EVOLUTION OF THE RHEIC OCEAN AND THE CONUNDRUM OF PANGEA

Two contrasting end-member modes of supercontinent formation, extroversion and introversion, are preserved in the geologic record. The ca. 0.8-0.75 Ga breakup of Rodinia was accompanied by preferential consumption of the exterior ocean that surrounded Rodinia. The exterior margins of the dispersing continental fragments collided resulting in the Late Neoproterozoic assembly of Pannotia by extroversion. The interior oceans generated between the dispersing continents formed part of Paleopacific oceanic lithosphere that surrounded Pannotia. The Late Neoproterozoic breakup of Pannotia initially led to consumption of the exterior Paleopacific Ocean, but at ca. 480 Ma to 460 Ma, this motion was reversed and preferential subduction of interior oceans (e.g. Iapetus, Rheic, Uralian) led to the formation of Pangea by introversion. These end-member models can be distinguished by comparing the Sm-Nd crust-formation ages of accreted mafic complexes (e.g., ophiolites) in the collisional orogens formed during supercontinent assembly with the breakup age of the previous supercontinent. Most commonly accepted geodynamic models predict the formation of supercontinents by extroversion. Models explaining the formation of supercontinents by introversion, including the formation of Pangea, are more problematic. During the assembly of Pangea, subduction was not only initiated within the new Paleozoic oceans, but the rates of subduction of this relatively young oceanic lithosphere must have exceeded those of the already well-established subduction zones within the exterior Paleopacific ocean. To understand the processes leading to the formation of Pangea, we need to investigate the potential geodynamic linkages between the evolution of the Rheic Ocean and the penecontemporaneous evolution of the exterior (Paleopacific) ocean. This evolution is recorded in the 18,000 km Terra Australis orogen which preserves episodes of subduction-related orogenesis from 570 Ma until 230 Ma.