The Iapetus suture, one of the more fundamental geologic boundaries of the British Isles, has been located beneath the North Sea by deep seismic reflection profiling carried out by the British Institutions Reflection Profiling Syndicate (BIRPS). Reflection profiling and magnetotelluric sounding have previously been used to infer the position of the Iapetus suture beneath the Irish Sea and the Southern Uplands, but the present study provides the first direct evidence for the location of the Iapetus suture east of Britain. A comparison of the seismic reflection profiles over the Iapetus suture east and west of Britain shows variations in the dip of the suture, which steepens to the northeast beneath the North Sea. The comparison also reveals unexpected changes in lower crustal reflectivity along the strike of the suture and suggests that there may be no straightforward relation in this area between the crustal reflectivity and the crustal age.

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