The North Iberian or Cantabrian margin, located at the southern flank of the Bay of Biscay, underwent successive tectonic regimes of rifting, passive margin, and compression from Mesozoic to Tertiary times. A complete crustal cross section of the North Iberian Margin, from the abyssal plain to the shoreline, and a reconstruction of its undeformed Upper Cretaceous structure are presented here. It is constrained after the compilation and interpretation of different geophysical data sets recently available in the area. The reflective pattern is provided by deep seismic reflection profile ESCIN‐4, complemented by commercial profiles and well information from the North Iberian platform, and the velocity‐depth control comes from an onshore‐offshore wide‐angle seismic profile, tested also with gravity modeling. The detailed cross section of the Meso‐Tertiary basins that fill the platform and abyssal plain shows structures from the three main tectonic events: (1) normal faults and asymmetric basins from the Permian to lower Cretaceous extensional stage; (2) Upper Cretaceous sediments deposited under stable conditions during the passive margin period; and (3) inverted faults, thrusts and folds related to the Tertiary compression. The deep structure of the crust beneath the margin is poorly constrained from ESCIN‐4 profile that shows reflectivity in the lower crust only at a small area under the platform. However, the corresponding wide‐angle experiment provides a conspicuous image of the Moho that shows a continued deepening toward the continent. On the basis of the geophysical results, we propose a new model for this margin where the lower crust is detached and underthrusted to the south as a result of the partial closure of the Bay of Biscay. An interpreted Mesozoic crustal‐scale extensional detachment could be the weakness zone that allowed the lower crust to “slide” southward under the upper crust, resulting in the indentation of the Cantabrian Margin lower crust into the Iberian crust. Plate convergence stopped at an early stage, making this a unique area to study the initial stages of deformation of a passive margin.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Geology. A B S T R A C TThe 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 (7Њ-8Њ) 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.

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