DOI: 10.1126/science.1148397 , 85 (2008); 319Science et al.Paul G. Silver,Intermittent Plate Tectonics? www.sciencemag.org (this information is current as of January 28, 2008 ):The following resources related to this article are available online at http://www.sciencemag.org/cgi/content/full/319/5859/85version of this article at: including high-resolution figures, can be found in the onlineUpdated information and services, http://www.sciencemag.org/cgi/content/full/319/5859/85/DC1 can be found at: Supporting Online Material http://www.sciencemag.org/cgi/content/full/319/5859/85#otherarticles, 6 of which can be accessed for free: cites 35 articlesThis article http://www.sciencemag.org/cgi/collection/geochem_physGeochemistry, Geophysics : subject collectionsThis article appears in the following http://www.sciencemag.org/about/permissions.dtl in whole or in part can be found at: this articlepermission to reproduce of this article or about obtaining reprintsInformation about obtaining registered trademark of AAAS. is aScience2008 by the American Association for the Advancement of Science; all rights reserved. The title CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience on January 28, 2008 www.sciencemag.orgDownloaded fromIntermittent Plate Tectonics?Paul G. Silver1* and Mark D. Behn2Although it is commonly assumed that subduction has operated continuously on Earth withoutinterruption, subduction zones are routinely terminated by ocean closure and supercontinentassembly. Under certain circumstances, this could lead to a dramatic loss of subduction, globally.Closure of a Pacific-type basin, for example, would eliminate most subduction, unless this loss werecompensated for by comparable subduction initiation elsewhere. Given the evidence for Pacific-typeclosure in Earth’s past, the absence of a direct mechanism for termination/initiation compensation,and recent data supporting a minimum in subduction flux in the Mesoproterozoic, we hypothesizethat dramatic reductions or temporary cessations of subduction have occurred in Earth’s history. Suchdeviations in the continuity of plate tectonics have important consequences for Earth’s thermaland continental evolution.Plate tectonic theory originated to explainhow oceanic lithosphere is created at spread-ing centers and consumed at subductionzones. The continental component of this theorywasaddedbyWilson(1), who proposed thatthere is an cycle of continental breakup and colli-sion that accompanies the opening and closing ofocean basins. One of the most intriguing aspectsof this theory is the formation of supercontinentsafter ocean closure. In particular , supercontinentassembly has the potential to dramatically reducesubduction flux by the termination of subductionvia continent-continent collision. For example,consider the Atlantic Ocean basin, which hasbeen growing in area at the expense of the Pacificsince its opening at 200 million years ago (Ma).If this trend continues, relative plate-motionmodels (2) predict that in ~350 million years(My), the Pacific will effectively close, leading towidespread continent-continent collisions and thetermination of subduction in the Pacific basin.This would eliminate most of Earth’s subductionzones, resulting in an order of magnitudereduction in global subduction flux.Despite the plausibility of such an event, it iscommonly assumed that reductions in globalsubduction flux of this magnitude do not occur(3). The goal of this study is to critically evaluatethis assumption and to consider the alternativehypothesis that there have been dramatic reduc-tions in subduction flux over Earth’shistory.There are two main factors that must be con-sidered when evaluating the continuity of sub-duction flux: the mode of ocean closure andthe characteristics of subduction initiation. If theAtlantic closed (A-type closure), instead of thePacific (P-type closure), then Pacific subductionwould survive intact (Fig. 1). A-type closureoccurs when an interior ocean closes after thebreakup of a previous supercontinent. In contrast,P-type closure represents the closing of an ex-ternal ocean (4). The key distinction betweenthese two modes, in terms of subduction flux, isthat P-type closure has the potential to dramati-cally reduce or even eliminate subduction flux,whereas A-type closure does not (Fig. 1). Closuremode can be determined by dating the formeroceanic crust (preserved as ophiolites in sutures)after supercontinent assembly . For A-type closure,the (former) ocean crust from the internal ocean isalways younger than the age of breakup of theprevious supercontinent (Fig. 1). For P-type clo-sure, this crust can be older (because subductionpredates the breakup of the previous supercon-tinent), as exemplified today by the circum-Pacificophiolite belts, some of which possess earlyPaleozoic ages (such as the Trinity ophiolite ofnorthern California) and predate the breakup ofPangaea (4).P-type and A-type closure represent two end-member cases, and in practice the mode ofclosure is likely to be a mixture of the two. Itnevertheless appears that supercontinent assem-bly is typically predominantly produced by oneor the other (4). The supercontinent Pangaea,which was formed by the closing of the IapetusOcean, appears to have formed primarily byA-type closure (4). In contrast, presently availableevidence suggests that both the supercontinentsPannotia (5, 4) and Rodinia formed primarily byP-type closure (6, 4). Fin a ll y, the earlier hypothe-sized supercontinent Nuna may have formed byA-type closure, although this is highly uncertain(4)(Table1).For Rodinia, the evidence for P-type closureis based on (i) the greater age of oceanic-affinityrocks (such as ophiolites), as compared to the ageof breakup of Nuna (4); (ii) the fact that theSuperior province faced an open ocean for ~ 0.8billion years (4); and (iii) that the Grenvilleorogen, which ultimately created Rodinia, ap-1Department of Terrestrial Magnetism, Carnegie Institution ofWashington, 5241 Broad Branch Road, NW, Washington, DC20015, USA.2Department of Geology and Geophysics, WoodsHole Oceanographic Institution, Mail Stop 22, Woods Hole,MA 02543, USA.*To whom correspondence should be addressed.Fig. 1. Schematic of two modesof ocean closure: P-type andA-type (4). (Top) Supercontinent(brown), surrounded