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Department of Geology, University of Sheffield, Sheffield S1 3JD, UK
Although the major late Caledonian (Acadian) deformation of the British slate belts south of the Iapetus suture took place in early Devonian time, two intra-Ordovician unconformities are present in the Lower Palaeozoic succession of NW England which have been interpreted as due to regional compressional events. These unconformities are at the base and top of the Caradocian Borrowdale Volcanic Group (BVG) and have been associated respectively with a pre-BVG orogenic cycle affecting the early Ordovician Skiddaw Group (SG) and with ‘pre-Bala’ folding of the BVG. A problem has been that no consistent sets of minor structures attributable to these deformations have been demonstrated in the Skiddaw Group.
Recent advances in understanding of the volcanic history of the Lake District lead to a radical reinterpretation of the significance of these two unconformities. In the ‘pre-Bala’ (intra-Caradocian) deformation of the BVG, flexuring is now seen to be much less important than volcano-tectonic faulting and block tilting associated with caldera collapse and the eruption of voluminous ash flows in the upper part of the pile. Foundering of the volcanic edifice accounts for the preservation of this thick subaerial sequence beneath the overstepping late Ordovician marine deposits.
More tentatively, we suggest that the unconformity beneath the Borrowdale Volcanic Group reflects regional uplift due to buoyancy effects associated with the generation of andesitic melt by Iapetus subduction and its rise through the over-riding wedge of continental lithosphere; several such mechanisms have been proposed to account for the topographic elevation of present day continental margin magmatic arcs.
The new model removes the evidence for Ordovician N–S regional compression from the geological record of NW England and this has geotectonic implications: the evidence for late Ordovician collision is weakened and the direction of Iapetus subduction beneath southern Britain may not have been southward.
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