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RESEARCH to Understand Subduction Initiation, Study Forearc Crust

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RESEARCH to Understand Subduction Initiation, Study Forearc Crust RESEARCH To understand subduction initiation, study forearc crust: To understand forearc crust, study ophiolites R.J. Stern1*, M. Reagan2*, O. Ishizuka3*, Y. Ohara4*, and S. Whattam5* 1GEOSCIENCES DEPARTMENT, UNIVERSITY OF TEXAS AT DALLAS, RICHARDSON, TEXAS 75083-0688, USA 2DEPARTMENT OF GEOSCIENCE, UNIVERSITY OF IOWA, IOWA CITY, IOWA 52242, USA 3INSTITUTE OF GEOSCIENCE AND GEOINFORMATION, GEOLOGICAL SURVEY OF JAPAN/AIST, CENTRAL 7, 1-1-1, HIGASHI, TSUKUBA, IBARAKI 305-8567, JAPAN 4HYDROGRAPHIC AND OCEANOGRAPHIC DEPARTMENT OF JAPAN, TOKYO 104-0045, JAPAN 5DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, KOREA UNIVERSITY, SEOUL 136-701, REPUBLIC OF KOREA ABSTRACT Articulating a comprehensive plate-tectonic theory requires understanding how new subduction zones form (subduction initiation). Because subduction initiation is a tectonomagmatic singularity with few active examples, reconstructing subduction initiation is challenging. The lithosphere of many intra-oceanic forearcs preserves a high-fi delity magmatic and stratigraphic record of subduction initiation. We have heretofore been remarkably ignorant of this record, because the “naked forearcs” that expose subduction initiation crustal sections are dis- tant from continents and lie in the deep trenches, and it is diffi cult and expensive to study and sample this record via dredging, diving, and drilling. Studies of the Izu-Bonin-Mariana convergent margin indicate that subduction initiation there was accompanied by seafl oor spread- ing in what ultimately became the forearc of the new convergent margin. Izu-Bonin-Mariana subduction initiation encompassed ~7 m.y. for the complete transition from initial seafl oor spreading and eruption of voluminous mid-ocean-ridge basalts (forearc basalts) to normal arc volcanism, perhaps consistent with how long it might take for slowly subsiding lithosphere to sink ~100 km deep and for mantle motions to evolve from upwelling beneath the infant arc to downwelling beneath the magmatic front. Many ophiolites have chemical features that indicate formation above a convergent plate margin, and most of those formed in forearcs, where they were well positioned to be tectoni- cally emplaced on land when buoyant crust jammed the associated subduction zone. We propose a strategy to better understand forearcs and thus subduction initiation by studying ophiolites, which preserve the magmatic stratigraphy, as seen in the Izu-Bonin-Mariana forearc; we call these “subduction initiation rule” ophiolites. This understanding opens the door for on-land geologists to contribute fundamentally to understanding subduction initiation. LITHOSPHERE; v. 4; no. 6; p. 469–483 | Published online 16 May 2012 doi: 10.1130/L183.1 INTRODUCTION there are few active examples, and (2) nearly in Eocene time (Ishizuka et al., 2011). Major all of the evidence for tectonic, magmatic, and subduction initiation episodes are hemispheric A better understanding of the mechanisms sedimentary responses to subduction initia- in scale and necessarily reorganize upper-man- by which new subduction zones form is criti- tion is preserved in forearcs, which are deeply tle fl ow, and in many cases are accompanied by cal for advancing the solid Earth sciences. Until submerged and buried beneath sediments. We widespread and voluminous igneous activity. we can reconstruct how and why this happens, would prefer to study subduction initiation in Here, we outline a strategy that promises to we cannot pretend to understand a wide range progress, but there are few places to do this. accelerate our understanding of processes asso- of important Earth processes and properties, One such active region however, is the Puysegur ciated with major subduction initiation episodes including lithospheric strength, composition, subduction zone off the coast of southern New by considering both the subduction initiation and density, and the driving force behind plate Zealand (LeBrun et al., 2003; Sutherland et al., record preserved in forearcs and insights from motions. In spite of this, our understanding of 2006). However, as only a narrow segment of studying well-preserved ophiolites. The record the subduction initiation process has advanced the Australia-Pacifi c transform plate margin is of subduction initiation is preserved in igneous slowly, for two important reasons: (1) Sub- affected, studies of Puysegur cannot capture all crust and upper-mantle residues and the associ- duction initiation is an ephemeral process, so of the processes that accompany major subduc- ated sediments on the overriding plate next to the tion initiation events, i.e., those that change the trench. These collectively comprise the forearc lithospheric force balance suffi ciently to cause (Dickinson and Sealey, 1979) and provide the changes in plate motion and stimulate volumi- best record of subduction initiation. Signifi cant *E-mails: [email protected]; mark-reagan@uiowa .edu; [email protected]; [email protected]; nous magmatism, as discussed herein. Such parts of forearcs may be lost by tectonic erosion [email protected]. episodes shaped the western margin of North (Scholl and von Huene, 2009); nevertheless, Editor’s note: This article is part of a special issue ti- America in Mesozoic time (Dickinson, 2004), whatever remains contains the best record of tled “Initiation and Termination of Subduction: Rock Re- established a convergent margin along SW the processes that accompanied subduction ini- cord, Geodynamic Models, Modern Plate Boundaries,” edited by John Shervais and John Wakabaya shi. The Eurasia in Late Cretaceous time (Moghadem tiation of that particular convergent margin. We full issue can be found at http://lithosphere.gsapubs and Stern, 2011), and engendered most of the explore why this record has been overlooked and .org/content/4/6.toc. active subduction zones of the western Pacifi c summarize recent studies of forearc crust and LITHOSPHEREFor permission to| Volumecopy, contact 4 | Number [email protected] 6 | www.gsapubs.org | © 2012 Geological Society of America 469 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/6/469/3038655/469.pdf by guest on 01 October 2021 STERN ET AL. upper mantle, and what the results reveal about of a convergent plate margin is provided by the some is scraped off to form an accretionary subduction initiation. The expense and diffi - Late Mesozoic of California, with the Francis- prism. Such situations of forearc thickening and culty of directly studying forearc igneous rock can mélange representing exhumed subduction- widening are globally unusual, because most exposures are huge obstacles to our progress, so zone material, the Great Valley Group repre- forearcs lose upper-plate crust to the subduc- we explore the potential of some ophiolites for senting the forearc basin, and the Sierra Nevada tion zone due to tectonic erosion, as a result of illuminating forearc composition and magmatic Batholith representing the roots of the magmatic normal faulting, oversteepening, and basal frac- stratigraphy. Ophiolites are exposed on land and arc. This is indeed an excellent example of a turing and abrasion along the plate interface. so are vastly easier and cheaper to study than sediment-rich convergent margin, but empha- Another misconception (due to bias toward forearcs. We conclude that those ophiolites that sis on California and other sediment-dominated studying sediment-rich convergent margins) is formed in a forearc provide important opportu- forearc examples has inhibited appreciation of that all inner-trench slopes have very low slopes nities for advancing our understanding of sub- forearc crust itself. (<3°), when, in fact, erosive margins, especially duction initiation. The strategy of comparative Many—but not all—continental forearcs those exposing igneous basement, are much study of igneous forearc crust and ophiolites, are excellent examples of convergent margins steeper, typically with slopes of 3°–7° (Clift and coupled with geodynamic modeling, promises affected by high sediment fl ux. Some continental Vannucchi, 2004). Estimates of the proportion of to lead to major advancements in our under- convergent margins—such as Peru-Chile and NE accretionary versus erosive convergent margins standing of subduction initiation processes. Japan—do not have high sediment fl ux, but these vary. According to Clift and Vannucchi (2004), have not been textbook examples because the 57% of the cumulative length of trenches is ero- FOREARCS interesting outcrops are in very deep water, and sive and 43% is accretionary, whereas Scholl thus are diffi cult and expensive to study. In con- and von Huene (2007) estimated that 74% and Forearcs comprise the bulk of any arc-trench trast, forearcs away from continents are mostly 26% are erosive and accretionary, respectively. system, occupying the ~150–200-km-wide sediment starved (Clift and Vannucchi, 2004). Thickness of sediment on the downgoing plate region above the subducted plate between the Such naked forearcs expose crust and upper is the single most important control on whether trench and the magmatic arc. Forearcs are rela- mantle, which are readily accessed by drilling a margin is erosive or accretionary. A sediment tively stable and low standing—intra-oceanic through thin sediment cover, as was done during thickness of ~500 m divides the two types of forearcs lie entirely below sea level—and are Deep Sea Drilling Project (DSDP) Leg 60 and margins. Other factors favoring tectonic erosion morphologically unimpressive compared
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