Tectonics and sedimentary environment of the North Scotia Ridge region revealed by side-scan sonar

ALEX P. CUNNINGHAM1, PETER F. BARKER1 & JEREMY S. TOMLINSON 1,2

Abstract: The North Scotia Ridge is a series of islands and submarine ridges extending 2000 km from Tierra del Fuego to South Georgia in the western South Atlantic. The ridge forms the elevated northern tectonic margin of the Scotia Sea, and accommodates E-W sinistral strike-slip motion at the South American Scotia plate boundary. Existing studies have shown that the northern flank of the North Scotia Ridge is a large and continuous accretionary prism, formed during presumed mid late Cenozoic N-S convergence. In this study, we present long-range side-scan sonar (GLORIA) images and seismic reflection profiles which show the structural style of the accretionary prism for the first time. The youngest accreted sediments show a uniform fabric of initial deformation (symmetric-gently asymmetric folds of 14 km wavelength), which has been subsequently disrupted at shallower depths by additional shortening and uplift. Between 52deg45'W and 50deg30'W, the deformation front is exposed at the sea floor, and the Falkland Trough retains the appearance of an active convergent margin. Elsewhere, however, the deformation front is buried beneath younger, undeformed drift sediments indicating that convergence has ceased. GLORIA sonographs also show geological features consistent with current-control of sedimentation, nondeposition, and erosion beneath the Antarctic Circumpolar Current. In particular, this study describes current-influenced sedimentation in the Falkland Trough, and steep-sided, eroded depressions and diffuse slope-parallel fabric on the elevated Falkland Plateau.

Keywords: GLORIA, Scotia Ridge, South Atlantic:, accretionary wedges.

Studies of arc accretion have shown that arc-continent collision may lead to a cessation of convergent deformation in an accretionary prism, and relocation of the zone of convergence elsewhere. The Banda arc collision zone provides an example of this process (Genrich et al. 1996; Snyder et al. 1996), where the locus of convergence between the Australian and SE Asian plates may have moved from a site south of Timor to the Wetar thrust after collision. This study describes the similar development of the North Scotia Ridge in the western South Atlantic (Fig. la), where mid-late Cenozoic north south convergence of the South American and Scotia plates led to collision of an E-W limb of the Scotia arc with the partly continental Falkland Plateau. Here, GLORIA and seismic reflection profiles show clearly that convergence has ceased, and reveal the style of pre-collision deformation. The Banda arc remains in a convergent tectonic setting, but collision of the North Scotia Ridge with the Falkland Plateau has given way to east west strike-slip motion some 50 100 km south of the original deformation front.

Tectonic setting

The North Scotia Ridge consists of a series of islands and submarine ridges extending 2000 km from Tierra del Fuego to South Georgia, in the western South Atlantic (Fig. la). Submerged parts of the ridge have appreciable bathymetric relief: summit regions lie above 1000 m, and slopes exceed 15deg along its southern margin (BAS 1985). The North Scotia Ridge is bordered to the N by the Falkland Trough, an E W bathymetric deep extending from the South American continental margin to the Malvinas Outer Basin, and to the S, by ocean floor exceeding 3000 m depth. The southern flank of the ridge forms the northern topographic boundary of the Scotia Sea.

The North Scotia Ridge is a component of the Scotia arc, an eastward-closing loop of submarine ridges and islands which connects the Andean Cordillera of South America to the Antarctic Peninsula, and encloses the Scotia Sea (Barker & Dalziel 1983; Barker et al. 1991). The ridge is principally composed of small, discrete crustal blocks, partly overlain and flanked to the N by an appreciable thickness of sediment (e.g. Ludwig et al. 1968; Davey 1972). The largest crustal blocks (Burdwood Bank, the Shag Rocks block and South Georgia, located in Fig. la, c) are continental in origin, and once formed part of a continuous continental connection between southern South America and the Antarctic Peninsula (e.g. Dalziel et al. 1975; Tanner 1982; Barker & Dalziel 1983, and references therein). However, the origin of smaller blocks identified between 55degW and 44degW remains uncertain.

Tectonic reconstructions (Barker & Griffiths 1972; Hill & Barker 1980; Barker et al. 1984, 1991) suggest that the modern North Scotia Ridge formed in connection with the eastward growth of the Scotia Sea during the Cenozoic. In these models, the dispersal of crustal blocks along the Scotia Ridge was accomplished by back-arc spreading (between 28 and 6 Ma), coupled to westward subduction at ancestors of the South Sandwich Trench. The initiation of westward subduction, and subsequent growth of the Scotia Sea, are viewed as complications of otherwise simple South American-Antarctic plate motion.

North Scotia Ridge accretionary prism