RESEARCH To understand subduction initiation ... - GeoScienceWorld

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* 1

GEOSCIENCES DEPARTMENT, UNIVERSITY OF TEXAS AT DALLAS, RICHARDSON, TEXAS 75083-0688, USA DEPARTMENT OF GEOSCIENCE, UNIVERSITY OF IOWA, IOWA CITY, IOWA 52242, USA 3 INSTITUTE OF GEOSCIENCE AND GEOINFORMATION, GEOLOGICAL SURVEY OF JAPAN/AIST, CENTRAL 7, 1-1-1, HIGASHI, TSUKUBA, IBARAKI 305-8567, JAPAN 4 HYDROGRAPHIC AND OCEANOGRAPHIC DEPARTMENT OF JAPAN, TOKYO 104-0045, JAPAN 5 DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, KOREA UNIVERSITY, SEOUL 136-701, REPUBLIC OF KOREA 2

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-fidelity 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 distant from continents and lie in the deep trenches, and it is difficult 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 seafloor spreading 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 seafloor 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 tectonically 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

INTRODUCTION

A better understanding of the mechanisms by which new subduction zones form is critical for advancing the solid Earth sciences. Until we can reconstruct how and why this happens, we cannot pretend to understand a wide range of important Earth processes and properties, including lithospheric strength, composition, and density, and the driving force behind plate motions. In spite of this, our understanding of the subduction initiation process has advanced slowly, for two important reasons: (1) Subduction initiation is an ephemeral process, so

*E-mails: [email protected]; mark-reagan@uiowa .edu; [email protected]; [email protected]; [email protected]. Editor’s note: This article is part of a special issue titled “Initiation and Termination of Subduction: Rock Record, Geodynamic Models, Modern Plate Boundaries,” edited by John Shervais and John Wakabayashi. The full issue can be found at http://lithosphere.gsapubs .org/content/4/6.toc.

there are few active examples, and (2) nearly all of the evidence for tectonic, magmatic, and sedimentary responses to subduction initiation is preserved in forearcs, which are deeply submerged and buried beneath sediments. We would prefer to study subduction initiation in progress, but there are few places to do this. One such active region however, is the Puysegur subduction zone off the coast of southern New Zealand (LeBrun et al., 2003; Sutherland et al., 2006). However, as only a narrow segment of the Australia-Pacific transform plate margin is affected, studies of Puysegur cannot capture all of the processes that accompany major subduction initiation events, i.e., those that change the lithospheric force balance sufficiently to cause changes in plate motion and stimulate voluminous magmatism, as discussed herein. Such episodes shaped the western margin of North America in Mesozoic time (Dickinson, 2004), established a convergent margin along SW Eurasia in Late Cretaceous time (Moghadem and Stern, 2011), and engendered most of the active subduction zones of the western Pacific

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doi: 10.1130/L183.1

in Eocene time (Ishizuka et al., 2011). Major subduction initiation episodes are hemispheric in scale and necessarily reorganize upper-mantle flow, and in many cases are accompanied by widespread and voluminous igneous activity. Here, we outline a strategy that promises to accelerate our understanding of processes associated with major subduction initiation episodes by considering both the subduction initiation record preserved in forearcs and insights from studying well-preserved ophiolites. The record of subduction initiation is preserved in igneous crust and upper-mantle residues and the associated sediments on the overriding plate next to the trench. These collectively comprise the forearc (Dickinson and Sealey, 1979) and provide the best record of subduction initiation. Significant parts of forearcs may be lost by tectonic erosion (Scholl and von Huene, 2009); nevertheless, whatever remains contains the best record of the processes that accompanied subduction initiation of that particular convergent margin. We explore why this record has been overlooked and summarize recent studies of forearc crust and

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upper mantle, and what the results reveal about subduction initiation. The expense and difficulty of directly studying forearc igneous rock exposures are huge obstacles to our progress, so we explore the potential of some ophiolites for illuminating forearc composition and magmatic stratigraphy. Ophiolites are exposed on land and so are vastly easier and cheaper to study than forearcs. We conclude that those ophiolites that formed in a forearc provide important opportunities for advancing our understanding of subduction initiation. The strategy of comparative study of igneous forearc crust and ophiolites, coupled with geodynamic modeling, promises to lead to major advancements in our understanding of subduction initiation processes. FOREARCS

Forearcs comprise the bulk of any arc-trench system, occupying the ~150–200-km-wide region above the subducted plate between the trench and the magmatic arc. Forearcs are relatively stable and low standing—intra-oceanic forearcs lie entirely below sea level—and are morphologically unimpressive compared to spectacular volcanoes of the flanking magmatic arc and the tremendous gash of the trench. For these reasons, it is understandable that forearcs were either overlooked or misunderstood when the geologic implications of plate tectonics were first being explored in the late 1960s and 1970s. During this time, thinking about forearcs was dominated by examples on or near continents, such as California, Japan, Alaska, and Indonesia (for an account of early thinking about what would come to be called forearcs, see Dickinson, 2001). Even today, the textbook example

of a convergent plate margin is provided by the Late Mesozoic of California, with the Franciscan mélange representing exhumed subductionzone material, the Great Valley Group representing the forearc basin, and the Sierra Nevada Batholith representing the roots of the magmatic arc. This is indeed an excellent example of a sediment-rich convergent margin, but emphasis on California and other sediment-dominated forearc examples has inhibited appreciation of forearc crust itself. Many—but not all—continental forearcs are excellent examples of convergent margins affected by high sediment flux. Some continental convergent margins—such as Peru-Chile and NE Japan—do not have high sediment flux, but these have not been textbook examples because the interesting outcrops are in very deep water, and thus are difficult and expensive to study. In contrast, forearcs away from continents are mostly sediment starved (Clift and Vannucchi, 2004). Such naked forearcs expose crust and upper mantle, which are readily accessed by drilling through thin sediment cover, as was done during Deep Sea Drilling Project (DSDP) Leg 60 and Ocean Drilling Program (ODP) Legs 125 and 126 in the Izu-Bonin-Mariana arc and ODP Leg 135 in the Tonga forearc (Bloomer et al., 1995). EROSIVE VERSUS ACCRETIONARY FOREARCS

Subordinate proportions of forearcs are accretionary, growing by deposition of large sediment loads from a flanking continent, which bury forearc crust beneath forearc basins and then overflow to the trench, where these sediments briefly ride on the subducting plate before

some is scraped off to form an accretionary prism. Such situations of forearc thickening and widening are globally unusual, because most forearcs lose upper-plate crust to the subduction zone due to tectonic erosion, as a result of normal faulting, oversteepening, and basal fracturing and abrasion along the plate interface. Another misconception (due to bias toward studying sediment-rich convergent margins) is that all inner-trench slopes have very low slopes (8 km in the Tonga Trench (Bloomer and Fisher, 1987) but can be found as shallow as 5800 m in the southern Mariana Trench (Michibayashi et al., 2009). Such depths are mostly beyond the reach of manned submersibles, which currently cannot descend below 6500 m, so forearc peridotites are rarely sampled except by dredging. Still, we know about intra-oceanic peridotite exposures in four trenches: Izu-Bonin, Mariana, Tonga, and South Sandwich (Figs. 3 and 4). In addition, mantle peridotite is brought up by serpentine mud volcanoes, which are common in the Mariana forearc and are also known from the Izu forearc (Fryer, 2002). Because of its importance for understanding the nature and origin of intra-oceanic forearcs, some basic concepts about mantle peridotite in general and forearc peridotite in particular are presented here. These are residues after partial melting and complement magmatic rocks such as lavas—especially basalts—which are more common subjects of marine petrologic study. Because Earth has had mantle since shortly after it formed, but this has been modified by melt extraction and mixing with subducted materials, idealized compositions are useful for this discussion. For example, “primitive” mantle (PM) refers to an idealized chemical composition after the core segregated but before the continental crust was extracted. PM is also known

an idealized upper-mantle composition, but one which acknowledges the extraction of the continental crust. “Pyrolite” is another idealized composition (Green and Falloon, 1998) that is very similar to FMM. FMM and pyrolite approximate the composition of upper mantle that partially melts to generate oceanic crust beneath divergent plate boundaries (spreading ridges) and to produce arc melts beneath convergent margins (note that other components such as pyroxenite exist in the mantle, but these melt almost completely, leaving no identifiable residue). If PM, BSE, FMM, and pyrolite were rocks instead of ideas, they would be classified as

Volcanics

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Peridotite Figure 4. Interpretive section of a typical intra-oceanic forearc as reconstructed from dredging of the Tonga Trench. Figure is from Bloomer and Fisher (1987), reproduced with permission of Journal of Geology. N- and E- MORB refer to normal and enriched mid-ocean ridge basalt. V.E.—vertical exaggeration.

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www.gsapubs.org | Volume 4 | Number 6 | LITHOSPHERE

Subduction initiation, forearcs, and ophiolites

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Figure 5. What happens to peridotites when they melt: (A) Changes in modal mineralogy (and thus rock type) with progressive melting, plotted on International Union of Geological Boninite Sciences (IUGS) classification for peridotites (LeBas and Streckeisen, 1991). Fertile midocean-ridge basalt (MORB)–type mantle (FMM) is mostly peridotite, consisting of olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx), Forearc peridotite comprising lherzolite. Melting to produce basalCr# tic melts depletes peridotite in clinopyroxene, so that residual peridotite after ~20% melting is harzburgite, with 5% clinopyroxene (cpx), the remainder being orthopyroxene and an aluminous phase (spinel or garnet; Fig. 5A). Changes in peridotite composition due to partial melting are simple and clear. Melt depletion diminishes abundances of cpx, CaO, and Al2O3 and increases Cr# (Cr/Cr + Al) in the residual spinel (Fig. 5). This is because basalts, which are rich in CaO and Al2O3 (~12 and 16 wt%, respectively), are generated by partial melting of lherzolite, which is much poorer in CaO and Al2O3 (~3 and ~4 wt%, respectively for FMM; Fig. 5B). Clinopyroxene contains nearly all the CaO in peridotite, whereas spinel in FMM contains ~35% Al2O3 (a few percent Al2O3 is also dissolved in pyroxene). Melt depletion decreases the proportion of cpx, so that the residue progressively changes from lherzolite to harzburgite (90% olivine; Fig. 5A). Even though most forearc peridotite exposures are serpentinized, there are robust mineral and whole-rock compositional characteristics that are remarkably unaffected by such alteration. These include changes in the proportions of minerals (Fig. 5A), major-element bulk chemistry (Fig. 5B), and spinel compositions (Fig. 5C). All of these reflect the amount of melt depletion, as discussed already. These approaches allow us to compare the “refractoriness” of peridotites from various tectonic settings, i.e., how much melt depletion they have experienced. This reveals that forearc peridotites are the most depleted ultramafic rocks from any modern tectonic environment, with the highest Cr# spinels and lowest proportion of cpx and whole-rock abundances of CaO and Al2O3 (Bonatti and Michael, 1989). Not all forearc peridotites are so depleted; for example, some from the South Sandwich forearc include lherzolites with up to 3.7% Al2O3 and 4.4% CaO, along with spinels with Cr# as low as ~0.4 (Pearce et al., 2000). Morishita et al. (2011) documented two populations of spinel in IzuBonin forearc dunites, one group with moderate Cr# (0.4–0.6), and the other with high Cr# (>0.8). Variations in spinel compositions notwithstanding, forearc peridotites are dominated by ultradepleted compositions rarely found in other tectonic environments. The unusually depleted nature of forearc peridotites requires unusual melting conditions: abnormally high temperature, volatile flux, or both. Whatever the cause, these depletions are all the more noteworthy because forearcs associated with mature arcs have unusually low heat flow (Stein, 2003) and rarely are volcanically active. Whatever conditions caused the unusually extensive melting beneath forearcs no lon-

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ger exist. Such transitory conditions are linked to subduction initiation in the next section. Igneous rocks of exposed forearc crust above peridotites (Fig. 4) are only now becoming the focus of geoscrutiny. At one time, forearc crust was thought to comprise oceanic crust that was trapped when subduction began (e.g., Dickinson and Sealey, 1979), so the origin of forearc igneous crust has not, until recently, been much studied. This is changing, partly because of what is now recognized as the unusually strong depletion of forearc peridotites and because of interest in boninites. Boninites are lavas with unusual combinations of high silica and magnesium coupled with low calcium and aluminum abundances. These compositional features reflect low-pressure melting of harzburgitic mantle (e.g., Falloon and Danyushevsky, 2000), which is not otherwise observed on modern Earth. In addition, boninites are enriched in large ion lithophile elements (LILEs; elements with low valence and large ionic radius) and light rare earth elements (LREEs) relative to high field

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strength elements (HFSEs; elements with high valence and small ionic radius). High LILE/ HFSE ratios in boninites reflect metasomatism of the source mantle by hydrous fluid released from subducted crust and sediments (Gill, 1981; Stern et al., 1991; Pearce et al., 1992). This fluid lowers peridotite melting temperature at the same time that it re-enriches it in fluid-mobile elements, including LILEs and LREEs. Not all naked forearcs have boninite, but at least one of them—the Izu-Bonin-Mariana arc system—does (Fig. 6A; Stern et al., 1991; Macpherson and Hall, 2001). Probably the best exposure of boninite in the world is found in the Bonin (Ogasawara) islands (Taylor et al., 1994). These boninites erupted on the seafloor ca. 46–48 Ma, shortly after subduction began ca. 52 Ma (Ishizuka et al., 2006, 2011). Recent studies of Izu-Bonin-Mariana forearc crust exposed in the inner-trench wall (Fig. 6B) reveal that boninite may be the uppermost component of forearc crust, underlain by thicker, slightly older tholeiitic basalts, which Reagan et al.

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Figure 6. (A) Schematic map of the Izu-Bonin-Mariana arc system, showing principal tectonic features and forearc crustal sections discussed in text. 1—study of Reagan et al. (2010); 2—study of Ishizuka et al. (2011); 3—study of DeBari et al. (1999). Asterisk marks location of Deep Sea Drilling Project (DSDP) Site 458; B is location of Bonin (Ogasawara) Islands. White region is seafloor 1.4 wt% TiO2. Forearc basalts can also be more rich in SiO2 than typical MORB (most forearc basalts contain 1% TiO2, variable depletion in LREEs, absence of HFSEs (e.g., Nb, Ta), depletions on spider (primitive mantle– or N-MORB–normalized) diagrams, La/Nb 1, etc.; Pearce, 2003). Recognition of both arc-like and MORBlike compositions in some ophiolites has encouraged some workers to infer formation in a backarc basin (e.g., Beccaluva et al., 2004) or a complex, multistage tectonic history, for example, eruption of the two suites in discrete tectonic environments (e.g., Saccani and Photiades, 2004; Godard et al., 2003). The reasons against formation as a backarc basin are outlined in the previous section. The conclusion here that the basaltic sections in ophiolites are forearc basalts (Reagan et al., 2010) is supported by the observation that most Izu-Bonin-Mariana forearc basalts have lower Ti/V ratios than MORB, which probably results from the enhanced melting that commonly occurs in nascent subduction settings. The presence of transitional lavas with compositions that progress from forearc basalts to boninite with time at DSDP Sites 458 and 459 also supports this contention. Some ophiolites may reflect complex tectonic histories, but this cannot serve as the general explanation for subduction initiation rule ophiolites. Furthermore, any inference of complex tectonic histories is inconsistent with the absence of unconformities or significant sedimentary horizons between

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The Subduction Initiation Rule Lava composition

Age Youngest

Harzburgitic mantle source Modified by fluids from subducted crust /sediments

1–3 km

Calc-alkaline Sometimes boninite Mostly LREE-depleted Lower ΣREE Higher TiO2, Y, Zr Lower Ti/V Higher Cr/Y

Mantle composition

The four Tethyan ophiolites mentioned here show subduction initiation rule relationships, with lower lavas being more MORB-like and upper lavas being more arc-like, as Whattam and Stern (2011) showed. Troodos and Semail are at either end of the 3000-km-long ophiolite belt that marks the Late Cretaceous forearc of SW Asia, including the inner and outer ophiolite belts of Zagros, Iran (Moghadam and Stern, 2011). The magmatic chemostratigraphies of the many Zagros ophiolites have not been worked out, but these ophiolites in many cases have strong compositional affinities in some respect to arc magmas, for example, with respect to La/ Nb (arc lavas usually have La/Nb >1.4 according to Condie, 1999). It is very likely that we will be able to work out magmatic stratigraphies in more ophiolites around the world, but this will require thoughtful integrated structural and petrologic studies. DISCUSSION AND CONCLUSIONS

Tholeiitic LREE-depleted Higher ΣREE Higher TiO2, Y, Zr Higher Ti/V Lower Cr/Y

Lherzolitic mantle source Unaffected by fluids from subducted crust/sediments

Oldest

Figure 11. The subduction initiation rule, simplified after Whattam and Stern (2011). Ophiolites (and forearc crust) that form as a result of subduction initiation preserve systematic variations in basalt compositions (left), from mid-ocean-ridge basalt (MORB)–like at the base to volcanic arc)– like basalt (VAB), even boninitic, at the top. This reflects changes in the source mantle as a new subduction zone starts, beginning with adiabatic upwelling of asthenosphere that is not influenced by subduction-related fluids (Fig. 9B) to form MORB-like basalts (forearc basalts) by seafloor spreading. Fluids from the sinking lithosphere become increasingly important with time, eventually reaching and metasomatically re-enriching the increasingly depleted mantle source of melts (Fig. 9C). Asthenospheric upwelling diminishes in importance with time and is ultimately replaced by induced convection as sinking lithosphere begins downdip motion, and true subduction begins (Fig. 9D). This isolates the forearc mantle wedge, leading to establishment of localized magmatic arc behind a cold, dead forearc. LREE—light rare earth element; ∑REE— total rare earth element concentrations.

ophiolite lavas with MORB-like versus volcanic arc–like basalt compositions. Such interruptions might be difficult to identify because of ophiolite deformation, but such an important and distinctive feature should sometimes be recognized if this interpretation is generally correct. Conversely, the interpretation of a single, rapidly evolving magmagenetic environment is favored because such a hiatus is generally not observed. In addition, lavas with compositions that are transitional between forearc basalts and boninites have been observed in some ophiolites (e.g., Dilek and Furnes, 2009). As a result, ophiolite compositional variability is increasingly ascribed to progressive depletion and metasomatism of the mantle source as the ophiolite magmatic crust forms (e.g., Shervais, 2001; Beccaluva at al., 2005; Dilek and Furnes, 2009). Ophiolite lava compositions thus are increas-

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ingly interpreted to be products of magmatism in a single (suprasubduction zone) tectonic setting, albeit one that changed rapidly. It is worth emphasizing that it is generally difficult to recognize a magmatic stratigraphy in ophiolites, because they are so often jumbled by faulting, and because lavas and associated intrusions with different chemistries appear similar in the field. We recognize four ophiolites that have been studied in sufficient detail to reconstruct their magmatic stratigraphies: Mirdita, Pindos, Troodos, and Semail (as summarized by Whattam and Stern, 2011). There are other examples of subduction initiation rule ophiolites that could also be considered, for example, the 485–489 Ma ophiolite belt that can be traced >1000 km from Newfoundland down into the Taconic suture of New York (Bédard et al., 1998; Schroetter et al., 2003; Pagé et al., 2009).


Subduction initiation rule ophiolites provide wonderful opportunities to better understand the crust and upper mantle of magmatic forearcs and the ways in which new subduction zones form. Identifying and studying subduction initiation rule ophiolites will provide easy access to forearc lithosphere samples and structures, allowing many scientific perspectives to be engaged cheaply and easily. Where ophiolites can be firmly linked to forearcs through application of the subduction initiation rule and other approaches, geodynamic models for subduction initiation can be more easily developed and tested. Because subduction initiation rule ophiolites are fossil forearcs, they often can be traced across strike for tens of kilometers and along strike for hundreds of kilometers. Erosion of deformed subduction initiation rule ophiolites exposes various levels, allowing fourdimensional reconstructions of timing as well as vertical, across-strike, and along-strike magmatic variations. Sampling for geochronology is easier than for in situ forearcs because of this exposure, and relations of such dated samples to the surrounding rocks and fabrics provide context for interpreting the ages. The multiple perspectives and levels of detail allowed by these approaches mean that many aspects of forearc crust structure and subduction initiation can be understood much better by indirect study of subduction initiation rule ophiolites than by studying forearcs directly, as can be seen by mentally comparing the experiences captured in Figure 1. Even as we recognize that studies of subduction initiation rule ophiolites are essential for understanding forearcs, studies of in situ forearc crust need to move forward. We are only



beginning to understand the igneous crust of a single forearc in any detail—that of Izu-BoninMariana—and this example seems to be unusual in terms of the abundance of early-arc boninites. We need to continue to test and refine what we know about forearc magmatic evolution. It is especially important that we drill and sample through the magmatic stratigraphy of an igneous forearc in order to better understand the sequence and timing of magmas at one location, allowing us to answer questions such as: What are the relative proportions of forearc basalts versus volcanic arc–like basalt/boninitic lavas? How does forearc basalt magmatism transition to volcanic arc–like basalt magmatism? Is it gradational or abrupt? How long does forearc volcanism last? Answering such questions for an in situ forearc and comparing these answers with those for subduction initiation rule ophiolites will be key tests of the ideas presented in this paper. Furthermore, there are aspects of forearc structure and evolution that cannot be understood without studying extant forearcs. For example, active serpentine mud volcanoes in the Mariana forearc serve as actualistic models for sedimentary serpentinite deposits associated with some forearcs (Fryer et al., 1995; Fryer, 2002). Knowing that these mud volcanoes exist has stimulated the search for ancient serpentine mudflows (e.g., Teklay, 2006). Another example is the problem of tectonic erosion, which can remove a few kilometers of forearc crust in a few million years. Studies of active systems are generally aware that some crust is missing, whereas studies of ophiolites rarely consider this complication. Another point worth emphasizing is that there may be a wide range of subduction initiation magmatic products. Our models now are heavily biased toward the Izu-Bonin-Mariana convergent margin, which has abundant boninitic lavas and serpentinite mud volcanoes. The Izu-Bonin-Mariana forearc may serve as a useful example of intra-oceanic subduction initiation, but it could be less useful as a model for continental margin subduction initiation. Subcontinental mantle lithosphere is compositionally distinct from that beneath the ocean basins (Bonatti and Michael, 1989), and continental and oceanic mantle may melt to different extents during subduction initiation. Because there are likely to be significant differences in magmatic products from that shown by IzuBonin-Mariana and ophiolites, we should keep an open mind when considering the origin of any magmatic forearc built about the time that subduction began at a given convergent margin. The Siletzia terrane of coastal Oregon and Washington (Duncan, 1982) could be an example of continental margin subduction initiation where the magmatic sequence is somewhat dif-

ferent than that seen for Izu-Bonin-Mariana, for example, by having upper alkalic lavas instead of volcanic arc–like basalt overlying tholeiites. Above all, combined studies of subduction initiation rule ophiolites and forearc crust need to be integrated with geodynamic modeling to learn about the ways in which new subduction zones form. Geodynamic modeling of subduction initiation needs to be firmly tethered in reality, and the more that such models attempt to explain the rocks making up a real forearc and ophiolite, the more rapidly our understanding of this fundamental Earth processes will advance. ACKNOWLEDGMENTS

We thank Dave Scholl for edifying comments on accretionary prisms and Steve Graham for the photo of a Franciscan chert exposure. Thoughtful reviews by Rod Metcalf, Jean Bédard, and editors John Shervais and John Goodge are much appreciated. This manuscript was written while Stern enjoyed a Blaustein Fellowship at Stanford University. Stern’s research on intraoceanic arcs has been supported by the National Science Foundation, most recently by grant OCE-0961352. Reagan acknowledges research support from National Science Foundation grant EAR-0840862. This is University of Texas at Dallas Geosciences contribution 1228. REFERENCES CITED Anonymous, 1972, Penrose field conference on ophiolites: Geotimes, v. 17, p. 24–25. Beccaluva, L., Coltori, M., Giunta, G., and Siena, F., 2004, Tethyan vs. Cordilleran ophiolites: A reappraisal of distinctive tectono-magmatic features of supra-subduction complexes in relation to the subduction mode: Tectonophysics, v. 393, p. 163–174, doi:10.1016/j.tecto.2004.07.034. Beccaluva, L., Coltorti, M., Saccani, E., and Siena, F., 2005, Magma generation and crustal accretion as evidenced by supra-subduction ophiolites of the AlbanideHellenide Subpelagonian zone: The Island Arc, v. 14, p. 551–563, doi:10.1111/j.1440-1738.2005.00483.x. Bédard, J.H., Lauzière, K., Tremblay, A., and Sangster, A., 1998, Evidence from Betts Cove ophiolite boninites for forearc seafloor-spreading: Tectonophysics, v. 284, p. 233–245, doi:10.1016/S0040-1951(97)00182-0. Bloomer, S.H., 1983, Distribution and origin of igneous rocks from the landward slopes of the Mariana Trench: Journal of Geophysical Research, v. 88, no. B9, p. 7411–7428, doi:10.1029/JB088iB09p07411. Bloomer, S.H., and Fisher, R.L., 1987, Petrology and geochemistry of igneous rocks from the Tonga Trench—A non-accreting plate boundary: The Journal of Geology, v. 95, p. 469–495, doi:10.1086/629144. Bloomer, S.H., Ewart, W., Hergt, J.M., and Bryant, W.T., 1994, Geochemistry of igneous rocks from ODP Site 841, Tonga forearc, in Hawkins, J.W., Parsons, L., Allan, J., et al., Proceedings of the Ocean Drilling Program, Scientific Results Volume 135: College Station, Texas, Ocean Drilling Program, p. 625–646. Bloomer, S.H., Taylor, B., MacLeod, C.J., Stern, R.J., Fryer, P., Hawkins, J.W., and Johnson, L., 1995, Early arc volcanism and the ophiolite problem: A perspective from drilling in the western Pacific, in Taylor, B., and Natland, J., eds., Active Margins and Marginal Basins of the Western Pacific: American Geophysical Union Geophysical Monograph 88, p. 1–30.


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