Contrasting modes of supercontinent formation and the conundrum of Pangea
Introduction
The Phanerozoic Eon is dominated by the assembly and amalgamation of Pangea in the Paleozoic (Fig. 1), followed by its breakup and dispersal in the Mesozoic and Cenozoic. Although to a first order, there is a consensus on the paleocontinental reconstruction and timing of these events, (e.g. McKerrow and Scotese, 1990, Cocks and Fortey, 1990, Scotese, 1997, van Staal et al., 1998, Cocks and Torsvik, 2002, Stampfli and Borel, 2002, Veevers, 2004), the mechanisms responsible for the amalgamation of this supercontinent are poorly understood. Furthermore, over the last 20 years, evidence has been amassing that Pangea is just the latest in a series of supercontinents that have formed since the Archean, only to breakup and reform again (e.g. Rogers and Santosh, 2004, Silver and Behn, 2008). Although the causes remain elusive, many geoscientists agree that repeated cycles of supercontinent amalgamation and dispersal (the “supercontinent cycle” of Worsley et al., 1984, Nance et al., 1988), have had a profound effect on the evolution of the Earth's crust, atmosphere, hydrosphere, and life (e.g. Worsley et al., 1984, Worsley et al., 1986, Nance et al., 1986, Veevers, 1990, Veevers, 1994, Condie, 1994, Condie, 1995, Hoffman et al., 1998, Ross, 1999, Condie, 2002, Knoll et al., 2004, Maruyama et al., 2007, Maruyama and Santosh, 2008, Stern, 2008, Meert and Lieberman, 2008, Rino et al., 2008).
In this paper, we review the development of this concept and then use the history of Pangea to gain insights into potential mechanisms that might account for episodic supercontinent formation. We first trace these ideas to show how the concept of supercontinent cycles originated. We point out that oceanic lithosphere created and destroyed during this cycle has different geodynamic properties depending on whether the lithosphere was located around the supercontinent (“exterior” ocean floor) or formed between dispersing continents (“interior” ocean floor). We then review the evidence that supercontinents may have formed by different mechanisms. Example, the late Neoproterozoic supercontinent Pannotia (Powell, 1995, Dalziel, 1997), which consists of the continental fragments of Gondwana and Laurentia, was formed by the preferential consumption of the exterior oceanic lithosphere (Mozambique Ocean), whereas Pangea was formed by the preferential subduction of interior oceanic lithosphere (Iapetus and Rheic oceans).
The implications of different relative ages of oceanic and continental lithosphere interaction are significant when viewed in the light of geodynamic models used to explain supercontinent formation. For example, the formation of Pangea cannot be explained by most widely accepted geodynamic models, since these models, when applied to the widely accepted paleocontinental reconstructions for the Early Paleozoic, do not yield Pangea in the correct configuration (Murphy and Nance, 2008). Hence, a fundamental disconnection exists between the geologic evidence for supercontinent formation, and the models purported to explain their assembly.
Finally, to provide constraints for future geodynamic models and to gain insight into the processes leading to the formation of Pangea, we investigate the geodynamic linkages between the Paleozoic evolution of the interior (Iapetus and Rheic) oceans, as recorded in the orogens (Ouachita, Appalachian, Caledonide, Variscan) produced by their closure (e.g. van Staal et al., 1998, Matte, 2001), and the pene-contemporaneous evolution of the exterior (paleo-Pacific) ocean, as recorded in the 18,000 km Terra Australis orogen, which preserves a record of subduction from ca. 570 to 230 Ma (e.g. Cawood, 2005). In doing so, we show that proxy records for oceanic lithosphere development, such as sea level and isotopic data (Hallam, 1992, Veizer et al., 1999, Condie, 2004, Miller et al., 2005, Barnes, 2004), can be used to identify global-scale changes in plate geodynamics, and so provide a complementary approach to the analysis of supercontinent formation throughout geologic time.
Section snippets
Development of concepts
The notion of episodic crustal development and orogenic activity actually predates the general acceptance of the plate tectonic paradigm. Holmes (1954) suggested that the development of continents took place through the episodic production of new crust. This concept was further developed by Gastil (1960), who drew attention to radiometric data suggesting that granite production throughout geologic time was episodic rather than continuous, by Sutton (1963), who used these data to define
Geodynamic framework
When a supercontinent breaks up, two geodynamically distinct tracts of oceanic lithosphere exist (Fig. 4). The new interior oceans that form between the dispersing continents are underlain by oceanic lithosphere that is younger than the age of supercontinent breakup, whereas the exterior ocean surrounding the supercontinent is underlain by older oceanic lithosphere, the vast majority of which predates the time of supercontinent breakup. Because of their contrasting ages, the interior and
Introversion and extroversion in the geologic record: Sm/Nd evidence
The geological record suggests that both introversion and extroversion have occurred at different times in the geologic past. For example, most paleocontinental reconstructions imply that the breakup of the Late Mesoproterozoic supercontinent, Rodinia, and subsequent assembly of the Late Neoproterozoic supercontinent, Pannotia, occurred by preferential subduction of the exterior ocean and, hence, is an example of extroversion (e.g. Hoffman, 1992, Dalziel, 1992, Dalziel, 1997, Murphy and Nance,
Geodynamic conundrum of Pangea
The “top–down” geodynamic models of Anderson, 1982, Anderson, 1994, Anderson, 2001) and Gurnis (1988), in which supercontinents breakup over geoid highs and migrate away from those highs to reassemble over geoid lows (represented by subduction zones, Fig. 8), predict that supercontinents form by extroversion, unless there is a fundamental change in the location of the geoid anomalies. Hence, they provide an adequate explanation for the formation of Gondwana/Pannotia from the 0.83–0.75 Ga
Proxy records
The dramatic change in tectonic environment recorded in the Terra Australis Orogen at precisely the time that the Rheic Ocean begins to subduct is consistent with a geodynamic linkage between events in the interior and exterior oceans. To examine this potential linkage further, we can look at proxy records that document on a broad scale, events within the global oceanic domain. For example, tectonic events can effect sea level by as much as 100 m on timescales varying from one million to
Conclusions
Geodynamic models of supercontinent cycles involve continental breakup over geoid highs and the movement and re-amalgamation of the continents over geoid lows (e.g. Anderson, 2001). Such models imply a top–down geodynamic driver in which continental amalgamation is controlled by surface plates and the location of subduction zones, which correspond to geoid lows. These models require that supercontinents form by extroversion in which the exterior ocean surrounding the supercontinent is consumed
Acknowledgments
We are grateful to Victor Ramos and an anonymous reviewer for their insightful and constructive comments. JBM acknowledges the continuing support of the Natural Sciences and Engineering Research Council, Canada through Discovery and Research Capacity grants. RDN is supported by National Science Foundation grant EAR-0308105 and a Baker Award from Ohio University, and PAC acknowledges the support of the Australian Research Council. This paper is a contribution to the International Geoscience
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