Late Caledonian (Acadian) transpression in north-west England: timing, geometry and geotectonic significance

Abstract

SUMMARY The late Caledonian structure of the Lower Palaeozoic slate belts which lie to the south of the Iapetus suture in Britain is not ‘Caledonoid’ (NE-SW) but characterised by arcuate trends. The significance of the major cleavage arc of northern England is the subject of this paper. Its exposed part, in the Lake District and adjacent inliers, is described and its regional extent inferred from the control exerted by Caledonian basement trends on early Carboniferous sedimentation patterns. The arc is shown to be a major feature of the orogen, marking a change from a NE-SW ‘Appalachian’ trend to the ESE-WNW ‘Tornquist’ trend of northern Germany and Poland. Evidence for the age of deformation in the British slate belts is reviewed. It is shown that the deformation was not ‘end-Silurian’ as previously supposed, but of early Devonian age, climaxing in the Emsian and approximately synchronous with the Acadian orogeny of Canada. The systematic variation in cleavage/fold transection angles around the arc is described and interpreted in terms of transpressive strains associated with the northward movement of a basement block, the Midlands Massif, which acted as a rigid indenter during accretion of the southern British terrane (Eastern Avalonia) onto the Laurentian margin. These new data on the timing and geometry of the Acadian accretion event in Britain go some way to resolving the current controversy concerning late Ordovician vs. Devonian closure of Iapetus.