MODES OF TECTONIC ACCRETION AND RECENT (< 10 MA) TECTONIC EXHUMATION IN THE SOUTHERN APENNINES FOLD AND THRUST BELT (ITALY): STRUCTURAL AND THERMOCHRONOLOGICAL CONSTRAINTS

Integrated structural, clay mineralogy, vitrinite reflectance, fluid inclusion and apatite fission track studies point out exhumation of sedimentary units from depths locally in excess of 5 km during the last 10 Ma in the southern Apennines. Here, Early Miocene docking of the convergent continental margins did not result readily in full continental collision; subduction of the leading edge of the thinned Adria continental margin is testified by the occurrence of HP-LT metasediments and was accompanied by NE-directed, detachment-dominated thrusting of cover rocks. These include both Mesozoic-Tertiary carbonate platform and pelagic (Lagonegro Basin) successions, together with unconformable Miocene siliciclastics. Collectively these units form a highly displaced allochthon that has been carried onto a footwall succession essentially continuous with that of the foreland Apulian Platform. The buried Apulian carbonates (6-8 km thick) are deformed by relatively low-displacement, high-angle reverse faults probably involving the basement. Therefore, a switch from thin-skinned to thick-skinned thrusting appears to have occurred as the Apulian carbonates – and the underlying thick continental lithosphere – were deformed. Apatite fission track data, showing cooling ages below the closure temperature (ca. 120 °C) ranging between 9.2±1.0 and 1.5±0.8 Ma, indicate that exhumation was associated with this final stage of orogenesis (full continental collision), probably involving buttressing of the allochthonous wedge against the outer crustal ramp of the inherited rifted margin of the Lagonegro Basin. Subsequent (Pliocene to Early Pleistocene) foreland advancing of the allochthonous units largely exceeds the total amount of slip that, based on cross-section balancing and restoration, could be transferred from the buried Apulian carbonates to the overlying allochthon. This suggests that emplacement of the allochthon on top of the foreland Apulian carbonates was followed by gravity spreading of the weak allochthonous wedge. Extensional faults tend to branch onto pre-existing thrust faults. These were likely reactivated as low-angle extensional detachments controlling tectonic exhumation, as suggested by thermal indicators locally recording tectonic omission across them.