The seismic reflection Moho beneath the United Kingdom and adjacent areas

Abstract

Onshore and offshore deep seismic reflection data have been integrated to produce two-way travel-time and depth maps of the seismic reflection Moho beneath the UK. Moho travel-times are mostly in the range 9.5–11.5 s, showing only weak correlation with specific upper crustal structural features, but decreasing generally northwestward towards the Atlantic margin. Moho depths show a much stronger correlation with upper crustal structure than do travel-times. There is pronounced crustal thinning (Moho depths <25 km) beneath the central and northern North Sea grabens and the basins of the northwest margin. Less marked crustal thinning is evident beneath the basins of the Irish Sea, English Channel and southern North Sea. Greatest Moho depths (>32 km) are found generally beneath basement massifs, particularly onshore. Short-wavelength topographic features on the Moho are quite rare, and are generally restricted to ancient zones of continental collision and/or subduction. Some of these ancient collision and/or subduction zones are spatially associated with a highly reflective Moho. This is particularly evident where low-angle shear-zones, in the lower crust or uppermost mantle, converge with or intersect the Moho. It is suggested that development of a Moho detachment surface can tectonically smooth the Moho and thereby markedly increase its reflectivity. A tendency in certain areas for the Moho to remain at roughly constant two-way travel-time (though not constant depth) beneath both sedimentary basins and intervening basement massifs is interpreted as indicating that structural features on the scale of several tens of kilometres are in regional isostatic equilibrium. Conversely, travel-time perturbations beneath smaller features (∼10 km across), indicate that these are at least partially uncompensated. The UK crust is therefore weak, but not sufficiently weak to attain local Airy isostasy. Basement massifs west of Scotland show systematically smaller Moho two-way travel-times than similar structures farther south. Within the constraints of regional isostasy, this can be interpreted in two main ways. Greater rigidity and strength of Lewisian rocks within the west Scottish massifs (manifest as higher seismic velocity relative to density) could be a cause of the observed travel-time differences. Alternatively, low-density material beneath the crust could buoy up the western Scottish massifs. If this buoyancy were the consequence of igneous underplating, then any underplated material would have to lie beneath the seismic reflection Moho.