SEDIMENTARY BASIN INVERSION CAUSED BY IGNEOUS UNDERPLATING - NORTHWEST EUROPEAN CONTINENTAL-SHELF

A considerable body of evidence indicates that many of the extensional sedimentary basins in the vicinity of the British Isles underwent per manent exhumation during the Tertiary. The most dramatic indicator of this process is the present-day absence of as much as 4 km of anticipa ted postrift thermal subsidence in basins just north and west of Scotl and. Any explanation of this observation must take into account the be t that the entire region has very small, long-wavelength, free-air gra vity anomalies. This important constraint implies either that the crus t has been thickened or that low-density material has been added to or formed from the lithosphere and rules out models that invoke flexural effects arising from the opening of the North Atlantic. Tertiary epei rogeny is often attributed to compression that is assumed to be relate d in a general sense to Alpine mountain building. However, to remove s imilar to 3 km of sedimentary rock from a basin similar to 100 km wide requires >15 km of shortening. Minor Tertiary compression is observed all over the continental shelf, but nowhere is it sufficient to accou nt for the required amount of uplift and erosion. In addition, exhumat ion dramatically increases from south to north, whereas the observed c ompression decreases markedly in the same direction. At the beginning of the Tertiary, rifting associated with the initiation of the Iceland plume generated substantial volumes of melt. Inversion of rare-earth- element concentrations of MgO-rich igneous rocks suggests that a minim um of similar to 5 km of melt was produced beneath at least part of th e continental shelf. We infer that much of this melt remains trapped w ithin the lithosphere, presumably close to the Moho, which acted as a density filter. Such underplating will generate rapid uplift.