Abstract - The Cenozoic tectonics of North West Europe is generally attributed to a combination of three major controls: Alpine compression, mantle plume-related uplift and North Atlantic opening. The relative influence of each is the subject of ongoing debate. North East Ireland hosts extensive exposures of Paleogene rocks of the North Atlantic Igneous Province and in combination with a number of high resolution geophysical data sets, provides a firm basis for considering the contribution of each factor. Evidence derived from analysis of the recently acquired Tellus aeromagnetic dataset suggests that the principal Paleogene-Oligocene structures of North East Ireland are conjugate strike-slip faults and regional dyke swarms. Their geometrical configuration and apparent synchroneity is not, however, consistent with a simple application of ‘Andersonian’ concepts, but instead supports a model in which North East Ireland was subjected to N-S Alpine compression, resulting in strike-slip faulting, a stress regime which was punctuated by distinct pulses of plume-related NE-SW to ENE-WNW extension.
Whilst Tertiary dyke swarms are an established feature of the tectonics of North East Ireland, newly acquired aeromagnetic data provides excellent definition of considerable numbers of associated WNW-NW trending dykes, the ages of which are constrained by cross-cutting relationships with stratigraphic units, intrusions or faults. Four distinct swarms have now been distinguished, each of which shows different amounts of lateral displacement across NE-SW trending sinistral strike-slip faults, a feature which attests to the broadly overlapping periods of dyke-related extension and faulting. Strike-slip faulting conforms to conjugate pairs of NE-trending sinistral and NNW-trending dextral faults, with displacements of up to 2.5km. Sinistral faults reactivate pre-existing Caledonian and Carboniferous structures, whilst dextral faults show no evidence of previous structure. NNW-trending dextral faults mapped from seismic data farther to the south in the Irish Sea are believed to be the lateral equivalent of onshore structures which extend into the Lough Neagh Basin. The latter is characterised by thick sequences of both Oligocene clays and earlier Palaeocene basalts, and appears to represent a pull-apart basin which is close to the intersection of major dextral and sinistral strike-slip faults. Similar, but smaller scale, evidence suggests that related strike-slip faulting may have controlled the location of other volcanic and sedimentary depocentres and of igneous intrusive complexes.
The activity of contemporaneous conjugate sinistral and dextral strike-slip faults with km-scale displacements which are attributed to approximately N-S Alpine compression may at first appear to conflict with the main orientations of dyke swarms. The pulsed nature of these swarms provides, however, a rationale for their formation in a tectonic regime which was otherwise characterised by Alpine-related deformation, with pulsed plume-related dynamic stresses and related deformation temporarily overwhelming the background far-field tectonic stresses and strain. The operation of both influences in the Paleocene nevertheless supports the model that strike-slip faulting could be partly responsible for the localisation of Paleocene volcanic activity and of Oligocene sedimentary depocentres.
Abstract of talk given to:
Anderson Conference: Stress controls on faulting, fracturing and igneous intrusion in the Earth's crust, Glasgow, Scotland, September 2010.