Rotational kinematics of a curved fold: The Balzes anticline (Southern Pyrenees)

Abstract

This study deals with how vertical-axis rotations are accommodated in thrust fronts. Studies on the rotational kinematics of fold and thrust belts are limited owing to the scarcity of the record of syntectonic rocks and other data required for 4D reconstruction approaches. We studied the Balzes anticline, a 17-km long, thrust-related, arc-shaped anticline with dense paleomagnetic sampling (75 sites) to unravel the origin of its curvature (about 50° of arc in map view). The availability of both synfolding and synrotational indicators enables us to decipher the complete kinematic history of the fold. Reliable paleomagnetic data (more than 500 specimens and about 1000 measurements) were obtained from Ypresian to Priabonian rocks, typically with six demagnetization steps. The ChRM is unblocked between 420 and 575°C in most samples (85%), and at 675°C in the others. The primary direction displays two polarities and passes the fold test. Vertical axis rotation values for the structure range from non-significant up to >80° clockwise. A strike vs. rotation diagram clearly relates this variability to the fold curvature (VAR=−46°+0.5∗TREND [R=0.97]) and reveals the addition of primary and secondary curvatures. The original (primary) curvature is related to the emplacement of the thrust sheet underlying the Balzes anticline; it shows an obliquity of approximately 15° with respect to the expected Pyrenean trend. Progressive unconformity in the field (Santa Marina site) provides a record of a Middle–Late Lutetian major folding event. The detailed analysis of the synrotational sedimentary record together with accurate temporal calibration based on previous magnetostratigraphic data allows us to date the rotation stage as Lutetian–Bartonian and to estimate the rotation velocity of the Balzes anticline to have been about 5.2°/Ma, both in agreement with previously published observations for the South Pyrenean front.