Onset and evolution of the scotia sea basins, antarcticatectonic, sedimentary and palaeoceanographic implications

Résumé

The Scotia Sea is a complex area located in the Southern Ocean, to the east of Drake Passage. Internally it is formed by two small tectonic plates of a mainly oceanic nature. Structural highs of a continental nature, from the former continental bridge between South-America and the Antarctic Peninsula, surround the southern abyssal plains. The main aim of this study is to describe the onset and stratigraphic evolution of the south Scotia Sea sedimentary basins. The interplay between tectonics, sedimentary processes, oceanography and climate is explored to trace a regional evolution linked to water mass interchange between the Weddell Sea and the Pacific and Atlantic oceans. The basins¿ stratigraphic architecture is studied in detail through a seismo-stratigraphic and morpho-structural analysis of available multichannel seismic profiles. Gravity and magnetic anomaly models are additionally considered to constrain the nature and age of the basins. Scan Basin is the easternmost one within the southern Scotia Sea. The results expounded here point to two different formation processes in its southern and northern sectors. South of this basin lies Bruce Passage, a 3000 m deep gateway that connects Scan Basin with Jane Basin and the Weddell Sea. This passage represents one of the worldwide-identified examples of overflow, allowing the very first incursions of Weddell Sea Deep Water into the Scotia Sea and subsequent interaction with the main water masses of the Southern Ocean, including the Circumpolar Deep Water. The NE-SW oriented Dove Basin is located west of Scan Basin, and its crust is primarily oceanic; a roughly N-S oriented ridge in the central part of the Dove Basin is interpreted as the remnant spreading centre. This basin straddles the junction between the Circumpolar Deep Water, the Weddell Sea Deep Water and the South Pacific Deep Water. To the west, Protector Basin developed through a S-N propagating rift. The tectonic events involved in the initial evolution of this basin would control the basal stratigraphic architecture, whereas the Weddell Sea Deep Water and Circumpolar Deep Water interaction would drive the sedimentary processes from middle Miocene to Present-day. The southwestern corner of the Scotia Sea is occupied by the Ona Basin. The so-called Ona High traces two discrete abyssal domains: the Western and Eastern Ona basins. The Eastern Ona Basin is defined as the oldest sector of the Scotia Sea. The sedimentary record of all these basins holds evidence of Mass Transport Deposits related to tectonic and sedimentary events. Furthermore, diverse fluid leaks and Bottom Simulating Reflectors can be related to the presence of free gas or gas hydrates in the southern Scotia Sea. These small basins distributed over the southern Scotia Sea opened in a back-arc context, although they present different types of fragmentation associated with the processes undergone in the two end members of passive margins. The older Ona and Scan basins present fragmentation processes of volcanic passive rifting, whereas the fragmentation in the younger Dove and Protector basins responds to the model followed in magma-poor passive riftings. The resulting evolutionary model of the Scotia Sea entails two main phases: the Paleogene witnessed the onset of formation of the Scotia Sea, whereas eastward progress of the Scotia Arc took place during Neogene and Quaternary times. Within the latter phase, a primary change in the tectonic stress field of the Scotia Sea occurred between the two types of basins. It is related to the end of the Weddell Sea subduction and can be seen as the first change in the sedimentary pattern, dating from early Miocene. The second major change in the stratigraphic pattern took place in the middle Miocene and is tied to alterations in the palaeoceanographic context due to the opening of main gateways around the Scotia Sea coeval with tectonic readjustments. This allowed the intrusion of Weddell Sea Deep Water into the Scotia Sea, shifting the former Circumpolar Deep Water to a northern position and establishing the former as the bottom flow of the southern Scotia Sea, up to the Present. This tectonic change has been globally documented and is reported here in the northwest part of the Scotia Sea. Late Miocene and Late Pliocene paleoceanographic modifications enhanced the Weddell Sea Deep Water circulation in conjunction with climatic and tectonic episodes. Regional changes are recorded in the sedimentary stacking pattern of the Scotia Sea, while regional tectonic and oceanographic events involved in phases of intense plate tectonic activity and ice-sheet oscillations in Antarctica can be related to major climatic events. The coeval occurrence of sedimentary, tectonic, climatic and palaeoceanographic events points to close interplay between these factors. Although the relationship between climatic and palaeoceanographic changes is broadly recognized, this contribution signals tectonics as a probable long- and short-term driver of both palaeoceanographic and climatic alterations.