Early Cretaceous syn-rotational extension in the Organya basin-New constraints on the palinspastic position of Iberia during its rotation

In this study we interpret the paleo-stress pattern in the Organya Basin (southern Pyrenees, northern Spain) as inferred from the anisotropy of magnetic susceptibility (AMS) of 39 sites distributed over the entire basin. Combined with information from other Cretaceous Iberian basins, such analysis adds to constrain kinematic reconstructions of the Cretaceous rotation of Iberia allied with the opening of the Bay of Biscay and the northward propagation of the North Atlantic. The Organya Basin is an inverted Cretaceous basin in the hanging wall of the Boixols thrust. The lithologies are mainly weakly deformed pelagic and hemi-pelagic limestones and marts which recorded the Aptian 35 degrees counterclockwise rotation of Iberia. Three types of AMS fabrics could be distinguished. all representing typical intermediate and tectonic fabrics. EW magnetic lineations dominate in the eastern part of the basin and are related to crustal shortening during the Pyrenean orogeny. This interpretation is consistent with structural cross-sections across the basin showing more intense shortening in the east. In the central part of the basin, approximately NS oriented magnetic lineations are observed, interpreted as the original extensional direction during basin foundering. So. in line with results from previous studies, AMS can still unveil the original extensional direction in an inverted sedimentary basin, something which may be difficult to reconstruct from geological data alone. The original extension direction in the Organya Basin is perpendicular to the Boixols thrust bounding the basin to the south. Correction for the Aptian rotation of Iberia leads us to infer a NE-SW oriented extension direction at the onset of the rotation of Iberia. This extension direction is inconsistent with current plate kinematic reconstructions of the Cretaceous rotation and motion of Iberia. We therefore suggest that, during the opening of the Bay of Biscay and related Iberian rotation, Iberia was in a much more westerly position than assumed in current models. and that the rotation of Iberia was followed by dominantly eastward translation with respect to southern France, prior to N-S convergence and shortening in the Pyrenees since the latest Cretaceous. (c) 2009 Elsevier B.V. All rights reserved.