SOUTH GEORGIA MICROCONTINENT AND THE SOUTHERNMOST ANDES: UPPER PLATE EXTENSION, MARGINAL BASIN INVERSION, ESCAPE TECTONICS

The mountainous, glaciated island of South Georgia is the crest of one of the most isolated fragments of continental crust on Earth. It is located approximately 1700 kilometres east of the southern termination of the Andean Cordillera of South America. The island is primarily composed of Lower Cretaceous turbidites, the infill of a marginal basin floored by stretched continental and ophiolitic crust. Remnants of a volcanic arc are preserved on offshore islets to the southwest. The Pacific hinterland of the southernmost Andes is missing in Tierra del Fuego, terminating at a submarine escarpment forming the continental margin immediately east of Cape Horn. The arc, marginal basin floor and back-arc basin infill rocks of South Georgia correspond exactly to part of the missing Cordilleran hinterland, the Rocas Verdes marginal basin and South Patagonian arc/batholith. The Fuegian-South Georgian Andes above the Pacific margin subduction zone underwent Late Jurassic – Early Cretaceous extension to form the Rocas Verdes basin followed by mid-Cretaceous compression and basin inversion initiating Andean uplift. The extension reflects the initial fragmentation of the Gondwana supercontinent and continued into the early stages of South Atlantic opening, while the compression and inversion accompanied rapid expansion of the ocean northwards during the 'Long Normal' geomagnetic interval, presumably with increased convergence rate along the Pacific margin. The mechanism of transport of the South Georgia microcontinent eastward relative to South America remains obscure. It likely involved some form of eastward ‘escape tectonics’ analogous to the westward 'orogenic stream' envisaged for the northernmost Alaskan Cordillera. South Georgia's escape was probably initiated during Rocas Verdes basin closure as the Pacific margin arc rotated counterclockwise towards the South American continental margin to form the Patagonian orocline.