Rotation velocity of a thrust:: a paleomagnetic study in the External Sierras (Southern Pyrenees)

An accurate calibration of chronologic and geologic data (biostratigraphic and magnetostratigraphic) with GPTS has been performed for the middle and upper Eocene syntectonic deposits in the surrounding areas of the Pico del Aguila anticline (South Pyrenean External Sierras). This calibration shows a variable sedimentation rate that changes from 4.8 cm/ka at the bottom to 34 cm/ka at the top, with a maximum of 58.6 cm/ka in the upper pail of the studied stratigraphic section, and gives a fine and continuous chronologic frame to understand the rotational kinematics of the infrajacent thrust system. At the same time, a detailed paleomagnetic study has been carried out (14 sites and a total of 157 thermally demagnetised samples) to know the amount of rotation in the area taking advantage the "tape-recording" effect of the syntectonic materials. These samples display a ChRM between 250 and 400 degreesC, both polarities (that agree very well with the sequence of magnetostratigraphic zones) and consistency with the magnetic reference. Due to the decreasing value of the magnetic declination (from + 46degrees to - 3degrees) of the primary components (checked by fold and reversal test), the calibration has been useful to assign an absolute age to every rotation value (from 40.32 to 37.05 Ma). A time versus rotation curve does not show significant differences of rotation between the flanks of the Pico del Aguila anticline (less than 8degrees). Thus, the clockwise rotations detected in the study area result from the kinematics of the imbricate basal thrust system, which is also responsible for the development of detachment folds (e.g. Pico del Aguila anticline). In a time against rotation graph, the simplest fit for the overall rotation values is given by a linear segment (rotation velocity of 10degrees/Ma); however, the best fit is given by a segment of a parabolic curve. This pattern implies that there were changes of the rotation velocities (between 21degrees/Ma and -5.3degrees/Ma) that in this segment of the External Sierras were caused by an acceleration of the rotational movement (between 9degrees/Ma(2) and 5.5degrees/Ma(2)). The thrust sheet shows maximum values of at least 40degrees of clockwise rotation. (C) 2002 Elsevier Science B.V. All rights reserved.