Stratigraphical and structural variations in central SW England: a critical appraisal

Abstract

Extensive remapping of central SW England has led to the recognition of significant, strike-parallel variations in stratigraphy and structure across major NNW-SSE trending Hercynian fault zones. Tectonostratigraphic units across the region are distinguished and the tectonic evolution interpreted. The NNW-SSE faults interacted with E-W trending faults, which extend down to the basement, to provide differential vertical movements affecting both the sedimentation and structural evolution. West of Dartmoor northwards translation of nappes, which thinned by gravitational spreading, commenced in the late Visean. This facilitated the northward progradation of flysch sedimentation which had already advanced across the Devonian limestones in the south. The flysch became intercalated with the upper parts of the nappe sequence during the late Lower Namurian and was translated northwards into the regional downwarp of N Devon (‘Culm Synclinorium’). At the southern boundary of this feature, part of the allochthon terminated against an E-W block which had profoundly affected Upper Palaeozoic sedimentation. The complex NNW-SSE high-angle fault zones acted as boundary zones within and between nappes permitting differential northward displacements. In contrast, S and E of Dartmoor sedimentation continued into Lower Namurian times and subsequent compression of a series of E-W trending basins and rises produced folding and variable degrees of local thrusting. These field data provide significant constraints for regional modelling and estimates of shortening around 26 km (34%) for central SW England and less than 68 km for the peninsula as a whole. Models invoking regional decollement, extensive translation and back thrusting are rejected.