Gravity and geomorphological evidence for a large crustal bulge cutting across Brittany (France): a tectonic response to the closure of the Bay of Biscay

Moho undulations from the analyses of the gravity anomalies over Brittany c omputed by incorporating various improvements in the classical filtering te chnique have been compared with deep seismic refraction data. By progressiv ely low-pass filtering the gravity anomalies with increasing cut-off wavele ngths, some distinct new anomaly trends associated with deer structures hav e become prominent. Comparisons with heat flow and wide-angle seismic refle ction results confirm that the sources of these anomalies are located at th e lowest part of the crust. In Brittany, the image of the Moho undulations reveals the presence of a pr ominent N130 degrees Moho downward cutting across the superficial geologica l trends. On the surface, this trend corresponds to an active topographic u plift and indicates that the deep crustal bulge is still building up. This structure is bounded by N130 degrees dextral faults that are known to accom modate the actual closure of the Bay of Biscay. West of Brittany, this shor tening is also at the origin of an east-west oriented bulge previously mapp ed with the same technique. This bulge limits and postdates the subsidence of Mesozoic and Cenozoic basins of the Celtic Sea. (C) 2000 Elsevier Scienc e B.V. All rights reserved.