The Pangea conundrum J. Brendan Murphy1, R. Damian Nance 2 1Department of Earth 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 supercontinent 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 oceans created by continental breakup in the early Paleozoic (e.g., Iapetus, Rheic) would have continued to expand as the continents migrated toward sites of mantle downwelling in the paleo-Pacifi c, reassembling an extroverted supercontinent as this ocean 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 Ordovician 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, supercontinents, Appalachian orogen, Terra Australis orogen. INTRODUCTION subduction 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 Laurentia, Baltica, 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 North America), Baltica (northwestern Europe), and we investigated the geodynamic linkages between the Paleozoic evolution northern Gondwana (South America–West Africa), 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 supercontinent cycle. 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 Terra Australis orogen, which pre- 90% of the force needed to drive plate tectonics (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 Iapetus Ocean 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 orogeny (Appalachians) and the Grampian them (interior oceans; Murphy and Nance, 2003) widen, and the exterior orogeny (Caledo nides) (van Staal et al., 1998). At about the same time ocean (Panthalassa-Pacifi c) that surrounded Pangea contracts. If current as these orogenic events, the Rheic Ocean formed as a result of the Late plate motions are projected into the future, then the next supercontinent Cambrian–Early Ordovician drift of peri-Gondwanan terranes (including (Amasia; Hoffman, 1999) should assemble as the exterior ocean closes, a Ganderia-Avalonia 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 Bohemian Massif fi guration. In the Late Neoproterozoic and Ediacaran, 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 rift-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 Silurian, 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 Passive margin Mafic terranes Ocean ridge Transform fault 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 Suture the Paleozoic, by which time the orogen was distributed along the periph- ery of Pangea. The orogen is interpreted to have formed by accre tionary 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 Australia (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.