The structure and rheological evolution of reactivated continental fault zones: a review and case study

Abstract

AbstractRepeated reactivation of structures and reworking of crustal volumes are characteristic, though not ubiquitous, features of continental deformation. Reactivated faults and shear zones exposed in the deeply exhumed parts of ancient orogenic belts present opportunities to study processes that influence the mechanical properties of long-lived fault zones at different palaeo-depths. Ancient basement fault systems typically comprise heterogeneous, superimposed assemblages of fault rocks formed at different times and depths for which down-temperature thermal histories are most common. Several lithological and environmental factors influence the evolution of fault rock fabrics and rheology, but most fault/shear zone arrays appear to develop as self-organized deformation systems. Once mature, the kinematic and mechanical evolution of the system is strongly influenced by the rheological behaviour of the interconnected fault/shear zone network.A case study from the crustal-scale Great Glen Fault Zone (GGFZ), Scotland, reveals a complex evolution of mid- to upper-crustal deformation textures formed adjacent to the frictional-viscous transition. Fluid influx in the mid-crust has led to reaction softening of the rock aggregate as strong pre-existing phases such as feldspar are replaced by fine-grained, strongly aligned aggregates of weak phyllosilicates. In addition, a grainsize-controlled switch to fluid-assisted diffusional creep occurs in the highest strain regions of the fault zone. It is proposed that this led to a shallowing and narrowing of the frictional-viscous transition and to long-term overall weakening of the fault zone relative to the surrounding wall-rocks. Cataclasis is particularly important in the deeper part of the frictional regime as it helps to promote retrograde metamorphism and changes in deformation regime, by both reducing grainsize and promoting pervasive fluid influx along fault strands due to grain-scale dilatancy. Equivalent processes are likely to occur along many other long-lived, crustal-scale fault zones.