Geoscience Frontiers 4 (2013) 185e194
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China University of Geosciences (Beijing) Geoscience Frontiers
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Research paper Speculations on the mechanisms for the formation and breakup of supercontinents
J. Brendan Murphy a,*, R. Damian Nance b a Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada b Department of Geological Sciences, Ohio University, Athens, OH 45701, USA article info abstract
Article history: The supercontinent cycle has had a profound effect on the Earth’s evolution since the Late Archean but Received 17 May 2012 our understanding of the forces responsible for its operation remains elusive. Supercontinents appear to Received in revised form form by two end-member processes: extroversion, in which the oceanic lithosphere surrounding the 9 July 2012 supercontinent (exterior ocean) is preferentially subducted (e.g. Pannotia), and introversion in which the Accepted 18 July 2012 oceanic lithosphere formed between dispersing fragments of the previous supercontinent (interior Available online 21 August 2012 ocean) is preferentially subducted (e.g. Pangea). Extroversion can be explained by “topedown” geo- dynamics, in which a supercontinent breaks up over a geoid high and amalgamates above a geoid low. Keywords: Supercontinent cycle Introversion, on the other hand, requires that the combined forces of slab-pull and ridge push (which Introversion operate in concert after supercontinent break-up) must be overcome in order to enable the previously Extroversion dispersing continents to turn inward. Introversion may begin when subduction zones are initiated along Pangea boundaries between the interior and exterior oceans and become trapped within the interior ocean. We Rodinia speculate that the reversal in continental motion required for introversion may be induced by slab Pannotia avalanche events that trigger the rise of superplumes from the core-mantle boundary. Ó 2012, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.
1. Introduction for this episodicity strengthened as the number and the precision of radiometric dates increased during the 1970s leading to the Our current understanding of the origin of supercontinents is hypothesis of a supercontinental cycle (Worsley et al., 1984, 1986; rooted in hypotheses about the potential link between orogenesis Nance et al., 1986, 1988; Fig. 1). This hypothesis proposed that the and the episodicity in the generation and destruction of continental supercontinent Pangea, which formed in the Late Paleozoic and crust. This linkage was proposed before the general acceptance of broke up in the Mesozoic (Cocks and Torsvik, 2002; Stampfli and plate tectonic principles and before the advent of precise U-Pb Borel, 2002; Veevers, 2004), is only the youngest of a number of geochronology (e.g. Holmes, 1954; Gastil, 1960; Sutton, 1963; supercontinents that have formed, only to breakup and reform, Armstrong, 1968, 1981), although the mechanisms responsible since the Late Archean. Although not held universally (e.g. Stern, were unclear. Application of plate tectonic principles to pre- 2005; Hamilton, 2011), there is an emerging consensus that these Mesozoic orogenic belts (Wilson, 1966; Dewey, 1969) linked “supercontinent cycles” have had a profound effect on the evolu- crustal generation and destruction to processes occurring at tion of the Earth’s systems for at least the last 2.5 Ga (e.g. Nance constructive and destructive plate margins, respectively. Evidence et al., 1986; Veevers, 1994; Hoffman et al., 1998; Condie, 2002; Knoll et al., 2004; Rogers and Santosh, 2004; Maruyama and Santosh, 2008; Meert and Lieberman, 2008).
* Corresponding author. However, an understanding of how supercontinents form is E-mail address: [email protected] (J.B. Murphy). lacking. For example, Murphy and Nance (2003, 2005) pointed out that ancient supercontinents have formed by different mecha- Peer-review under responsibility of China University of Geosciences (Beijing). nisms throughout geologic time. They proposed two end member mechanisms to account for these differences: (1) introversion (e.g. Nance et al., 1988), in which the oceanic lithosphere formed between dispersing fragments of the previous supercontinent (the Production and hosting by Elsevier interior ocean) is preferentially subducted to form the next
1674-9871/$ e see front matter Ó 2012, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gsf.2012.07.005 186 J.B. Murphy, R.D. Nance / Geoscience Frontiers 4 (2013) 185e194
Figure 1. The supercontinent cycle of Worsley et al. (1984) summarizing episodic events in biogenesis (supercontinent breakup), collisional orogenesis (supercontinent amal- gamation), mafic dike swarms (supercontinent rifting), and major glacial intervals (supercontinent stasis), the pattern of which suggested the existence of five supercontinents in the past 2 Ga. supercontinent; and (2) extroversion (e.g. Hartnady, 1991; vestiges of oceanic lithosphere incorporated into orogenic belts Hoffman, 1991), in which the ocean surrounding a supercontinent imply that Pangea formed by preferential subduction of interior (exterior ocean) is preferentially subducted (Fig. 2). Paleogeo- oceans (i.e. by introversion) during the Paleozoic, but that Pan- graphic reconstructions combined with Sm-Nd isotopic data from notia formed by the preferential subduction of exterior oceans (i.e.
Figure 2. Schematic diagrams (modified from Murphy and Nance, 2003; Murphy et al., 2009) showing introversion and extroversion models for the development of a super- continent. AeC show stages in supercontinent breakup, with relatively old oceanic lithosphere (blue) surrounding the supercontinent (exterior ocean), and the progressive development of relatively new oceanic lithosphere (yellow) between the dispersing continents (interior ocean). Note the high angle between the spreading ridges in the interior ocean and its plate boundaries with the exterior ocean. In B and C, subduction initiates along these boundaries (red ellipses). D: Configuration of a supercontinent formed by introversion (i.e. preferential subduction of the interior oceanic lithosphere). E: Configuration of a supercontinent formed by extroversion (i.e. preferential subduction of the exterior oceanic lithosphere). F: An intermediate case in which one ocean is closed by introversion and the other by extroversion. For orthoversion, see Fig. 1 of Mitchell et al. (2012). J.B. Murphy, R.D. Nance / Geoscience Frontiers 4 (2013) 185e194 187 by extroversion) during the latest Neoproterozoic (Murphy and Continued convergence and subduction eventually leads to Nance, 2003, 2005). consumption of the interior oceans, amalgamation of the super- More recently, Mitchell et al. (2012) proposed an orthoversion continent by a series of terminal continent collisions, and the model, in which the succeeding supercontinent forms at an angle of development of interior orogenic belts that are stranded within the 90 relative to its predecessor (i.e. on the great circle of a subduction newly formed supercontinent (Fig. 2; Murphy and Nance, 1991; system that encircled the previous supercontinent). These authors Nance and Murphy, 1994). Complexities in deciphering the illustrate the differences between the three models by showing geological records of interior orogens are profoundly influenced by reconstructions that have different predictions for the configuration the irregular geometry of the colliding continental margins, which of the next supercontinent: introversion implies that this super- inevitably include promontories and re-entrants. For example, continent would form by the closure of the Atlantic Ocean, extro- collision of opposing promontories leads to intense deformation, version by closure of the Pacific Ocean, and orthoversion by closure including processes such as crustal thickening and tectonic of the Arctic Ocean and Caribbean Sea. From the standpoint of wedging, while at the same time, re-entrants contain oceanic mechanism, however, orthoversion is a form of introversion since lithosphere that is progressively eliminated by localized and short- both the Arctic and Caribbean are interior oceans of Pangea breakup. lived subduction systems, such as those that dominate the late As an additional complication, the application of well- Cenozoic evolution of the eastern Mediterranean (e.g., Harangi established geodynamic models to an Early Paleozoic Earth fails et al., 2006; Dilek and Sandvol, 2009). to yield Pangea in the correct configuration (Murphy and Nance, Continentecontinent collisions lead to elimination of the 2008; Murphy et al., 2009). This failure points to our lack of subduction zones between the converging continents and may be understanding of the fundamental forces that influence the long- accompanied, or followed by, the foundering of the subducting slab term motion of the plates. into the mantle. Compensatory upwelling of warm asthenosphere In this paper, we briefly review the evolution of the oceanic contributes to the uplift and melting of the previously thickened lithosphere affected by supercontinent breakup and amalgamation, crust as well as the formation of a more youthful sub-continental the contrasting styles in orogenic activity recorded at the leading lithospheric mantle. Such slab-break off and compensatory and the trailing edges of the amalgamating continents, the different asthenospheric upwelling is now widely recognized as one of the ways in which supercontinents may form, and the potential last stages in collisional orogenesis. The thickened crust may also mechanisms for episodic supercontinent formation. We emphasize undergo extensional collapse as gravitational potential energy is at the outset that our paper is an attempt to provide a broad released (e.g. Dewey, 1988; Rey et al., 2001). In the core of Pangea, template in order that the many exceptions to the template can be one of the final stages resulted in regional oroclinal development identified as such and investigated in the correct context. and coeval lithospheric delamination (Gutierrez-Alonso et al., 2011).
2. Supercontinent assembly and amalgamation 2.2. Trailing edges
Prior to amalgamation, the assembly of a supercontinent begins The geologic record along the trailing edges of continents during when several continents begin to converge on one another, implying supercontinent assembly is dominated by the formation of arc and subduction of the intervening oceanic lithosphere. At this stage in the back-arc complexes as the continental margins retreat relative to cycle, continental margins exhibit contrasting histories depending on their subduction zones. This retreat may be due to subduction zone whether they are located on the leading edge (i.e. facing the con- roll-back within the expanding (exterior) ocean that these margins tracting oceans) or trailing edge of the converging continents. face and/or motion of the continents themselves away from their subduction zones (Chase, 1978; Dewey, 1980; Hynes and Mott, 2.1. Leading edges 1985). This scenario typically results in a collage of ensimatic and ensialic island arcs, which may be geographically separated from Along the leading edges of the converging continents, evidence of the retreating continental margins by expanding marginal backarc contracting oceans includes the widespread foundering of passive basins that are typically floored by oceanic lithosphere. If the marginsaswellasthedevelopmentofmagmaticarcsandmélanges.If continental margins become sufficiently distant from the subduc- the rate of continent convergence exceeds the rate of subduction zone tion complexes, non-volcanogenic sequences resembling those roll-back, orogenesis associated with Andean-type subduction typical of passive margins may develop along the continental predominates. If the rate of roll-back exceeds the rate of continental margin side of the marginal basins. Sequences deposited near the convergence, western Pacific-type subduction predominates, with volcanic side of the backarc basin are typically dominated by vol- episodes of superimposed orogenesis potentially induced by the canogenic deposits as sediment supply becomes progressively cut- collision of terranes or the subduction of plateaux or ridges, or by off from the retreating continent. changes in the profile of the subduction zones (e.g. Collins, 2002). As the supercontinent amalgamates, subduction between the Additional complications are evident from recent syntheses and geo- colliding continents becomes progressively more localized in re- dynamic models which suggest that fore-arc complexes may become entrants, and eventually ceases. At the same time, subduction entrained (either piecemeal or in large tectonic slices) in the subduc- becomes more prevalent along the supercontinent’s periphery, as tion zone, a process called subduction erosion (e.g. von Huene et al., exemplified by widespread arc development along the periphery of 2004; Scholl and von Huene, 2007; Keppie et al., 2009; Stern, 2011). Pangea in the Late Paleozoic (Nance and Murphy, 1994; Scotese, The leading edges are also those likely to experience ophiolite 1997, 2002, 2007) and the periphery of Pannotia in the late Neo- emplacement. Ophiolite complexes are commonly viewed as proterozoic early Cambrian (Murphy and Nance, 1989, 1991; vestiges of subducted oceanic lithosphere. However, geochemical Cawood, 2005 ; van Staal et al., in press). data (e.g. Saunders and Tarney, 1979; Jenner et al., 1991; Dilek and Furnes, 2009; van Staal et al., 2009) suggest that the majority of 3. Supercontinent breakup these complexes have broad arc affinities, implying that most ophiolites originated in the upper plate and that the vast majority Using Pangea as a precedent, Meert (2012) proposed that of oceanic lithosphere in the lower plate is subducted and lost to a supercontinent should consist of at least 75% of the preserved the geological record. continental crust prior to initial breakup. Because of its lower 188 J.B. Murphy, R.D. Nance / Geoscience Frontiers 4 (2013) 185e194 density relative to the surrounding oceanic lithosphere, a super- Regardless of the mechanism, two geodynamically distinct continent at this stage of the cycle is likely to be almost entirely tracts of oceanic lithosphere must exist once a supercontinent encircled by inward dipping subduction zones that represent plate breaks up: (1) that of the interior oceans, which lie between the boundaries between continental and oceanic lithosphere. This dispersing continents and are underlain by oceanic lithosphere that inference is supported by regional syntheses that show Pangea cannot be older than the age of breakup, and (2) that of the exterior (Scotese, 2001, 2002; Stampfli and Borel, 2002) and Pannotia (de ocean, which surrounded the supercontinent following its amal- Wit et al., 1988; Murphy and Nance, 1989, 1991; Cawood, 2005; gamation and so is largely underlain by oceanic lithosphere that is Cawood and Buchan, 2007; Murphy et al., 2009) encircled by significantly older than the age of breakup (Murphy and Nance, subduction zones in the late Paleozoic and late Neoproterozoic, 2003, 2005). Immediately following breakup, there is an impor- respectively. tant geodynamic distinction between these two types of oceans The mechanisms responsible for supercontinent breakup are because of the contrasting average age of their lithosphere. The poorly understood, highlighted by the ongoing controversy interior oceans are underlain by relatively young and buoyant between the relative roles of active rifting (initiated by upwelling lithosphere, the presence of which is a major factor in the global magma) and passive rifting (initiated by crustal extension). Sengör rise in sea level that occurs after supercontinent breakup (e.g. Bond and Burke (1978) proposed that rifting occurred above mantle et al., 1984; Hallam, 1992; Fig. 3). On the other hand, much of the plumes, and this model has commonly been invoked to explain the exterior ocean contains tracts of lithosphere that formed prior to breakup of ancient supercontinents (e.g. Pisarevsky et al., 2008). supercontinent amalgamation and must therefore be relatively old. According to Anderson (1982, 2001), supercontinents have in-built The distinction between the two types of ocean diminishes with obsolescence in that they insulate the mantle, resulting in the time as: (1) the interior oceans age, and (2) subduction preferen- generation of thermal upwellings and geoid highs that result in tially removes the oldest lithosphere from the exterior ocean. supercontinent breakup. Condie (1998) proposed that geoid highs However, sea level curves suggest that this difference is geo- may be related to superplumes that form in response to an dynamically significant for 60e80 Ma following breakup (e.g. Miller avalanche of subducted slabs from the 660 km discontinuity to the et al., 2005 and references therein). core-mantle boundary. By analyzing the motion of Pangea in During this 60e80 Ma interval, the interior oceans are flanked several reference frames, Hynes (1990) concluded that the breakup by passive margins upon which thick shelf successions are depos- of Pangea occurred in two stages, the first involving passive rifting ited. The development of these margins is accompanied by subsi- during rapid supercontinental motion in the early Jurassic, whereas dence that reflects: (1) cooling and subsidence of the continental the second (during which major dispersal occurred) was accom- margins as they migrate away from ocean ridges, and (2) a rise in panied by active rifting during a Cretaceous supercontinental sea level related to the presence of new oceanic ridge systems stillstand. This scenario is consistent with Anderson’s model for within the relatively young interior oceans, whose oceanic litho- continental breakup in which mantle upwelling is facilitated by sphere is buoyant and therefore elevated. These margins become a stationary supercontinent. the trailing edges of the dispersing continents. Irrespective of the mode of its initiation, there is strong evidence Over the same time interval, subduction of the exterior oceanic that pre-existing structures play a guiding role in the geometry of lithosphere around the periphery of the supercontinent becomes supercontinent breakup. This implies that the configuration of more vigorous as the continental margins facing the exterior ocean supercontinents may be profoundly influenced by structures become the leading edges of the dispersing continents. These formed during their earlier convergence. leading continental margins are characterized by ongoing
Figure 3. Global sea level curve for the Phanerozoic (after Hallam, 1992). Sea level rise occurs throughout the Cambrian, consistent with the opening of the Iapetus Ocean. Global sea level drop at the end of the Cambrian is attributed to the onset of subduction within the Iapetus Ocean. The long-term drop in sea level beginning at ca. 440 Ma is broadly coincident with the onset of subduction within the Rheic Ocean, which eventually closed with the amalgamation of Pangea. The rise in sea level in the Mesozoic coincides with the opening of the Atlantic Ocean. J.B. Murphy, R.D. Nance / Geoscience Frontiers 4 (2013) 185e194 189 orogenesis associated with subduction that typically produces particular, it is clear that Pangea, the supercontinent for which we continental arc magmatism (e.g. Kay et al., 2005; Tatsumi, 2005), as have the best constraints, formed by preferential subduction of the well as episodic orogenesis related to changes in the dip of the interior Iapetus and Rheic oceans (i.e. by introversion, Fig. 4), an subduction zone and/or the accretion of outboard terranes (arcs, outcome not predicted by topedown geodynamic models (Murphy microcontinents, oceanic plateaux etc.), many of which formed and Nance, 2008). during the previous stage of supercontinent assembly (e.g. Coney For introversion to occur, the combined forces of slab-pull and et al., 1980; Cawood and Buchan, 2007). Accretion of outboard ridge push, which operate in concert after supercontinent breakup, terranes is typically followed by relocation of the subduction zone must be overcome. Not only must new subduction zones initiated outboard of the accreted terrane, resulting in the re-instatement of in order to consume the interior oceans, the previously dispersing Andean-style orogenesis. Magmas produced beneath the expanded continents must reverse their direction of motion. In addition, the continental margin have Sr and Nd isotopic compositions that rates of subduction of interior oceanic lithosphere must exceed the reflect recycling of accreted oceanic mantle lithosphere (e.g., Kistler rates of spreading. and Peterman, 1973; DePaolo, 1981; Murphy et al., 2009). The geodynamic evolution of the interior oceans in this context Recent syntheses show that these processes are often accom- is far from clear. In the extroversion model, progressive closure of panied by the transfer of crust in the fore-arc region from the upper the exterior Pacific Ocean over the next 80 Ma would imply that: (1) plate to the lower plate, a process known as subduction erosion the Atlantic Ocean (an interior ocean relative to the breakup of (e.g. Stern, 2011). This crust is transported down the subduction Pangea) would continue to be flanked by passive margins, and (2) zone and into the mantle, from whence it may subsequently be the oceanic lithosphere along these passive margins would be ca. extruded, either back up the subduction channel (e.g. Gerya et al., 250e260 Ma old by the time the extroverted supercontinent 2002; Ernst, 2006; Federico et al., 2007) or upward into the amalgamated. But this is a non-uniformitarian outcome because upper plate (Stöckhert, and Gerya, 2005; Warren et al., 2008; oceanic lithosphere of this antiquity is unknown in the modern Keppie et al., 2009). Some of this subducted crustal material may be world. Indeed, it is widely believed that subduction of oceanic recycled, such that its chemical components affect the geochem- lithosphere this old is inevitable because of its significant negative istry of the arc-related magmas (von Huene et al., 2004; Keppie buoyancy relative to the underlying asthenosphere (e.g. Hynes, et al., 2009; Stern, 2011). 1982; Mueller and Phillips, 1991). However, subduction initiation at passive margins is geodynamically challenged. According to 4. Supercontinent re-assembly and amalgamation geodynamic models (Cloetingh et al., 1989), the negative buoyancy of the oceanic lithosphere is trumped by its increasing strength as it Once a supercontinent has fragmented and dispersed, the key ages. So while old oceanic lithosphere may be significantly denser question is how does the next supercontinent assemble? Prefer- than the underlying asthenosphere, its rigidity prevents its ential destruction of the exterior oceanic lithosphere leads to subduction. This argument is strengthened by the observation that supercontinent amalgamation by extroversion whereas preferen- there is no clear example in the MesozoiceCenozoic of a passive tial consumption of the interior oceanic lithosphere leads to margin that evolved to become a subduction zone. Yet the geologic introversion. Since orthoversion must involve the preferential record indicates that oceanic lithosphere is typically subducted consumption of either the exterior or interior oceans, or a combi- within 200 Ma of its formation, and the evidence of modern ocean nation of both (Murphy and Nance, 2003; Mitchell et al., 2012), it is basins suggests that, regardless of the location of their initiation, not discussed further here. subduction systems ultimately migrate to continental margins. According to Anderson (1994, 2001), subduction of cold litho- An important (and often overlooked) consequence of super- sphere provides 90% of the force needed to drive plate motions. The continent breakup is that plate boundaries must exist between the dispersing continents migrate away from geoid highs (i.e. regions of interior and exterior oceans (Fig. 2), across which there are likely to mantle upwelling) toward geoid lows (existing subduction zones at be signi ficant contrasts in the age (and therefore the buoyancy and sites of convective downwelling). For example, since the breakup of relative elevation, see Sclater et al., 1980) of their respective oceanic Pangea, the continents have migrated away from the geoid high lithospheres (Murphy et al., 2009, 2011). Furthermore, these beneath Africa toward geoid lows located adjacent to the subduc- boundaries are likely to be at a high angle to the spreading direction tion zones of the western Pacific. If the current rates of plate in the interior oceans, and so are likely to be major transform faults. motions are maintained, the Pacific Ocean will close in ca. 80 Ma to Although the details may be different, similar juxtapositions of form the next supercontinent. Since the Pacific Ocean is a vestige of interior and exterior oceanic lithospheres with contrasting ages, Panthalassa (i.e. the exterior ocean that surrounded Pangea in the elevations and relative buoyancies are also likely to develop soon Early Mesozoic), its closure in this fashion would be a case of after supercontinent breakup. For example, subduction initiation extroversion. The forces primarily responsible for the expansion of along the IsueBonineMariana subduction zone occurred along the Atlantic and contraction of the Pacific since the breakup of a transform fault that brought relatively young, buoyant oceanic Pangea can be viewed as a combination of ridge push from the Mid- lithosphere into contact with older and denser oceanic lithosphere Atlantic ridge, and slab pull associated with the subduction zones in (Casey and Dewey, 1984; Stern and Bloomer, 1992; Stern, 2004). the western Pacific. As subduction is viewed as the main driving Geodynamic models (Gurnis et al., 2004) support these interpre- force (e.g. Anderson, 2001), this process has been called tations and suggest that transform faults that juxtapose oceanic “topedown” tectonics, and it provides an elegant explanation for lithospheres of contrasting age are likely sites for subduction zone the formation of a supercontinent by extroversion. The extrover- initiation. In these situations, subduction initiation was followed by sion of Pangea into the next supercontinent (dubbed Amasia by rapid roll-back and voluminous mafic magmatism (Fig. 5). Hynes Hoffman, 1992) would be an inevitable outcome should these (2005) pointed out that if melt material floods the upper surfaces forces continue to combine into the future. of plates, the plates become negatively buoyant with a drive for However, available reconstructions (e.g. Cocks and Torvik, 2002, subduction similar to slab pull. These calculations support argu- 2011; Scotese, 2002; Stampfli and Borel, 2002), together with Sm- ments that subduction preferentially initiates at oceanic fracture Nd isotopic data from accreted mafic complexes in collisional zones, where the flooding of the plates by melt is most likely. orogens, indicate that previous supercontinents have formed by Because it would have been subducted, the age of the exterior both extroversion and introversion (Murphy and Nance, 2003). In oceanic lithosphere along plate boundaries with the interior oceans 190 J.B. Murphy, R.D. Nance / Geoscience Frontiers 4 (2013) 185e194
Figure 4. Paleozoic reconstructions (modified from Scotese, 1997; Cocks and Torsvik, 2002; Stampfli and Borel, 2002; Murphy et al., 2006; Murphy and Nance, 2008) showing how Pangea formed by preferential subduction of the interior Iapetus and Rheic oceans. Fig. 4A shows the Iapetus Ocean, which had formed between Laurentia and Gondwana by 540 Ma. At 505e480 Ma, AvaloniaeCarolinia-Ganderia (AeC) separated from Gondwana thereby creating the Rheic Ocean. Fig. 4B shows these terranes ca. 2000 km north of the Gondwanan margin by ca. 460 Ma. The Iapetus Ocean closed by the end of the Silurian. By 440 Ma (Fig. 4C), Laurentia, Baltica and AeC had collided to form Laurussia, and the Rheic Ocean was subducting, resulting in convergence (Fig. 4D) and closure (Fig. 4E) by 280 Ma to form Pangea. Tectonic activity within the paleo-Pacific Ocean during the Paleozoic is preserved in the Terra Australis orogen, located along the periphery of Pangea (Cawood, 2005).
is likely to remain unknown, but it is likely to be significantly older arc development (Fig. 5). These arc sequences should develop than that of the youthful interior ocean. If so, subduction of exterior within the interior oceans along much of their boundaries with the oceanic lithosphere beneath that of the interior oceans should exterior ocean and may occur adjacent to passive margin succes- commence soon after supercontinent breakup, leading to ensimatic sions in that ocean. In this scenario, the interior ocean ridge would be oriented at a high angle to the subduction zone, imparting a strong strike-slip component to arc magmatism. In addition, as the continents continue to diverge, the ensimatic arc complexes would become trapped within the geographic realm of the interior ocean in a manner analogous to the MesozoiceCenozoic transfer of oceanic lithosphere around the Caribbean plate from the Pacific to the Atlantic realm (e.g. Pindell et al., 2006). Regardless of the mechanism, the onset of subduction within the interior oceans and its eventual migration to the margins of these oceans would significantly alter the geodynamic relation- ship between the interior and exterior oceans. Subduction around the margins of the Atlantic Ocean, for example, would both sever the Americas from their present westward ridge push from the mid-Atlantic, and ultimately create a geoid low centered over the new sites of mantle downwelling. The effects Figure 5. Subduction initiation model (from Stern, 2004) applied to the boundaries between interior and exterior oceanic lithospheres. A subduction zone initiates preferentially of such a scenario on the motion of the Americas and the where two oceanic lithospheres of differing density are juxtaposed across a transform fault. continued closure of the Pacific Ocean have not been modeled, Such conditions are likely to occur along the boundary between interior and exterior oceans but are clearly relevant to the predicted assembly path of the soon after supercontinent breakup. A and B show initial conditions in cross-section and map next supercontinent and raise the possibility of a future intro- view, respectively. In C and D, old, dense lithosphere (reflecting subduction of exterior verted supercontinent (dubbed Pangea Ultima and, more oceanic lithosphere beneath the interior oceanic lithosphere) sinks asymmetrically. The asthenosphere then migrates above the subducting lithosphere (C and D). Extension then recently, Pangea Proxima by Scotese, 2000, 2007)formedas occurs (E and F) resulting in ocean-floor magmatism and the development of an infant arc. a consequence of Atlantic Ocean closure. J.B. Murphy, R.D. Nance / Geoscience Frontiers 4 (2013) 185e194 191
5. Discussion dramatically change when the interior ocean starts to contract. In the early to middle Silurian, for example, subduction began in the “Topedown” geodynamic models (Anderson, 1994, 2001; interior Rheic Ocean, and there was a dramatic change in the style Gurnis, 1988), in which supercontinents breakup over geoid highs of subduction in the exterior paleo-Pacific Ocean (represented by and reassemble over geoid lows (represented at the surface by the Terra Australis Orogen, which records subduction events subduction zones; Fig. 6) provide a plausible explanation for throughout the Paleozoic, Cawood, 2005; Cawood and Buchan, supercontinents that form by extroversion (e.g. the formation of 2007) as a series of ensimatic arcs associated with roll-back star- Pannotia from the 0.83e0.75 Ga breakup of Rodinia). However, ted to form (Gray and Foster, 2004). Dramatic changes are also without redistribution of geoid lows (potentially created by the supported by global sea level curves for the Paleozoic (e.g. Hallam, onset of subduction in the interior oceans), they do not provide an 1992), which peak in the early Silurian. explanation for the formation of introverted supercontinents, and Sea level fall from the early Silurian to the Carboniferous implies so cannot explain the formation of Pangea. For example, if that: (1) the global average elevation of the oceanic crust decreased topedown geodynamic models are applied to a 500 Ma paleoge- during this time period, (2) the global average age of the oceanic ography (i.e. before the onset of subduction in the Iapetus Ocean lithosphere increased as the Rheic Ocean was contracting and and before the opening of the Rheic Ocean), Pangea would have Pangea was forming, and (3) increased sea floor spreading in the formed by closure of the paleo-Pacific exterior ocean rather than by paleo-Pacific Ocean, which must be an outcome of convergence in closure of the Iapetus and Rheic interior oceans, an outcome that is the interior Rheic Ocean and is indicated by the relatively low initial totally incompatible with the configuration of Pangea as deduced 87Sr/86Sr in Late Paleozoic ocean waters (Veizer et al., 1999), was by a wealth of diverse geological data sets. primarily compensated by subduction of newer Rheic Ocean lith- This conundrum focuses attention on the importance of under- osphere rather than old paleo-Pacific lithosphere. The latter standing the mechanisms responsible for subduction initiation within scenario is incompatible with subduction-driven topedown interior oceans. Many published models, which imply subduction tectonics and, instead, suggests the convergence across the inte- initiation at passive margins, have no MesozoiceCenozoic analogs, rior oceans that led to the closure of the Rheic Ocean was imposed and are not supported by geodynamic models that suggest the most by some other mechanism. likely sites for subduction zone initiation are adjacent to transform Introversion leading to the amalgamation of Pangea would have faults (e.g. Stern and Bloomer, 1992; Stern, 2004). required a reversal in the direction of motion of the continents. If The boundaries between interior and exterior oceanic litho- continents migrate from geoid highs to geoid lows as implied by spheres (Murphy and Nance, 2008) provide a favorable geodynamic topedown geodynamic models, this reversal implies that a funda- setting for onset of subduction, in which ensimatic arc complexes mental change in the Earth’s geoid patterns occurred between ca. form in the overriding interior oceanic lithosphere. The strike of the 480e450 Ma. Beyond the possibility of geoid low development arc complexes would be oriented at a high angle to the interior within the interior oceans with the onset of their subduction, ocean ridge. As the continents continued to diverge, these which seems inadequate in the case of the Rheic Ocean, identifi- complexes become stranded within the interior oceanic realm in cation of the event that triggered this reversal remains elusive. But a manner broadly analogous to the MesozoiceCenozoic evolution it must have been global in scale. Based on geodynamic models (e.g. of the Caribbean plate (e.g. Pindell et al., 2006). Once initiated, the Zhong and Gurnis, 1997; Tan et al., 2002; Zhong et al., 2007), new subduction zones may migrate to the margins of the interior tomographic images (e.g. Hutko et al., 2005) and geochemical data oceans, creating new geoid lows as they do so. If events in the from oceanic basalts (e.g. Tatsumi, 2005), Murphy et al. (2009) exterior ocean are geodynamically linked to those in the interior speculated that this reversal might be induced by slab avalanche ocean, then the style of subduction in the exterior ocean should events (Condie, 1998), in which subducted slabs that have accu- mulated at the 660 km discontinuity, plunge through the mantle to accumulate at the core-mantle boundary. Such avalanche events may trigger the rise of superplumes from the core-mantle boundary. A superplume event centered in the paleo-Pacific Ocean would produce a geoid high that would result in a reversal in the direction of motion of the continents, especially if subduction of the interior oceans was associated with the development of a geoid low. In this scenario, “topedown” tectonics of the Early Paleozoic may have temporarily been trumped by “bottom-up” tectonics beginning in the early Ordovician. This scenario is compatible with the global Sm/Nd isotopic database, which suggests major additions of juve- nile crust at ca. 450 Ma (Condie et al., 2009), and with the strati- graphic record, which preserves evidence of enhanced biological activity, global warming events, the formation of ironstones and black shales, and elevated sea level (Barnes, 2004; Condie, 2004) during the Ordovician.
6. Application to older supercontinents
Figure 6. Time-dependent numerical models (after Gurnis, 1988) showing supercon- tinent breakup and dispersal followed by assembly and amalgamation. A: Supercon- Insights into understanding how older supercontinents were tinents insulate the mantle beneath them, eventually leading to breakup over a geoid formed are complicated by decreasing resolution of paleo- high. The resulting continents disperse toward subduction zones in the exterior ocean continental reconstructions with time and by a lack of sufficient (B), where they amalgamate to form a new supercontinent (C), and the cycle begins isotopic data from many orogenic belts. However, where sufficient again (D). This model predicts that supercontinents form by extroversion. In A and B, the subduction zone arrowed is the same one. It has been centered in B to show the data do exist, the approach used to study the formation of younger continents converging toward it. supercontinents can provide important insights (Murphy and 192 J.B. Murphy, R.D. Nance / Geoscience Frontiers 4 (2013) 185e194
Nance, 2005). To illustrate this, we provide examples from the produce Pangea in the wrong configuration. Pangea was assembled mafic complexes accreted to the Superior craton of North America by the preferential subduction of interior oceanic lithosphere, in the Mesoproterozoic (Grenville orogen to the east) and Paleo- suggesting that “topedown” geodynamics was temporarily trum- proterozoic (Trans-Hudson orogen to the north and west). ped by a different global-scale process. We speculate that super- Mesoproterozoic accretionary events produced episodes of plumes, perhaps driven by slab avalanche events, may occasionally orogenesis at 1.9e1.6 Ga (Penokean), 1.68e1.67 Ga (Labradorian), overwhelm “topedown” geodynamic processes by creating a geoid 1.50e1.45 Ga (Pinwarian) and 1.25e1.19 Ga (Elzeverian), which high that causes the dispersing continents to reverse their direc- collectively imply that the eastern flank of the Superior craton faced tions, resulting in the preferential subduction of interior oceanic an open ocean to the east (present coordinates) for nearly 0.8 Ga. lithosphere. Accretionary orogenesis was followed at 1.09e1.02 and 1.00e0.98 (Rivers, 2009; Hynes and Rivers, 2010) by terminal continental Acknowledgments collision that produced the Grenville orogen, a major event in the amalgamation of the supercontinent Rodinia. The Penokean and We thank Prof. M. Santosh for inviting us to submit this Labradorian arcs are juvenile with very limited crustal contami- manuscript. JBM acknowledges the continuing support of the þ þ nation, and have 3 (Nd) values of 1to 3 and TDM model ages of ca. Natural Sciences and Engineering Research Council, Canada 1.9 Ga (Dickin, 1998, 2000). Similar Sm-Nd isotopic signatures have through Discovery and Research Capacity grants. RDN is supported been documented in Scandinavia, Greenland, Scotland, and Colo- by National Science Foundation grant EAR-0308105 and a Baker rado (DePaolo, 1981; Patchett and Bridgewater, 1984; Patchett and Award from Ohio University. This paper is a contribution to the Kuovo, 1986; Marcantonio et al., 1990). These data support the International Geoscience Program, IGCP Project 597. interpretation (Dickin and McNutt, 1989) of widespread arc accre- tionary events in response to global-scale changes in plate geom- References etry. 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