Deep structure of the Aquitaine shelf: constraints from expanding spread profiles on the ECORS Bay of Biscay transect

Abstract

This paper describes the analysis and interpretation of six Expanding Spread Profiles (ESP) which were shot approximately perpendicular to a 300 km long vertical reflection profile along the eastern continental margin of the Bay of Biscay (Aquitaine shelf) by the French ECORS program in association with Hispanoil. This transect crosses various tectonic features of different ages: the Armorican shelf, the Parentis basin and the Cantabria shelf. Velocity—depth models have been derived from the ESPs by the combination of two complementary methods using time-distance and intercept-slowness domains. They provide important constraints for the analysis of the vertical reflection data. The velocities allow definition of crustal layering with a 5.8-6.2 km s −1 upper crust and a 6.5–7.1 km s −1 lower crust. This layering matches the change of reflectivity observed on CDP data with a relatively transparent upper crust and upper mantle in opposition to a highly layered lower crust. Important variations of the thickness of these two layers are revealed by this study. The most important one occurs beneath the Parentis basin with a 15 km shallowing of the upper mantle, the velocity distribution suggesting that major crustal thinning has taken place at the cost of a large part of the lower crust.