Crustal Structure of the British Isles: A Receiver Function Perspective

The crustal structure of the British Isles has been investigated using teleseismic receiver functions. Data from 25 short--period three component seismic recording stations across Britain have been used to study variations in crustal and upper mantle velocity structure. The most densely covered area straddles the Iapetus Suture zone, the inferred boundary between the continents of Baltica and Avalonia fused during the Caledonian orogeny. Inspection of the receiver function data reveals that there is considerable azimuthal variation in both crustal and upper mantle structure beneath several stations. The data have been projected on to a 2--D profile in order to evaluate any lateral variation in signature. This shows a continuous Moho \textit{Ps} conversion that varies between 2.5--4s. Synthetic receiver functions generated using the velocity model from a previous deep seismic wide--angle reflection/refraction survey, show comparable \textit{Ps} and \textit{PpPms} phases to the observed data. The data from individual stations are analysed using crustal thickness--V_{p/V_s} ratio stacking. The results show crustal thicknesses that range from 25km in north--west Scotland, to 36km in central England. The V_{p/V_s} ratios range between 1.66 and 1.74 where the stations are located on Lower Paleozoic rocks; however where the stations are located on younger sediments, ratios of up to 1.82 have been found. The variation in crustal thickness is consistent with the inferred terrane structure of the British Isles, although there is little variation over the Iapetus Suture with Moho depths of \sim30km. Investigation of the data from station GIM close to the Iapetus Suture shows a sub--Moho phase, which is only found on data from northerly back--azimuths. Phase modelling is consistent with the presence of a 5km thick low velocity zone (V_p 6.5kms^-1) at a depth of 35km. This large scale heterogeneity is interpreted as being caused by structures associated with the Iapetus Suture. However, modelling has been performed using 1--D approximations, while the phase signature could result from complex 3--D morphology. In summary the results show that significant modification of the sub--crustal structure has occurred.