Shock effects and pre-shock microstructures in hydrothermal quartz veins from the Rochechouart impact structure, France

Microfabrics in hydrothermal quartz veins in gneisses of the Massif Central, France, from the Rochechouart impact structure and St. Paul la Roche, ∼40 km to the SE of the centre of the structure, have been investigated. In the quartz veins from the Rochechouart impact structure, planar deformation features in the basal plane and cataclastic zones indicate low shock pressures (<c. 8 GPa), and high shock-induced differential stresses. Recrystallized grains, subgrains and undulating deformation lamellae are interpreted as pre-shock features. In quartz veins from St. Paul la Roche outside the impact structure no shock effects occur. There, the microfabric is characterized by healed microcracks, undulating deformation lamellae, subgrains and recrystallized grains aligned along fractures. These microstructures are similar to the pre-shock features of quartz veins within the impact structure. They indicate initial high stress glide-controlled deformation accompanied by microcracking and subsequent modification by recovery and recrystallization at low stress. Such a microfabric development is characteristic for coseismic loading and postseismic stress relaxation in the middle crust below the seismogenic layer. The microfabric of the St. Paul la Roche quartz vein is considered as potential “starting material” for the deformation of quartz veins during the late Triassic meteorite impact at Rochechouart.