An early miocene ridge crest-trench collision on the South Scotia Ridge near 36°W

Preliminary interpretation of marine geophysical data and geochemical analyses of dredged rocks has led to the identification of a ridge crest-trench collision zone southeast of the South Orkney Islands. The existence of a series of such collision zones along the South Scotia Ridge had earlier been hypothesised to explain changes in Scotia Sea evolution. The collision zone takes the form of a double ridge with dissecting trough, aligned NE-SW and about 200 km long. Oceanic magnetic lineations in the northern Weddell Sea young northwestward towards the zone, with the youngest (identified as anomaly 6, −20 Ma) reaching the central trough. Volcanics dredged from Jane Bank, the northwestern ridge, are chemically very similar to the more siliceous of the low-K tholeiite series which characterises the presently active South Sandwich arc. Jane Bank is interpreted as an in situ remnant of the arc and upper fore-arc of an intra-oceanic arc produced by subduction of South American oceanic lithosphere (like the present South Sandwich Islands). Part of the upper fore-arc and all of any accretionary wedge appear to have been subducted. Unlike ocean floor at collision zones off Chile and the Antarctic Peninsula, the seabed in the trough and on the ridge to the southeast does not conform to the normal oceanic age-depth relationship. The estimated collision time (20 Ma) is close to the time of onset of N-S extension in the Central Scotia Sea. The limited data set described here, however, does not establish the synchroneity of the collision over the entire 200 km of Jane Bank, so an assessment of the extent of any causal relationship between the two events must await the analysis of a more extensive survey.

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