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[1] We report on a paleomagnetic study of the southern sector of the Olevano-Antrodoco-Sibillini (OAS) thrust front, which corresponds to the southern limb of the Northern Apennines (Italy) orogenic salient. A lively debate has developed regarding the oroclinal/progressive-arc versus non-rotational nature of the OAS, which has been alternatively interpreted as a dextral strike-slip fault, dextral transpressive fault, or frontal to oblique ramp that reactivated pre-existing Jurassic normal faults. Here, we document the paleomagnetism, integrated with biostratigraphic and structural data, of 52 new sites from both the OAS hanging wall and footwall. On the basis of 39 retained sites, we find a peculiar pattern of tectonic rotations along the OAS thrust that evidences four rotational domains. The thrust footwall is characterized by a southern domain that undergoes an approximately 30° counterclockwise rotation with respect to the stable foreland, and an approximately non-rotated domain. The data from the hanging wall indicate the occurrence of a dextral strike-slip component along the southern sector of the OAS thrust supported by a strong clockwise rotation close to the NE-SW lateral ramp, which rapidly fades 1 km from the thrust front. A slight but significant CW rotation observed in the remaining sites from the hanging wall confirms the progressive nature of the OAS, and its structural position as the southern limb of the Northern Apennines salient. Our detailed paleomagnetic study is crucial in discriminating between progressive-arc- and strike-slip-related components in the main curved orogenic front of the Northern Apennines.

This work reports the results of our analysis of the brittle-ductile shear zone associated with the Olevano-Antrodoco-Sibillini (OAS) curved-shape thrust in the Central/Northern Apennines of Italy. Its southern sector is characterized by NNW–SSE trending footwall anticlines, which also affect the OAS thrust surface. S tectonites in the NNE–SSW trending sector are crenulated by conjugate extensional surfaces (extensional crenulation cleavage). The NW–SE trending segment of the OAS thrust is characterized by SC tectonites developed along the thrust-related shear zone. Both the SC and S tectonites are consistent with a top-to-the N60–70°E direction of tectonic transport. The SC tectonites are associated with a simple-shear-dominated deformation (Wn = 0.86–0.98). On the other hand, the S tectonites and their related extensional shear planes document two stages of strain: (i) sub-simple shear (Wn = 0.34–0.50, related to OAS thrusting contemporaneous to growing incipient footwall anticlines, and (ii) pure-shear-dominated deformation (Wn = 0.17–0.00), subsequent to the OAS thrusting and caused by the definitive growth of footwall anticlines within an in-sequence deformation context. The present study proposes the analysis of shear zones to: (i) discriminate in-sequence against out-of-sequence evolution, and (ii) use as a tool to constrain the modes and timing of the curved belt's development.

The relationships between thrusts and normal faults represent primary constraints in the reconstruction of the modes and timing of pre-, syn- and post-orogenic deformation events in fold-and-thrust belts. Such relationships are well exposed in curved orogenic belts where the thrusts are oblique to the trend of normal faults. We study the NNE–SSW-trending Olevano-Antrodoco-Sibillini oblique thrust and its crosscutting relationships with NW–SE-trending normal faults in order to constrain the Neogene–Quaternary deformation history of the Central-Northern Apennine (Italy). The analysis of structural and geological data allowed us to reconstruct the geometric and kinematic constraints of two inversion events: 1 – During the Pliocene, positive inversion reactivated the NNE–SSW-trending pre-existing Ancona-Anzio normal fault as the Olevano-Antrodoco-Sibillini oblique thrust ramp and caused the shortcut of NW–SE-oriented normal faults; 2 – During the Quaternary, negative inversion reactivated NW–SE-trending pre-thrusting normal faults. The growth of the NW–SE Quaternary normal faults causes seismicity and is responsible of the development of wide Quaternary intramontane basins. Their distribution and the related seismicity have been controlled and compartmentalized by NNE–SSW-trending oblique thrusts. Thus, the crosscutting relationships between thrusts and normal faults are crucial in seismic hazard assessment.