Plate tectonic development of the Scotia Sea

The Scotia Sea opened to fill the widening space created between the dominantly-westward motion of the South American plate and the dominantly-eastward motion of the South Sandwich Trench and its ancestors, as seen from Antarctica. Presently this space opens along an east-west azimuth, and is partitioned into complex transform motions on two large fault zones at the southern and northern edges of the Scotia Sea, and post-17 Ma seafloor spreading in the East Scotia Sea back-arc basin just west of the trench. A patchwork of basins and extended microcontinents within the floor and margins of the Scotia Sea show that, in the past, other divergent plate motions saw to the growth of much of the area west of the back-arc basin by seafloor spreading and continental extension. Two different assumptions about the tectonic history of the Scotia Sea steer interpretations of these basins and microcontinents. The first is predicated on a 19th Century view of the regional geology in which ‘the Andes are to be seen again in Graham Land’; that is, on the assumption that the Scotia Sea grew by the disruption of a compact segment of an initially-continuous convergent Antarctic-South American continental margin. The second assumes that the Scotia Sea grew within the space generated by the relative motions of the major plates behind the active continental margin. The oldest remnants of the Scotia Sea’s growth are Jurassic and Cretaceous rocks of Tierra del Fuego and South Georgia. These rocks can be interpreted in terms of the growth of a back-arc basin, the Rocas Verdes basin, at the active continental margin of Gondwana. They are alternatively interpretable as records of basins that formed during NE-directed South American–Antarctic relative plate motions in the supercontinent interior. In Tierra del Fuego, remnants of small extensional basins with Paleogene sedimentary fills record the onset of WNW-directed motion of the South American plate at ~50 Ma. Ongoing plate motion in this sense culminated in the development of a large and organised mid-ocean ridge in the west Scotia Sea, expressing divergence of the South American plate from a plate overriding at the South Sandwich Trench, and the smaller Powell Basin between this overriding plate and the Antarctic plate. Magnetic anomaly records show that the West Scotia Ridge was active at 30–6 Ma and that the Powell Basin was opening at 30–22 Ma. Powell Basin has alternatively been interpreted as a back-arc basin and the west Scotia Sea as ‘back-arc in the broad sense’. A long history of interpretations concern the growth of the remainder of the Scotia Sea floor, in the small Protector, Dove and Scan basins and the central Scotia Sea at the same time as spreading on the West Scotia Ridge. They are constrained by short sequences of magnetic polarity reversal anomalies, which can be modelled to show changes in spreading rate at the same times as changes in the more complete west Scotia sequence of reversal anomalies. Interpretations like these require a complex circuit of small plates in the Scotia Sea after 30 Ma, a state that is suggested to have been driven by a similarly complex pattern of trench migration with both northerly and southeasterly components away from the northern and southern edges of the Scotia Sea. More recent interpretations of these basins are tectonically simpler, taking as their starting point knowledge of the bounding major plate motions. They fall into two time categories. In the younger category, the smaller basins are related to relative motions between an east-moving (and much slower) trench, and the South American plate. Their growth happens to accommodate regional plate divergence that was ongoing in the period between the development of Paleogene continental extensional basins in Tierra del Fuego and the ~30 Ma onset of spreading at the West Scotia Ridge. In the older category, the central Scotia Sea is related to N-S directed Jurassic–Cretaceous divergence of the South American and Antarctic plates in Gondwana breakup and subsequent excision from the South American plate by development of the South Sandwich Trench at a collision zone between oceanic floor of that plate and the continental Antarctic Peninsula