The switch in subduction polarity at the transition from the Pyrenees to the Cantabrians implies an abrupt change in the gross direction of plate movement. The Pyrenees formed in response to the northward subduction of continental crust beneath Europe during the Palaeogene whereas the Cantabrians developed during the southward subduction of Biscay oceanic crust beneath Spain. As yet, no attempt has been made to explain the mechanism and geometry of this apparent switch in plate kinematics. The transition from northward to southward subduction occurs across a discrete lineament, the Pamplona Fault, oriented transversely with respect to the long axis of the adjacent mountain belts. Regional mapping reveals a 180° switch in the facing direction of thrusts and associated folds across the Pamplona Fault. Balanced structural cross-sections, constructed either side of the Pamplona Fault, exhibit marked contrasts in bulk shortening, depth to detachment, gross structural geometry, and the thickness and completeness of the pre-orogenic Mesozoic succession. The Pyrenean-Cantabrian transition zone is thus envisioned as a fundamental crustal lineament of pre-orogenic origin.

A new model for the Cenozoic tectonic evolution of north Spain has the Pamplona Fault separating a series of transtensional basins in the Pyrenees from a passive margin in the Cantabrians. Subsequent plate collision between Spain and Europe reactivated the faults that controlled pre-orogenic 'syn-rift' sedimentation in the present Pyrenean area. In the Cantabrians, however, plate collision signalled a shift from a passive margin to an active margin consuming Biscay oceanic crust beneath a northern Spain subduction zone. An implication of this model is that the foreland crust was affected variably by 'backarc' heating during the early stages of Spain-Europe collision. This is expressed by the changing depth of the Ebro Basin, the foreland basin that developed in response to Pyrenean-Cantabrian orogenesis, whose thickness of the Tertiary succession varies laterally according to changes in flexural rigidity.

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