A structural analysis of the Orielton anticline Pembrokeshire

Abstract

The results of a detailed investigation into the relationships between folds, faults and joints in the Orielton anticline are presented. The study continues the early structural work of Dixon (1921) and the stress analysis of the area made by Anderson (1951). The Orielton anticline is a compound and faulted Armorican fold largely affecting Upper Palaeozoic rooks. The structural pattern of the anticline developed during two major deformation phases: the first essentially corresponding to a period of folding and thrusting, the second to a period of wrench faulting. Within each phase, which is divisible, faulting occurred before jointing with joint sets not necessarily lying parallel to equivalent faults. The attitudes of both faults and joints depend on fold geometries. Faults are oriented relative to fold axial planes and axes, whilst joint attitudes are largely controlled by bedding dip and the plunge of the bedding - fracture cleavage intersection. It is tentatively suggested that the dependence of fracture attitudes upon fold geometries is due to the operation of residual stress systems. The dihedral angle between complementary shear planes has been investigated and shown to be consistently low, usually less than 50º. Regional tension joints appear to be absent. Joint orientations in collapsed blocks of Carboniferous Limestone enclosed in Triassio breccias show that all phases of the deformation belong to the Armorican orogeny.