The Origin of Supercontinents and the Pangea Conundrum

Repeated amalgamation and dispersal of supercontinents have had a profound effect on the Earth's evolution since the end of the Archean. However, the mechanisms responsible for these events are unclear. Following supercontinent breakup, there are two geodynamically distinct types of ocean; interior oceans formed between the dispersing continents with lithosphere whose age is younger than the time of supercontinent breakup, and an exterior ocean that surrounded the supercontinent prior to breakup and so is dominated by lithosphere that predates the time of breakup. A variety of data indicate that the Late Neoproterozoic supercontinent, Pannotia, formed by closure of an exterior ocean (extroversion), whereas Pangea formed in the Late Paleozoic by interior ocean closure (introversion), implying that supercontinents can be assembled by fundamentally distinct geodynamic processes. Although the assembly of Pannotia is broadly consistent with top-down geodynamic models, whereby the dispersing continental fragments of a supercontinent breakup and migrate from geoid highs to reassemble at geoid lows, the better documented record of Pangea assembly runs counter to such models. 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-Pacific, reassembling an extroverted supercontinent as this ocean closed. Instead, Pangea assembled as a result of the closure of the younger Iapetus and Rheic oceans. Geodynamic 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 superplume that produced a geoid high in the paleo-Pacific. 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.