Rotational velocity for oblique structures (Boltaña anticline, Southern Pyrenees)

Despite the large amount of paleomagnetic and structural studies on fold and thrust belts, many key questions about rotational kinematics remain unsolved (pace of rotation, subsequent accommodation of the hanging wall, etc.). Excellent exposure conditions and syntectonic sedimentation related to the growth of oblique structures in the Southern Pyrenees allow an accurate investigation that sheds light on the rotational kinematics of thrust systems. Fifty-nine paleomagnetic sites (819 specimens) are homogeneously distributed in both limbs along the marine and continental sedimentary sequences contemporary with the uplift of the Boltaña anticline and its post-folding evolution. They indicate a clockwise rotation of about 52° during Ypresian to Priabonian times. Primary, stable (350°-580 °C), pre-folding magnetization in the Eocene rocks is attested by a positive fold test and the antipodality proof, with magnetite and iron sulphide traces as magnetic carriers. Rotation velocity fits a logarithmic model and shows a low rate during the Ilerdian-Middle Lutetian interval (ca. 1°/m.y.) and much higher (up to 10°/m.y.) in the Late Lutetian-Priabonian interval. Most of the clockwise rotation of the Boltaña anticline can therefore be constrained within the 42-35 M.a. interval (partially post-folding), indicating a period of strong differential displacement in the southern Pyrenean Zone, probably related to the emplacement of the underlying External Sierras thrust sheets (Tozal-Alcanadre).