LISPB -- IV. Crustal structure of Northern Britain

Abstract

This paper presents those results from the 1974 Lithospheric Seismic Profde in Britain (LISPB) which relate to the compressional velocity structure of the crust and uppermost mantle beneath Northern Britain. A combination of interpretation techniques suitable for modelling laterally inhomogeneous media, including twodimensional ray-tracing and timeterm analysis, has resulted in a detailed seismic cross-section across the Caledonian orogenic belt. The main features of this section are a possible horizontal discontinuity in the Pre-Caledonian' basement, a change in the relationship between the lower crust and the uppermost mantle from north to south and a considerable thickening of the crust beneath the Caledonian fold belt. These results place considerable constraints upon tectonic models for the evolution of the Caledonides in particular in their implication of differing crustal structures north and south of the Southern Uplands and their indication of the primary significance of the Southern Uplands Fault. 44 mantle beneath Northern Britain. D. Barn ford et al .This paper presents LISPB results for the Pvelocity structure of the crust and uppermost 2 The LISPB experiment: data and travel-time correlationsThe LISPB experiment has been fully described in Bamford e l al. (1976) and Kaminski et al. (1976); that part of the experiment relevant to studies of the crust and uppermost mantle beneath Northern Britain in shown in Fig. I(a).In brief, during 1974 July and August, 60 German and British seismic stations (recording three components of ground motion on magnetic tape) occupied at different times the three segments ALPHA, BETA and GAMMA. Shots were fired at the various shotpoints to build up a series of reversed and overlapping crustal profiles (Fig. I@)) with observations out to at least 180 km distance, that is, sufficient for penetration to the Moho. In addition, a local earthquake (at KEQ - Fig. l(a)) was well recorded whilst the stations were occupying segments ALPHA and BETA and a single test profile had been completed in 1973 August using land shotpoint 2 and recording slightly to the east of GAMMA (Fig. l(a)).

Seismic interferometry and ambient noise tomography in the British Isles

“…Interpreted depths of the Moho across Scotland range from 36 km in the Midland Valley to 22 km off Cape Wrath (Trewin, 2002), and this is also consistent with the results of the LISPB experiments in Scotland (e.g. Bamford et al, 1978;Barton, 1992). The sharp increase in crustal thickness observed by Di Leo et al (2009) west of the Moine thrust is not observed in our results.…”

Sources of post-orogenic calcalkaline magmas: The Arrochar and Garabal Hill–Glen Fyne complexes, Scotland

“…Thus, the deep Grampian crust could contain Lewisian, Grenvillian and Caledonian rocks. There may also be a rock layer below the Lewisian, with a density consistent with granulite-facies intermediate to felsic rocks (Bamford et al, 1978). Evidently, the crust below the Grampian terrane is potentially highly complex.…”

Local earthquake tomography of Scotland

“…This level of detail is not reached by the tomography discussed here and so it is reasonable to compare directly with the 1978 model for clarity. Figure 11 shows the location of the LISPB line and a copy of the line drawing by (Bamford et al, 1978). A profile through the 3-D tomography model is shown for comparison.…”

“…Eighteen shots of between two and six tons of explosive were recorded by up to 40 stations along this line. One of the products of this experiment was a 2-D velocity profile along the line (Bamford et al, 1978), that has formed the basis of the velocity model used for earthquake location in Scotland ever since. (Barton, 1992) refined the 1978 LISPB model but her model remains broadly similar with the most obvious difference the addition of intermediate layers.…”

“…However, most of these studies have been offshore, where a dense array of seismic profiles has been gathered by BIRPS (British Institutions Reflection Profiling Syndicate) (Klemperer and Hobbs, 1992). Onshore, crustal structure is constrained by a much smaller number of profiles: LISPB, a north-south profile through Scotland and northern England (Bamford et al, 1978, Barton, 1992; LISPB-Delta, a profile from the north coast of Wales to the English Channel (Edwards andBlundell, 1984, Maguire et al, 2011); CSSP, an east-west profile along the Iapetus Suture line (Bott et al, 1985); and, ICSSP, an extension of CSSP into Ireland (Jacob et al, 1985). (Kelly et al, 2007) attempted to combine data from a number of controlled source experiments, both onshore and offshore, to produce a 3-D seismic velocity model for the crust under the UK and surrounding area but found that velocities were poorly constrained for areas away from the original input data.…”