Quartz veins record earthquake-related brittle failure and short term ductile flow in the deep crust

Discordant quartz veins in a retrograded eclogite-amphibolite (Western Gneiss Region, Norway) formed at temperatures of about 500 to 550 °C. The parental fractures crosscut the foliation and show branching, indicating rapid crack propagation at high differential stress. The shape of the quartz veins indicates intense inhomogeneous ductile deformation of the host rock during opening and sealing, controlled by the presence of the cohesionless fluid-filled fractures. The vein quartz microfabrics indicate crystallization in an open cavity, implying a stage of very high permeability and fluid flow. The event of brittle failure at 500 to 550 °C, followed by an episode of inhomogeneous ductile deformation, is attributed to stress changes related to the earthquake cycle, i.e. coseismic loading and postseismic creep. The record suggests seismic activity at the nearby low angle normal fault at 15 to 25 km depth and provides insight into transient crustal properties and processes near active faults at great depth.