The Start-Dodman-Lizard Boundary-Zone in Relation to the Alpine Structure of Cornwall

Abstract

Previous opinion sums up in favour of the continuity of the three boundaries, the entire line being one of dislocation in the east, dislocation and intrusion in the west. Previous opinion also favours the view that, inside the boundary-zone, not only are the Start-Lizard schists continuous, but they have features comparable with some of the Alpine “Schistes lustrés”, with recumbent folding at the Lizard.The boundary-zone is defined as a belt of variolite (ophiolite), exotic blocks, and serpentine intrusive in, and contemporaneous with, a great thrust.Recent work by the writer has shown that some of the beds outside the boundary-zone are a “facies flysch”. Younger beds come in towards the west, in association with the westerly pitch visible in the older beds in South Devonshire; geanticlines occur in the Lower and Middle Devonian respectively, of South Devon and South Cornwall. In an area further north some of the beds show an accumulation of folds comparable with those of the “autochthonous sedimentary” in front of some of the great Alpine nappes; recumbent folding is prevalent in both Devon and Cornwall. The Cornish Flysch was deposited under conditions such as might have preceded a great nappe movement; it is described in relation to the exotic blocks and serpentine in the fracture.The much younger Alpine Flysch also includes ophiolites and exotic blocks, with serpentine intrusive in the fractures upon which the great nappes advanced. According to some authors, the nappes arose from recumbent folds based on geanticlines.The similar direction of both the Cornish (Variscan) and the Alpine movements makes possible a comparison in certain effects as, for instance, the southward increase in albitization and metamorphism, between the great Alps and the similar but older, comparatively tiny, flattened, and denuded relic in south-west England.These cases are also strikingly paralleled by a zone of basic igneous rocks and exotic blocks which overlies the Himalayas.The evidence appears sufficient basis for the suggestion that the Start-Dodman-Lizard boundary-zone may be the relic of a great Variscan nappe, associated with the fan-structure of Cornwall.