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Home , Baltica, Ganderia, Iapetus Ocean, Rheic Ocean, Terra Australis Orogen

The Pangea conundrum

J. Brendan Murphy1, R. Damian Nance 2 1Department of Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada 2Department of Geological Sciences, Ohio University, Athens, Ohio 45701, USA

ABSTRACT Geodynamic models for assembly, whereby the dispersing continen- tal fragments of a supercontinent break up and migrate from geoid highs to reassemble at geoid lows, fail to account for the amalgamation of Pangea. Such models would predict that the created by continental breakup in the early (e.g., Iapetus, Rheic) would have continued to expand as the migrated toward sites of mantle downwelling in the paleo-Pacifi c, reassembling an extroverted supercontinent as this closed. Instead, Pangea assembled as a result of the closure of the younger Iapetus and Rheic Oceans. Geo- dynamic linkages between these three oceans preserved in the rock record suggest that the reversal in continental motion may have coincided with the emergence of a super- plume that produced a geoid high in the paleo-Pacifi c. If so, the top-down geodynamics used to account for the breakup and dispersal of a supercontinent at ca. 600–540 Ma may have been overpowered by bottom-up geodynamics during the amalgamation of Pangea.

Keywords: Pangea, geodynamics, , Appalachian orogen, orogen.

INTRODUCTION zones in the exterior paleo–Pacifi c Ocean, and it is this ocean Although the existence of Pangea is one of the cornerstones of geol- that should have closed. Instead, a wealth of geologic evidence indi- ogy, the mechanisms responsible for its amalgamation are poorly under- cates that subduction commenced in the relatively newly formed interior stood. To a fi rst order, we know where and when it formed (e.g., Scotese, ( Iapetus and Rheic) oceans located between , , and Gond- 1997; Cocks and Torsvik, 2002; Stampfl i and Borel, 2002), but not why. wana, and that Pangea assembled as the interior oceans closed, a process The formation of Pangea is primarily recorded in the origin and evolu- termed introversion (Fig. 2; Murphy and Nance, 2003). tion of the Paleozoic oceans (e.g., Iapetus, Rheic, Prototethys) between To gain insight into the processes leading to the formation of Pangea, Laurentia (ancestral ), Baltica (northwestern ), and we investigated the geodynamic linkages between the Paleozoic evolution northern (–West ), the closure of which of the interior (Iapetus and Rheic) oceans, as recorded in the orogens pro- resulted in its amalgamation (e.g., van Staal et al., 1998). Although well duced by their closure (Ouachita, Appalachian, Caledonia, Variscan), and documented, this record runs counter to many widely accepted geo- the penecontemporaneous evolution of the exterior (paleo-Pacifi c) ocean. dynamic models for the forces that drive the . There Oceanic domains at the leading (exterior) edges of the dispersing conti- is a broad consensus that sinking of cold lithosphere (slab pull) provides nents are recorded in the 18,000 km , which pre- 90% of the force needed to drive (e.g., Anderson, 2001) and serves a record of subduction until at least 230 Ma (e.g., Cawood, 2005). is indirectly responsible for upwelling beneath mid-ocean ridges (Stern, 2004). This scenario is compatible with geodynamic arguments (Gurnis, GEOLOGICAL FRAMEWORK 1988), which propose that supercontinents break up and migrate from Initial rifting of the occurred in stages from ca. 600 Ma sites of mantle upwelling (geoid highs) toward subduction zones, and they to 550 Ma between Laurentia, Baltica, and Amazonia (Fig. 1A; Cawood reassemble at sites of mantle downwelling (geoid lows). et al., 2001). The onset of subduction in the Iapetus realm is recorded The Mesozoic breakup and dispersal of Pangea, for example, by the development of ca. 500–480 Ma arc-related ophiolitic com- occurred above sites of mantle upwelling, and its fragments are currently plexes, which were obducted soon after onto the Laurentian margin, an being dispersed toward sites of mantle downwelling as the oceans between event assigned to the Taconic (Appalachians) and the Grampian them (interior oceans; Murphy and Nance, 2003) widen, and the exterior orogeny (Caledonides) (van Staal et al., 1998). At about the same time ocean (-Pacifi c) that surrounded Pangea contracts. If current as these orogenic events, the formed as a result of the Late plate motions are projected into the future, then the next supercontinent –Early Ordovician drift of peri-Gondwanan (including (; Hoffman, 1999) should assemble as the exterior ocean closes, a - and Carolinia) away from the northern Gondwanan process termed extroversion (Fig. 1; Murphy and Nance, 2003). margin, which preserves a ca. 500 Ma passive-margin assemblage and However, if these geodynamic arguments are applied to the early coeval voluminous bimodal magmatism in southern Mexico (Acatlán Paleozoic, they fail to predict the formation of Pangea in the correct con- Complex; Nance et al., 2006), Britain, Iberia, and the fi guration. In the Late and , most reconstructions (e.g., Sánchez-García et al., 2003; Kryza et al., 2003). An Early Ordo- show Laurentia joined to West Gondwana, while the eastern Gondwanan vician breakup unconformity and widespread -related subsidence are blocks converged and ultimately collided with West Gondwana. This pro- indicated by the distribution of the Armorican Quartzite across much of duced a hemisphere dominated by the assembly of continental fragments mainland Europe (Quesada et al., 1991; Linnemann et al., 2004). surrounded by the hemispheric paleo–Pacifi c Ocean. By the early Paleo- By the Middle Ordovician, the peri-Gondwanan terranes were zoic, Laurentia, Baltica, and Gondwana had separated (Fig. 1A), resulting positioned ~2000 km north of the Gondwanan margin and separated the in the opening of the Iapetus and Rheic Oceans and convergence in the Iapetus Ocean to the north from the Rheic Ocean to the south (Fig. 1). paleo-Pacifi c. However, for Pangea to have assembled at a site of mantle The Iapetus Ocean was closed in stages between the Late Ordovician and downwelling, the dispersing continents should have migrated toward the Middle , in the north by the collision of Laurentia and Baltica to

© 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, September September 2008; 2008 v. 36; no. 9; p. 703–706; doi: 10.1130/G24966A.1; 2 fi gures. 703 High angle between plate boundary and ridge A Laurentia A Supercontinent B Breakup C Drift 540 Ma 460 Ma Laurentia

A-C

Iapetus D Introversion E Extroversion F Combination Gondwana

A-C South Pole Laurussia Mafic terranes Ocean ridge Transform Interior arc Variscan Subduction zone Collisional orogeny Exterior arc A-C orogen Alleghanian- Figure 2. A–C: Schematic diagrams (modifi ed from Murphy and Rheic Ocean Ouachita Nance, 2003) showing stages in breakup of a supercontinent, fate orogen of relatively old oceanic lithosphere that surrounds supercontinent before breakup (exterior ocean, dark shade), and creation of rela- 280 Ma tively new oceanic lithosphere between dispersing blocks (interior ocean, pale shade). In the case of Pangea, paleo-Pacifi c represents 370 Ma exterior ocean, and Iapetus and Rheic Oceans are examples of interior oceans. In B and C, note subduction commences along the boundaries between interior and exterior oceans. D: Confi guration of a supercontinent formed by introversion (i.e., preferential subduc- tion of interior oceanic lithosphere). E: Confi guration of a superconti- nent formed by extroversion (i.e., preferential subduction of exterior oceanic lithosphere). F: An intermediate case in which one ocean is North America closed by introversion and the other is closed by extroversion. B 280 Ma BM A A C ? ARM ? Ch-Oax-Y ? NW-I FI NW Africa OM WEurope America (Fig. 1; Cawood, 2005). Subduction along this margin began at South ca. 580 Ma (Cawood and Buchan, 2007) and continued until the end of America Rheic the Paleozoic, by which time the orogen was distributed along the periph- ery of Pangea. The orogen is interpreted to have formed by accretionary Figure 1. Paleozoic reconstructions (modifi ed from Scotese, 1997; Cocks and Torsvik, 2002; Stampfl i and Borel, 2002; Murphy et al., processes without the involvement of major continental collisions. Its 2006; Cawood and Buchan, 2007). A: By 540 Ma, Iapetus Ocean had evolution is best documented in the Adelaide, New England, and Lachlan formed between Laurentia and Gondwana. By 460 Ma, Avalonia- fold belts of eastern (Collins, 2002; Gray and Foster, 2004). The Carolinia (A–C) had separated from Gondwana, creating the Rheic most complete record of continental-margin evolution is preserved in the Ocean. By 370 Ma, Laurentia, Baltica, and A–C had collided to form Laurussia, and Rheic Ocean began to contract, closing by 280 Ma, Adelaide fold belt, where Neoproterozoic to Cambrian passive-margin to form Pangea. Terra Australis orogen at 280 Ma was located sedimentary successions occur in a series of rift-sag basins. Abundant along periphery of Pangea, representing vestige of tectonic activity early Paleozoic volcanic complexes and intra-oceanic sequences are pre- within paleo–Pacifi c Ocean. B: Location of Rheic Ocean suture within served in the New England fold belt, and the 700-km-wide Lachlan fold Alleghanian-Ouachita and Variscan orogens and early Mesozoic belt is dominated by 520–480 Ma oceanic volcanic rocks overlain by a position of peri-Gondwanan terranes mentioned in text (see Keppie et al., 2003). Ch-Oax-Y—Chortis, Oaxaquia-Yucatan; F—Florida; blanket of sediments analogous to the modern Bengal fan. Between 440 C—Carolina; A—Avalonia; O-M—Ossa Morena; NW-I—Northwest and 400 Ma, intra-oceanic arc and basin development recorded in these Iberia, ARM—Armorica; BM—Bohemian massif. belts by sedimentary and mafi c volcanic rocks is interpreted to refl ect the retreat of the subducted slab during episodes of upper-plate extension and basaltic magmatism, and transient fl at subduction attributed to the arrival of buoyant oceanic plateaus (Collins, 2002). form Laurussia, and in the south by the accretion of Ganderia, Avalonia, Intra-oceanic subduction in the paleo–Pacifi c Ocean was clearly and Carolina to Baltica and then Laurentia (van Staal et al., 1998; Hibbard well established ~80 m.y. before subduction commenced in the Iapetus et al., 2002). Northerly directed subduction of the Rheic Ocean beneath Ocean (Cawood and Buchan, 2007) and before the Rheic Ocean opened. Laurussia commenced in the Silurian and continued until its closure and According to conventional geodynamic models, slab-pull forces associ- the amalgamation of Pangea in the . ated with this subduction should have resulted in the migration of the In the early Paleozoic, Laurentia and Gondwana were surrounded dispersing continents toward the paleo-Pacifi c subduction zones, as was by the paleo–Pacifi c Ocean, vestiges of which are preserved in the Terra the case when the Iapetus Ocean opened. However, this motion reversed Australis orogen, which extends from eastern Australia and Tasmania into itself when subduction commenced in the relatively newly formed Iapetus , the Transantarctic Mountains, southern Africa, and South and Rheic Oceans between Laurentia, Baltica, and Gondwana. During the

704 GEOLOGY, September 2008 assembly of Pangea, therefore, subduction was not only initiated within Global reconstructions (e.g., Scotese, 1997; Stampfl i and Borel, the new interior oceans, but the rates of subduction of this relatively young 2002) imply that assembly of Pangea began after the closure of the Iapetus Paleozoic lithosphere must also have overpowered those of the already Ocean. They also imply that the rate of subduction of Rheic Ocean litho- well established subduction zones within the exterior (paleo-Pacifi c) sphere exceeded subduction rates in the paleo–Pacifi c Ocean despite ocean. Thus, rather than forming by extroversion, as most geodynamic the fact that subducted Rheic oceanic lithosphere was a maximum of models would predict, the formation of Pangea was actually achieved by 60 m.y. old, and that subduction zones were already well established in the introversion (see Fig. 2). paleo–Pacifi c Ocean. Although the causes are hotly debated, a geodynamic linkage GEODYNAMIC SCENARIO between tectonic events in the Rheic and paleo–Pacifi c Oceans is sug- A key problem that needs to be addressed in resolving this conundrum gested by the dramatic change in tectonic environment of paleo-Pacifi c is the geodynamic scenario that facilitated the initiation of subduction in subduction zones between 440 Ma and 420 Ma, best documented in the Iapetus and Rheic Oceans and generated suffi cient slab pull to drive the Lachlan fold belt of eastern Australia (e.g., Gray and Foster, 2004), the the dispersing continental fragments toward the Pangea confi guration. time at which the Iapetus Ocean was closing and subduction within Subduction initiation is controversial, although it is generally accepted the Rheic Ocean was commencing. that initiation at passive margins is mechanically diffi cult because of Proxy records for oceanic lithosphere evolution are also consistent the fl exural rigidity of the oceanic lithosphere (Cloetingh et al., 1989). with this linkage. Rapid sea-level rise in the Cambrian and Early Ordovi- No simple modern analogues for such a process have been documented. cian (e.g., Hallam, 1992), generally attributed to the development of rela- On the other hand, Stern and Bloomer (1992) and Stern (2004) showed tively young, elevated Iapetan oceanic lithosphere, gave way to a drop in that intra-oceanic subduction initiation along the Isu-Bonin-Mariana arc sea level from the Late Ordovician to the Carboniferous, implying that system occurred along a transform system where younger, less dense the average age of the oceanic lithosphere increased during the middle to oceanic lithosphere was in contact with older, denser lithosphere. This late Paleozoic. This trend is compatible with preferential subduction of was followed by rapid trench retreat and voluminous mafi c (including relatively young Rheic Ocean lithosphere over older paleo-Pacifi c oceanic boninitic) magmatism as the older lithosphere subducted beneath the lithosphere. Similarly, compared to the early Paleozoic, the lower initial younger, and it is consistent with geodynamic models that show subduc- 87Sr/86Sr value deduced for ocean waters in the late Paleozoic (e.g., Veizer tion initiation followed by rapid back-arc extension (Gurnis et al., 2004). et al., 1999) implies an increase in the rate of seafl oor spreading. Taken A modern analogue for this process may be occurring along the Atlantic together, the sea-level and Sr isotope data suggest that any increased margin beneath Gibraltar, where subduction initiation of the Atlantic has seafl oor spreading in the paleo–Pacifi c Ocean during Pangea assembly been documented at the eastern end of the Azores-Gibraltar transform was compensated, not by subduction of old paleo-Pacifi c lithosphere, but fault (Gutscher et al., 2002). by subduction of newer Rheic Ocean lithosphere. Within the Iapetan realm, such a scenario would have existed in the Top-down geodynamic models adequately account for the early early Paleozoic along boundaries been the young Iapetan and old paleo- Paleozoic dispersal of continents, but they cannot account for the middle Pacifi c oceanic lithosphere (see Figs. 2B and 2C). Such boundaries are to late Paleozoic introversion that resulted in the amalgamation of Pan- a requirement of supercontinent breakup. The high angle between these gea. This conundrum suggests that top-down geodynamic forces can be boundaries and the Iapetan ocean ridge suggests that they had a strong overridden by other forces. Assuming that continents migrate from geoid strike-slip component and, hence, were likely sites for transform faults highs to geoid lows, the reversal of their movement in the middle Paleo- to nucleate. The age of the paleo-Pacifi c oceanic lithosphere adjacent zoic implies that the geoid lows became geoid highs. The mechanisms for to these transform faults may never be known, but it is almost certain this are unclear. However, one possibility is that the descent of subducted to have been older than the adjoining interior (Iapetus) oceanic litho- slabs in the paleo–Pacifi c Ocean to the core-mantle boundary thermally sphere. The age and density contrast between them, and therefore the insulated that boundary, leading to the ponding of hot mantle material that most favorable conditions for subduction initiation, would have been built up and rose toward the surface as a superplume (Zhong and Gurnis, most pronounced when the Iapetus Ocean was young. We view this to be 1997; Condie, 1998; Tan et al., 2002). Such an event would result in a the most favorable geodynamic scenario for the origin of the arc-related geoid high and enhanced seafl oor spreading in the paleo–Pacifi c Ocean oceanic complexes that were respectively obducted onto the margins of that could have been responsible for the reversal in the direction of motion Laurentia and Baltica during the ca. 500–480 Ma Taconic and Grampian of the continents. In this scenario, “top-down” tectonics of the early Paleo- (see van Staal et al., 1998). The implication of this interpre- zoic gave way to “bottom-up” tectonics in the late Paleozoic. An Ordovi- tation is that these mafi c arc-related complexes were formed from the cian superplume has been proposed by Condie (2004) to account for the subduction of paleo-Pacifi c, rather than Iapetan, lithosphere in a manner period’s high sea levels, high paleotemperatures, abundant black shales, analogous to the Scotia arc and the Mesozoic-Cenozoic “capture” of the and strong biological activity. subduction zones around the within the Atlantic realm (e.g., Pindell et al., 2006). The evolution of the Iapetan–paleo-Pacifi c CONCLUSIONS plate boundaries would have depended on a number of factors, includ- The amalgamation of Pangea cannot be explained by top-down ing the spreading rate at the Iapetan ridge, the drift of the dispersing geodynamic models. If these principles are applied to an Early Ordo- continents, and the rate of rollback of the subduction zone. However, vician , they would not generate Pangea in the correct confi gura- the presence of boninites and mafi c sheeted dikes in Iapetan ophiolitic tion. Instead, the geologic record implies that the rates of subduction in complexes (e.g., Bédard et al., 1998) indicates that forearc extension, the relatively new interior oceans (e.g., the Iapetus and Rheic Oceans) and hence slab rollback, occurred. The strike of an arc produced in this exceeded those in the exterior paleo–Pacifi c Ocean. Geodynamic linkages fashion would have been oriented at a high angle to the Iapetan ocean between subduction in the interior and exterior oceans are implied by dra- ridge, and this geometry would have facilitated its subsequent obduction matic changes in the style of subduction in the paleo-Pacifi c that coincide (see Suhr and Cawood, 1993). However, as pointed out by Gurnis et al. with the onset of subduction in the Rheic Ocean. Assuming continents (2004), subduction zones initiated in this manner cannot pull the large migrate from geoid highs to geoid lows, then the reversal in continen- overriding plates toward them, and so cannot be considered a mecha- tal motions that led to the formation of Pangea may have coincided with nism that began the assembly of Pangea. the emergence of a geoid high in the paleo-Pacifi c during the Ordovician.

GEOLOGY, September 2008 705 We tentatively suggest that this may refl ect the formation of a superplume, Laurentia and Laurussia: Tectonophysics, v. 365, p. 195–219, doi: 10.1016/ for which there is independent, supporting evidence in the rock record. S0040-1951(03)00037-4. Kryza, R., Mazur, S., and Pin, C., 2003, Subduction- and non-subduction-related If so, the preferential destruction of interior oceans, required to produce igneous rocks in the Central-European Variscides: Geochemical and Nd introverted supercontinents such as Pangea, may occur when top-down isotope evidence for a composite origin of the Kodzko Metamorphic Com- tectonics give way to bottom-up tectonics. plex, Polish : Geodinamica Acta, v. 16, p. 39–57, doi: 10.1016/ S0985-3111(02)00004-9. ACKNOWLEDGMENTS Linnemann, U., McNaughton, N.J., Romer, R.L., Gehmlich, M., Drost, K., and Murphy acknowledges the continuing support of the Natural Sciences and Tonk, C., 2004, West African provenance for Saxo-Thuringia ( Bohemian Engineering Research Council, Canada, through Discovery and Research Capacity Massif): Did Armorica ever leave pre-Pangean Gondwana?—U/Pb- grants. Nance is supported by National Science Foundation grant EAR-0308105. SHRIMP zircon evidence and the Nd-isotopic record: International Journal We are indebted to Peter Cawood, Michael Gurnis, and Joe Meert for their insight- of Earth Sciences, v. 93, p. 683–705, doi: 10.1007/s00531-004-0413-8. ful reviews. This paper is a contribution to the International Geological Correlation Murphy, J.B., and Nance, R.D., 2003, Do supercontinents introvert or extro- Programme (IGCP) Projects 453 and 497. vert?: Sm-Nd isotopic evidence: Geology, v. 31, p. 873–876, doi: 10.1130/ G19668.1. 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