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Pseudotachylites of the Vredefort Dome and the surrounding Witwatersrand Basin, South Africa

1992; Geological Society of America; Linguagem: Inglês

10.1130/spe293-p177

2331-219X

W. U. Reimold, W. P. Collision,

Geotechnical and construction materials studies

The Vredefort Structure is the type locality for pseudotachylite, a type of clast-laden melt breccia often observed in tectonic zones of brittle or brittle-ductile deformation and generally believed to be the result of frictional melting. Pseudotachylitic breccia is also abundant in the Sudbury Structure and the Roter Kamm impact crater, as well as in the northern part of the Witwatersrand Basin surrounding the Vredefort Dome. In order to facilitate comparison between pseudotachylites from Vredefort and from impact structures and tectonic settings, the existing data base is reviewed and recent field and laboratory observations summarized. Modes of breccia occurrences, petrographic appearance, the distribution and orientation data for Vredefort and Witwatersrand pseudotachylite, as well as the complex temporal relationships between Vredefort pseudotachylite and other deformation phenomena in the structure are reviewed. Geometrically, Vredefort pseudotachylites are generally similar to those in tectonic settings. Comparison of chemical compositions of pseudotachylite and host rock pairs leads to the conclusion that preferential melting of certain minerals (first hydrous ferromagnesian minerals, then feldspar minerals and some quartz) is involved in pseudotachylite formation. This process has also been recognized for the formation of tectonic pseudotachylites and pseudotachylite from the Rochechouart impact structure. The Ar chronological record and field observations for Vredefort and Witwatersrand pseudotachylite strongly suggest multiple pseudotachylite-forming events in the Vredefort Dome.