Origin and evolution of the Burgalesa Platform salient (Western Basque Pyrenees, northern Spain): Preliminary paleomagnetic results

The Basque Pyrenees involves a large and thick Jurassic to Upper Cretaceous basin (the Basque Cantabrian Basin) that was inverted during the building of the Pyrenees. Its southern frontal structure, detached at Upper Triassic evaporites, is a roughly E-W trending major thrust which defines a broad thrust salient with the eastern and western edges trending NE-SW and NW-SE respectively. To the east, the footwall of this Basque-Cantabrian thrust front corresponds to the Ebro foreland basin. However, to the west another structural unit occupies an intermediate position between the foreland and the main Basque-Cantabrian thrust front. This unit, known as Burgalesa Platform, also shows a thrust salient concave to the north, although asymmetric and more pronounced than the previously described one. It developed during the Oligocene and Early Miocene times and inverted an Upper Jurassic-Lower Cretaceous extensional basin. In order to better understand and constrain the evolution of these thrust salients and the role played by the initial configuration of the rift basins, a paleomagnetic study focused on identifying and quantifying vertical-axis rotations has been carried out. This study includes 62 sampling sites, with a mean of 10 samples per site, widely distributed in the Burgalesa Platform Domain and surroundings. The sampled materials include Lower Cretaceous fluvio-deltaic fine grained sandstones, Upper Cretaceous marine limestones and marls and Cenozoic syn-orogenic and post-orogenic fluvio-lacustrine red clays and limestones. Laboratory procedures include stepwise thermal demagnetization and subsequent measurement of the NRM up to 680°C, IRM acquisition and 3 axis IRM demagnetization. In addition, fold tests have been performed in order to test the stability of magnetization. IRM measurements reveal that hematite is the main remanence carrier in the Lower Cretaceous sandstones and the Cenozoic red clays, whereas (titano)magnetite dominates in the Upper Cretaceous marls and limestones. Characteristic components have been used to calculate the mean directions at site level revealing significant clockwise and counterclockwise vertical-axis rotations within the bended tips of the thrust salient. The preliminary results of this study denote that the present salient shape of the Burgalesa Platform records a complex evolution with the inversion of pre-existent Mesozoic arched structures and vertical-axis rotation during the early stages of the Pyrenean contractional deformation of salt-cored folds developed during the Mesozoic extension.