collision takes place between the ofthe process ofplate subduction and to study Pacific plate and the Philippine Sea plate, mak­ them . Only in this way can we begin to deter­ ing interoceanic collision zones. The island arc mine how these components are related to one systems generated in this region ofthe Pacific, another and how subduction relates to the the Mariana and Izu-Bonin arc systems , are of evolution of the earth's lithosphere. particular interest to marine geologists because The physical effects of subduction on the such convergent plate boundaries provide an wedge of cru st and mantle overlying the de­ opportunity to study several fundamental pro- scending lithospheric plate, in what is now termed the supra-subduction zone, are ofpar­ ticular concern to those interested in producing models of the nature of interactions between 1 Hawaii Institut e of Geophysics Contribution 2221. Manuscript accepted I July 1989. lithospheric plates. The type ofmodel consid­ 2 Hawaii Institute of Geophysics, 2525 Correa Rd., ered for each con vergent margin depends on Honolulu, Hawaii 96822. the subduction system. There are two end 95 96 PACIFIC SCIENCE,Volume 44, April 1990 member types ofconvergent margins in the in­ on a small scale than is that of the seamount­ teroceanic settings, accretionary and erosional. poor plate being subducted beneath the Izu­ Accretionary systems are those in which Bonin system. Input to the Mariana arc system portions of the down-going plate are detached, from constituents driven off the down-going usually including part ofthe sedimentary col­ plate may therefore be more heterogeneous. umn and sometimes part of the underlying The line of active volcanoes in the arc is igneous basement. These pieces of the sub­ known as the volcanic front. Behind the vol­ ducting plate are then accreted as slivers, canic front, in many convergent margins, such wedges, or en masse as exotic terranes onto the as the Mariana and Izu-Bonin systems, rifting toe of the overriding plate . Erosional systems, of the arc produces young ocean basins . As such as the Izu-Bonin system, are those in the basins evolve they progress from a series of which the toe of the overriding plate is me­ rift graben to a system of volcanic rift seg­ chanically eroded by normal faulting as the ir­ ments offset by transform faults. The backarc regular surface of the down-going plate passes basin regions of the Mariana and lzu-Bonin beneath it. Although it is generally accepted systems are particularly useful because they that both types of convergent plate margins can provide information regarding the tec­ currently exist within the ocean basins, there is tonic evolution of the backarc basins from the growing acceptance of the concept that a given earliest stages of rifting (both Mariana and convergent margin may change from one mode Izu-Bonin) to the more fully evolved (Mari­ to the other with time. A convergent margin ana). The results of the tectonic studies of may even exhibit both modes , varying with these two backarc environments are being local environments along strike. Such a situa­ compared with studies of continental rifting tion has been suggested for the Mariana sys­ as well as with studies of midoceanic ridge tem, which is erosional over most ofits length, rifting in an effort to contrast the rifting styles but is possibly accreting slivers of the Pacific of convergent margin settings with those of plate in the southernmost latitudes. To under­ purely divergent margin settings (the conti­ stand the various factors that control the na­ nental rifts and midocean spreading centers) . ture of subduction in any given convergence The backarc basins produce magmas that situation, a detailed model based on field evi­ are similar to midocean ridge basalts in some dence as well as theoretical considerations regards, but have a unique compositional must be produced and tested. overprint from the arc that distinguishes them Understanding the nature ofthe interactions as a new class ofbasalt. Clearly, as the backarc between the subducted slab and the mantle basins evolve, the structural similarities to through which it descends is critical for pro­ midocean ridges increase. If the magmas ducing working models of subduction systems. maintain a distinct composition, then even if Of obvious interest among these interactions the structural components of an exposed sec­ is the generation of magmas in the supra­ tion through a backarc basin terrane appear to subduction zone environment and the forma­ represent a midocean ridge setting, it would be tion ofisland arc volcanoes. Study of the evo­ possible to trace the origin ofsuch an exposure lution of the island arc volcanoes can docu­ to an arc environment. Geochemical studies ment not only the timing of subduction and of some subaerial suites of rocks that include the structural evolution ofthe arc system, but oceanic sediments, basalts, gabbros, and as­ also provide a means by which to trace aspects sociated mantle rocks (ophiolite sequences) of the chemical interaction between the down­ show that these rocks formed in the supra­ going plate and the supra-subduction zone subduction zone environment, either in a mantle. The Mariana and Izu-Bonin systems volcanic front setting or in a backarc setting. are very different in the types of oceanic plates Volcanological studies of the Mariana and being subducted. The plate being subducted Tzu-Bonin arc volcanic fronts and backarc beneath the Mariana system contains numer­ basin spreading centers provide the oppor­ ous large seamounts. Thus, the composition tunity to investigate eruptive processes, both of the plate is likely to be more heterogeneous subaerial and submarine, and the generation Review Article: Marine Geological Research-FRYER 97 of several types of economical1y valuable de­ Mariana and Tzu-Bonin systems; oflarge, ac­ posits, such as porphyry ore deposits, Kuroko­ tive backarc basin volcanoes erupting arc type deposits, and hydrothermal vent deposits. lavas; of the occurrence in close proximity to Recently, attention has been drawn to a far one another ofmagmas generated from differ­ less obvious locus for generation ofmagmas, ent sources in neovolcanic zones of the inci­ the region trenchward of the volcanic front, pient backarc rifting portions of both systems; termed the forearc. Located between the trench and of active hydrothermal processes in the axis and the line of volcanic islands, the fore­ arc and backarc regions of both systems pro­ arc region had long been considered too cold to vided the impetus to perform in situ studies of permit the generation of magmas. Petrologists these regions. A series of deep submersible now recognize the importance ofthe evolution (Alvin) dive cruises and two cruises of the of magma types specific to the forearc envi­ Ocean Dril1ingProject were completed within ronment (boninites, named for the Bonin the last two years in the Mariana and Izu­ Islands where they were first described) and Bonin systems to study these new phenomena. have begun to provide detailed studies oftheir This paper documents these latest advances petrogenesis and distribution. To understand in marine geology of the Mariana and Tzu­ the process of magma production in the supra­ Bonin arc systems. Itprovides a technical sum­ subduction zone region , particularly in the mary of the development of ideas
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