Structural arcuation in the Berwyn Hills, North Wales

Abstract

New mapping has better defined the well known arc in Acadian (late Caledonian) structures centred in the Berwyn Hills. This arc, convex to the north-west and about 20 km across, is defined by the strikes of bedding and cleavage and the trends of fold hinges. In contrast, a mineral lineation within the cleavage has a north-north-westerly plunge throughout the arc. It approximates to the long (X) axes of measured strain ellipsoids. A primary origin for the arcuation is implied by this consistency, by the absence of secondary cleavages and by opposing senses of fold transection across the arc. The new data favour the hypothesis of shortening of the Lower Palaeozoic cover against and over a pre-existing basement high inferred from geophysical data and from metamorphic and sedimentary facies.