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 detai led structural work and structural modelling to gain new insights on the pr ocesses of tectonic rotation in fold-and-thrust belts. Measured paleomagnet ic declinations indicate two different patterns from adjacent areas. Paleom agnetic declinations from the coastal (i.e. northeastern) sites show a limi ted amount (mean value of 11 degrees) of anticlockwise rotation compared wi th the expected Late Miocene direction, whereas large anticlockwise rotatio ns (mean value of 63 degrees) 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 bel t, suggests that processes other than large-scale lateral bending of the or ogen played an important role. Although no major faults are exposed that ca n account for such large rotations, the two areas of contrasting rotation p atterns are notably located in structurally different portions of the thrus t belt. The coastal sites lie in the frontal part, which is characterized b y broad folds and relatively low-displacement, basement-involved ('thick-sk inned') thrusts. On the other hand, most southwestern sites overly an obliq ue ramp segment of the major thrust fault (Cesana Thrust) occurring in the internal area. Displacement of the detached sedimentary cover across this t hrust ranges from a maximum of about 4 km (frontal ramp area) to zero at th e northwestern termination of the oblique ramp segment mentioned above. Cro ss-section balancing and structural modelling carried out independently fro m paleomagnetic analysis indicates that limited (10-12 degrees) rotations c an be associated with laterally variable displacements across the external thrusts, whereas much larger rotations (in excess of 45 degrees) must be at tributed to complex strain patterns and rotations in the northwestern tip z one to the Cesana Thrust. Although a uniform counterclockwise rotation of a n entire sector of the thrust belt cannot be ruled out, our results indicat e that this is quite unlikely, and anyway would not exceed a maximum of app roximately 12 degrees for the study area. Therefore, large-scale lateral be nding of the chain appears to be subordinate, in controlling tectonic rotat ions about a vertical axis, with respect to the role played by lateral/obli que ramps, non-cylindrical folding, and displacement gradients in 'thick-sk inned' or detachment-dominated thrusting. As these features are common in m ost fold-and-thrust belts, our results might be of interest in the applicat ion and interpretation of paleomagnetic studies in similar structural setti ngs elsewhere. (C) 2001 Elsevier Science Ltd. All rights reserved.