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.