Geodynamic evolution of the Scotia sea (Antarctica), paleoceanographic implications and global change

Resumen

The opening of the Drake Passage marked the final break-up of Gondwana Supercontinent, with the creation of the Scotia Arc. The Scotia Sea, located between South America and Antarctica, is constituted at present by the Scotia and Sandwich plates. To the west of the Scotia Plate, the Shackleton Fracture Zone accommodates the Phoenix Plate and Scotia Plate movements. The development of the Scotia Sea is considered a key gateway for both asthenospheric and oceanic currents. The opening of the Drake Passage is evoked as the last Southern Ocean gateway, which led to the instauration of the Antarctic Circumpolar Current, isolating the Antarctic Continent and bearing a major influence on climatic and global changes. In this Ph.D. Thesis it is shown that the opening of the passage meant a gateway for Pacific mantle outflow because of the absence of deep lithospheric roots in the passage (Chapter 5). The mantle material fed the West Scotia Ridge until the uplift of the Shackleton Fracture Zone in the middle Miocene (Chapter 8). The Shackleton Fracture Zone relief formed a lithospheric root in the Drake Passage, serving as a barrier for the asthenospheric flows, avoiding the mantle material fed the West Scotia Ridge properly. Simultaneous to this asthenospheric setting, the uplift of the Shackleton Fracture Zone together with the initial incursions of the Weddell Sea Deep Water in the Scotia Sea forced the Circumpolar Deep Water and the Polar Front to move even further from Antarctica. This had profound climatic implications, favoring the thermal isolation of Antarctica and the growing and permanent ice-sheets. As the Scotia Arc is considered a small ocean formed as a back-arc, and fed mostly by the Pacific mantle, the small convection cells responsible for oceanic spreading lend the West Scotia Sea a very different thermal behavior than large oceans (Chapter 6). The mantle dynamics also control the tectonic distribution of the Scotia Arc and surrounding areas. The subduction of the Phoenix Plate in the western margin of the Antarctic Peninsula formed the batholithic body known as the Pacific Margin Anomaly, which is used to propose an initial distribution of the continental blocks in the Antarctic Peninsula and in the Scotia Arc, and an opening model of the Scotia Sea (Chapter 7). As a consequence of the Phoenix Plate subduction, due to the roll-back of the South Shetland Islands Block, the Bransfield Strait developed as a back-arc basin. This basin is found to be in its final stages of continental extension in some areas, and in the early stages of oceanic spreading in others (Chapter 9). The Bransfield Strait is characterized by a central neovolcanic axis, the Deception Island active volcano being located in its southern part. The volcano is a consequence of the active extensional setting of the Bransfield Strait, analyzed here using a new methodology (Chapter 10) in order to assess its volcanic activity over a 20-year period. In summary, an interdisciplinary analysis of the Scotia Arc and surrounding areas is accomplished in this Ph.D. Thesis in order to improve in the knowledge and understanding of the study area evolution and the implications in global scale.