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In the northeast Atlantic, DSDP drilling results, combined with intensive geophysical surveys, permit a proposed model of the structural evolution of a starved, passive continental margin. Environment and tectonics of the rifting phase have been established. Active rifting took place in Early Cretaceous time in a pre-existing marine basin in contrast to many subaerial rift systems. The overall tectonic style is characterized by a series of tilted fault blocks bounded in many cases by listric faults. The rotation of the blocks (20-30°) along listric faults reduced the thickness of the upper continental crust from 6 to 8 km to 4 to 5 km. Close to the near horizontal base of the listric faults, a strong horizontal reflector corresponding to the 6.3 to 4.9 km/s refraction interface has been interpreted as the boundary between the upper brittle and the lower ductile continental crusts. The Moho discontinuity, 25 km deep in the vicinity of the shelf break, is 12 km deep in the lower part of the margin. In this area the ductile part of the crust (6.3 km/s) is only 3 km thick. Drill, dredge, and seismic reflection data allow reconstruction of the topography of the sea floor at the end of rifting in Aptian time. In the axis of the rift system, submarine troughs 2.5 km deep existed. The mechanism of riftingis discussed. The thinning of the continental crust cannot beexplained by the 10 to 15 per cent of extension estimated for the upper brittle part. It is suggested that the ductile part of the crust is thinned by creep in response to tension in the continental plate. Knowing the topography of the sea floor at the end of rifting and the present depth of the Aptian datum, the absolute amount of subsidence can be determined on a transect of the margin after the beginning of accretion (late Aptian time). This value decreases continuously from the oceanic/ continental crust boundary (4000 m) to the shelf break. For each point of the margin, the subsidence versus time curve is an exponential, the time constant of which increases with depth. The post-rifting subsidence is essentially an isostatic adjustment to cooling of the lithosphere in which the continental crust previously has been thinned during the rifting process.

Rifting and Subsidence of the Northern Continental Margin of the Bay of Biscay