Clues to the deep structure of the European Variscides from crustal seismic profiling in North America

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Abstract
COCORP data from two areas of North America where late Palaeozoic deformation occurred provide clues to the deep structure of the Variscides of Europe. These areas are: (1) the southern Appalachians, where deformation culminated in the Alleghenian orogeny, and (2) the Ouachita Mountains, formed during the Ouachita orogeny. The COCORP data show deep structures which were probably formed during collision and overriding of the early Palaeozoic edge of the North American Grenville basement. Two types of reflector sequences are characteristic: (1) subhorizontal or gently dipping reflectors defining the base of crystalline or sedimentary nappes, which in the interior of the orogen pass into: (2) more steeply dipping (25–40°) zones of reflectors, 10–20 km thick, that are possibly offshelf, basinal facies metasedimentary rocks, and slices of basement, stacked and imbricated against, and hence marking the edge of, the North American continent. Similar reflector sequences are observed in seismic data recorded from the European Variscides, suggesting that here similar processes occurred at the southern edge of the Old Red Sandstone continent. Changes in the character of the Variscan front can perhaps be explained in terms of the extent of overthrusting of the continental edge (i.e. the extent of nappe formation).