Crustal velocity structure of the British Isles; a comparison of receiver functions and wide-angle seismic data

Abstract

SUMMARY Magmatic underplating at the base of the continental crust is thought to be an important consequence of mantle plume activity. Geochemical evidence supports the hypothesis of underplating beneath the British Tertiary Igneous Province during Palaeocene times, a process that has been invoked to explain at least part of the observed pattern of Cenozoic denudation and consequent offshore deposition. This study uses receiver functions to independently test the results of modelling of four wide‐angle seismic lines across the British Isles, several of which indicate the presence of a lens of high‐velocity material at the base of the crust. This lens has been interpreted as evidence of Palaeogene magmatic underplating. Receiver functions were calculated for three permanent and two temporary three‐component broad‐band seismometers. In order to detect P‐to‐S conversions from intracrustal velocity discontinuities, high frequencies were preserved in the calculation of the receiver functions. Receiver functions were modelled using the delay times of the P‐to‐S converted arrivals only, with constraints on the VP and VP/VS structure imposed from controlled source data. This pragmatic method of interpreting receiver function data agrees well with the results of applying an established inversion technique. Resulting crustal velocity models are broadly consistent with the velocity models from the wide‐angle lines, showing evidence for a four‐layer crust with laterally varying Moho depths of between 33 ± 2.5 km beneath southwest Ireland, 32 ± 2.5 km beneath northern and eastern Ireland, 31 ± 2 km beneath the Isle of Man and 35 ± 3 km beneath northern England. While these velocity models are consistent with the presence of a lower crustal layer, receiver functions are insensitive to the P‐wave velocity structure, and so unfortunately they cannot be used to independently constrain the P‐wave velocity of the lower crust, which is an important diagnostic of the presence of magmatic underplating.

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

“…The crustal model for the Irish crust, presented in this study, shows a subdivision of the lower crust into two distinct layers with an aggregate thickness of about 10 km and with a uniform Poisson's ratio (σ= 0.257–0.265) diagnostic of an unusual felsic composition (Figs 9, 10 and 12). There is no unambiguous seismological evidence for a significant mafic (underplated) component within the lower crust or near its base, as has been inferred on the basis of receiver function studies, which are partially constrained by wide‐angle seismic data beneath Ireland (Shaw Champion et al 2006). The rare enigmatic mafic granulite xenoliths, which the thermobarometric studies indicate are sourced from close to or below the present‐day seismic refraction Moho (Van den Berg et al 2005), have uncertain geochemical affinities.…”

A New Seismic Data Set on the Depth of the Moho in the Alps

“…The ZK technique has been applied worldwide in multiple studies (e.g., FRANÇA and ASSUMPÇ AO 2004;GEISSLER et al 2008;PIANA AGOSTINETTI and AMATO 2009;LLOYD et al 2010;GALLACHER and BASTOW 2012) and is a well-established method to estimate depths and V p /V s ratios for simple crustal structures. Complex tectonic environments prove to be a challenge for ZK technique, since the RF time series are composed of a multitude of converted phases due to the occurrence of several seismic discontinuities in the crust (e.g., CHAMPION et al 2006). Due to the intricate structure of the Eastern Alps, we chose to constrain the results by keeping the V p /V s ratio constant and invert for Moho depths only.…”

The influence of microstructure on seismic wave speed anisotropy in the crust: computational analysis of quartz-muscovite rocks

“…Seismic anisotropy in the continental crust has been observed in numerous seismological studies, and many workers seek to relate the observed seismic anisotropy to crustal deformation and metamorphism (e.g. Okaya et al 1995; Weiss et al 1999; Godfrey et al 2000; Okaya & Christensen 2002; Okaya & McEvilly 2003; Vergne et al 2003; Ozacar & Zandt 2004; Shapiro et al 2004; Sherrington et al 2004; Lloyd & Kendall 2005; Champion et al 2006; Mahan 2006; Mainprice 2007; Xu et al 2007; Lloyd et al 2009; Readman et al 2009).…”