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The Cameros basin (Iberian Chain) was developed under a very subsident extensional regime during the latest Jurassic-Early Cretaceous. Its sedimentary fill constitutes a megasequence of more than 5000 m of vertical thickness which contains six depositional sequences. Most of the sediments are continental (alluvial and lacustrine) with only minor marine intercalations. A lateral accretionary geometry at the basin scale shows a SSW-NNE migration of the depocentre and an onlap to the north of the sedimentary sequences on the Mesozoic substratum of the northern boundary of the basin.The overall Mesozoic structure of the basin, as seen from field studies and seismic profile interpretation, is a gentle WNW-ESE synclinorium 30-70km wide and 150kin long. Extension in the pre-basin Mesozoic rocks is very small. The basin boundaries are defined by an unconformity of the basin filling rocks on the former Mesozoic substratum; it is bounded only locally by major faults at surface. The basin is interpreted as an extensional-ramp basin produced over an S-dipping ramp in a blind, flat-lying extensional fault some kilometres deep in the basement. The extensional displacement on the fault produced a synclinal basin which progressively widened with time. The depocentres of the successive depositional sequences were always located above the ramp and migrated to the north, inside the basin, as a result of the hangingwall displacement to the south. From a computer-modelled cross-section, we estimate the total displacement on the extensional fault to be about 33 km. This extension, in the hangingwall, would have taken place in the Pyrenean basin, north of the Cameros basin.Tertiary compression (Palaeogene to Lower-Middle Miocene) inverted the basin by means of a newly formed E-W north-directed thrust. Overthrusting in the Tertiary Ebro basin fill, this new fault formed in the Keuper beds, in a weakness zone with uniform dip within the extensional hangingwall (about 30 km in cross-section). During the deformation it expanded to the north and south until it branched to the Iberian sole thrust, which might coincide with the Mesozoic extensional sole fault. Although this thrust produced a slight (2-3 km) inversion of the basement-Mesozoic cover unconformity, its maximum displacement was of about 28km. Pre-basin Mesozoic rocks overlie the thrust surface and are overlain by the basin-filling rocks. A south-directed imbricate-fan thrust system developed in the southern margin. The total shortening was about 38 km, leading to the complete inversion of the basin.

The Iberian Chain developed within the Iberian plate during the Palaeogene and Early Miocene as a result of convergence between the African and Eurasian plates. It is a fold-and-thrust belt, which involves the Hercynian basement and the Mesozoic and Cenozoic cover. A generalized cross-section of the NW part of the Chain, of 195 km length, is presented. Mesozoic basins, developed on the Hercynian basement, display thickness variations across normal faults that bounded them and that can be recognized in the field. The Tertiary contraction deformed and inverted the Mesozoic basins, and it is inferred to have produced a thrust sheet about 5 km thick in its frontal (northeastern) part, which thickens to the SW. The total Tertiary shortening in the section is 66.6 km (26%). The structure and the crustal shortening and thickening of the Iberian Chain are explained by a major upper-crustal thrust system with simple flat-and-ramp geometry, which may branch to the Pyrenees or the Betics. This is combined with internal deformation of the thrust sheet. The contribution of the Iberian Chain shortening to the convergence of the African and Eurasian plates during the Tertiary is about one-half of that of the Pyrenees, and should be taken into account in any reconstruction of the kinematics of these plates.

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