Deformation mechanisms accommodating faulting of quartzite under upper crustal conditions

Abstract

Analysis of the deformation microstructures associated with a high-level fault in quartzite (Skiag Bridge, Assynt, NW Scotland) reveals a complex variation in the deformation mechanisms active during faulting. The different mechanisms have been identified using an integrated study involving optical, cathodoluminescence and electron (both SEM and TEM) microscopy. The specific mechanisms identified include: intragranular cleavage fracture (types 1, 2 and 3), brittle intergranular fracture (types 1 and 2), low-temperature ductile fracture, diffusion mass transfer and low-temperature crystal plasticity. Fracturing dominates the deformation (faulting), initially via intragranular extension cleavage fractures due to stress concentrations at grain contacts (although many of these may be healed by quasi-simultaneous diffusive mass transfer processes). These eventually link and are then exploited as shear fractures, leading to the development of microbreccia-cataclasite zones which define a three-dimensional fracture array. Quasi-simultaneous diffusive mass transfer processes may heal these through-going fractures. Continued fault zone deformation involves the development of a damage ('wake') zone along the displacement zone borders where low-temperature plasticity and subsequent low-temperature ductile fracture processes aid the expansion of the fault zone. This study emphasizes that the evolution of the Skiag Bridge fault zone has involved three main categories of deformation mechanisms: fracture, crystal plasticity and diffusion mass transfer. The interrelationship between these categories, and the transition between individual fracture mechanisms, are significant aspects of this evolution. The examples presented demonstrate the complex interrelationships which exist between a group of deformation mechanisms and emphasize the potential importance of low-temperature plasticity and low-temperature ductile fracture processes during faulting under upper crustal conditions. © 1992.