Syntectonic quartz vein evolution during progressive deformation

Abstract

Two models to explain the progressive deformation of syntectonic quartz veins are derived from conventional theories for simple and pure shears. The simple shear model is based on reorientation and changes in length of linear vein elements and predicts initial orientations of veins for imposed shear strains, elongations and strain ratios. The pure shear model considers changes in length of lines variably oriented relative to the maximum compression direction and yields estimates of elongation strains and strain ratios. Expectations of both models are different, as illustrated by analysis of quartz veins from the Rhoscolyn Anticline, Anglesey, NW Wales. The simple shear model recognises three distinct initial orientations, which predict different strains across the fold; the pure shear model suggests veins were initially subparallel to the principal compression direction and predicts effectively constant strains across the fold. In addition, both models predict different patterns of fold vergence: for simple shear, vergence depends on magnitude and direction of shearing and may exhibit complex patterns; for pure shear, vergence patterns are predicted to be essentially constant. In general, the predictions of either model are critically dependent on the origin of the veins, particularly relative to the formation of the Rhoscolyn Anticline. The publication of 'Folding and Fracturing of Rocks' (Ramsay 1967) represented a step change in Structural Geology. It cemented the combination of rigorous field investigations augmented by mechanical and numerical analyses in the structural geological psyche and prepared the ground for ever more sophisticated investigations that persist to this day. However, it must be recognised that historically, the book represented a continuous progression of ideas that had developed albeit slowly over more than a century. Although folds and folding are central themes of Ramsay's book (Chapters 7-10), fracturing is actually of relatively minor consequence (i.e. there are no chapters dedicated to it specifically); in essence, the presentation is predicated on the significance of stress and particularly strain during geological deformation (Chapters 1-6). This contribution follows the philosophy inherent in 'Folding and Fracturing of Rocks' to interpret the evolution of syntectonic quartz veins during progressive deformation. The veins occur within (semi-)pelitic units folded by the Rhoscolyn Anticline, Anglesey, NW Wales (Fig. 1), a well-known location for both structural geology research and teaching. It begins with a brief description of the general geology of Rhoscolyn, including the recognition of various models to explain the evolution of the kilometre scale Rhoscolyn Anticline (e.g.