Understanding progessive arc and strike-slip rotations in curve-shaped orogenic belts: the case of the Olevano-Antrodoco-Sibillini thrust (Northern Apennines, Italy)

We report on a paleomagnetic study of the southern sector of theOlevano-Antrodoco-Sibillini (OAS) thrust front, which corresponds to the southern limb ofthe Northern Apennines (Italy) orogenic salient. A lively debate has developed regarding theoroclinal/progressive-arc versus non-rotational nature of the OAS, which has beenalternatively interpreted as a dextral strike-slip fault, dextral transpressive fault, or frontal tooblique ramp that reactivated pre-existing Jurassic normal faults. Here, we document thepaleomagnetism, integrated with biostratigraphic and structural data, of 52 new sites fromboth the OAS hanging wall and footwall. On the basis of 39 retained sites, we find a peculiarpattern of tectonic rotations along the OAS thrust that evidences four rotational domains. Thethrust footwall is characterized by a southern domain that undergoes an approximately 30counterclockwise rotation with respect to the stable foreland, and an approximately nonrotateddomain. The data from the hanging wall indicate the occurrence of a dextral strike-slipcomponent along the southern sector of the OAS thrust supported by a strong clockwiserotation 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 wallconfirms the progressive nature of the OAS, and its structural position as the southern limb ofthe Northern Apennines salient. Our detailed paleomagnetic study is crucial in discriminatingbetween progressive-arc- and strike-slip-related components in the main curved orogenicfront of the Northern Apennines.