A paleomagnetic study of the Basque Arc (Basque-Cantabrian Basin,Western Pyrenees)

The Basque Arc constitutes the northern segment of the Basque-Cantabrian basin, in the western part of the Pyrenees. The main goal of the present study was to find out by means of paleomagnetic analysis if the arched shape of the Basque Arc has a primary origin, due to the development of sedimentary basins related to the opening of the Bay of Biscay or a secondary origin due to rotations about vertical axes. Nine volcanic flows of late Albian to Santonian age (100–83.5 My) were sampled together with 10 sedimentary sites (marls, limestones, calcarenites and sandstones) of lower Jurassic to early Eocene age in order to carry out paleomagnetic and rock-magnetic experiments, 15 paleodirections being obtained. In order to analyse these results together with data from previous studies, the studied area was subdivided into a western, a central and an eastern sector. While inclinations of all three sectors show a similar value, declinations differ. The western sector displays a 37 ± 16° clockwise rotation of its mean paleodeclination, the central sector is not rotated (4 ± 9°) and both sedimentary sites which make up the eastern sector show counter-clockwise rotations (−25 ± 11° and −68 ± 9°). These results suggest that the shape of the Basque Arc does not have a primary origin, but a secondary origin due to rotations about vertical axes as a result of differential shortening related to post-Lutetian compressive tectonics which resulted in the formation of the Pyrenees.