Shear zone fabrics and their significance in curved, inverted basin-derived thrust systems

In this study, we report how criteria inferred from structural and kinematic analysis of shear zone fabrics can be instrumental in deciphering events of positive or negative inversion tectonics. Mesoscopic structural and kinematic analyses are performed on well-exposed brittle-ductile thrust shear zones within curved thrust systems in the Central-Northern Apennines of Italy. The structural and kinematic features of thrust-related tectonites make it possible to outline a history of positive inversion. S/C- and S-tectonites/fabrics are distinctive of frontal or oblique thrust ramps, respectively. In particular, S-fabrics result from selective transpressional-reactivation of pre-thrusting normal faults. By contrast, more complex composite fabrics, characterized by folded cleavage, reveal a history of negative inversion tectonics. Mesocopic cleavage folding is ascribed to shear-sense reversal along the shear surfaces during extensional reactivation of pre-existing thrusts; or to synthetic reactivation by listric foreland-dipping normal faults, merging downwards on former thrust surfaces during protracted episodes of gravitational collapse. Shear zone fabrics identification criteria, here illustrated from thrusts of different size, if systematically used, may be diagnostic of different deformation paths, and hence provide useful tools to elucidate the tectonic history of curved thrust systems in orogens that have experienced multiple episodes of inversion.