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Shear wave seismic velocities ( Vs) when used together with compressional wave velocities ( Vp) are a powerful diagnostic of the chemical composition and mineralogy of the continental lithosphere. In this paper, we present whole crustal models of Vs and Vp velocities from two wide-angle seismic profiles from southwest Ireland, which straddle the Eastern Avalonian Terrane within the Iapetus Suture Zone. These models are based on traveltime interpretations of primary and reflected P- and S-wave seismic phases generated by explosive underwater sources. The quality of the shear wave data is exceptional when compared to similar data sets recorded elsewhere and allows the distribution of Poisson's ratio (σ) within the entire crust to be accurately determined. Models of Poisson's ratio, Vp and Vs are then used to constrain crustal composition at various depths down to the Moho ( ca. 32 km depth) using published laboratory velocity measurements from rocks of different metamorphic grades and chemistry. The results indicate that the Irish crust in this region of the Caledonian orogenic belt is unusually felsic in bulk composition with a σ value of 0.247, which is below the global average value of 0.265. Variations in σ within the upper ca. 5 km of crust are greatest (0.20–0.28) and can be correlated with siliciclastic and carbonate sediments in Devonian and Carboniferous extensional sedimentary basins that were formed during the early part of the Variscan orogenic cycle. The typically more uniform variation in the mid crust down to 15 km depth is consistent with a granite/granodiorite or metagreywacke composition of greenschist to amphibolite facies mineralogy. Within the lower crust the value of σ (0.257–0.265) is significantly less than the global mean value of 0.281 predicted from an average crustal petrology model. This suggests a bulk silica content (%SiO2) of ∼64 per cent for the lower crust, based on an empirical relationship between σ and %SiO2, determined from compilations of laboratory data. These results are consistent with petrophysical, geochemical and petrological data from an unusually well-preserved lower crustal xenolith suite from the Irish Midlands, hosted in early Carboniferous volcanic rocks. The xenoliths (mainly partially melted granulite facies metapelites) evidently represent the bulk composition of the lower crust, which was largely derived by accretion of sedimentary material, derived from oceanic, island arc and continental margin sources, during oblique Caledonian collision. [ABSTRACT FROM AUTHOR]

Ireland and neighbouring Britain share much of their tectonic history and are both far from active plate boundaries at present. Their seismicity shows surprising lateral variations, with very few earthquakes in Ireland but many low-to-moderate ones in the adjacent western Britain. Understanding the cause of these variations is important for our understanding of the basic mechanisms of the intraplate seismicity distributions and for regional hazard assessment. The distribution of microseismicity within Ireland and its underlying causes have been uncertain due to the sparsity of the data sampling of the island, until recently. Here, we use the data from numerous recently deployed seismic stations in Ireland and map its seismicity in greater detail than previously. The majority of detectable seismic events are quarry and mine blasts. These can be discriminated from tectonic events using a combination of the waveform data, event origin times, and the epicentres' proximity to quarries and mines, catalogued or identified from the satellite imagery. Our new map of natural seismicity shows many more events than known previously but confirms that the earthquakes are concentrated primarily in the northernmost part of the island, with fewer events along its southern coast and very few deeper inland. Comparing the seismicity with the recently published surface wave tomography of Ireland and Britain, we observe a strong correspondence between seismicity and the phase velocities at periods sampling the lithospheric thickness. Ireland has relatively thick, cold and, by inference, mechanically strong lithosphere and has very few earthquakes. Most Irish earthquakes are in the north of the island, the one place where its lithosphere is thinner, warmer and, thus, weaker. Western Britain also has relatively thin lithosphere and numerous earthquakes. By contrast, southeastern England and, probably, eastern Scotland have thicker lithosphere and, also, few earthquakes. The distribution of earthquakes in Ireland and Britain is, thus, controlled primarily by the thickness and mechanical strength of the lithosphere. The thicker, colder, stronger lithosphere undergoes less deformation and features fewer earthquakes than thinner, weaker lithosphere that deforms more easily. Ireland and Britain are tectonically stable and the variations in the lithospheric thickness variations across them are estimated to be in a 75–110 km range. Our results thus indicate that moderate variations in the lithospheric thickness within stable continental interiors can exert substantial control on the distributions of seismicity and seismic hazard—in Ireland, Britain and elsewhere around the world. [ABSTRACT FROM AUTHOR]

Studies of the fine-scale structure of the continental lithosphere using seismic coda, associated with reflections from within the crust and the uppermost mantle, have almost solely been based on the compressional wavefield, observed at near vertical to wide-angle incidence. Here the results from a full waveform analysis of both P- and S-wave coda, obtained during a crustal scale wide-angle refraction experiment, from southwest Ireland are presented. The basic foundation for the study is a 2-D model for the P- and S-wave structure of the crust and its chemical composition, based on traveltime inversion of first arrivals and reflections for the Irish Caledonides. This sector of the Caledonian orogenic belt straddles the boundary between the Laurentian and Avalonian tectonic plates, along the Iapetus Suture Zone. The excellent quality and relatively broad frequency range (e.g. 1–20 Hz) of the P- and S-wave coda permits finer structure in the upper 40 km of lithosphere to be resolved than would be possible using just primary seismic phases. A series of 1-D waveform calculations are performed to systematically build a model for the fine-scale structure of the 32-km-thick crust and the uppermost mantle lithosphere. Petrophysical and geochemical data from felsic lower-crustal xenoliths from Central Ireland place limits on the permitted velocity fluctuations. Additional constraints are imposed by vertical P-wave reflections observed on contiguous offshore seismic reflection profiles. The results indicate that the uppermost mantle to a depth of ca. 40 km consists of a vertically stacked sequence of alternating high ( Vp∼ 8.0 km s−1) and low ( Vp∼ 7.25 km s−1) velocity layers. Each layer is about 500 m thick with a Vp/ Vs ratio ∼1.73. These layers are interpreted as a series of lens-shaped mafic sill complexes, intruded into the subcrustal lithosphere. The fine structure within the lower crust consists of similar seismic velocity fluctuations ( Vp∼ 6.4 and 7.1 km s−1) but on a scale of 200–300 m thickness. Velocity variations within the lower crust are consistent with the petrophysical properties of metapelitic, orthogneissic and psammitic lithologies, from lower-crustal xenoliths in Central Ireland, and with similar exhumed sections of lower crust in southern Europe. These results support tectonic theories for the origin of the crust by tectonic accretion of predominantly sedimentary and volcaniclastic material derived from oceanic, island-arc and continental margin sources during the Caledonian orogenic cycle. A conceptual model for crustal accretion and growth involves partial melting of the accreted sedimentary wedge, producing granites and a chemically differentiated crust during the late Caledonian (latest Silurian/early Devonian). The heat source that drove melting of the accreted sediments may have been provided by the interpreted mafic sills intruded into the uppermost mantle during oblique late Caledonian (sinistral) collision of the Laurentian and Avalonian plates. [ABSTRACT FROM AUTHOR]