Instability and localization of deformation in lower crust granulites, Minas fault zone, Nova Scotia, Canada

Abstract

Abstract Blocks of granulite from within the megabreccia at Clarke Head, Nova Scotia, Canada contain extremely well preserved mylonitic and ultramylonitic textures developed in mineral assemblages for which thermobarometic calculations have indicated temperatures and pressures between 700–860 °C and 750–950 MPa. Deformation within these rocks is characterized by localization at several discrete length scales associated with the development of new microstructures comprising finer-grained material. Mylonitized granulite exhibits dislocation creep microstructures, with development of intense S-C fabrics and shear bands during the transition to ultramylonite. Dynamically recrystallization of plagioclase can be followed through progressive grain size reduction to about 5 μm, but there remain extensive zones with grains less than 1 μm in diameter. Localization of the these finest-grained ultramylonites occurs by transient frictional events associated with the introduction of partial igneous melts and formation of pseudotachylyte which produces abrupt decreases in grain size that cannot arise during dislocation mediated grain size reduction. The heterogeneous response of these rocks demonstrates the importance of considering characteristic length scales when assigning evidence from the rock record (e.g. palaeopiezometry) to bulk behaviour of the lithosphere. Associated with the localization of strain and subsequent strain softening is the observation that microstructures formed during the event that initiated the instability can be an obliterated by ductile flow. In instances where critical components of the microstructural evolution are known to have been largely overprinted, it becomes possible to reconcile contradictions in the rock record, such as production of ultra-fine-grained superplastic aggregates in what otherwise appears to be a dominantly dislocation creep regime.