Regional conductivity data used to reassess Early Palaeozoic structure in the Northern Ireland sector of the Southern Uplands-Down-Longford terrane

A high-resolution, airborne conductivity survey has proved spectacularly successful in delineating the zones of carbonaceous mudstone (Moffat Shale Group) that form the structural and stratigraphical base of various sandstone-dominated tracts within the Northern Ireland sector of the Southern Uplands–Down–Longford Ordovician–Silurian accretionary terrane. The anomalies associated with mudstone both at outcrop and concealed allow the major tract boundary faults to be plotted across large areas of poor exposure to reveal a large-scale regional swing in the strike of these faults from their prevalent ENE–WSW trend into a NE–SW orientation. The fault traces defined by the geophysical anomalies cut across those, deduced by extrapolation into an area of sparse bedrock exposure, that are illustrated on the current geological map. A substantial revision of the regional fault pattern is thus required. In places the major tract boundary faults appear to anastomose into strike-slip duplexes, suggesting transpression in the accretionary regime. However, the wholesale realignment of the strike trend is likely to have had a later, post-accretion origin, perhaps involving a releasing bend on a major, strike-parallel fault that controlled emplacement of the Newry granitoid pluton (425 Ma), and might also be associated with Au mineralization in Armagh and Monaghan.