New evidence for block and thrust sheet rotations in the central northern Calcareous Alps deduced from two pervasive remagnetization events

Abstract

We present 81 paleomagnetic sites (Early Triassic to Early Cretaceous) from the central sector of the northern Calcareous Alps (NCA, Eastern Alps, Austria and Germany). Stepwise thermal demagnetization defines three magnetic directions mostly carried by low unblocking temperature and low‐coercivity minerals: J3, 350°; J2, 500°; J1, 575° and 680°. J3 and J2 show positive inclinations, whereas J1 (very seldom) is of dual polarity. The fold tests show that a J3 can be interpreted as a postfolding and posttilting remagnetization and J2 as a postfolding and pre(syn)tilting. J1 can be considered as primary because of the occurrence of two polarities and evidence presented by other authors in the area. All three components show a systematic and significant clockwise rotation after comparing with the expected European references. J2 or J1 are marked by higher rotation values than J3. J2 shows different inclinations depending on the structural position (north or southward dips). Considering the structural evolution and the observed inclinations, the first postfolding and pretilting remagnetization event (J2) could have taken place between Late Cretaceous and Eocene times but certainly before the thrusting of the NCA over the Rhenodanubian Flysch and northward tilting caused by the stacking of the lower Austroalpine nappes. The second postfolding and posttilting remagnetization (J3) would have been acquired after the final thrusting of the NCA over Penninic units. From then, the NCA behaved as a set of rigid blocks recording the main stage of vertical axis clockwise rotation (65° in average) associated with the continental collision. The variable degree of rotation in the different positions (from 40° to 134°) can be explained by individual vertical axis rotation in a block system trying to adjust to space problems. The constant declination differences between J2 and J3 (25° in average) would reflect the rotation related with the lateral differences of shortening during the oblique thrust of the Austroalpine units over the Penninic units.