Origin of pseudotachylite in the lower Witwatersrand Supergroup, Vredefort Dome (South Africa): Constraints from metamorphic studies

Small, sub-millimetre to centimetre thick, pseudotachylite veins in metapelitic rocks of the lower Witwatersrand Supergroup in the collar of the Vredefort Dome are recrystallized to a biotite+/-cordierite paragenesis, indicating the attainment of lower amphibolite facies metamorphic conditions (T greater than or equal to 500 degrees C) following their formation. The veins occur along ubiquitous anastomosing fracture networks displaying millimetre-to centimetre-scale displacements and truncate, and incorporate fragments of, an older, ca. 2.05 Ga, mid-amphibolite facies, peak metamorphic assemblage. The post-pseudotachylite metamorphism is attributed to the same event that caused highly variable annealing of ca. 2.02 Ga shock microdeformation features in rocks across the dome. Together with the disproportionately large volume of pseudotachylite in the rocks relative to that found in tectonic provinces, and the absence of evidence of genetically related large-scale shear or fault zones, this metamorphic evidence suggests an origin for the pseudotachylite related to the 2.02 Ga Vredefort bolide impact event that produced the dome. Possible origins for the pseudotachylite include shock brecciation and melting, post-shock decompression melting, and friction melting associated with the post-impact high-velocity excavation and modification phases of crater formation. The post-pseudotachylite metamorphism is attributed to the combined effects of an elevated pre-impact crustal geotherm (similar to 25 degrees C/km) related to the waning stages of the 2.05 Ga peak metamorphism, release of elastic strain energy from the impact shock event and contact effects beneath the impact melt body.