Passive Margin Formation by Low-Angle Faulting Within the Upper Crust: the Northern Bay of Biscay Margin

We use a detailed 6500 km commercial seismic reflection survey over the lower northern margin of the Bay of Biscay to discuss the structure of the thinned lower margin crust. Our study demonstrates that the "S" reflecting horizon, at the base of the tilted block layer, progressively cuts into higher levels of the upper continental crust oceanward. This S horizon is best interpreted as a low-angle normal fault within the upper crust with an original dip toward the continent. We use previously acquired seismic refraction work to demonstrate that the S horizon is the top of a highly thinned high-velocity lower continental crust layer. The crustal layers above and below S both thicken toward the continent. We consequently interpret the lower crust as a ductile zone of decoupling between the upper brittle crust and the upper presumably brittle lithospheric mantle. We present a model which incorporates those features.