The development of a regional-scale intraplate strike-slip fault system; Alpine deformation in the north of Ireland

A paucity of displacement markers can make constraining the kinematics and interaction of regional-scale strike-slip faults difficult. In this study, a high-resolution aeromagnetic survey allows quantitative analysis of kilometre-scale Paleogene faulting in the north of Ireland. Mapping offset dykes and igneous centres reveals the presence of two orientations of Cenozoic strike-slip faults attributed to broadly N-S Alpine shortening arising from the convergence of Africa and Europe. Reactivated NE-SW striking faults accommodated up to 2.3 km of sinistral displacement mainly during the Paleocene. Following tectonic quiescence, up to 2.5 km of Oligocene-aged dextral movement was accommodated on newly-formed NNW-SSE striking faults. Displacement analysis suggests that the faults were kinematically linked and experienced a switch in polarity of the predominantly active fault orientation during the Paleogene, forming an apparent conjugate system with asymmetric displacement distributions. The switch in fault polarity is attributed to a rotation in the regional maximum horizontal stress orientation and the locus of deformation moving from pre-existing reactivated structures to newly formed faults. The analysis also illustrates that, despite low strain rates, the interaction of the principal components of the fault system controlled the formation of sedimentary basins and defined the present-day geography of the region, far from an active plate boundary.