A new paleogeographic model for the Neoproterozoic Adamastor and Khomas oceans and the tectonic context for Snowball Earth

The Neoproterozoic Adamastor and Khomas oceans separated the Congo craton from South American cratonic blocks and the Kalahari craton, respectively. These paleo-oceans are key features in plate reconstructions of the break-up of Rodinia, and their closures are associated with major Pan-African orogens and the assembly of Gondwana. Sedimentary records from the margins of these basins include the iconic Cryogenian Snowball Earth sequences of the Otavi/Swakop Group, which are preserved in the Kaoko and Damara belts of Namibia. Despite the central role of these paleo-oceans in Neoproterozoic tectonics and Earth History, their existence has been questioned and disparate tectonic and paleogeographic models persist for the tectonic setting of the Otavi/Swakop Group. The classic model for deposition of Cryogenian glacial deposits of the Otavi/Swakop Group is along rifted passive margins; however, recent models propose Cryogenian subduction under the Kaoko margin of Congo. Here we use new geological mapping, stratigraphy, Nd-, Hf-, and C-isotope geochemistry, and U-Pb zircon geochronology from the distal margins of the Congo and Kalahari cratons and allochthonous terranes in the Damara and Kaoko belts in Namibia as input to a new continuous plate model for the opening and closure of these former oceans. We compare two different models: 1) the Coastal Terrane collided with the Kaoko margin of Congo ca. 650-630 Ma followed by slab breakoff and reversal; 2) the Coastal Terrane is part of a peri-Kalahari ribbon continent that includes the Ugab and Swakop Terranes, which did not collide with the Congo until ca. 590 Ma. The new data and paleogeographic model provide tectonic context for paleoenvironmental interpretations of Cryogenian glacial successions on the Kalahari and Congo margins.