Understanding paleomagnetic rotations in Sicily: Thrust vs. transpressive structures

Since the 1970s, paleomagnetic data collected in Sicily have documented large magnitude clockwise (CW) rotations around vertical axis with respect to Africa and the Hyblean foreland. Many Authors argued that rotations arise from rotational thrusting of large coherent nappes coinciding with paleogeographic units. In the forward thrust propagation process, each nappe rotates the overlying nappe stack. This would explain the stepwise decrease of rotation magnitudes from the internal Panormide unit (90°-140°) to the external Saccense unit, yielding no rotation. However, other Authors later proposed that rotations of Sicily are the consequence of dextral shear occurring since late Miocene times along EW to NW-SE strike-slip faults. To understand the tectonics responsible of paleomagnetic rotations in Sicily, we paleomagnetically investigated 29 sites and a stratigraphic section from Meso-Cenozoic sediments belonging to the Imerese and Trapanese successions exposed in the Piana degli Albanesi area, Mt. Kumeta, and Rocca Busambra. In the study area the fold and thrust belt is characterized by the occurrence of two main sets of subsequent tectonic structures: 1) the early thrusts, producing imbricate-fan and duplex since early Tortonian (deep-water Imerese Units thrust over carbonate-platform Trapanese units); 2) the superimposed wedging at depth of carbonate platform units (since late Tortonian), that produced the most striking (and studied) structural highs of Kumeta and Busambra ridges, bounded by transpressive faults. In order to test the effect of the latter faults on the cumulated CW rotation, we collected data along several transects perpendicular to both Kumeta and Busambra ridges. In fact, rotations are expected to diminish progressively moving away from faults located at the northern ridge edges, in a way that is related to fault offset. The main results of our study are as follows: 1. Six new sites (and one site from previous study) show that the Imerese unit rotated ≈130°, similarly to the Panormide unit at the Monti di Palermo. This evidence requires updated discussion on the tectonic and paleogeographic relations between the Panormide and Imerese domains. 2. At Mt. Kumeta the rotations are effectively greater (120°) along the dextral fault plane, but they decrease to 80° (normal value of the Trapanese unit) at only 300-400 m from the fault. Thus we calculate that the lateral offset of the Kumeta transpressive fault is definitely less than 1 km. 3. At both Mt. Kumeta and Rocca Busambra, rotations from Scaglia sites surprisingly increase moving southward (i.e. far from fault). This suggests a differential rotational and tectonic behavior of the Scaglia with respect to the underlying carbonate backbones of the Trapanese ridges. As a conclusion, paleomagnetic rotations in Sicily are almost entirely due to thrust tectonics, while transpressive fault activity induced local rotations that fade out at only few hundreds of meters from fault planes.