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Seismic and paleomagnetic constraints on the development of the Burgalesa Platform salient (Western Basque Pyrenees)

Abstract

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 Mesozoic rift basins, interpretation of seismic lines and a paleomagnetic study have been carried out. The seismic data are located in the Burgalesa Platform and in the adjacent Ebro and Duero foreland basins. The paleomagnetic study was focused on identifying and quantifying vertical-axis rotations from 62 sampled sites, with a mean of 10 samples per site, widely distributed in the Burgalesa Platform 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 measurement of the NRM, 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 results of this study denote that the present salient shape of the Burgalesa Platform and its associated foreland basins records a complex evolution linked to the inversion of pre-existent Mesozoic arched structures. Our interpretations indicate that salt-cored folds, developed during the Mesozoic extension, were later deformed during early stages of the Pyrenean contraction. This inherited configuration limited the southward displacement forcing the southeast lateral extrusion of the Burgalesa Platform.