Constraints on crustal structure and composition within a continental suture zone in the Irish Caledonides from shear wave wide-angle reflection data and lower crustal xenoliths

Abstract

S U M M A R YShear wave seismic velocities (V s ) when used together with compressional wave velocities (V p ) are a powerful diagnostic of the chemical composition and mineralogy of the continental lithosphere. In this paper, we present whole crustal models of V s and V p 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, V p and V s 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 (%SiO 2 ) of ∼64 per cent for the lower crust, based on an empirical relationship between σ and %SiO 2 , 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.

From Continental Hyperextension to Seafloor Spreading: New Insights on the Porcupine Basin From Wide‐Angle Seismic Data

“…The thickness of the crust varies from ~7 km in the center of the basin (~15 km) to ~8 km at 30 km (beneath the PMR), and ~5.5 km beneath the eastern flank of the basin (~50 km). The crustal velocity (5.0–7.1 km/s) beneath the continental shelf (>75 km) is close to continental velocities observed onshore Ireland (Hauser et al, ).…”

Comparison of lithosphere structure across the Orphan Basin–Flemish Cap and Irish Atlantic conjugate continental margins from constrained 3D gravity inversions

“…The Irish continental shelf and many of the basins therein have been the focus of seismic reflection, seismic refraction and potential field investigations for several decades (Holder & Bott 1971;Whitmarsh et al 1974;Bunch 1979;Jacob et al 1985;Makris et al 1988;Roberts et al 1988;Lowe & Jacob 1989;O'Reilly et al 1991O'Reilly et al , 1995O'Reilly et al , 2010Vogt et al 1998;Landes et al 2000Landes et al , 2003Morewood et al 2005;Hauser et al 2008). Coupled with sparse well control, the resulting studies have allowed for the piecing together of the detailed tectonostratigraphic basin evolution of the margin (Shannon 1991;Naylor & Shannon 2009.…”

Lithospheric density variations and Moho structure of the Irish Atlantic continental margin from constrained 3-D gravity inversion

“…The unstretched crust beneath Ireland is approximately 30 km thick (Lowe & Jacob 1989; Hauser et al 2008), while beneath the Irish and Celtic Sea regions it is approximately 25 km thick, with evidence of crustal decoupling to accommodate the slight thinning beneath the basins (O'Reilly et al 1991). The nature and the thickness of the crust beneath the large Atlantic basins such as the Porcupine and Rockall basins has been the subject of considerable debate (see Smythe Smythe 1989; Shannon et al 1999).…”

“…The thickness of the crust on the continental shelf has been well constrained by deep‐seismic profiles (Morewood et al 2005; O'Reilly et al 2006). Recent studies onshore (van den Berg et al 2005; Hauser et al 2008) have suggested that the main phase of crustal accretion and growth occurred during the Caledonian orogenic cycle. However, little else is known about the composition or detailed nature of the crust or its age.…”

“…The Moho discontinuity marks the seismic boundary between the crust and mantle. Below this boundary, which can be abrupt or gradational, seismic P‐wave velocity is generally greater than 7.9 km s −1 for the Irish crust (Lowe & Jacob 1989; Hauser et al 2008). Determining the location and structure of the Moho can be accomplished using wide‐angle reflection/refraction profiling (WARRP).…”