Passive Continental Margins Formation in the Scotia Sea Basins, Antarctica

The Scotia Sea is a complex geological area located in the Southern Ocean, to the east of Drake Passage (Fig. 1). Its onset occurred during the last steps of Gondwana breakup when the continental bridge between South-America and the Antarctic Peninsula fragmented. The structural highs of continental nature generated during this fragmentation are at present surrounding the abyssal plains of the south Scotia Sea and restricting small isolated and deep sedimentary basins (Fig. 1). The onset and stratigraphic evolution of these basins is approached in the present study through seismo-stratigraphic and morpho-structural analysis of the available multichannel seismic profiles.

The southern Scotia Sea basins opened in a back-arc context, although the structural margins of the five small basins and their intrinsic features are different and allow to divided them into two groups: one formed by the two end basins, Ona and Scan; and other formed by the two central basins, Dove and Protector, together with the slightly north Pirie Basin (Fig. 1). The margins of Ona and Scan basins do not resemble the classical continental blocks of typical rifting, as do the well-known margins of the North Atlantic Ocean. Otherwise, Protector, Dove and Pirie, present asymmetric margins comprising several tilted blocks that diminish in height oceanwards and rotate on a progressively rising detachment fault as are typically seen in passive margins. Such margins structure, together with other structural differences, highlight two different types of fragmentation along the south Scotia Sea.

The two basins located in the Scotia Sea extremes (Ona and Scan) present fragmentation processes typical of passive volcanic rifted margins and the onset of their formation would have entailed crustal thinning and diffuse spreading, with abundant volcanic intrusions. Otherwise, the central basins of south Scotia Sea (Dove, Protector and Pirie) reflect the architectural evolution of magma-poor margins and the rift of previously thinned continental crust could be reconstructed since the entire crust broke up before exhumation of the mantle.

The resulting evolutionary model of the Scotia Sea entails three major stages, divided by two main events, which have deeply conditioned the sedimentary stacking pattern. During Paleogene the onset of the Scotia Sea started through WNW-ESE extension. Ona and Scan basins formed during this extension (Fig. 1). But the extensional direction changed during early Miocene due to the end of the Weddell Sea subduction and the subsequent regional change in the extensional field stress. Dove, Protector and Pirie basins started their formation after this change, during the eastwards progression of the Scotia Arc (Fig. 1). The next tectonic readjustment during middle Miocene allowed the opening of major gateways around the Scotia Sea and the instauration of a new palaeoceanographic setting is reflected in the onset of the sedimentary homogenization along the southern Scotia Sea.

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This work was funded through projects CTM2011-30241-C01/02ANT and CTM2014-60451-C2-1/2. It was also partially supported by the COMPASS consortium.