Upper Palaeozoic successions and nappe structures in north Cornwall

Abstract

For many years the geology of north Cornwall has been interpreted in terms of a south-facing overfold model, which predicts a major confrontation at Padstow with the north-facing structures of south Cornwall. New evidence indicates the existence of contrasting successions separated by flat-lying thrusts. In upward succession the Port Isaac, Tredorn and Boscastle nappes are identified; the first appears to have been emplaced by gravity sliding in late Viséan times, followed by the Tredorn and Boscastle nappes in the early Namurïan. Strata represented in the inverted limb of the Port Isaac Nappe, not previously distinguished, are described as the Jacket's Point Slate Formation. Throughout the nappe pile there is consistent evidence of north-facing and northward transport for distances measured in tens of kilometres. Not more than 3 km of south-facing strata may now exist at the Padstow confrontation, and this feature is unlikely to be of great regional significance. The allochthon extends northwards to the Rusey Fault Zone where the autochthon was locally backfolded over the allochthon in late Westphalian times; this appears to be the only significant confrontation of facing directions in north Cornwall.