A case study of the Rusey Breccia : thrust zone brecciation, reorganisation and mineralisation
The Rusey Breccia, 80m long and 2-3m thick, occurs in the footwall of the Rusey Thrust, beneath the launch point of a series of thrust ramps. The thrust is a major Variscan structure exposed in the Boscastle shales on the north Cornish coast. The breccia forms a series of size graded layers, fining towards each layer boundary. The aligned clasts are surrounded in three dimensions by radial euhedral quartz. No fault gouge is present. The layering and re-brecciated clasts indicate repeated episodes of organisation and mineralisation. Clast organisation occurred in a dynamic environment resulting from episodic fluid injection associated with fault slip events, causing fluidisation of the clasts along the injection pathways. Mineralisation textures indicate relatively slow growth in a stable saturated medium capable of maintaining the clasts in suspension. Fluidisation is the dominant mechanism in the organisation of the breccia. Scale dependent differences in the behaviour of fluidised suspensions create the mineralising medium by forming a colloidal suspension of the fault gouge. The Rusey Breccia is compared with a similar breccia outcropping in Kimmeridge Bay, Dorset, formed in a thrust system developed at shallow depths in a carbonate system. The literature on Variscan and Comubian geology is reviewed and the structural geology of the Rusey Headland is described and explained as part of the Variscan orogenic system. Photoelastic modelling of the Rusey Thrust geometry is used to support the model proposed for the formation of the Rusey Breccia. The processes involved in the reorganisation and mineralisation of the breccia are described and reviewed. The internal structure and matrix morphology of the Rusey and Kimmeridge breccias is described and a statistical analysis of the Rusey data presented. The field, lab and analytical evidence presented in the thesis is synthesized to produce an evolutionary model for the Rusey Breccia.