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Reconstructing Pre-Pangean Supercontinents 1888 2013

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Reconstructing Pre-Pangean Supercontinents 1888 2013 Reconstructing pre-Pangean supercontinents 1888 2013 CELEBRATING ADVANCES IN GEOSCIENCE † David A.D. Evans Invited Review Department of Geology & Geophysics, Yale University, New Haven, Connecticut 06520, USA ABSTRACT to routine dating by U-Pb on baddeleyite, the tive cells due to oceanic closure (Zhong et al., global abundance of Paleo-Mesoproterozoic 2007; O’Neill et al., 2009). Twenty-fi ve years ago, initial plans for dike swarms might make Nuna more immi- Some form of a supercontinent cycle is reconstructing the Rodinia supercontinent nently solvable than Rodinia. generally invoked to explain a number of were being drafted, based on the growing rec- Prior to the assembly of Nuna, various global-scale phenomena in the long-term geo- ognition of correlatable mid-Neoproterozoic “supercraton” connections such as Vaalbara, logic record. Following an initial compilation (0.8–0.7 Ga) rifted passive margins, many of Superia, and Sclavia are only beginning to of global peaks and troughs in isotopic ages which were established on the eroded rem- take form. Unmetamorphosed, early Paleo- (Gastil , 1960), early concepts of orogenic cyclic- nants of late Mesoproterozoic (1.3–1.0 Ga) proterozoic (2.5–2.0 Ga) mafi c dike swarms ity (e.g., Sutton, 1963; Wilson, 1966) planted orogenic belts. The 1990s witnessed a surge are commonplace features across the in- fertile seeds of thought regarding pre-Pangean of interest in Rodinia, with many regional teriors of Archean cratons, and their joint supercontinents. The concept of “two Phanero- studies of tectonostratigraphy and U-Pb paleo magnetic and geochronologic study can zoic supercycles” (Fischer, 1984) subsequently geochronology generally conforming to the help reassemble the cratons into their super- organized many scholars’ ideas on processes “inside-out” reconstruction model: juxtapo- craton parent landmasses. Progressively leading to long-term sea-level variations and ice sition of west Laurentia with east Australia/ older geologic times require consideration of ages (Worsley et al., 1984; Nance et al., 1986; Antarctica, north Laurentia with Siberia, a greater number of potentially independent Veevers, 1990) and possible links to geomag- and east Laurentia with Baltica and cratons terranes, each needing individual kinematic netic superchrons through putative superplume that would later form West Gondwana. This constraints. Furthermore, the initial stabi- events (Larson, 1991). The “supercycle” idea standard model of Rodinia appeared to be lizing events of most extant cratons during has been extended through geologic time, and converging toward a solution with only minor Neoarchean time (3.0–2.5 Ga) therefore ren- according to some researchers, with a constant variations by the turn of the millennium, but der global reconstructions older than that in- periodicity (Krapez, 1999; Bozhko, 2011) that new paleomagnetic data and tectonostrati- terval improbable. would seem astonishing given the stochastic graphic information obtained in the succeed- nature of many well-dated orogenic events in ing decade chipped away at various aspects INTRODUCTION the Phanerozoic, and also given likely scenarios of the reconstruction; several cratons seemed for secular changes in the thermal evolution of to require exclusion from the supercontinent This paper attempts to summarize the past the planet (e.g., Korenaga, 2008). The origi- (thus questioning its very validity), or the 25 years of research on the confi gurations of nally proposed cycle focuses on the history of landmass might have assembled much later ancient supercontinents. There is both empirical closing and opening of the Atlantic Ocean and (≤0.9 Ga) than originally envisaged (thus and theoretical evidence to suggest that the most its predecessor, Iapetus (Wilson, 1966; Har- weakening the link to global Mesoprotero- recent supercontinent, Pangea, is merely the lat- land and Gayer, 1972). Today’s wide Atlantic zoic orogenesis). Although a consensus model est of a series of large continental aggregations, Ocean would be akin to a perhaps equally wide of Rodinia’s assembly and fragmentation separated in time by ~500–600-million-year Iapetus in the Cambrian–Ordovician time, and has arisen from the International Geoscience intervals (Fig. 1). The empirical data include the Pangea’s predecessor would have existed prior Programme Project 440 working group, the well-established global maxima and minima in to establishment of those earliest Paleozoic pas- reconstruction is supported by rather sparse isotopic age determinations and number of oro- sive margins (Bond et al., 1984). Upon critical defi nitive-quality data. gens (Condie, 1998, 2002; Campbell and Allen, examination, however, the Pangean supercycle As the quest for Rodinia matures to a third 2008), the number and durations of ancient pas- concept fails to account for many of the other decade of scrutiny, the search for its predeces- sive continental margins (Bradley, 2008), geo- geologic features of Phanerozoic Earth his- sor Nuna (a.k.a. Hudsonland or Columbia) is chemical trends in sedimentary rocks (Shields, tory. For example, sea level (Fig. 1C) has fallen only now reaching a stage of global synthesis 2007), isotopic proxies for mantle extraction through the Cenozoic Era, despite continued between tectonostratigraphic and paleomag- and continental growth (Collins et al., 2011), widening of the Atlantic. Sea level may bet- netic data. According to most defi nitions, and other proxies for the amalgamation of ter refl ect the broad pattern of ice ages (Fig. Nuna assembled at 1.9–1.75 Ga, or perhaps landmasses (reviewed by Bradley, 2011). Theo- 1D), but those appear equally divorced from as late as 1.6 Ga, and fragmented during retical expectations for a supercontinent “cycle” the supercontinental record, because they are the interval 1.5–1.2 Ga. Because mafi c dike arise from numerical modeling of mantle con- temporally related to latest Precambrian rift- swarms are ideal targets for paleomagnetic vection that alternatively posit a thermal insu- ing (cf. Young, 1995), Pangea assembly, and study, and because they are now amenable lation effect under the supercontinent (Phillips the present wide-Atlantic stage. Geomagnetic and Coltice, 2010, and references therein), or a superchrons (Fig. 1E) appear unconnected to †E-mail: [email protected] reorganization of subduction loci and convec- either the existence of supercontinents or the GSA Bulletin; November/December 2013; v. 125; no. 11/12; p. 1735–1751; doi:10.1130/B30950.1; 6 fi gures. For permission to copy, contact [email protected] 1735 © 2013 Geological Society of America Evans A B C D E F notia, Rodinia, Nuna/Columbia, and Kenorland (reviewed by Nance et al., 2013). Among these Proposed Proposed Sea Ice Super- Seawater hypothetical landmasses, only Pannotia is char- Supercon- Supercon- Level Ages chrons 87Sr 86Sr tinents tinents -25 250 acterized by an unambiguous reconstruction Age (Ma) m m .706 .709 Age (Ma) (Dalziel, 1997), and all of them are question- able even in principle. Nonetheless, the proxy Pangea evidence for pre-Pangean supercontinents for at least the latter half of Earth history has 500 100 motivated substantial efforts to create plausible reconstructions of their cratonic arrangements. Rodinia (old) 200 1000 (new) WHAT IS A SUPERCONTINENT? Pangea 300 Identifi cation of ancient supercontinents (new) requires a standard by which the large sizes 1500 Nuna (old) of former landmasses merit the prefi x “super.” 400 Pangea is the quintessential supercontinent, yet during its brief tenure it still excluded some 2000 (alternative) Artejia? ??? eastern Asian cratons, which at the time were 500 on a northward track across Tethys (Fig. 2A), lying on distinct tectonic plates even if ephemer- 2500 Kenorland? ally connected via land bridges. Bradley (2011) opted for a fl exible defi nition of a superconti- Pannotia? 600 (original) nent as merely “a grouping of formerly dis- 3000 persed continents,” but this fl exibility comes at 700 the cost of ambiguity. Meert’s (2012) proposed minimum requirement of 75% of the preserved crustal area from a given age seems reasonable, 3500 ??? ??? Rodinia 800 as it conforms to most earlier usage (i.e., Pan- (new) gea is, but Gondwana in itself is generally not, ??? considered to be a supercontinent). It is also 4000 900 simple and quantifi able in an objective manner. ??? Most reconstructions of Rodinia (Figs. 2B–2E) ??? appear to satisfy the 75% minimum area cri- 4500 1000 terion for at least brief intervals of kinematic evolution, although some tectonic blocks with limited or no paleomagnetic data and largely Figure 1. Supercontinental hypotheses versus time, and fi rst-order geological events of unconstrained evolution are usually included Neoproterozoic–Phanerozoic time. (A) Entire Earth timeline, with Pangea and putative arbitrarily within the landmasses for the mere pre-Pangea supercontinents, in both original or old conceptual ages of assembly and dis- sake of completeness (e.g., Li et al., 2008). persal, and updated ages as necessary. (B) Enlargement of the last billion years of Earth Other measures of a supercontinent’s impor- history; pre-Pangea supercontinents are as summarized by Dalziel (1997). (C) Estimates of tance could include its effects on the broader sea level, relative to that at present, based on varying continental hypsometry, from Algeo Earth system, whether in terms of the surface and Seslavinsky (1995). (D) Ice ages scaled horizontally according to latitude distribution environment
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