The nature, distribution and genesis of the coesite and stishovite associated with the pseudotachylite of the Vredefort Dome, South Africa

Structural analysis of the collar of the Vredefort Dome, South Africa—Significance for impact‐related deformation and central uplift formation

“…The latter may indicate formation of these fractures in conjunction with the large-scale folds and faults, while the former might be compatible with either the shock compression stage or the final collapse of the central uplift (see Discussion section). Morphologically similar breccia-filled fractures in the core of the dome ) and the Central Rand group quartzites in the northeastern collar (Martini 1991) have been identified as shock features because of the localization of shock metamorphic effects along their margins. Thin psammitic layers within the pelitic rocks locally show an orthogonal to rhomboidal fracture pattern on bedding surfaces involving up to three sets of closely spaced (typically mm-to cm-scale spacing and tens of centimeters long) fractures, similar to the pattern seen in thinly bedded quartzite units (Fig.…”

“…1a and 1c). Evidence of the impact origin of the dome is largely restricted to rocks within a 30-35 km radius of its center and includes shatter cones (Hargraves 1961;Manton 1962Manton , 1965Albat 1988;Albat and Mayer 1989;Nicolaysen and Reimold 1999), planar microdeformation features in quartz (e.g., Lilly 1978;Fricke et al 1990;Grieve et al 1990; Leroux et al 1994) and zircon (Kamo et al 1996;Gibson et al 1997), coesite and stishovite (Martini 1978(Martini , 1991, microdeformation features, recrystallized diaplectic glass and shock melts in feldspars , impact melt breccia dikes (Reimold et al 1990;Koeberl et al 1996), and extremely voluminous pseudotachylitic breccia dikes (Dressler and Reimold 2004;Reimold and Gibson 2005). While these features have been studied in some detail, only limited investigation has been conducted of the larger-scale structures (faults and folds) in the collar of the dome by, among others, Manton (1962Manton ( , 1965 and Lilly (1978).…”

Original size of the Vredefort Structure: Implications for the geological evolution of the Witwatersrand Basin

“…In the original paper citing evidence for an impact origin, Dietz (1961) estimated the structure diameter at 1200 km, based on a width of -60 km for a "great ring syncline" surrounding the core. Martini (1991) estimated the diameter of the structure at -150 km, based on a "circular fault scarp", while Bishopp (1962) proposed a diameter of -160 km, based on the outer diameter of the surrounding "dolomite" (Transvaal Supergroup) basin. Grieve and Masaitis (1994) estimated the original diameter at -300 km, based on the occurrence of roughly concentric structural features as shown by McCarthy et al (1990).…”

“…The metasedimentary and metavolcanic rocks occur as a semicircular series of prominent hills in the northern half of the area, while in its southern half, the Vredefort structure s partially covered by younger sediments and volcanics (Carboniferous to Permian) of the Karoo Supergroup. The generally circular form of the structure, its uplifted central core, the occurrence of shatter cones, stishovite, coesite, and planar deformation features (PDFs:l in quartz and zircon, have been used to identify the Vredefort structure as an eroded remnant of a very large, complex impact structure (e.g., Dietz, 1961;Hargraves, 1961;Carter 1965Carter , 1968Manton, 1965;Martini, 1978Martini, , 1991Grieve et a/., 1990;Leroux et a/., 1994;Kamo et a/., 1995).…”

Preferred orientation in experimentally deformed stishovite: implications for deformation mechanisms

“…Stishovite was first produced experimentally (Stishov and Popova 1961) and found to have a rutile structure (space group P4 2 /mnm, Stishov and Belov 1962;Sinclair and Ringwood 1978;Ross et al 1990) with its silicon in octahedral rather than tetrahedral coordination, as is characteristic of lower-pressure SiO 2 polymorphs. Stishovite has since been documented in meteorites such as the Shergotty (El Goresy et al 2004); at meteorite impact sites such as Ries Crater, Germany (Shoemaker and Chao 1961;Chao and Littler 1963), Meteor Crater, Arizona, USA (Chao et al 1962), and Vredefort crater, South Africa (Martini 1978(Martini , 1991; and in high-pressure metamorphic environments (Liu et al 2007). First principles calculations (e.g., Cohen 1991; Karki et al 1997;Teter et al 1998;Lee and Gonze 1997), Landau theory (Carpenter et al 2000), Raman spectroscopy experiments (e.g., Kingma et al 1995), and X-ray diffraction experiments (e.g., Tsuchida and Yagi 1989;Ross et al 1990;Kingma et al 1996;Andrault et al 1998;Hemley et al 2000) find stishovite to be stable from ~10 to 50 GPa, corresponding to 300-1,200 km depth in the Earth, so a significant portion of the upper and lower mantle.…”