GEOLOGY, September 2008 703

INTRODUCTIONAlthough the existence of Pangea is one of the cornerstones of geol-

ogy, the mechanisms responsible for its amalgamation are poorly under-stood. To a fi rst order, we know where and when it formed (e.g., Scotese, 1997; Cocks and Torsvik, 2002; Stampfl i and Borel, 2002), but not why. The formation of Pangea is primarily recorded in the origin and evolu-tion of the Paleozoic oceans (e.g., Iapetus, Rheic, Prototethys) between Laurentia (ancestral North America), Baltica (northwestern Europe), and northern Gondwana (South America–West Africa), the closure of which resulted in its amalgamation (e.g., van Staal et al., 1998). Although well documented, this record runs counter to many widely accepted geo-dynamic models for the forces that drive the supercontinent cycle. There is a broad consensus that sinking of cold lithosphere (slab pull) provides 90% of the force needed to drive plate tectonics (e.g., Anderson, 2001) and is indirectly responsible for upwelling beneath mid-ocean ridges (Stern, 2004). This scenario is compatible with geodynamic arguments (Gurnis, 1988), which propose that supercontinents break up and migrate from sites of mantle upwelling (geoid highs) toward subduction zones, and they reassemble at sites of mantle downwelling (geoid lows).

The Mesozoic breakup and dispersal of Pangea, for example, occurred above sites of mantle upwelling, and its fragments are currently being dispersed toward sites of mantle downwelling as the oceans between them (interior oceans; Murphy and Nance, 2003) widen, and the exterior ocean (Panthalassa-Pacifi c) that surrounded Pangea contracts. If current plate motions are projected into the future, then the next supercontinent (Amasia; Hoffman, 1999) should assemble as the exterior ocean closes, a process termed extroversion (Fig. 1; Murphy and Nance, 2003).

However, if these geodynamic arguments are applied to the early Paleozoic, they fail to predict the formation of Pangea in the correct con-fi guration. In the Late Neoproterozoic and Ediacaran, most reconstructions show Laurentia joined to West Gondwana, while the eastern Gondwanan blocks converged and ultimately collided with West Gondwana. This pro-duced a hemisphere dominated by the assembly of continental fragments surrounded by the hemispheric paleo–Pacifi c Ocean. By the early Paleo-zoic, Laurentia, Baltica, and Gondwana had separated (Fig. 1A), resulting in the opening of the Iapetus and Rheic Oceans and convergence in the paleo-Pacifi c. However, for Pangea to have assembled at a site of mantle downwelling, the dispersing continents should have migrated toward the

subduction zones in the exterior paleo–Pacifi c Ocean, and it is this ocean that should have closed. Instead, a wealth of geologic evidence indi-cates that subduction commenced in the relatively newly formed interior ( Iapetus and Rheic) oceans located between Laurentia, Baltica, and Gond-wana, and that Pangea assembled as the interior oceans closed, a process termed introversion (Fig. 2; Murphy and Nance, 2003).

To gain insight into the processes leading to the formation of Pangea, we investigated the geodynamic linkages between the Paleozoic evolution of the interior (Iapetus and Rheic) oceans, as recorded in the orogens pro-duced by their closure (Ouachita, Appalachian, Caledonia, Variscan), and the penecontemporaneous evolution of the exterior (paleo-Pacifi c) ocean. Oceanic domains at the leading (exterior) edges of the dispersing conti-nents are recorded in the 18,000 km Terra Australis orogen, which pre-serves a record of subduction until at least 230 Ma (e.g., Cawood, 2005).

GEOLOGICAL FRAMEWORKInitial rifting of the Iapetus Ocean occurred in stages from ca. 600 Ma

to 550 Ma between Laurentia, Baltica, and Amazonia (Fig. 1A; Cawood et al., 2001). The onset of subduction in the Iapetus realm is recorded by the development of ca. 500–480 Ma arc-related ophiolitic com-plexes, which were obducted soon after onto the Laurentian margin, an event assigned to the Taconic orogeny (Appalachians) and the Grampian orogeny (Caledo nides) (van Staal et al., 1998). At about the same time as these orogenic events, the Rheic Ocean formed as a result of the Late Cambrian–Early Ordovician drift of peri-Gondwanan terranes (including Ganderia-Avalonia and Carolinia) away from the northern Gondwanan margin, which preserves a ca. 500 Ma passive-margin assemblage and coeval voluminous bimodal magmatism in southern Mexico (Acatlán Complex; Nance et al., 2006), Britain, Iberia, and the Bohemian Massif (e.g., Sánchez-García et al., 2003; Kryza et al., 2003). An Early Ordo-vician breakup unconformity and widespread rift-related subsidence are indicated by the distribution of the Armorican Quartzite across much of mainland Europe (Quesada et al., 1991; Linnemann et al., 2004).

By the Middle Ordovician, the peri-Gondwanan terranes were positioned ~2000 km north of the Gondwanan margin and separated the Iapetus Ocean to the north from the Rheic Ocean to the south (Fig. 1). The Iapetus Ocean was closed in stages between the Late Ordovician and Middle Silurian, in the north by the collision of Laurentia and Baltica to

Geology, September 2008; v. 36; no. 9; p. 703–706; doi: 10.1130/G24966A.1; 2 fi gures.© 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.

The Pangea conundrumJ. Brendan Murphy1, R. Damian Nance 2

1Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada2Department of Geological Sciences, Ohio University, Athens, Ohio 45701, USA

ABSTRACTGeodynamic 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.

704 GEOLOGY, September 2008

form Laurussia, and in the south by the accretion of Ganderia, Avalonia, and Carolina to Baltica and then Laurentia (van Staal et al., 1998; Hibbard et al., 2002). Northerly directed subduction of the Rheic Ocean beneath Laurussia commenced in the Silurian and continued until its closure and the amalgamation of Pangea in the Carboniferous.

In the early Paleozoic, Laurentia and Gondwana were surrounded by the paleo–Pacifi c Ocean, vestiges of which are preserved in the Terra Australis orogen, which extends from eastern Australia and Tasmania into New Zealand, the Transantarctic Mountains, southern Africa, and South

America (Fig. 1; Cawood, 2005). Subduction along this margin began at ca. 580 Ma (Cawood and Buchan, 2007) and continued until the end of 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 processes without the involvement of major continental collisions. Its evolution is best documented in the Adelaide, New England, and Lachlan fold belts of eastern Australia (Collins, 2002; Gray and Foster, 2004). The most complete record of continental-margin evolution is preserved in the Adelaide fold belt, where Neoproterozoic to Cambrian passive-margin sedimentary successions occur in a series of rift-sag basins. Abundant early Paleozoic volcanic complexes and intra-oceanic sequences are pre-served in the New England fold belt, and the 700-km-wide Lachlan fold belt is dominated by 520–480 Ma oceanic volcanic rocks overlain by a blanket of sediments analogous to the modern Bengal fan. Between 440 and 400 Ma, intra-oceanic arc and basin development recorded in these 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).

Intra-oceanic subduction in the paleo–Pacifi c Ocean was clearly well established ~80 m.y. before subduction commenced in the Iapetus Ocean (Cawood and Buchan, 2007) and before the Rheic Ocean opened. According to conventional geodynamic models, slab-pull forces associ-ated with this subduction should have resulted in the migration of the dispersing continents toward the paleo-Pacifi c subduction zones, as was the case when the Iapetus Ocean opened. However, this motion reversed itself when subduction commenced in the relatively newly formed Iapetus and Rheic Oceans between Laurentia, Baltica, and Gondwana. During the

Rheic Ocean

Iapetus

A-C

540 Ma

A-C

Gondwana

South Pole

460 Ma

280 Ma370 Ma

Laurussia

Laurentia

A-C

SouthAmerica

NW Africa

North America

Rheic Suture

? ?

?

C

280 Ma

A ABM

ARMNW-IOMFI W Europe

Ch-Oax-Y

Variscanorogen

Alleghanian-Ouachitaorogen

Laurentia

A

B

Figure 1. Paleozoic reconstructions (modifi ed from Scotese, 1997; Cocks and Torsvik, 2002; Stampfl i and Borel, 2002; Murphy et al., 2006; Cawood and Buchan, 2007). A: By 540 Ma, Iapetus Ocean had formed between Laurentia and Gondwana. By 460 Ma, Avalonia-Carolinia (A–C) had separated from Gondwana, creating the Rheic Ocean. By 370 Ma, Laurentia, Baltica, and A–C had collided to form Laurussia, and Rheic Ocean began to contract, closing by 280 Ma, to form Pangea. Terra Australis orogen at 280 Ma was located along periphery of Pangea, representing vestige of tectonic activity within paleo–Pacifi c Ocean. B: Location of Rheic Ocean suture within Alleghanian-Ouachita and Variscan orogens and early Mesozoic position of peri-Gondwanan terranes mentioned in text (see Keppie et al., 2003). Ch-Oax-Y—Chortis, Oaxaquia-Yucatan; F—Florida; C—Carolina; A—Avalonia; O-M—Ossa Morena; NW-I—Northwest Iberia, ARM—Armorica; BM—Bohemian massif.

A Supercontinent B Breakup

D Introversion

Subduction zoneInterior arcTransform fault

Collisional orogeny Exterior arc

E Extroversion

Mafic terranes

F Combination

Ocean ridge

Passivemargin

C Drift

High angle between plate boundary and ridge

Figure 2. A–C: Schematic diagrams (modifi ed from Murphy and Nance, 2003) showing stages in breakup of a supercontinent, fate of relatively old oceanic lithosphere that surrounds supercontinent before breakup (exterior ocean, dark shade), and creation of rela-tively new oceanic lithosphere between dispersing blocks (interior ocean, pale shade). In the case of Pangea, paleo-Pacifi c represents 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 closed by introversion and the other is closed by extroversion.

GEOLOGY, September 2008 705

assembly of Pangea, therefore, subduction was not only initiated within the new interior oceans, but the rates of subduction of this relatively young Paleozoic lithosphere must also have overpowered those of the already well established subduction zones within the exterior (paleo-Pacifi c) ocean. Thus, rather than forming by extroversion, as most geodynamic models would predict, the formation of Pangea was actually achieved by introversion (see Fig. 2).

GEODYNAMIC SCENARIOA key problem that needs to be addressed in resolving this conundrum

is the geodynamic scenario that facilitated the initiation of subduction in the Iapetus and Rheic Oceans and generated suffi cient slab pull to drive the dispersing continental fragments toward the Pangea confi guration. Subduction initiation is controversial, although it is generally accepted that initiation at passive margins is mechanically diffi cult because of the fl exural rigidity of the oceanic lithosphere (Cloetingh et al., 1989). No simple modern analogues for such a process have been documented. On the other hand, Stern and Bloomer (1992) and Stern (2004) showed that intra-oceanic subduction initiation along the Isu-Bonin-Mariana arc system occurred along a transform system where younger, less dense oceanic lithosphere was in contact with older, denser lithosphere. This was followed by rapid trench retreat and voluminous mafi c (including boninitic) magmatism as the older lithosphere subducted beneath the younger, and it is consistent with geodynamic models that show subduc-tion initiation followed by rapid back-arc extension (Gurnis et al., 2004). A modern analogue for this process may be occurring along the Atlantic margin beneath Gibraltar, where subduction initiation of the Atlantic has been documented at the eastern end of the Azores-Gibraltar transform fault (Gutscher et al., 2002).

Within the Iapetan realm, such a scenario would have existed in the early Paleozoic along boundaries been the young Iapetan and old paleo-Pacifi c oceanic lithosphere (see Figs. 2B and 2C). Such boundaries are a requirement of supercontinent breakup. The high angle between these boundaries and the Iapetan ocean ridge suggests that they had a strong strike-slip component and, hence, were likely sites for transform faults to nucleate. The age of the paleo-Pacifi c oceanic lithosphere adjacent to these transform faults may never be known, but it is almost certain to have been older than the adjoining interior (Iapetus) oceanic litho-sphere. The age and density contrast between them, and therefore the most favorable conditions for subduction initiation, would have been most pronounced when the Iapetus Ocean was young. We view this to be the most favorable geodynamic scenario for the origin of the arc-related oceanic complexes that were respectively obducted onto the margins of Laurentia and Baltica during the ca. 500–480 Ma Taconic and Grampian orogenies (see van Staal et al., 1998). The implication of this interpre-tation is that these mafi c arc-related complexes were formed from the subduction of paleo-Pacifi c, rather than Iapetan, lithosphere in a manner analogous to the Scotia arc and the Mesozoic-Cenozoic “capture” of the subduction zones around the Caribbean plate within the Atlantic realm (e.g., Pindell et al., 2006). The evolution of the Iapetan–paleo-Pacifi c plate boundaries would have depended on a number of factors, includ-ing the spreading rate at the Iapetan ridge, the drift of the dispersing continents, and the rate of rollback of the subduction zone. However, the presence of boninites and mafi c sheeted dikes in Iapetan ophiolitic complexes (e.g., Bédard et al., 1998) indicates that forearc extension, and hence slab rollback, occurred. The strike of an arc produced in this fashion would have been oriented at a high angle to the Iapetan ocean ridge, and this geometry would have facilitated its subsequent obduction (see Suhr and Cawood, 1993). However, as pointed out by Gurnis et al. (2004), subduction zones initiated in this manner cannot pull the large overriding plates toward them, and so cannot be considered a mecha-nism that began the assembly of Pangea.

Global reconstructions (e.g., Scotese, 1997; Stampfl i and Borel, 2002) imply that assembly of Pangea began after the closure of the Iapetus Ocean. They also imply that the rate of subduction of Rheic Ocean litho-sphere exceeded subduction rates in the paleo–Pacifi c Ocean despite the fact that subducted Rheic oceanic lithosphere was a maximum of 60 m.y. old, and that subduction zones were already well established in the paleo–Pacifi c Ocean.

Although the causes are hotly debated, a geodynamic linkage between tectonic events in the Rheic and paleo–Pacifi c Oceans is sug-gested by the dramatic change in tectonic environment of paleo-Pacifi c subduction zones between 440 Ma and 420 Ma, best documented in the Lachlan fold belt of eastern Australia (e.g., Gray and Foster, 2004), the time at which the Iapetus Ocean was closing and subduction within the Rheic Ocean was commencing.

Proxy records for oceanic lithosphere evolution are also consistent with this linkage. Rapid sea-level rise in the Cambrian and Early Ordovi-cian (e.g., Hallam, 1992), generally attributed to the development of rela-tively young, elevated Iapetan oceanic lithosphere, gave way to a drop in sea level from the Late Ordovician to the Carboniferous, implying that the average age of the oceanic lithosphere increased during the middle to late Paleozoic. This trend is compatible with preferential subduction of relatively young Rheic Ocean lithosphere over older paleo-Pacifi c oceanic lithosphere. Similarly, compared to the early Paleozoic, the lower initial 87Sr/86Sr value deduced for ocean waters in the late Paleozoic (e.g., Veizer et al., 1999) implies an increase in the rate of seafl oor spreading. Taken together, the sea-level and Sr isotope data suggest that any increased seafl oor spreading in the paleo–Pacifi c Ocean during Pangea assembly was compensated, not by subduction of old paleo-Pacifi c lithosphere, but by subduction of newer Rheic Ocean lithosphere.

Top-down geodynamic models adequately account for the early Paleozoic dispersal of continents, but they cannot account for the middle to late Paleozoic introversion that resulted in the amalgamation of Pan-gea. This conundrum suggests that top-down geodynamic forces can be overridden by other forces. Assuming that continents migrate from geoid highs to geoid lows, the reversal of their movement in the middle Paleo-zoic implies that the geoid lows became geoid highs. The mechanisms for this are unclear. However, one possibility is that the descent of subducted slabs in the paleo–Pacifi c Ocean to the core-mantle boundary thermally insulated that boundary, leading to the ponding of hot mantle material that built up and rose toward the surface as a superplume (Zhong and Gurnis, 1997; Condie, 1998; Tan et al., 2002). Such an event would result in a geoid high and enhanced seafl oor spreading in the paleo–Pacifi c Ocean that could have been responsible for the reversal in the direction of motion of the continents. In this scenario, “top-down” tectonics of the early Paleo-zoic gave way to “bottom-up” tectonics in the late Paleozoic. An Ordovi-cian superplume has been proposed by Condie (2004) to account for the period’s high sea levels, high paleotemperatures, abundant black shales, and strong biological activity.

CONCLUSIONSThe amalgamation of Pangea cannot be explained by top-down

geodynamic models. If these principles are applied to an Early Ordo-vician world, they would not generate Pangea in the correct confi gura-tion. Instead, the geologic record implies that the rates of subduction in the relatively new interior oceans (e.g., the Iapetus and Rheic Oceans) exceeded those in the exterior paleo–Pacifi c Ocean. Geodynamic linkages between subduction in the interior and exterior oceans are implied by dra-matic changes in the style of subduction in the paleo-Pacifi c that coincide with the onset of subduction in the Rheic Ocean. Assuming continents migrate from geoid highs to geoid lows, then the reversal in continen-tal motions that led to the formation of Pangea may have coincided with the emergence of a geoid high in the paleo-Pacifi c during the Ordovician.

706 GEOLOGY, September 2008

We tentatively suggest that this may refl ect the formation of a superplume, for which there is independent, supporting evidence in the rock record. If so, the preferential destruction of interior oceans, required to produce introverted supercontinents such as Pangea, may occur when top-down tectonics give way to bottom-up tectonics.

ACKNOWLEDGMENTSMurphy acknowledges the continuing support of the Natural Sciences and

Engineering Research Council, Canada, through Discovery and Research Capacity grants. Nance is supported by National Science Foundation grant EAR-0308105. We are indebted to Peter Cawood, Michael Gurnis, and Joe Meert for their insight-ful reviews. This paper is a contribution to the International Geological Correlation Programme (IGCP) Projects 453 and 497.

REFERENCES CITEDAnderson, D.L., 2001, Top-down tectonics?: Science, v. 293, p. 2016–2018, doi:

10.1126/science.1065448.Bédard, J.H., Lauziere, K., Tremblay, A., and Sangster, A., 1998, Evidence for

forearc seafl oor-spreading from the Betts Cove ophiolite, Newfoundland: Oceanic crust of boninitic affi nity: Tectonophysics, v. 284, p. 233–245, doi: 10.1016/S0040-1951(97)00182-0.

Cawood, P.A., 2005, Terra Australis orogen: Rodinia breakup and development of the Pacifi c and Iapetus margins of Gondwana during the Neoproterozoic and Paleozoic: Earth-Science Reviews, v. 69, p. 249–279, doi: 10.1016/j.earscirev.2004.09.001.

Cawood, P.A., and Buchan, C., 2007, Linking accretionary orogenesis with supercontinent assembly: Earth-Science Reviews, v. 82, p. 217–256, doi: 10.1016/j.earscirev.2007.03.003.

Cawood, P.A., McCausland, P.J.A., and Dunning, G.R., 2001, Opening Iapetus: Constraints from the Laurentian margin of Newfoundland: Geological Soci-ety of America Bulletin, v. 113, p. 443–453, doi: 10.1130/0016-7606(2001)113<0443:OICFTL>2.0.CO;2.

Cloetingh, S., Wortel, R., and Vlaar, N.J., 1989, On the initiation of subduc-tion zones: Pure and Applied Geophysics, v. 129, p. 7–25, doi: 10.1007/BF00874622.

Cocks, L.R.M., and Torsvik, T.H., 2002, Earth geography from 500 to 400 million years ago: A faunal and palaeomagnetic review: Journal of the Geo-logical Society of London, v. 159, p. 631–644.

Collins, W.J., 2002, Hot orogens, tectonic switching, and creation of continen-tal crust: Geology, v. 30, p. 535–538, doi: 10.1130/0091-7613(2002)030<0535:HOTSAC>2.0.CO;2.

Condie, K.C., 1998, Episodic continental growth and supercontinents: A mantle avalanche connection: Earth and Planetary Science Letters, v. 163, p. 97–108, doi: 10.1016/S0012–821X(98)00178–2.

Condie, K.C., 2004, Supercontinents and superplume events: Distinguishing sig-nals in the geologic record: Physics of the Earth and Planetary Interiors, v. 146, p. 319–332, doi: 10.1016/j.pepi.2003.04.002.

Gray, D.R., and Foster, D.A., 2004, Tectonic evolution of the Lachlan orogen, southeast Australia: Historical review, data synthesis and modern per-spectives: Australian Journal of Earth Sciences, v. 51, p. 773–817, doi: 10.1111/j.1400-0952.2004.01092.x.

Gurnis, M., 1988, Large-scale mantle convection and the aggregation and dispersal of supercontinents: Nature, v. 332, p. 695–699, doi: 10.1038/332695a0.

Gurnis, M., Hall, C., and Lavier, L., 2004, Evolving force balance during incipi-ent subduction: Geochemistry, Geophysics, Geosystems, v. 5, p. Q07001, doi: 10.1029/2003GC000681.

Gutscher, M.-A., Malod, J., Rehault, J.-P., Contrucci, I., Klingelhoefer, F., Mendes-Victor, L., and Spakman, W., 2002, Evidence for active subduction beneath Gibraltar: Geology, v. 30, p. 1071–1074, doi: 10.1130/0091-7613(2002)030<1071:EFASBG>2.0.CO;2.

Hallam, A., 1992, Phanerozoic Sea Level: New York, Columbia University Press, 266 p.

Hibbard, J.P., Stoddard, E.F., Secor, D.T., and Dennis, A.J., 2002, The Carolina zone: Overview of Neoproterozoic to early Paleozoic peri-Gondwanan ter-ranes along the eastern fl ank of the southern Appalachians: Earth-Science Reviews, v. 57, p. 299–339, doi: 10.1016/S0012-8252(01)00079-4.

Hoffman, P.F., 1999, The break-up of Rodinia, birth of Gondwana, true polar wander and the snowball Earth: Journal of African Earth Sciences, v. 28, p. 17–33, doi: 10.1016/S0899-5362(99)00018-4.

Keppie, J.D., Nance, R.D., Murphy, J.B., and Dostal, J., 2003, Tethyan, Medi-terranean, and Pacific analogues for the Neoproterozoic-Paleozoic birth and development of peri-Gondwanan terranes and their transfer to

Laurentia and Laurussia: Tectonophysics, v. 365, p. 195–219, doi: 10.1016/S0040-1951(03)00037-4.

Kryza, R., Mazur, S., and Pin, C., 2003, Subduction- and non-subduction-related igneous rocks in the Central-European Variscides: Geochemical and Nd isotope evidence for a composite origin of the Kodzko Metamorphic Com-plex, Polish Sudetes: Geodinamica Acta, v. 16, p. 39–57, doi: 10.1016/S0985-3111(02)00004-9.

Linnemann, U., McNaughton, N.J., Romer, R.L., Gehmlich, M., Drost, K., and Tonk, C., 2004, West African provenance for Saxo-Thuringia ( Bohemian Massif): Did Armorica ever leave pre-Pangean Gondwana?—U/Pb-SHRIMP zircon evidence and the Nd-isotopic record: International Journal of Earth Sciences, v. 93, p. 683–705, doi: 10.1007/s00531-004-0413-8.

Murphy, J.B., and Nance, R.D., 2003, Do supercontinents introvert or extro-vert?: Sm-Nd isotopic evidence: Geology, v. 31, p. 873–876, doi: 10.1130/G19668.1.

Murphy, J.B., Gutierrez-Alonso, G., Nance, R.D., Fernandez-Suarez, J., Keppie, J.D., Quesada, C., Strachan, R.A., and Dostal, J., 2006, Origin of the Rheic Ocean: Rifting along a Neoproterozoic suture?: Geology, v. 34, p. 325–328, doi: 10.1130/G22068.1.

Nance, R.D., Miller, B.V., Keppie, J.D., Murphy, J.B., and Dostal, J., 2006, The Acatlan Complex, southern Mexico: Record spanning the assembly and breakup of Pangea: Geology, v. 34, p. 857–860, doi: 10.1130/G22642.1.

Pindell, J., Kennan, L., Stanek, K.-P., Maresch, W.V., and Draper, G., 2006, Foun-dations of Gulf of Mexico and Caribbean evolution: Eight controversies resolved, in Iturralde-Vinent, M.A., and Lidiak, E.G., eds., Caribbean Plate Tectonics: Geologica Acta, v. 4, p. 303–341.

Quesada, C., Bellido, F., Dallmeyer, R.D., Gil Ibarguchi, J.I., Oliveira, J.T., Pérez Estaún, A., Ribeiro, A., Robardet, M., and Silva, J.B., 1991, Terranes within the Iberian Massif: Correlations with West African sequences, in Dallmeyer, R.D., and Lecorché, J.P., eds., The West African Orogens and Circum–Atlantic Correlations: Berlin, Springer-Verlag, p. 267–294.

Sánchez-García, T., Bellido, F., and Quesada, C., 2003, Geodynamic setting and geochemical signatures of Cambrian-Ordovician rift-related igneous rocks (Ossa Morena zone, SW Iberia): Tectonophysics, v. 365, p. 233–255, doi: 10.1016/S0040-1951(03)00024-6.

Scotese, C.R., 1997, Continental Drift (7th edition): Arlington, Texas, PALEO-MAP Project, 79 p.

Stampfl i, G.M., and Borel, G.D., 2002, A plate tectonic model for the Paleo-zoic and Mesozoic constrained by dynamic plate boundaries and restored synthetic oceanic isochrones: Earth and Planetary Science Letters, v. 196, p. 17–33, doi: 10.1016/S0012-821X(01)00588-X.

Stern, R.J., 2004, Subduction initiation: Spontaneous and induced: Earth and Planetary Science Letters, v. 226, p. 275–292, doi: 10.1016/S0012-821X(04)00498-4.

Stern, R.J., and Bloomer, S.H., 1992, Subduction zone infancy: Examples from the Eocene Izu-Bonin-Mariana and Jurassic California: Geological Society of America Bulletin, v. 104, p. 1621–1636, doi: 10.1130/0016-7606(1992)104<1621:SZIEFT>2.3.CO;2.

Suhr, G., and Cawood, P., 1993, Structural history of ophiolite obduction, Bay of Islands, Newfoundland: Geological Society of America Bulletin, v. 105, p. 399–410, doi: 10.1130/0016-7606(1993)105<0399:SHOOOB>2.3.CO;2.

Tan, E., Gurnis, M., and Han, L., 2002, Slabs in the lower mantle and their modu-lation of plume formation: Geochemistry, Geophysics, Geosystems, v. 3, p. 1067, doi: 10.1029/2001GC000238.

van Staal, C.R., Dewey, J.F., MacNiocaill, C., and McKerrow, W.S., 1998, The Cambrian-Silurian tectonic evolution of the Northern Appalachians and British Caledonides: History of a complex, west and southwest Pacifi c-type segment of Iapetus, in Blundell, D.J. and Scott, A.C., eds., Lyell: The Past is the Key to the Present: Geological Society of London Special Publica-tion 143, p. 197–242.

Veizer, J., Davin, A., Azmy, K., Bruckschen, P., Buhl, D., Bruhn, F., Carden, G., Diener, A., Ebneth, S., Godderis, Y., Jasper, T., Korte, C., Pawellek, F., Podlaha, O., and Strauss, H., 1999, 87Sr/86Sr, δ13C and δ18O evolution of Phanerozoic seawater: Chemical Geology, v. 161, p. 59–88, doi: 10.1016/S0009-2541(99)00081-9.

Zhong, S., and Gurnis, M., 1997, Dynamic interaction between tectonic plates, subducting slabs, and the mantle: Earth Interactions, v. 1, p. 1–18, doi: 10.1175/1087-3562(1997)001<0001:DIBTPS>2.3.CO;2.

Manuscript received 18 March 2008Revised manuscript received 16 May 2008Manuscript accepted 20 May 2008

Printed in USA