Abstract - A recent model for the Palaeogene tectonics of Northern Ireland envisages a broadly N-S compressive regime, attributed to Alpine compression, which is punctuated intermittently by plume-related NE – SW extension and associated dyke emplacement (Cooper et al. 2010). This model mainly derives from new evidence provided by the Tellus aeromagnetic survey revealing that Palaeocene dykes, which can be assigned to four main swarms that both pre- and post-date the extensive Antrim Lava Group, are offset by NE-SW trending faults. Here we examine the extent to which both pre- and post-lava swarms have been sinistrally displaced along previously recognised Caledonian and Carboniferous NE – SW trending faults in Counties Fermanagh and Tyrone. We show that the Tempo Six Mile Cross, Omagh and Clogher Valley faults, are characterised by decreasing maximum displacements respectively (with 2km, 500m and 200m) and differences in their growth history. All three faults have time-averaged fault displacement rates which are relatively slow compared to other well studied strike-slip fault systems, presumably reflecting associated low regional strain rates and the intra-plate location of Northern Ireland in Paleogene times. Analysis of the displacements of different dyke swarms on each fault suggests however that the locus of fault activity migrated with time. Faulting appears to have initiated along the Tempo Six Mile Cross fault, with the the Omagh Thrust and Clogher Valley faults becoming active later. Post-dyke fault activity is responsible for the generation of pull-apart basins along the lateral equivalent of the Omagh Thrust, the Tow Valley fault, during Oligocene times. We also investigate the possibility that fault activity along both the Tempo Six Mile Cross and the Clogher Valley faults, may have combined with complementary dextral displacements along the Newry-Camlough faults, leading to the Oligocene basin of Lough Neagh and perhaps even the thickening of the Antrim Lava Group (Quinn 2006). Our preliminary studies suggest that Northern Ireland was subjected to two principal tectonic influences, reflecting the complex tectonic regime influencing NW Europe during the Palaeogene, and that associated tectonic structures could have had a major influence on the localisation of Palaeocene igneous activity and Oligocene basins.
Abstract of talk presented at:
Irish Geological Research Meeting, Natural History Museum, Belfast, February 2010.