450 km of deep seismic reflection data have been acquired over the UK landmass. A qualitative assessment of lower crustal reflectivity (based on the amplitude and continuity of deep crustal events) has been made. In the cratonic area of central England the lower crust is generally poorly reflective, while in the foldbelt terrains of southern, eastern and northern England it is more strongly reflective. Beneath the Variscides of southern England an important distinction can be made between poorly reflective ‘foreland-type’ lower crust lying beneath the Variscan Front thrust and strongly reflective ‘orogenic’ lower crust farther south. A brief review is given of hypotheses for the origin of lower crustal reflectivity, and the relationship of the latter to crustal tectonic regime, both in the UK and elsewhere. It is concluded that the types of lower crustal heterogeneity required to produce the observed reflections are more easily generated in orogenic provinces (fold/thrust/metamorphic belts) than in areas which have undergone only low to moderate crustal extension (beneath typical sedimentary basins). The generation of granites by partial melting within the lower crust, and processes of metamorphic differentiation and recrystallization which accompany orogenic activity, are likely to play a significant role in the development of such heterogeneities. In addition, post-orogenic processes of magmatic underplating and low-viscosity ordering may also have played a part.

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