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)).

The planning, execution and preliminary results of a major Anglo-German explosion seismic project are presented in this, paper I of a series. This Lithospheric Seismic Profile in Britain (LISPB) was planned as a reversed 1000 km line between two major sea-shot points off Cape Wrath in Scotland and one in the English Channel; additional sea-shots and intermediate land-shots were fired to give reversed and overlapping crustal coverage (to 180400 km distance) along the line. In all, 29 shots were fired and 60 mobile magnetic tape stations recorded three-components of ground 'motion. The resulting 14 crustal and three long-range profiles have observations at intervals of typically 2-4 km. Recordings have been digitized and four examples of filtered, computer-plotted record sections are presented to illustrate data quality. In a preliminary analysis, phase correlations are discussed and some models presented; the latter especially are more relevant to future interpretations than to geological or tectonic problems. However, significant variations in crustal thickness and in the nature of the crust-mantle transition do seem to occur beneath the British Isles.

Interpretation of upper crustal data obtained during the LISPB seismic experiment reveals the velocity structure of the pre-Caledonian basement in northern Britain. Lewisian-like basement with a relatively high seismic velocity (> 6.4 km/s) extends from the Caledonian foreland into the Midland Valley and probably terminates at the Southern Uplands fault. To the south, beneath northern England, the basement has a lower velocity (< 6.3 km/s). We suggest that a horizontal discontinuity may exist in the pre-Caledonian basement between the Southern Uplands fault and the Stublick Line though we cannot yet determine the exact nature of this discontinuity.

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The Kenya Rift International Seismic Project (KRISP) seismic refraction-wideangle reflection experiments carried out between 1985 and 1994 show abrupt changes in Moho depths and Pn phase velocities as the rift boundaries are crossed. Beneath the rift flanks, normal Pn phase velocities of 8.0-8.3 kms -1 are observed, except for the Chyulu Hills volcanic field, east of the rift, where it is 7.9-8.0kms -I . Also to the east, some of the thickest crust (38-44km) encountered so far beneath Kenya has been observed over a distance of c. 300 km. However, beneath the surface expression of the rift itself, the uppermost mantle velocity of the Pn phase is anomalously low at 7.5-7.8 km s -1 throughout its length.Beneath the rift itself, there are major differences in crustal thickness, extension and upper mantle velocity structure between the north and the south. Beneath the section from the centre of the Kenya Dome southwards, where the extension is estimated to be 5-10km, the crust is thinned by c. 10 km to a thickness of 35 km, and the narrow low-velocity zone in the mantle extends to a depth of at least 65 km. However, in the north beneath Turkana, where the extension is 35-40km, the crust is only c. 20km thick and two layers with velocities of 8.1 and 8.3 km s -l are embedded in the low velocity mantle material at depths of 40-45 km and 60-65 km. This mantle velocity structure indicates that the depth to the onset of melting is at least 65 km beneath the northern part of the rift and is thus not shallower than the corresponding depth (45-50km) in the south. These results, taken together with those from teleseismic studies, petrology and surface geology, have been used to deduce that anomalously hot mantle material appeared below the present site of the Kenya Rift c. 20-30 Ma ago. This led to widespread volcanism along the whole length of the rift and modification of the underlying crust by mafic igneous underplating and intrusion.