Frontal collapse during thrust propagation in mountain belts: a case from the Lucanian Apennines, southern Italy

In thrust belts, low-angle tectonic contacts are common, and are associated with the stacking of tectonic units and the resulting regional shortening. The Southern Apennines of Italy, where basin and platform sediments are stacked along low-angle regional thrusts caused by the shortening of the Adria passive margin, are no exception. We studied a portion of a north-south-trending, low-angle regional thrust that separates Apennine platform sediments from Lagonegro basin rocks. To the east of the thrust, klippen composed of platform sediments overlying Lagonegro rocks along a low-angle tectonic contact are present. The klippen were first interpreted as remnants of the regional thrust. We performed a detailed structural analysis of the regional thrust, and we examined the structural setting of the klippen. Field evidence and analysis of map patterns revealed that emplacement of low-angle, foreland-propagating thrusts was followed by extensional deformation accommodated by regional east-dipping, low-angle normal faults. At the base of the klippen, we identified low-angle tectonic contacts with an extensional kinematics. We conclude that the klippen were the result of movements of platform sediments and Lagonegro rocks along low-angle normal faults, and not thrusts as previously interpreted. These faults are cut by more recent, high-angle normal faults associated with the opening of the Agri basin. Collectively, we show that evidence of changes in the tectonostratigraphic architecture, fault geometry and kinematics, and fabrics in the thrusts can reveal the presence of low-angle normal faults, the result of an extensional regime. We expect that our findings will contribute to the understanding of the deformation history of part of the Southern Apennines, and of other mountain belts. The results are also important to understand the transition from compression to extension in the Southern Apennines and in similar orogenic belts