Folding and pressure solution in a laminated calcite-quartz vein from the Silurian slates of the Llangollen region of N Wales

Abstract

SummaryFolds of a 12 mm thick calcite-rich vein, embedded in slate and with a primary planar fabric of parallel, thin and plate-like calcite crystals, or laminae, have a vein thickness/arc length ratio near 1. Folds were first entirely dependent for development on intra-laminar strain and inter-laminar sliding. As limb dips increased, however, pressure solution began to thin laminae in limb regions only. As adjacent inter-laminar surfaces met at fold inflexion points, where laminae were first cut through, loss on fabric-cutting surfaces began to replace inter-laminar pressure solution. The resulting stylolites grew across the vein in fold limbs, at the limit eliminating them. In contrast, in folded equigranular calcite rocks stylolites are widely developed in both limbs and hinges, and eventual fold collapse through late stage concentration of pressure solution takes place in fold hinges and not limbs.