Grain boundary contact effects during faulting of quartzite: an SEM/EBSD analysis

During low-temperature faulting of Cambrian quartzite (Assynt, NW Scotland), stress concentrations develop at grain contacts either at the onset of deformation, prior to the establishment of a through-going fault plane, or within the damage zone remote from the main displacement segment. Such concentrations contribute to the development of intragranular microfractures, cataclastic microstructures and fault rocks. This contribution considers the progressive deformation sequence that precedes microfracturing and cataclasis. The complexity of this deformation is revealed by scanning electron microscope (SEM) electron backscattered diffraction (EBSD). Dauphiné twinning is a widespread feature associated with grain contact stress concentration and forms distinctive EBSD microstructures. Automatic SEM/EBSD analysis reveals that whilst initial indentation causes dauphiné twinning of many grains, continued indentation results in the formation of an arcuate array of subgrains via low temperature plasticity and/or microcracking, which overprint the dauphiné twins. These observations are consistent with transmission electron microscopic analysis of quartz crystals used for microhardness indentation tests, which reveal that indentation causes an intensely deformed region to develop, comprising a high density of microfractures and a submicron scale ‘blocky’ microstructure that accommodates any ‘plastic’ deformation. Deformation mechanisms and associated microstructures develop sequentially with progressive indentation and may provide sites of microfracture nucleation via low-temperature ductile fracture. The new microstructures assist diffusive mass transfer (DMT) processes by the formation of a cellular or subgrain array that represents a reduction of several orders of magnitude in apparent grain size and hence in diffusion path length. Concomitantly, associated microfracturing perturbs local thermodynamic equilibrium, leading to enhanced DMT, crack healing and cementation overgrowths. Together, these processes form the aseismic creep and sealing components of fault zone development.