Comparing magnetic fabrics in in-basin and out-of-basin domains: The Mesozoic western Cameros Basin (N Spain)

Anisotropy of magnetic susceptibility (AMS) is a very sensitive and accurate technique for determining deformation in rocks that show no other markers of deformation. In the Mesozoic-Cenozoic history of the Iberian plate, two stages have been clearly defined: an extensional period in which large sedimentary basins were generated (Late Jurassic-Early Cretaceous), and a tectonic inversion stage during the Cenozoic compression. However, details on the development and evolution of these stages, as well as the transition from extensional to compressional regime are not yet well understood. The western Cameros Basin provides the opportunity for testing the applicability of magnetic fabric to determine the tectonic regime at the transition between the syn-rift and the post-rift sequences in an area close to the out-of-basin domain and therefore, suitable for recording the far-field and the near-field constraints. Furthermore, the influence of the subsequent inversion on the magnetic fabrics in these two sequences can be also discussed. In this work, we have focused on the Jurassic (mostly limestones), Lower Cretaceous (mostly shales) and Upper Cretaceous (mostly limestones) rocks, taking a total of 575 samples from 47 sites. AMS results show the record of the Alpine compressional stage for most of the sites of the post-rift materials, while the syn-rift materials record a NE-SW stretching direction. This indicates that the extensional tectonic imprint remains in in-basin domains even after compression (and probably layer parallel shortening) provided that the intensity of extensional deformation is significant (i. e. linked to basin formation). Conversely, out-of-basin domains generally record compressional fabrics, both in the thinned syn-rift and post-rift sequences. Another key point in this work is the analysis of the out-of-phase anisotropy of magnetic susceptibility, which has allowed to improve to some extent the definition of the magnetic fabric in the, a priori, unfavorable lithologies for AMS studies, such as the limestones of the Jurassic and Upper Cretaceous.