The Tertiary Almaz¿n Basin was a topographic high during the Mesozoic rifting at the northern part of the Iberian plate. It was bordered by two Mesozoic basins: (1) the Castilian Basin (with a maximum thickness of more than 2000 m) to the south and (2) the Cameros Basin (with a maximum thickness of 9000 m) to the north. To the east the Almaz¿n Basin is bordered by Paleozoic rocks of the NW-SE trending Aragonian Range, which underwent little subsidence during the Mesozoic. The two Mesozoic basins show an E-W direction. During the Tertiary NNE-SSW compression (mainly late Eocene-early Miocene), both Mesozoic basins were inverted and uplifted, and the Almaz¿n Basin, with an overall synclinal geometry, was filled with syn-tectonic terrestrial sediments, reaching a maximum thickness of 3500 m. Deformation during the Tertiary was strongly controlled by preexisting structures, both Mesozoic extensional faults and reactivated Variscan folds. The depocenter of the Tertiary basin migrated southward during the Paleogene. This migration is consistent with a piggyback transport of the whole basin northward, due to the movement on the underlying, crustal-scale north verging Cameros thrust. The geometry and sedimentary evolution of the Almaz¿n thick skinned piggyback basin was controlled by the uplift associated with the main crustal-scale thrust, which was the main source area, and was separated from the foreland basin since the early stages of filling (late Eocene). This makes the main difference with thin-skinned piggyback basins, controlled by the foreland thrust belt and separated from the foreland basin only at their late stages. ¿ 2000 American Geophysical Union