Vein quartz microfabrics indicating progressive evolution of fractures into cavities during postseismic creep in the middle crust

Abstract

Discordant quartz veins in metamorphic rocks formed at temperatures similar to 300 °C in an extensional tectonic regime. The vein shape is characterized by an aperture to length ratio exceeding the maximum value feasible for purely elastic deformation by orders of magnitude. Non-elastic ductile deformation of the host rock must have controlled widening of the arrested cracks. Fracture parallel shortening is low. Due to the low accumulated strain, the microstructural record of this stage of deformation cannot be isolated in the polyphase host rock. Information is exclusively obtained from the microfabrics of the vein sealing quartz. There, the density of geometrically necessary dislocations, derived from orientation maps created by electron backscatter diffractometry (EBSD), decreases markedly from the vein wall to the centre. The gradient is attributed to progressive cavity formation and buckling of the vein walls during sealing. Early grown crystals at the vein margins record a prolonged deformation history compared to lately grown grains in the centre of the vein. The gradient and the lack of significant crystal plastic deformation of the crystals in the vein centre show that deformation of the completed vein is insignificant compared to deformation during progressive opening and sealing. In view of the precursory brittle deformation and the intense crystal plastic deformation at the vein margins, the monogenetic veins are interpreted to record a single mid-crustal stress transient, related to the earthquake cycle. Fracturing is proposed to be a consequence of coseismic loading, while progressive opening and sealing record the deformation during postseismic stress relaxation by creep.