Precise U-Pb dating of incremental calcite slickenfiber growth: Evidence for far-field Eocene fold reactivation in Ireland

The Variscan orogen in southern Ireland and Britain is characterized by an intensely deformed, E-W-trending fold-and-thrust belt. Farther north in Ireland, the Carboniferous North Dublin Basin exhibits tight chevron folds and kinematically linked en echelon vein sets, along with bedding-parallel veins with slickenfibers. This deformation is assumed to be Variscan in age, despite lying 150 km north of the supposed Variscan front. The laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of these calcite veins undertaken for this study showed that relict Variscan U-Pb ages are very poorly preserved. Instead, late Eocene ages were obtained from many calcite veins, including fold hinge breccias and bedding-parallel slickenfiber veins associated with N-S shortening (flex-ural slip). Also, U-Pb ages from one bedding-parallel vein showed protracted flexural slip over similar to 5 m.y. during late Eocene times. Detection of the growth domains within this vein was facilitated by the imaging approach to LA-ICP-MS U-Pb dating adopted in this study, which can identify age-homogeneous domains by integrating spatial U-Pb isotopic information with maps of petrogenetically diagnostic major and trace elements. The late Eocene fold reactiva-tion phase was hitherto undetected on the Irish mainland, but regional Cenozoic N-directed shortening has been documented in Mesozoic-Cenozoic sequences of the southern Irish Sea, Celtic Sea, southern England, and the Paris Basin. We attribute late Eocene fold reactivation to far-field, N-directed shortening associated with the Alpine/Pyrenean orogenies. It is likely that many Variscan or Caledonian folds, particularly in southern onshore Ireland, were reactivated during Eocene-Oligocene shortening, which has not been recognized to date be-cause of the lack of post-Variscan markers (e.g., dikes, Mesozoic-Cenozoic cover sequences).

Automated summary

The Variscan orogeny in southern Ireland and Britain is characterized by intense folding and thrusting. Late Eocene fold reactivation was detected in calcite veins, which suggests N-directed shortening associated with the Alpine/Pyrenean orogenies. This reactivation has not been recognized previously due to the lack of post-Variscan markers. The imaging approach to LA-ICP-MS U-Pb dating helped identify age-homogeneous domains and revealed the late Eocene fold reactivation.