Seismological evidence for shallow crystalline basement in the Southern Uplands of Scotland

Abstract

New evidence is presented, and old data re-appraised, to indicate that the Southern Uplands of Scotland contain crystalline rocks of continental affinity at shallow depth (1–5 km) in at least two zones roughly parallel to the Caledonoid strike. This is in contrast with earlier interpretations (especially of the LISPB seismic profile^1,2) which suggested that the lack of such rocks implied a major discontinuity across the Caledonides near the Southern Uplands Fault. The new interpretations suggest that basement indistinguishable seismologically from that below the Midland Valley continues 15–20 km southwards below the Southern Uplands, and that another such crustal block underlies the Eskdalemuir (EKA) seismological array further to the south. The existence of such blocks radically affects plate tectonic reconstructions of the Caledonides especially the margins of the lapetus Ocean^3–6. Only drilling is likely to show whether the contact between basement and the overlying Lower Palaeozoic greywackes and shales is structural, igneous or, least likely, simple overstep.