The tectonics of the Tintagel area, North Cornwall

Abstract

Summary The paper describes the structure of the coastal region of North Cornwall in the neighbourhood of Boscastle, Tintagel and Trebarwith Strand. The area has been remapped, and the tectonics originally outlined by Dewey (1909) have been reinterpreted. The revision has shown that the epidiorite at Tintagel is more extensive than hitherto believed. Massive epidiorite grades into chlorite-schist which is often very similar in appearance to the sheared volcanic rocks of the area; but the schist derived from the intrusive rock can be identified by its local thermal metamorphism of the phyllites which lie above it. The important tectonic episode in the area was that of thrusting, in which the upper beds moved from the south-south-east towards the north-north-west. The direction and sense of the translation is indicated by various minor structures, e.g. slickenside striations, elongations of lava pillows, amygdules etc., in a (the movement direction); and by drag-folds, zig-zag folds, boudinage structure, tension fractures etc. in b (normal to a ). Three main thrust-zones are recognized, and they separate the rocks into three tectonic units or thrust-slices which lie on an autochtonous foundation. The higher thrust-slices lie to the west of the lower ones because they have been preserved from erosion by the local westerly dip aided by normal faulting which downthrows to the west-north-west. The more important of these normal faults pass through Tintagel Haven and form, inter alia , the great fault complex seen in the cliffs of the Island. Similar faulting occurs at Trebarwith Strand, Lill Cove and Hole Beach. In each case the downthrow is such that the flat thrust-plane there is dropped to the west. The general direction of strike in the area is controlled by the gentle doming due to the Davidstow Anticline. This hemidome pitches north-westerly, and it developed later than the thrusting episode. The evidence shows that there is no connexion between the thrust movements which carried the beds forward at least 2½ miles (or more) and the warping of the anticline. The tectonics at Tintagel are thus no longer to be considered the result of thrusting from the west-north-west, but are rather the result of a major movement from the south-south-east, and so become an integral part of the Hercynian tectonic pattern of Cornwall.