Mid-crustal strain localisation triggered by localised fluid influx and activation of dissolution–precipitation creep

Understanding mid-crustal deformation is vital for determining the spatial and temporal distribution of strain localisation, with implications for upper-crust deformation including seismic hazard. Here, we conduct fieldwork and microstructural and minerochemical analyses on the amphibolite-facies, 100-m-wide Upper Badcall shear zone in northwest Scotland, which deforms initially anhydrous quartzofeldspathic gneiss and a mafic dyke. We show that with increasing strain, m-scale strain distribution and mineral chemistry become increasingly homogeneous, while hydrous phases and syntectonic quartz veins become more abundant. With increasing strain there is an overall increase in grain size, grain boundary alignment and shape preferred orientation in amphibole, plagioclase and quartz. Only amphibole and large grained quartz exhibit crystallographic preferred orientation in strained areas. Subtle microstructures that may be overlooked elsewhere, particularly in felsic gneiss, indicate dominant activity of dissolution–precipitation creep and equivalent rheological weakening in both mafic and felsic rocks. We propose that brittle fractures, now preserved as syntectonic quartz veins, allowed localised fluid-infiltration in previously anhydrous crust. This triggered local retrogressive reactions and introduced sufficient grain boundary fluid for deformation to favour dissolution–precipitation creep over dislocation creep. Our study suggests that dissolution–precipitation creep may be more dominant in mid- to lower-crustal localised zones of deformation than previously thought.