Shock veins in the central uplift of the Manicouagan impact structure: Context and genesis

We describe the development of shock veins that penetrate the anorthositic central uplift of the Manicouagan impact structure. They occur as thin (< 2.5 mm wide), linear micro-fault systems that can be traced for several meters in length, and which predominantly trend radially from the point of impact. The shock veins are distinguished by the development of maskelynite along vein margins and stishovite in vein matrices, which are otherwise absent in non-veined regions. These phases define a shock excursion of up to 30 GPa, in contrast to bulk shock effects of <= 12 GPa defined by development of shatter cones, planar fractures and planar deformation features in various minerals. The shock veins at Manicouagan share many similarities with vein systems developed in meteorites. They also provide an in situ context with which to better understand meteoroid source and lofting conditions. In addition to containing high pressure phases, the shock veins exhibit evidence for high temperature partial melting of host silicate clasts, with the generation of flow-textured fragments and glasses. The formation of microcrystallites and dendrites from some melts indicates rapid cooling. We propose a two-stage generation mechanism comprising an initial high-pressure shock excursion (estimated to last < 0.5 s based on projectile size considerations) followed by a longer high-temperature pulse of a few seconds duration. We suggest that the shock excursion is initiated by target heterogeneities that cause distortions in the hemispherically propagating shock front. This results in radially oriented tearing and vein formation with shock amplification occurring via intra-vein shock reverberation. High-speed displacement along the veins is driven by stress release on rarefaction, which results in frictional melting via adiabatic heating. (C) 2011 Elsevier B.V. All rights reserved.