Abstract

The limits of 3 distinct upper Palaeozoic to lower Mesozoic tectono-stratigraphic zones of the ancestral Pacific margin of Gondwana in southern South America are documented. These are: a fore-arc region consisting in part of ‘oceanic’ components; a magmatic arc that is coincident with Carboniferous to Triassic continental sedimentation; and a back-arc region comprising the upper Palaeozoic epicratonic sequences of the ‘Samfrau Geosyncline’. These zones, traceable 2300 km from 29–56°S (Cape Horn), document the semi-continuous subduction of the ancestral Pacific floor from the Middle Devonian to the Triassic. Between 29°S and 37°S the palaeo-subduction zone was roughly coincident with the present Pacific coast line. Between 37°S and Cape Horn, the subduction zone migrated progressively westward as an ‘Alaskan’ style accretionary prism grew to a width of 250 km. While in the northern sector, Jurassic are intrusions are coincident with the upper Palaeozoic are intrusions, in the S they intruded the upper Palaeozoic to lower Mesozoic accretionary prism 200 km W of their Palaeozoic equivalents. No post middle Palaeozoic ‘sutures’ have been identified in southern South America. Hence, there is no evidence of late Palaeozoic, Mesozoic, or Cenozoic accretion of discrete microplates.

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