Vredefort Structure Research Articles

Occurrence and orientation of shatter cones in Pretoria group quartzites in the collar of the Vredefort ‘dome’: Impact origin precluded

Shatter cones have been found cutting completely indurated fault breccia associated with the down‐faulting of previously overturned Pretoria Group quartzites around the Vredefort structure. The evidence indicates that the shatter cones formed later than the overturning. Hence they cannot be considered to have been formed by an impact‐generated shock wave which penetrated the strata just before overturning.

Radioelement concentrations in the deep profile through Precambrian basement of the Vredefort structure

The Vredefort basement granite in South Africa forms the core of an updomed and overturned sequence of Precambrian strata. Slawson (1976) proposed that the granite core has also been overturned, exposing to view a thick section through the Archean crust, increasing in depth toward the center of the dome. Profiles across the Vredefort basement provide an opportunity for studying the distribution of U, Th, and K with depth in the upper and middle crust. Detailed radioelement concentration profiles across the granitic basement showed that (1) the central part of the basement core is depleted in Th and U relative to the perimeter, and the concentration of Th and U falls off regularly from the basement margin inward, (2) the distribution of the radioelements and the distribution of heat production with depth over the outer 8 km are consistent with an exponential depth function, (3) the felsic rocks of the central basement are characterized by high K/U and Th/U ratios and the ratios K/U, K/Th, and Th/U increase from the margin inward, and (4) mafic granulites, which occur in the central regions of the dome, have ‘normal’ K/U, K/Th, and Th/U ratios. High‐grade metamorphic environment resulted in the depletion of both U and Th relative to K in the greater proportion of the felsic rocks from the central regions of the Vredefort basement. Depletion of U was far greater than that of Th. The normal Th/U, K/U, and K/Th ratios in mafic rocks, in close proximity to the felsic rocks, suggest that mineral assemblages have a strong control on the behavior of the radioelements during regional metamorphism in the Vredefort basement.

Faulting mechanics in the collar rock of the vredefort ring structure

The fault pattern in the Vredefort collar is explained, in part, by a mechanical model using the theory of hollow cylinders. Centripetal movement of the collar along bounding concentric faults appears to have generated concentric, horizontal maximum principal stresses within the collar rocks which, in turn, have produced the observed primary and secondary fault patterns in the western, northwestern and part of the northern arcs of the collar. Most of the northern arc, however, does not have a fault pattern which conforms to this model but this may be explained if the Vredefort structure is open to the south.

The composition of spherules and other features on shatter cone surfaces from the Vredefort structure, South Africa

Minute spherules and other glassy objects have been found on the surfaces of shatter cones in the sedimentary rocks forming the collar around the Vredefort ring structure. The composition of twenty-five of these spherules was determined using an energy-dispersive X-ray analysis system attached to a scanning electron microscope. These results show that spherules can be classified into a Si-, Al-rich variety, an Fe-rich variety and an intermediate variety. At first, the origin of the spherules appeared enigmatic: we considered that they could have been of organic origin; that they may represent dust or fly ash particles due to atmospheric pollution; or that they may result from processes operating during shatter cone formation. For analytical and logical reasons, we argue that the first two possible modes of origin can be discounted. Instead, it is suggested that the spherules result from melts generated by friction on the host rock surfaces during the formation of shatter cones. The variations in composition of the individual spherules may indicate selective melting of different parts of the rock surface or that the melts may have been affected by liquid immiscibility processes. This argument is reinforced by the striking analogy between the particles discussed here and those found in lunar samples.

Clinoeulite (magnesian clinoferrosilite) in a eulysite of a metamorphosed iron formation in the vredefort structure, South Africa

A unique clinopyroxene (En19Fs78Wo3), clinoeulite, space group P21/c, $${\text{(Fe}}_{{\text{1}}{\text{.48}}} {\text{Mg}}_{{\text{0}}{\text{.37}}} {\text{Mn}}_{{\text{0}}{\text{.08}}}^{{\text{2 + }}} {\text{Ca}}_{{\text{0}}{\text{.05}}} {\text{Al}}_{{\text{0}}{\text{.01}}} {\text{)}}_{{\text{1}}{\text{.99}}} {\text{ [Si}}_{{\text{2}}{\text{.01}}} {\text{O6],}}$$ contains sharp exsolution lamellae of ferroaugite (En17Fs43Wo40) from which the former presence of a ferropigeonite near En17Fs70Wo13 can be calculated. This two-pyroxene intergrowth is the main component of a eulysite containing also magnetite, olivine (Fo9Fa86Te5), quartz, oligoclase-K feldspar inter-growth, and retrograde cummingtonite with about 76 % grunerite end member.

Symplectitic cordierite-orthopyroxene-garnet assemblages as products of contact metamorphism of pre-existing basement granulites in the Vredefort Structure, South Africa, and their relations to pseudotachylite

Former idioblastic garnet crystals of an Archaean granulite with Mg/(Mg+Fe+Mn)-ratios (= M) near 0.40 were converted to symplectitic pseudomorphs consisting mainly of cordierite (M=0.61), orthopyroxene (M=0.40), and a relic garnet with a new composition (M=0.18), during a static metamorphism which is related in time to the Vredefort event. On the basis of experimental data for the continuous reaction garnet+quartz=orthopyroxene+cordierite the conditions of metamorphism were near 5 kb, 700 °C. Orthopyroxenes crystallized initially as minute, myrmekitic grains with metastable excess Al contents up to 13 mol% Al2O3 and, through grain growth and Al exsolution, transformed into dense aggregates of coarser crystals with equilibrated compositions near 3 mol% Al2O3. In the absence of free silica hercynitic spinel appears as an additional phase that coexists with cordierite, orthopyroxene, and garnet of more magnesian compositions than in the case of silica saturation. Pseudotachylite veins crosscutting the hornfelsed granulite consist of pyroxene of variable Al contents, two feldspars, opaques, and quartz, and are finely recrystallized. There is textural and mineral chemical evidence from both the Al-contents of the pyroxenes and the Mg/Fe distribution among the phases adjacent to the pseudotachylite that these veins were emplaced late during the static metamorphism, which was probably caused by a local, deep-seated magma diapir. On this basis, the meteorite impact hypothesis for the origin of the Vredefort Structure is considered less likely than a mechanism of internal origin which can be made responsible for the close link in time and space between static metamorphism and the high strain rate deformation that produced the pseudotachylite.

Spherules on Shatter Cone Surfaces from the Vredefort Structure, South Africa

Spherical particles of silicate composition occur on the surface of some shatter cones from the collar rocks around the Vredefort structure, South Africa. They are best developed on shatter cones from a shale horizon but are also found on more arenaceous rocks and banded ironstones. They have not been found on shatter cones from the purer quartzites.

Paleomagnetic Evidence Relevant to the Origin of the Vredefort Ring

The time of emplacement of three masses of alkali granite and associated syenites, intruded into the outward-overturned collar of sedimentary rocks is crucial to choosing between a meteorite-impact or terrestrial origin for the Vredefort structure. These intrusives contain pseudotachylite, shatter cones, and microscopic evidence of shock deformation, and their solidification must be preimpact if the structure is an astro-bleme. However their field relationships are considered to indicate that they were emplaced after the overturning, and hence that their shock features are unrelated to the mechanism of overturning. Radiometric age measurements have shown that the alkali granites are at most 70 m.y. older than the basic granophyre (1,970 m.y.) of a poststructure ring-dike complex. Relatively little polar wander might be expected in such a short interval. The paleomagnetism of this granophyre, the alkali granites, and other relevant intrusive bodies has been investigated to determine whether the paleomagnet...

Die Vaalrivierdreineringstelsel in Die Parysgebied

Abstract The erosional history of the superimposed Vaal River in the Parys area started on the horizontal Karroo strata as a normal consequent stream and later exposed more resistant and older rocks. Several terraces are found in the area. These result from erosion to local base levels, which are formed by quartzites transvert to the river course. The trellis drainage of the sedimentary part of the Vredefort structure results from structural control while jointing influences streams on the central granite.