Paleomagnetic rotations in thrust belts: a case study from the Marche Romagna area (Northern Apennines, Italy)

Paleomagnetic analyses of Upper Miocene synorogenic strata from the Marche–Romagna foothills of the Northern Apennines have been integrated with detailed structural work and structural modelling to gain new insights on the processes of tectonic rotation in fold-and-thrust belts. Measured paleomagnetic decli- nations indicate two different patterns from adjacent areas. Paleomagnetic declinations from the coastal (i.e. northeastern) sites show a limited amount (mean value of 11) of anticlockwise rotation compared with the expected Late Miocene direction, whereas large anticlockwise rotations (mean value of 63) are detected from more internal (southwestern) sites. The large amount of mean differential rotation, detected from two adjacent areas located in the same strike-length position of the thrust belt, suggests that processes other than large-scale lateral bending of the orogen played an important role. Although no major faults are exposed that can account for such large rotations, the two areas of contrasting rotation patterns are notably located in structurally different portions of the thrust belt. The coastal sites lie in the frontal part, which is char- acterized by broad folds and relatively low-displacement, basement-involved (‘thick-skinned’) thrusts. On the other hand, most southwestern sites overly an oblique ramp segment of the major thrust fault (Cesana Thrust) occurring in the internal area. Displacement of the detached sedimentary cover across this thrust ranges from a maximum of about 4 km (frontal ramp area) to zero at the northwestern termination of the oblique ramp segment mentioned above. Cross-section balancing and structural modelling carried out independently from paleomagnetic analysis indicates that limited (10–12) rotations can be associated with laterally variable displacements across the external thrusts, whereas much larger rotations (in excess of 45) must be attributed to complex strain patterns and rotations in the northwestern tip zone to the Cesana Thrust. Although a uniform counterclockwise rotation of an entire sector of the thrust belt cannot be ruled out, our results indicate that this is quite unlikely, and anyway would not exceed a maximum of approxi- mately 12 for the study area. Therefore, large-scale lateral bending of the chain appears to be subordinate, in controlling tectonic rotations about a vertical axis, with respect to the role played by lateral/oblique