Exhumation history of the Northern Apennines (Italy) recorded by low temperature thermochronology of Epiligurian wedge-top basins

Abstract

The Northern Apennines accretionary wedge has been extensively investigated by means of thermochronological studies to constrain its thermal history with respect to burial and exhumation. However, the Epiligurian wedge-top basins, which represent the shallowest portions of the orogenic wedge, have received less attention. These basins exhibit an internal complex structural architecture formed in response to the progressive growth of the Northern Apennines accretionary wedge during its progressive involvement in the fold-and-thrust belt. In this study, we combine a new structural characterisation of the Epiligurian stratigraphic succession with preliminary thermochronological data, with the aim to constrain the low-temperature thermal history of the Epiligurian system formation. We investigated the coarser arenaceous components of different middle Eocene to the upper Miocene Epiligurian formations (Loiano, Antognola, Pantano and Cigarello formations). We use both apatite fission-track (AFT) and (U-Th)/He (AHe) analyses. The majority of the AFT central ages cluster between 53 and 65 Ma (Paleocene-Lower Eocene). None of the samples passed the χ2 test, indicating the presence of different population of grains. For all but one sample three to four detrital populations are characterised by the same range of ages, which varies from 140 to 41 Ma (Early Cretaceous-upper Eocene). The fact that these detrital populations are older than the depositional age of the hosting strata suggests minimal resetting of the AFT system and T generally lower than 120°C post deposition. We interpret the AFT detrital populations consistent throughout the stratigraphic features as representative of cooling ages of the sediment source (alpine derivation source) that fed the Epiligurian basins. AHe ages show a more variable single grain age distribution ranging from 104 to 13 Ma (Late Cretaceous-middle Miocene) suggesting a significant degree of thermal resetting for the AHe system post deposition. AHe ages for the lowest part of the Epiligurian Units (Loiano and Antognola Fms) suggest a possible cooling/exhumation event of the basin at around 30-20 Ma. These preliminary results suggest that the Epiligurian basins experienced T ranging between >120 to 80°C post deposition. AFT data suggest rapid exhumation of the sediment source in the middle to late Eocene recorded by relatively short lag time of the youngest detrital population (~41 Ma). AHe data suggest subsequent Oligocene-Miocene exhumation consistent with deformation of the Northern Apennine.