THE ASSEMBLY OF PANGEA: GEODYNAMIC CONUNDRUMS REVISITED

Although the existence of Pangea is a keystone of modern geology, geodynamic models purporting to explain its assembly are problematic because they require a knowledge of Paleozoic mantle convection patterns. Although subduction provides ~90% of the force (slab pull) needed to drive modern global plate motions, application of “top-down” tectonics to late Neoproterozoic–Early Paleozoic plate reconstructions fails to yield Pangea in the correct configuration. Top-down tectonics predicts that Pangea should have formed by consumption of the exterior paleo-Pacific Ocean (extroversion). However, virtually all viable reconstructions show Pangea assembly by consumption of the interior Paleozoic Iapetus, Rheic, and Proto-Tethys oceans (introversion) between Laurentia (ancestral North America), Baltica (northwestern Europe), and Gondwana. Such an outcome implies top-down tectonics was not the dominant force leading to Pangea amalgamation.

Key to solving the Pangea conundrum is understanding Paleozoic mantle convection patterns and the mantle legacy of the Late Neoproterozoic–Cambrian amalgamation of Gondwana (which was possibly part of a larger though fleeting entity known as Pannotia). A wealth of proxy data indicates that Gondwana amalgamation imparted a legacy on mantle convection patterns that must be factored into models for Pangea amalgamation. These proxy data suggest that the mantle downwelling driving Pan-African collisions and Gondwana assembly evolved into a mantle upwelling as evidenced by the interplay between subduction-related and plume-related tectonics around Gondwana’s periphery.

Application of orthoversion theory suggests that Gondwana may have amalgamated above an intense downwelling along a meridional subduction girdle that bisected two antipodal sub-equatorial LLSVPs. Several processes beneath and around Gondwana would have reduced the intensity of the downwelling, as plume-related activity along its margins initiated subduction zone roll-back and the export of terranes from Gondwana that eventually collided with the margin of Laurentia–Baltica (e.g., Acadian orogeny). As upwelling beneath Gondwana intensified, Gondwana migrated along the girdle until it collided with Laurentia–Baltica, resulting in the final assembly of Pangea.