Do Supercontinents Turn Inside-in or Inside-out?

Abstract Supercontinent amalgamation and dispersal has occurred repeatedly since the Archean. However, the mechanisms responsible for these events are unclear. Following supercontinent breakup, two geodynamically distinct oceans may be distinguished: an interior ocean formed between the dispersing continents, whose lithosphere is younger than the time of supercontinent breakup (TR); and an exterior ocean that surrounded the supercontinent prior to breakup, and consequently is dominated by lithosphere that is older than the time of breakup. In order to evaluate geodynamic models for supercontinent formation, it is essential to determine which of these two types of ocean is consumed during supercontinent amalgamation. Although much of the evidence needed is destroyed by subduction, vestiges of oceanic lithosphere are preserved in mafic complexes accreted to continental margins prior to terminal collision. Because the age contrast between interior and exterior oceans diminishes as the continents drift apart, the ages of the earliest accreted complexes are the most diagnostic of the ocean in which they formed. Constraints on the age of the mantle lithospheric sources (TDM) that give rise to these accreted complexes can be derived from Sm-Nd isotope systematics. In the case of Pangea, for example, the North American Cordillera represents an accretionary orogen along the leading edge of a dispersing supercontinent. Within this orogen, the oldest accreted oceanic terranes, characterized by high εNd values close to contemporary depleted mantle values, show similar crystallization and TDM model ages that imply crustal formation and arc activity during the lifespan of Pangea, that is, within the exterior Panthalassa ocean (i.e., TDM>TR). This example suggests that a similar approach applied to older orogens may constrain the relationship between continental margins and their accreted mafic complexes. Pangea was formed by closure of Paleozoic oceans (e.g., Iapetus and Rheic) that were formed after the ca. 550 Ma breakup of Pannotia. Uncontaminated mafic rocks from both oceans that have εNd values close to depleted mantle values at their respective times of emplacement show closely matching crystallization and depleted mantle model ages that do not exceed the age of rifting (i.e., TDM ≥ TR). This indicates that the oceanic lithospheric source of these suites was generated after the rifting of Pannotia, such that Pangea was formed by the closure of interior oceans (introversion). In contrast, mafic terranes accreted in orogens that terminated in the formation of the Late Neoproterozoic supercontinent Pannotia have Sm-Nd TDM model ages between ca. 1.2 and 0.71 Ga, implying that much of the oceanic lithosphere that was subducted and recycled to yield these complexes was formed before the ca. 755 Ma breakup of the supercontinent Rodinia (i.e. TDM > TR). These mafic complexes are therefore vestiges of oceanic lithosphere that formed within the peri-Rodinian ocean, such that Pannotia was formed by the closure of an exterior ocean (extroversion). This analysis suggests that Pangea and Pannotia were assembled by fundamentally distinct geodynamic processes. Hence, the "supercontinent cycle" may have a more complex origin than previously considered.

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