Formation of central uplift structures in large, complex impact craters: The role of pseudoachylitic in the development of the Vredefort Dome, South Africa

• Applicant: Professor Dr. Wolf Uwe Reimold
• Subject Area: Palaeontology
• Term: from 2007 to 2013
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 41469035

This proposal is for a continuation of the previous application (RE 528/5-1), with respect to both the proposed goals and the work schedule. Pseudotachylitic breccias (PTB) are the most prominent impact-induced deformation phenomenon in the Vredefort Dome, the eroded central uplift of the 2.02 Ga, originally 250 km wide Vredefort Impact Structure, South Africa. Such melt breccias occur in microscopic veins and up to kilometer long and 10s of meters wide breccia zones. The topic central to this project is the controversy about the origin of these melt breccias, by either (1) shearing (friction melting); (2) shock compression melting; (3) decompression melting immediately after shock propagation; (4) combination of these processes; and (5) intrusion of allochthonous impact melt. Resolving this problem requires detailed multidisciplinary (field and laboratory, structural geological, microdeformation, petrographic, and geochemical) analysis to characterize the enigmatic breccias and modes of occurrence. Understanding the genesis of PTB allows then to assess the role of PTB during formation and collapse of the uplift structure. This study uses a different approach from the previous, largely macroscopic outcrop analysis by microstructural and (micro)chemical analysis of a polished 3 x 1.5 m granite slab from a dimension stone quarry in the core of the Vredefort Dome and comparative macro- to mesoscopic quarry analysis. Investigation of microfracture and melt breccia orientation, density and 3D geometry, as well as chemical, mineralogical and textural observations from the groundmass of pseudotachylitic breccia and respective host rock samples from the Rand Granite Quarry and the granite slab has been carried out. Microchemical analysis has shown that variable and localised chemical compositions in very thin PTB veinlets support local (grain scale) origin of these melts. What remains is to unravel the problem of how this melt originated in the first place. First microchemical investigations of microfractures and PTBs have indicated that formation of very small veinlets involved local melting, likely during the early shock compression phase. Evidence for friction melting is very limited, and our results exclude PTB generation by intrusion of impact melt. Additional structural and microchemical evidence to assign the exact mechanism(s) for breccia genesis is needed, especially with respect to melt breccia generation in intermediate to mafic host rock. It is planned to extend the work to samples from other quarries in the northern and western part of the Vredefort Dome. Samples already exist and are ready for analysis. Furthermore, the process of decompression melting as a viable mechanism for melt breccia formation has not been investigated to date, and we propose to test this hypothesis by numerical modelling. We apply for a 1-year extension of this project in order to (1) complete the petrographic-microchemical data acquisition for further investigation of the role of lithological control on melt breccia chemistry and the possible link between impact-induced shock features in the host rocks to the breccias and, thus, the occurrence of PTB veins; and (2) to develop a numerical model of melt formation by decompression melting at different radial distances from the center of the dome.

• DFG Programme: Research Grants
• International Connection: Austria
• Participating Person: Professor Dr. Christian Koeberl