A complete arc-trench system recognized in Gondwana sequences of the Antarctic Peninsula region

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Abstract
Summary: Prior to Late Triassic–Early Jurassic times, the geological history of the Antarctic Peninsula region was dominated almost entirely by Gondwana sequences that together comprised a major arc-trench system. Subduction complex, trench-slope-break and fore-arc basin sedimentation can all be recognized, and deposition was at least partly on early Palaeozoic or older continental crust. The only evidence for a contemporaneous magmatic arc situated in the Antarctic Peninsula at this time consists of patchy occurrences of metavolcanic rocks, possibly representing the frontal edge of the arc, and the major outcrop area of these rocks is believed to lie under the broad shallow continental shelf E of the Antarctic Peninsula. This is contrary to most current hypotheses in which a marginal basin, presumably floored by oceanic crust, is thought to crop out close to the E coast of the Antarctic Peninsula. However, the complete absence of substantial outcrops of pre-Jurassic volcanic rocks anywhere in eastern Antarctica and South Africa, which are the closest and most likely places in which these should exist, supports the new proposal. Moreover, the identification of back-arc elements of the arc-trench system (foreland fold-thrust belt and retro-arc foreland basin) in eastern Antarctica and South Africa greatly strengthens the likelihood of the arc cropping out in the area suggested.Towards the end of the Triassic and during the Early Jurassic Periods, an intense diastrophic event, or culmination of events, of orogenic magnitude occurred (Gondwanian orogeny), causing substantial redistribution of the pre-existing elements of the arc-trench system. In particular, all the fore-arc sequences were strongly deformed, some possibly for the first time (e.g. in the fore-arc basin), and became firmly accreted to the continental margin. Moreover, the magmatic foci migrated trenchwards to intrude the deformed rocks in the Antarctic Peninsula. Because many of the plutons were emplaced synkinematically, they often closely resemble ‘basement’ gneisses. The distinction between these rocks remains a serious problem in the Antarctic Peninsula and it can be solved only partially by radiometric dating.