Anisotropic magnetic susceptibility record of the kinematics of the Boltaña Anticline (Southern Pyrenees)



Anisotropic magnetic susceptibility (AMS) analysis of 1454 samples evenly distributed along an 1800 m section in the South Pyrenean Zone is presented. The sampled rocks are pre- and syn-tectonic limestones, marls and sandstones of Ypresian-Lutetian age. They are weakly deformed rocks sampled from both limbs of the N–S trending Boltaña Anticline (oblique to the WNW–ESE main Pyrenean trend). Rock magnetism analyses, including low temperature and high field magnetic susceptibility measurements confirm a dominant paramagnetic signal and hence allow the interpretation of AMS in terms of preferred rock fabric or finite strain orientation. Therefore, this study was focused on obtaining an independent kinematic information from a vast set of AMS data from particular structural positions. The three directional maxima for the maximum axis of the magnetic ellipsoid (KMAX, magnetic lineation) can be tentatively related to major tectonic events in the Central Pyrenees. A first stage (recording a N–S magnetic lineation) is related to an oblique ramp of the Cotiella Thrust, emplaced during Ypresian times. A pseudo-progressive variation turns into a second stage (linked to a NW–SE KMAX orientation maximum) that can be related to Pyrenean compression. Both stages witness a far-field imprint of active structures on the foreland sediments. Once the Boltaña Anticline was nucleated (during the Lutetian), a predominant, more constant NW–SE trend is observed, this near-field magnetic fabric being the only lineation imprinted in the rocks. A final stage arrived in the course of vertical axis rotation during the Late Lutetian-Bartonian. This latest event was responsible for the present configuration of the Boltaña Anticline (N–S). Copyright © 2010 John Wiley & Sons, Ltd.