Research Proposal
on the
Vredefort Structure


Cover Letter

To all friends of the Kaapvaal craton: understand The Vredefort Structure and understand the Kaapvaal craton.

The Vredefort structure (widely believed to be an impact crater) is uniquely situated near the centre of the Kaapvaal craton. Large scale updoming and overturning provides a window into the deep Archaean geology of the Kaapvaal craton. We believe that the rocks exposed in the Vredefort structure can provide information on key questions relating to the issues raised in the joint US-South African project, "The anatomy of an Archaean craton: The evolution of the South African Continental Lithosphere" Four project proposals are attached: We invite all members of the Kaapvaal craton "team" to participate in any of these projects and would welcome any suggestions on how best to achieve our goals.

Please note that should any participant of the Kaapvaal craton project wish to visit Vredefort, a one or two day field trip can be arranged. Contact: HART@SchonLAN.SRC.Wits.ac.za

Current Kaapvaal participants: Rodger Hart, Maarten de Wit and Marian Tredoux.

Other associated collaborators from already existing research projects the inception of which predates the start of the K.V.C. project: Marco Andreoli and Desmond Moser.

The Vredefort crust on edge profile: its contribution to the Kaapvaal craton program.

Project A

Investigation of the coupling between craton crust and mantle and the timing of craton assembly.

Background

The Vredefort structure consists of a semicircular series of ridges of Precambrian strata, surrounding a central flat area consisting of Archaean granitic basement (See Fig.1 ). Research in the basement over the last decade has led to the conclusion that the crystalline basement rocks may have been overturned, exposing a substantial section of Archaean middle and lower crust which increase in metamorphic grade towards the centre of the structure. Detailed major, trace and isotopic chemical profiles (Hart et al. 1981a; Hart et al. 1981b; Hart et al., 1990a) across the Vredefort basement, are consistent with the premise that a section of the Archaean crust may be standing on edge (Fig.1 ). The deepest crustal levels, exposed near the centre, consists of a complex and heterogeneous granulite facies terrain made up of charnockites, leucogranofels (quartz feldspar granulites) and mafic and ultramafic rocks. The mafic and ultramafic rocks are described as high-grade meta-igneous rocks (mainly two pyroxene granulites and metapyroxenites) and metasedimentry (mainly banded ironstones, metapelites and calcsilicates) supracrustals. The metasediments and meta-igneous rocks occur as discrete units inter-layered with the felsic rocks (leucogranofels). Gravity data (Stepto, 1989) and borehole data suggest that the ratio of mafic and ultramafic rocks relative to the leucogranofels increases towards the centre of the structure. Rb-Sr and Th-Pb ages of ~ 3500 Ma are recorded for the mafic granulite and the supracrustal relics, whereas the surrounding leucogranofels record both a ~ 3500 Ma age (using Th-Pb) and a ~ 2800 Ma resetting of the Rb-Sr isotopic system (Hart et al. 1981b). Welke and Nicolaysen (1981) provide evidence of two uranium depletion events which affected both mafic and felsic rocks in the central terrain at ~3800 and ~2800 Ma.

Proposed Research

The mafic and ultramafic rocks found in the core region of the Vredefort crust may represent some of the oldest preserved fragments of simatic crust in the Kaapvaal craton. How these rocks were emplaced into the lower crust and their relationship to the surrounding quartzo-feldspathic granulites is unclear. However, the presence of supracrustal rocks in the high-grade granulite terrain of the Vredefort lower crust provide strong evidence that tectonic processes might have been involved in the formation of granulite facies rocks and the transport of supracrustal lithologies to lower crustal depths. Although much of the Rb/Sr, Th/Pb and Pb/Pb isotopic analyses (Hart et al. 1981b) done on these rocks has indicated mid-to late-Archaean ages, dating systems using incompatible elements have not been able to distinguish the mafic and ultramafic "xenoliths" from events that have affected the surrounding felsic rocks. The Re- Os dating system however, has proved to be successful in dating old crust forming events involving mafic and ultramafic lithologies (e.g. Reisberg and Lorand 1995; Carlson, 1995), and may provide a more "robust" dating system which may see through the complex thermal and tectonic history of the Vredefort lower crust. A Re/Os and U/Pb zircon dating program on a suite of the mafic and ultramafic xenoliths may provide valuable information on the coupling between cratonic crust and mantle and the timing of craton assembly.

Project B

Investigation of mid-crustal (Conrad?) discontinuity and tectonic stacking of continental lithosphere slabs.

Background

The rocks of the upper (and outer) part of the Vredefort crystalline crust are very different from those found in the core of the structure (see above) and petrographic, chemical and isotopic evidence all suggest that the upper and the lower crust have undergone very different styles of evolution. The upper crust is made up of a differentiated sequence of strongly layered granites, granodiorites and trondhjemites in amphibolite facies. The age patterns in the upper Vredefort crust are significantly different to those seen in their deeper counterparts. Whole rock isochrons of ~3050 Ma were obtained from three decay schemes, viz. Rb-Sr, U-Pb, and Th-Pb and significantly, there is no evidence of the ~ 3500 Ma age recorded in the centre of the structure. In general however, the relationship between the upper and lower Vredefort crust is not well constrained, as these two major rock segments are found in close proximity at only one locality, termed the Vredefort discontinuity" (Fig.1 @www) which can be traced over a distance of a few km. The boundary zone is relatively sharp (40 m) and is characterized by highly brecciated rocks and the extensive development of pseudotachylite. The rocks adjacent to the discontinuity show intense ductile deformation and shearing (Hart et al. 1990a). Hart et al. (1990a) suggest that the upper and the lower parts of the Vredefort crystalline crust were juxtaposed during intracratonic thrusting circa 2.8 Ga ago, and that the Vredefort discontinuity, may represent a possible mid-crustal ("Conrad") seismic discontinuity. Reflection seismology carried out in the Welkom area some 100 km south-west of Vredefort (Durrheim 1986), indicate the presence of a relatively strong reflector underlying the Witwatersrand basin at depths of about 7.5 km into the basement, which is about the same distance of the Vredefort discontinuity from the basement-sediment contact along the crust on edge profile.

Proposed Research

Although limited in aerial extent, the Vredefort discontinuity may provide one of the few examples of a mid-crustal discontinuity exposed at surface. In this current research program we propose to undertake a detailed zircon dating program and structural fault mapping across the type area of the Vredefort discontinuity:

1) To confirm the age difference between the upper and lower Vredefort crust using precise dating techniques.
2) To establish the time of tectonic juxtaposition between these two terrains.

Project C

Three-dimensional study of a large impact crater using reflection seismology

Background

The Vredefort structure is one of the largest known impact craters on Earth. The morphology of this structure resembles that of the deep central zone of a large complex crater. Therriault et al., (1993) estimates an original diameter of as much as ~300 km. However, the south-eastern portion of Vredefort is hidden beneath a cover of Palaeozoic Karoo rocks and the total geometry and size of the structure is still unresolved. Reimold and Gibson (1996) assumed that, in cross-section, the structure is roughly symmetrical, and resembles a cross-section through a small crater with central uplift and shallow dipping strata. They conclude that the central uplift is about 13 km. A 30 milligal positive gravity anomaly near the centre of the structure (indicative that the central region is underlain by a body of dense mafic or ultramafic rock) is interpreted by Reimold and Gibson (1996) as an intrusive body possibly related to the Bushveld complex. Hart et al. (1990a) demonstrated that, in the exposed northwestern sector of the crater, the rim strata as well as the outer parts of the basement rocks are all vertical to overturned. They provide evidence to suggest that the dips in the remainder of the basement are also near-vertical, and there is no change in dip from the north-west basement margin inwards towards the core of the structure, where the section is truncated by a southeast trending fault. Thus almost the entire ~ 36 km section of crust (including a palaeo-Moho) is thought to be exposed in the core of the structure. A borehole located close to the peak of the central gravity anomaly indicates that ultramafic rocks occur just beneath the surface, and persist to a depth of at least 300m (Hart et al. 1990b). The lithologies sampled by the borehole are Archaean hornblende-bearing harzburgites that bear no resemblance to Bushveld material. These rocks are interpreted by (Hart et al. 1990b) to represent the upper mantle beneath the Kaapvaal crust. Proposed Research The position of the Vredefort structure near the centre of the Kaapvaal craton, and consequently near the centre of the array of seismic stations deployed across the craton provide a unique opportunity to study the structure of a large impact crater in three dimensions. Available seismic (vibroseis) data over the northwestern segment of the crater (Durheim, 1986) reveals very little structural detail within the central core region of the structure. The reasons for the lack of reflectors is unclear however, it is possible that catastrophic breakup of the geological structure during impact might have severely affected any pre-impact fabric. Alternatively, it is possible that much of the structure in the central region is now vertical and therefore does not show up on the seismic profile. We propose that experiments could be designed to use the current seismic array across the craton to investigate the deep roots of the Vredefort structure in three dimensions, with the aim of answering three key questions:

1). What is the size of the Vredefort structure both aerial and in depth (i.e. what is the volume of rock that has been catastrophically disrupted by impact. ?

2). What is the geometry of the lithologies in the core region of the structure and to what extent have lower crustal and upper mantle rocks been uplifted to surface?

3). Is Vredefort an impact structure.??

Project D

Investigation of heat flow and temperature distribution in the continental lithosphere.

Background

The postulated ~36 km section exposed in the Vredefort structure, provide an opportunity to study the distribution of the heat producing elements (U, Th and K) with depth in the continental crust (Hart et al., 1981b; Nicolayson et al., 1981) . Radioelement profiles across Vredefort have been used to calculate the contribution of crustal radioactive heat production to surface heat flow in the Kaapvaal craton (average estimated at ~ 46 mWm-2). These measurements indicate that about 70% of the total heat flow comes radioactive heat generation in the crust, leaving about 14 mWm-2 to come from the mantle beneath the Kaapvaal craton. Proposed Research The radioelement profiles taken from margin to centre showed that the concentration of U, Th and K decreased exponentially inwards towards the centre of the structure (Hart et al., 1981b) and that the central region is depleted in the heat producing elements (< 0.3 ppm U and < 7 ppm Th) compared to crustal average (U~3ppm and Th ~15ppm). Boreholes drilled in the central region of the Vredefort structure indicate that much of this region is underlain by mafic and ultramafic rocks that are even further depleted in the heat producing elements. In short the centre of the Vredefort structure is characterized by a region about 20 to 30 km in diameter that has extremely low heat production compared to the remainder of the Kaapvaal craton. Boreholes drilled in the central region of the Vredefort structure provide the opportunity for heat flow measurements in a region that has been stripped of its heat producing elements during the 2.0 Ga impact event. Heat flow measurements in the central region of the Vredefort structure could provide important information on the mantle contribution to surface heat flow beneath beneath the Kaapvaal craton.

References


Last Updated: June 12 1997

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