Back-arc extension in the Scotia Sea

Abstract

The nature of back-arc extension in the East Scotia Sea is re-examined with the use of an enlarged geophysical data set. Well developed oceanic magnetic lineations confirm that the present spreading episode started about 8 Ma ago, that spreading is asymmetric, and that the total rate increased from 50 to 70 m m /a about 1.5 Ma ago. Most of the currently active South Sandwich volcanic island arc lies upon ocean floor only 6-8 M a old and generated at the current spreading ridge. Subsequent extension has not modified the curvature of the arc. East-west magnetic lineations of Miocene age in the Central Scotia Sea and contemporaneous low-K arc tholeiites dredged from the eastern South Scotia Ridge (Discovery Bank) indicate a regime of coupled subduction and back-arc extension preceding that occurring now. A speculative model involving a series of collisions of parts of this earlier Discovery trench with ridge crest sections of the South American-Antarctic plate boundary explains the transformation of this earlier regime into the present, self-contained Sandwich plate regime. The considerable small-scale variability observed in the back-arc region may be seen as an inevitable consequence of the action of the ridge-trench collision mechanism. The entire Scotia Sea could have formed by a similar kind of back-arc extensional modification of the South American-Antarctic plate boundary.

The tectonic regime along the Andes: Present‐day and Mesozoic regimes

“…The South Sandwich Arc is entirely intra-oceanic, far-removed from any continental crust and, on the basis of magnetic anomalies, is situated on 10 Ma old oceanic crust of the Sandwich Plate (Barker and Hill 1981). A new multichannel seismic reflection survey identified arc-ward tilted blocks in the mid-forearc indicating large scale gravitational collapse, consistent with earthquake data indicating normal extension at shallow depth in this area (Vanneste et al 2002).…”

Collisional model for rapid fore‐arc block rotations, arc curvature, and episodic back‐arc rifting in subduction settings

“…Moreover, rifting rates on the east Scotia Ridge remain fairly constant along its length (6-7 cm/a) supporting the assertion that the Sandwich Plate rotates about a distant pole relative to the Scotia Plate [Thomas et al, 2003]. The dominant influence on Scotia Basin rifting is likely to be rollback of the subducting South American slab [Barker and Hill, 1981;Barker et al, 1991]. The slab subducting beneath the South Sandwich arc is quite narrow ($600 km wide, in the alongstrike direction) in comparison to other subducting slabs which are typically 1000s of km wide.…”

“…6 Ma ago (coinciding with reinitiation of Nankai Trough subduction [Kamata and Kodama, 1994]) [Barker and Hill, 1981] No [Skerlec and Hargraves, 1980] Bering Sea North of western AleutiansEocene-Oligocene/Miocene [Worrall, 1991] [Zhou et al, 1995] None apparent None apparent Uncertain South Fiji Basin Australia/New Zealand region ( Tasman Sea) 25 -34 Ma ago [Watts et al, 1977;Malahoff et al, 1982b] None apparent None apparent…”

Anomalous geomagnetic variations on the island of South Georgia.

“…The distribution of plate boundaries in the region together with their motion is summarised by BARKER and DALZIEL (1980) and by BARKER and HILL (1981) as shown in Fig. 1.…”

“…The Scotia Sea is thought to have formed over the past 40Ma as an increasingly intricate modification of the South American (SAM) and Antartic (ANT) plate boudaries. Marine geophysical investigations (BARKER, 1972;BARKER and HILL, 1981) have shown that active sea-floor spreading is taking place some 440km west of the South Sandwich Trench. Such back-arc extension has given rise to the Scotia and Sandwich plates as defined by BARKER and DALZIEL (1980).…”