Goldilocks and the three craters: Finding conditions that are just right for shock vein formation
University of New Brunswick
Hypervelocity impacts on rocky planetary bodies generate many shock effects within the target rock on both microscopic and macroscopic scales. These features allow impact geologists to estimate the spatial shock pressures realized within the target rock during an impact event. Shock veins are unique among these features as both the pressure and temperature of their formation can be deciphered leading to a critical variable in understanding shock wave passage: time. Shock veins are typically thin (<2 mm wide) anastomosing veins that display textures ranging from glassy to a fine-grained crystalline matrix; indicative of melting followed by rapid cooling. Shock veins commonly contain locally derived clasts that are suspended within the once molten matrix. High-pressure/temperature polymorphs may be developed within shock vein systems crystallizing from the matrix or developing via solid-state transition within the suspended clasts or adjacent wall rock. Polymorphs within shock vein systems are key indicators of the pressure/temperature conditions realized by shock veins during their formation. This thesis focusses on obtaining a relationship between shock vein pressure-temperature-time (P-T-t) conditions and crater morphology for all three in situ terrestrial shock vein-bearing impact structures: Vredefort (South Africa), Steen River (Canada), and Manicouagan (Canada). The relationship is then explored further by calculating the P-T-t path of the shock vein-bearing paired lunar meteorites NWA 3163 et al. to estimate the crater size from which the meteorite originated on the lunar surface. A finite difference model has been developed (via MathWorks MATLAB) to calculate the P-T-t cooling paths of shock veins. The developed code simulates the passage of a shock wave through the target, while simultaneously forming and cooling the shock veins via 2-D steady-state conduction. The model was applied to all three in situ terrestrial shock vein systems. Calculated P-T-t results agree with petrologic observations of high-pressure/temperature polymorphs present within the studied shock vein systems. A relationship between the impact crater diameters and shock waves that formed them was then established. This relationship was applied to paired lunar meteorites NWA 3163 et al., and the origin crater diameter was determined to be approximately 216 km.