The Fossil Accretionary Wedge of the Bay of Biscay: Critical Wedge Analysis on Depth-Migrated Seismic Sections and Geodynamical Implications

摘要

The accretionary wedge of the Bay of Biscay is an east-west compressive belt buried under recent sediments of the abyssal plain at the north Iberian margin. This structure formed through the partial closure of the previously extended Biscay basin during the Cenozoic north-south collision between Europe and Iberia, the same collision that produced the Cantabrian-Pyrenean range on land. Three north-south seismic sections have been prestack depth migrated, showing a narrow-tapered wedge (7°-8°) whose internal structure corresponds to a set of south-dipping thrusts converging toward a basal decollement. There are differences along strike within the wedge: thrust spacing, the dip of the basal thrust, and the thickness of the sediments at the trough augment toward the east, increasing its overall size. The two-dimensional velocity models obtained through migration analysis reflect values between 2000 km/s at the sea floor (4500 m) and 5000 km/s at 12-km depth. The syntectonic package thickness varies from 1.5 to 3 km, while the posttectonic cover attains 1.5-2 km. A simple analysis based on critical wedge theory approaches suggests that the Biscay wedge formed in similar conditions to active submarine wedges, the strength of the decollement being lower than the strength of the wedge itself. Further comparison with other examples indicates high basal stress, which could be an added factor in the convergence stopping at this margin. The eastward size increase is attributed to the provision of extra sediments by the coetaneous rising of the cordillera on land. This weight steepens the basal angle without affecting the overall taper. Surprisingly, the eastward change from an oceanic to a transitional basement does not seem to be crucial in its geometry.