DEFINITION OF KAAPVAAL CRATON, SOUTH AFRICA

The Archean Kaapvaal craton of southern Africa contains some of the world’s oldest

and most intensely studied Archean rocks, yet nearly 86 percent

of the craton is covered by younger rocks. The craton

covers approximately 363,000 square miles (585,000 km2)

near the southern tip of the African continent. The craton is

bordered on the north by the high-grade Limpopo mobile

belt, initially formed when the Kaapvaal and Zimbabwe cratons

collided at 2.6 billion years ago. On its southern and

western margins the craton is bordered by the Namaqua-

Natal Proterozoic orogens, and it is overlapped on the east

by the Lebombo sequence of Jurassic rocks recording the

breakup of Gondwana.

Most of the rocks comprising the Archean basement of

the Kaapvaal craton are granitoids and gneisses, along with

less than 10 percent greenstone belts known locally as the

Swaziland Supergroup. The oldest rocks are found in the

Ancient Gneiss complex of Swaziland, where a 3.65–3.5-

billion-year-old bimodal gneiss suite consisting of interlayered

tonalite-trondhjemite-granite and amphibolite are

complexly folded together with migmatitic gneiss, biotitehornblende

tonalitic gneiss, and lenses of 3.3–3.0-billionyear-

old quartz monzonite. Several folding and deformation

events are recognized from the Ancient Gneiss complex,

whose history spans an interval of 700 million years, longer

than the entire Phanerozoic.

There are six main greenstone belts in the Kaapvaal craton,

the most famous of which is the Barberton greenstone

belt. Although many studies have attempted to group all of

the greenstone sequences of the Kaapvaal craton into the term

Swaziland Supergroup, there is little solid geochronologic or

other evidence that any of these complexly deformed belts are

contemporaneous or related to each other, so this usage is not

recommended. Other greenstone belts include the Murchison,

Sutherland, Amalia, Muldersdrif, and Pietersburg belts. U-Pb

isotopic ages from these belts span the interval of 3.5–3.0 billion

years ago, a period of 500 million years. The greenstone

belts include structurally repeated and complexly folded and

metamorphosed sequences of tholeiitic basalts, komatiites,

picrites, cherts (or metamorphosed felsic mylonite), felsic lava,

clastic sediments, pelites, and carbonates. Possible partial

ophiolite sequences have been recognized in some of these

greenstone belts, particularly in the Jamestown section of the

Barberton belt.

One of the long-held myths about the structure of greenstone

belts in the Kaapvaal craton is that they represent steep

synclinal keels of supracrustal rocks squeezed between diapiric

granitoids. However, detailed structural studies of the

Murchison greenstone belt have established that there is a

complete lack of continuity of strata from either side of the

supposed syncline of the Murchison belt, and that the structure

is much more complex than the pinched-synform model

predicts. Downward-facing structures and fault-bounded

panels of rocks with opposing younging directions have been

documented, emphasizing that the “stratigraphy” of this and

other belts cannot be reconstructed until the geometry of

deformation is better understood; early assumptions of a simple

synclinal succession are invalid.

Detailed mapping in a number of greenstone belts in the

Kaapvaal craton has revealed early thrust faults and associated

recumbent nappe-style folds. Most do not have any associated

regional metamorphic fabric or axial planar cleavage,

making their identification difficult without very detailed

structural mapping. Furthermore, in some cases, late intrusive

rocks have utilized the zone of structural weakness provided

by the early thrusts for their intrusion.

Elucidation of the structure of South African greenstone

belts has undergone a recent revolution. Several differ-

ent tectonic events are responsible for the present structural

geometry of the greenstone belts. Early regional recumbent

folds, thrust faults, inverted stratigraphy, juxtaposition of

deep and shallow water facies, nappes, and precursory olistostromes

related to the northward tectonic emplacement of

the circa 3.5 Ga Barberton greenstone collage on gneissic

basement have been documented. The thrusts may have

been zones of high fluid pressure resulting from hydrothermal

circulation systems surrounding igneous intrusions, and

are locally intruded by syn-tectonic 3.43–3.44-billion-yearold

felsic igneous rocks. Confirmation of thrust-style age

relationships comes from recent U-Pb zircon work, which

has shown that older (circa 3.482 ± 5 Ga) Komatii Formation

rocks lie on top of younger (circa 3.453 ± 6 Ga)

Theespruit Formation.

The Pietersburg greenstone belt is located north of the

Barberton and Murchison belts, near the high-grade Limpopo

belt. Greenschist to amphibolite facies, oceanic-affinity pillow

basalts, gabbros, peridotites, tuffs, metasedimentary rocks,

and banded iron formation are overlain unconformably by a

terrestrial clastic sequence deposited during D2 northwarddirected

thrusting between 2.98 billion and 2.69 billion years

ago. Coarse clastic rocks deposited in intermontaine basins

are imbricated with the oceanic affinity rocks and were carried

piggyback on the moving allochthon. Syn-thrusting

depositional troughs became tightened into synclinal structures

during the evolution of the thrust belt, and within the

coarse-clastic section it is possible to find thrusts that cut

local unconformities, and unconformities that cut thrusts.

The granite-greenstone terrane is overlain unconformably

by the 3.1-billion-year-old Pongola Supergroup,

which has been proposed to be the oldest well-preserved continental

rift sequence in the world. Deposition of these shallow

water tidally influenced sediments was followed by a

widespread granite intrusion episode at 3.0 billion years ago.

The next major events recorded include the formation of the

West Rand Group of the Witwatersrand basin on the cratonward

side of an Andean arc around 2.8 billion years ago,

then further deposition of the extremely auriferous sands of

the Central Rand Group in a collisional foreland basin

formed when the Zimbabwe and Kaapvaal cratons collided.

This collision led to the formation of a continental extensional

rift province in which the Ventersdorp Supergroup was

deposited at 2.64 billion years ago, with the extension occurring

at a high angle to the collision. The latest Archean

through Early Proterozoic is marked by deposition of the

2.6–2.1-billion-year-old Transvaal Supergroup in a shallow

sea, perhaps related to slow thermal subsidence following

Ventersdorp rifting. The center of the Witwatersrand basin is

marked by a large circular structure called the Vredefort

dome. This structure, several tens of kilometers wide, is associated

with shock metamorphic structures, melts, and

extremely high-pressure phases of silica suggesting that it represents

a meteorite impact structure.

The Bushveld complex is the world’s largest layered

mafic-ultramafic intrusion, located near the northern margin

of the Kaapvaal craton. The complex occupies an area of

40,000 square miles (65,000 km2) and intrudes Late

Archean–Early Proterozoic rocks of the Transvaal Supergroup.

Isotopic studies using a variety of methods have yielded

age estimates of 2.0–2.1 billion years, with some nearby

intrusions yielding ages as young as 1.6 billion years. The

complex consists of several lobes with a cone-like form and

contains numerous repeating cycles of mafic, ultramafic, and

lesser felsic rocks. Several types of ores are mined from the

complex, including chromite, platinum group metals, cobalt,

nickel, copper, and vanadiferous iron ores. Nearly 70 percent

of the world’s chrome reserves are located in the Bushveld

complex. The mafic phases of the complex include dunite,

pyroxenite, harzburgite, norite, anorthosite, gabbro, and

diorite. The center of the complex includes felsic rocks,

including granophyres and granite.

Much of the Kaapvaal craton is covered by rocks of the

Karoo basin, including fluvial-deltaic deposits and carbonaceous

deposits including coal. The top of the Karoo Sequence

includes mafic and felsic lavas that were erupted soon before

the breakup of Gondwana 200 million years ago.

See also ARCHEAN; CRATONS; GREENSTONE BELTS; PRECAMBRIAN;

WITWATERSRAND BASIN.

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