Modeling the pressure–temperature–time evolution of in situ shock veins: A terrestrial case study from Vredefort

Abstract

Numerical computing software (via MathWorks MATLAB) has been developed to understand the relationship between shock wave passage in geological targets (i.e., heterogeneous media) and the formation of shock veins and associated high-pressure/ temperature polymorphs. This approach takes into consideration the pressure due to the passage of the shock front, subsequent rarefaction unloading pressures, and associated heating and cooling rates. The model is applied to calculate pressure-temperature-time conditions for coesite-and stishovite-bearing shock veins within metaquartzites of the Vredefort impact structure of South Africa. To constrain the model, the position of the host metaquartzites at the time of impact is first reconstructed. The developed code then passes the appropriate shock conditions through the target to re-create the shock wave, while simultaneously forming and cooling the shock veins via 2-D steady-state conduction. We have found that (1) at the time of shock vein formation (2.4 s following the initial contact of the projectile), the shock front pressure was 13.8 GPa and the width of the shock wave was of 27 km; (2) the melt within the shock veins initially reached ~3000 °C, which corresponds to the melting temperature of the target rock at 13.8 GPa. Simulation results indicate that conditions reach the stishovite stability field within 2 ms of vein formation (~10-14 GPa; 2000-3000 °C), followed by coesite within 1.29 s (~3-10 GPa; 600-2000 °C).The dwell time of the modeled shock vein system is 4.35 s. The shock vein system is completely solidified 33.4 s after the initial shock front passage. The calculated P-T-t path of the model indicates that the polymorphs within the shock veins of the metaquartzites at Vredefort formed under their normal stability field conditions following rarefaction wave decompression.

Pressure–temperature–time controls on shock vein formation within the Steen River impact structure

“…At Steen River, the detached shock wave at the sample location was found to have a peak shock pressure and wave width of 18 GPa and 830 m, respectively. In contrast, Hopkins and Spray (2022) determined that the Vredefort detached shock wave at the location of the investigated shock melt vein-bearing metaquartzites had a peak shock pressure and wave width of $14 GPa and 27 km, respectively (Figure 8). This difference is a product of several key variables.…”

“…Shock veins are shock-generated features that form within the target rocks of hypervelocity impact structures. To date, they have been identified in situ within the central uplifts of three terrestrial impact structures: Vredefort, South Africa (Hopkins & Spray, 2022;Martini, 1978Martini, , 1991Martini, , 1992Spray & Boonsue, 2018;White, 1993), and two in Canada: Manicouagan (Biren & Spray, 2011;Spray & Biren, 2021) and Steen River (Walton et al, 2016(Walton et al, , 2018. Additionally, they have been recognized ex situ within lithic clasts from suevites in two terrestrial impact structures: Ries, Germany (Stähle et al, 2008(Stähle et al, , 2022 and Xiuyan, China (Yin et al, 2021).…”

“…Temperatures of shock‐induced friction melts are 2000–3000°C for peak shock pressures >10 GPa (Hopkins & Spray, 2022; Sharp & DeCarli, 2006; Stähle et al., 2011). These temperatures do not represent the superheating of the rock, but rather the melting point of target mineral phases at the prevailing shock pressure.…”

“…Veins may widen slightly via the melting and assimilation of wall rock, although the contribution is negligible since the wall rock is at the same pressure as the newly formed melt, and thus requires higher temperatures to undergo melting during loading. Contact heating commonly results in the generation of microcrystallization textures, glasses, and high‐temperature‐facilitated phase transformations along vein walls and within suspended clasts (Biren & Spray, 2011; Hopkins & Spray, 2022; Martini, 1991; Spray & Boonsue, 2018; Walton et al., 2016; White, 1993). A depiction of shock vein formation is provided in Figure 1.…”