Birth and consumption of Neoproterozoic oceans and possible scenarios of Ed... - Dr. Fabricio Caxito

Описание: Birth and consumption of Neoproterozoic oceans and possible scenarios of Ediacaran-Cambrian ecosystems in western Gondwana

Fabricio Caxito
Federal University of Minas Gerais, Brasil

Western Gondwana formed by the amalgamation of landmasses through consumption of the intervening Neoproterozoic oceanic domains. Accretionary processes dominated during the Tonian-early Cryogenian (ca. 900-720 Ma), forming a string of island-arcs whose remnantsextend from southern Brazil up to the Sahara, and present typically juvenile isotope and elemental geochemistry characteristics. Protracted closure of the Charrua-Goiás-Pharusian and Adamastor oceanic realms led to docking of the island-arc terranes, subduction polarity reversal and development of typical Andean-type continental arcs at ca. 660-620 Ma. This was followed by collision of the “Central African Block”, which comprised the Congo-São Francisco paleocontinent and its reworked margins, with probable Rodinia offspring such as the West African-São Luís, Parnaíba, Paranapanema and Rio de La Plata paleocontinents at 630-600 Ma, and finally by collision of the Amazonian paleocontinent in the Ediacaran-Cambrian transition. Thus, the process of supercontinent building in this case is the archetypical “extroversion”, whereupon collision happens in distinct paleocontinental margins that were not conjugated prior to rifting. At the same time, restricted oceanic basins developed along the northern and southern Congo-São Francisco paleocontinental margins. They correspond to the Transnordestino-Central African Ocean to the north and the Khomas Ocean to the south, which seem to have formed and closed through classical Wilson Cycles, i.e. “introversion” processes, characterized by collision of previously conjugated rifted margins. Some V-shaped oceanic terminations connected to the extroverted system, such as the precursor basin of the Araçuaí-West Congo orogen, also closed in an introverted nutcracker mode. Mountain building was probably unprecedented in Earth’s story, with formation of long and wide orogenic belts containing the first widespread record of retro-eclogites and related HP-UHP rocks. Ediacaran-Cambrian foreland basins of western Gondwana contain the first records of complex life forms, suggesting a link between orogenesis and favorable conditions for colonization such as nutrient delivery and O2 production. However, some of the internal basins became progressively restricted as they were surrounded by the developing orogenic belts, hampering the full development of typical Ediacaran-Cambrian ecosystems. Basin restriction could have led to the development of widespread anoxia and the delivery of high amounts of greenhouse gases such as CH4 to Earth’s atmosphere.