Structure and tectonic development of the southern margin of the Scotia Sea

LODOLO E.; CIVILE, D.; TASSONE A.

BOLLETTINO DI GEOFISICA TEORICA ED APPLICATA

ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA

Lugar: Trieste; Año: 2010 vol. 51 p. 146 - 147

0006-6729

The southern margin of the Scotia Sea corresponds to the Scotia-Antarctica plate boundary, along which the relative motion between the two plates is mostly left-lateral. Available multichannel seismic reflection profiles (see Fig. 1), integrated with earthquake data, and literature information, have allowed to define the general structural architecture of the margin, and propose a tectonic evolutionary history.             Three main tectonic segments have been identified along the margin, from the Elephant Island to the Herdman Bank. Along the arcuate western segment (from the Elephant Island to the South Orkney microcontinent), seismic data have shown the presence of a buried, scarcely developed accretionary body, and an evident deepening of the oceanic crust beneath the crustal blocks forming the South Scotia Ridge. Along this segment of the margin, the transition between the continental elevated blocks and the oceanic crust is abrupt. The central part of the south Scotia margin is occupied by the northern margin of the South Orkney microcontinent, where a quite developed S-verging subduction zone of the Scotia sea oceanic crust beneath the continental block, is present. The sector to the east of the South Orkney microcontinent till the Herdman Bank shows a very complex structural assemblage, due to the presence of several bathymetric continental highs separated by deep troughs and restricted oceanic basins. An ENE-oriented basin (the Bruce Deep) was found to the E of the South Orkney microcontinent. To the south of the Bruce Deep, a wide deformation zone with N-verging folds and thrusts (here named Jane Thrust Belt), has been identified. The easternmost segment of the plate boundary is structurally the less constrained, and may be composed by a series of tectonic lineaments of different lengths.             Analyzed data in general have shown that in the western sector of the southern Scotia margin, the Scotia oceanic crust seems to have subducted beneath the Antarctic plate, whereas the Weddell Sea subducted beneath Scotia plate, in the eastern sector. The activation of left-lateral transtensional strike–slip lineaments generated narrow pull-apart basins in the eastern sector in correspondence of the fore-arc of the convergent zones. These evidence suggest that the southern Scotia margin may represent an example of an opposite subduction polarity environment, for many aspects similar to the tectonic setting of the central sector of the northern Carribbean margin where a double subduction with N- and S-vergence is documented in correspondence of the Hispaniola Island.             Three main tectonic phases have been recognized in the deformational history of the Scotia-Antarctica plate boundary. The first phase (Lower Miocene) was characterized by a north-directed convergence of the Weddell Sea beneath the series of bathymetric highs now distributed along the south-eastern part of the Scotia Sea. A second phase (about 12 Ma) was characterized by the presence of the two possibly coeval and opposite-vergence subduction zones of the Scotia Sea and Weddell Sea oceanic crust. The subduction of the Scotia Sea is testified by the presence of some small and scarcely developed accretionary prisms. In the eastern zone the system of NNE-trending dextral transform faults was active and separated zones with different direction or rate of movement. These faults dismembered and partly deactivated the subduction zone and facilitated the process of fragmentation and dispersion of the crustal blocks. During this phase, an accretionary prism (Jane Thrust Belt), formed to the south of the Bruce Bank due to a presence of a S-verging subduction zone. The final phase was characterized by the deactivation of the subduction zones and by the activation of left-lateral strike-slip regional lineaments. These structures generated several narrow pull-apart basins in the eastern sector of the Scotia margin, while an abrupt contact between continental and oceanic crust, without the presence of a transitional crust, is observed along the western margin.