AAPG Annual Convention and Exhibition

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Volcanic Continental Margins and the Formation of Super Volcanoes: Examples from Argentina, Uruguay and Brazil

Abstract

The volcanic extensional margin of southern South America is characterised by small sediment-filled rifts which are limited to a narrow zone of 20-40 km width and only 1-2 km of fill. During early Cretaceous rifting, the crust underwent a limited amount of extension (ß<2) after which major magmatic underplating and volcanic eruptions filled the extending accommodation space instead of sediment. This produces spectacular seaward dipping reflector sequences. These are interpreted to be the result of magmatic chamber collapse and eruption of vast quantities of volcanic material. Where the mantle is anomalously hot and crust is actively extending, this creates favourable circumstances for super volcanoes to develop. Three such structures have been identified on seismic lines shot by ION GeoVentures along the South American margin. The largest super volcano in the Pelotas Basin covers an area of 225,000 km and consists of at least three overlapping collapsed calderas. The Pelotas super volcano produced internal reflections similar to seaward dipping reflector sequences that are 20 km in thickness and occupy most of the crust. The moho is visible only several kilometres (1-3 km) below the layered reflectors which are interpreted as lava flows. The crust is substantially thicker (25 km) below the super volcano compared to the adjacent stretched crust (15 km). The central areas of the volcanoes are non-reflective and interpreted to be frozen magma chambers which are now intrusive gabbro/diorite bodies. This talk will present seismic data which provide unique insights into the development of seaward dipping reflectors on volcanic margins and for the first time illustrate the internal structure of super volcanoes.