601.full

Geological Society of America Bulletin

doi: 10.1130/0016-7606(1976)872.0.CO;2 1976;87, no. 4;601-611Geological Society of America Bulletin

M. P. COWARD, P. R. JAMES and L. WRIGHT

Northern margin of the Limpopo mobile belt, southern Africa

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Northern margin of the Limpopo mobile belt, southern Africa

M . P. C O W A R D P R JAMES* R Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, United Kingdom L. W R I G H T

ABSTRACT

The Limpopo mobile belt is a zone of complexly deformed Archean gneiss units and rocks of high metamorphic grade. The Limpopo belt separates Archean greenstone belts and gneiss units of lower metamorphic grade in the Rhodesian craton to the north from similar lower grade rocks in the Kaapvaal craton to the south. At the north-ern edge of the Limpopo belt (that is, the southern edge of the Rhodesian craton), volcanic and sedimentary rocks of the greenstone belts, dated at 2,600 m.y. B.P., were deposited on gneissic basement, dated at 3,500 m.y. B.P. The greenstone belts were intensely deformed (F,) into recum-bent folds and thrust slices before being in-truded by large diapiric masses of granite.

The subsequent deformation (F2 and F3) can be considered in terms of intracratonic block tectonics. During the main phase of deformation, which produced a regional finite strain fabric throughout the Limpopo belt and the southern part of the Rhodesian craton, the deformation can be considered as having been produced by the Rhodesian block moving to the southwest, parallel to (not across) the trend of the Limpopo belt. This movement produced horizontal short-ening across the cleavage, normal to the Limpopo trend, by about 50 percent in southwest Rhodesia and northeast Bots-wana; in southeast Rhodesia, however, the result was northeast-trending zones of heterogeneous simple shear with nearly horizontal sinistral movement parallel to the Limpopo trend.

During a later (F4) phase, the deforma-tion involved moving the Messina block, located in the central part of the Limpopo belt, west against the Rhodesian block, again nearly parallel to (not across) the Limpopo trend. In southern Rhodesia the junction between these two blocks is a zone of gently dipping mylonite, but in Botswana the junction is a steep shear zone. From the intensity of deformation within the mylo-nite zone, estimates of more than 40 km of

* Present address: Department of Geology, University of Adelaide, Adelaide, South Australia 5001.

displacement during this phase have been made. Key words: structural geology, strain analysis, Precatnbrian tectonics.

INTRODUCTION

In 1970 the Research Institute of African Geology at the University of Leeds began a project whose aim was to quantify the strain and displacement across Archean orogenic belts in Africa. The Limpopo belt which crosses parts of Rhodesia, Bots-wana, and South Africa (Fig. 1) was con-sidered ideal for such a study. The Limpopo belt was defined as a complexly folded belt of gneiss units of high metamorphic grade dated at approximately 2,600 m.y. B.P. (van Breemen and Dodson, 1972); the belt separates the Rhodesian Archean craton in the north from the geologically similar Kaapvaal craton in the south (Fig. 1). Both margins of the belt were exposed and not obscured by later orogenesis as in other Af-rican mobile belts of similar age (Clifford, 1970).

According to the literature (Cox and others, 1965; Mason, 1968, unpub. data; 1970), the Limpopo belt was thought to have symmetrical structure, with a central area of complexly folded gneiss units and sedimentary rocks in the amphibolite facies or hornblende granulite facies and with flanking, marginal areas defined by major shear zones (compare, Hepworth, 1967) and zones of granulite-facies rocks.

During the present project, however, the Limpopo belt was found to be more difficult to define than had been previously supposed. The main deformation that af-fects the Limpopo belt has been traced as far south as Murchison (Graham, 1974) well into the Kaapvaal craton (Fig. 1) and a long way north into Rhodesia, where it was impossible to distinguish structures of the Rhodesian craton from those of the Limpopo belt. It is the diffuse northern edge, between Rhodesian craton and Lim-popo belt, that will be described in this paper.

The Rhodesian craton is an area of greenstone belts separated by granite and gneiss units of various ages. From geochem-

ical evidence Viljoen and Viljoen (1969) and Anhaeusser and others (1969) consid-ered that the greenstone belts were origi-nally deposited on the ocean floor without basement granitic crust; all the granite now exposed between the greenstone belts was considered to be material later derived from the mantle. However, several geologists of the Rhodesian Geological Survey reported older gneissic basement material between the belts. Macgregor (1951) and Oldham (1970) reported a clear unconformity of greenstone belt material resting on base-ment, and Stowe (1971) suggested that there were several orogenic episodes before deposition of the main greenstone belt.

All the greenstone belts of the Rhodesian craton show a characteristic sequence from ultramafic and mafic volcanic rocks to silicic volcanic rocks and sedimentary rocks. On the basis of unconformities and similar rock types Macgregor (1947, 1951) subdivided these rocks into three systems: the Sebakwian, Bulawayan, and Sham-vaiian (in ascending order). This subdivi-sion is still retained by the Rhodesian Geological Survey but as lithostratigraphic "groups" rather than "systems" (Wilson, 1973). Hawksworth and others (1975) dated four of the greenstone belts in south-ern Rhodesia at around 2,600 m.y. B.P. This is considerably younger than the dates of 3,300 m.y. B.P. from granite units that intrude the Barberton greenstone belt in South Africa (Anhaeusser, 1973), and hence, the Rhodesian and South African greenstone belts are of different ages.

Within the Limpopo belt, the sedimen-tary rocks that are termed the Messina Formation (Sohnge and others, 1948) from their type occurrence near Messina in the northern Transvaal (Fig. 1) are of different facies from that of the sedimentary rocks on the edge of the Rhodesian craton. The Mes-sina Formation contains more limestone and more quartzite (which often bears fuchsite) and also carries anorthosite and gabbro-anorthosite layers (Bahnemann, 1973). According to Mason (1970, 1973) and Mackie and Oosthuizen (1973), the sedimentary rocks of the Messina Forma-tion were deposited on top of a granulite-

Geological Society of America Bulletin, v. 87, p. 6 0 1 - 6 1 1 , 12 figs., April 1976, Doc. no. 60413.

601

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6 0 2 COWARD AND OTHERS

Figure 1. Regional map of southern Africa showing the position of the main Ar-chean greenstone belts and the margins of the Limpopo mobile belt (after Mason, 1973). Post-Witwatersrand supracrustal rocks (about 2,600 m.y. old) are shown as "cover."

facies basement; Mason (1970, 1973) sug-gested a possible correlation of the Messina Formation with the Transvaal system of sedimentary rocks in South Africa (dated at 2,300 to 2,600 m.y. B.P.). Van Breemen and Dodson (1972), however, suggested that the sedimentary rocks of the Messina Formation may be considerably older and may be correctable with parts of the green-stone belts. Recent field work during the Leeds project supports the suggestion of van Breemen and Dodson; the Messina Formation has been through the same de-formation sequence as the main Rhodesian greenstone belts. A discussion of the rela-tive stratigraphic positions of the green-stone belts and the Messina Formation is, however, outside the scope of this paper.

The northern region of the Limpopo belt is marked by a granulite-facies terrane (Fig. 1) consisting of reworked cratonic Archean granite and greenstone material (Cox and others, 1965; Mason, 1973). A minimum age for the granulite-facies metamorphism is given by satellite dikes of the Great Dyke system that intrude the granulite terrane and that were dated at 2,580 m.y. B.P. (Robertson and van Breemen, 1970). A maximum age for the deformation is 2,600 m.y. the age of some granite units and greenstone belts of southern Rhodesia (Hawksworth and others, 1975), and hence, all the deformation must have taken place within a relatively short period of time.

It is possible to describe the structure of the Rhodesian greenstone belts, the Mes-sina Formation, and many of the gneiss, granulite, and granite units in terms of four

major deformation phases (F[ to FJ . In dis-cussing this deformation sequence, em-phasis is placed on the interpretation of fabrics rather than folds.

A penetrative fabric, which is designated F2, can be traced northward from the South African border well into the Rhodesian era-ton. F2 need not necessarily be synchronous throughout this area and is a finite strain fabric made up from numerous separate in-crements of deformation. Locally, the fab-ric is very weak; elsewhere it is intensified within shear zones. It is, however, the first major fabric to affect many of the granite and metasedimentary units and, as such, can be traced continuously across all the greenstone belts and most of the interven-ing granite and gneiss units.

Episodes of deformation and intrusion have been related to this fabric-producing deformation. Many of the granite units carry the F2 fabric throughout, and their emplacement must therefore predate this deformation. There are also important folds and thrusts that predate the F2 cleav-age; these have been designated F,.

In southwest Rhodesia and northern Botswana, a later fold phase (F3) has been recognized that has a trend similar to that of the F2 structures but that folds the F, fab-ric and locally produces a new axial planar schistosity.

Deformations postdating F3 are grouped as F. Again, there is no claim that the de-fo rma t ion termed F4 is synchronous throughout the Limpopo belt. Indeed, there is new geochronologic evidence (Hickman and Wakefield, 1975) to suggest that the F4 deformation in central Botswana is much

later than that in central Rhodesia. An upper limit to the deformation in Rhodesia is given by the Great Dyke, dated at 2,580 m.y. B.P. (Allsopp, 1965), that cuts across all the F2 structures (Robertson, 1968) and postdates local F4 folds and fabrics. In Botswana, deformation may have con-tinued much later. Mineral ages of approx-imately 2,000 m.y. (Nicolaysen, 1962) are supported by a Rb-Sr whole-rock isochron of 2,100 m.y. (Hickman and Wakefield, 1975), and these ages may indicate the time of the F4 deformation in Botswana.

STRUCTURAL EVOLUTION OF THE LIMPOPO BELT

Basement Older Than the Greenstone Belts

Anhaeusser (1973, p. 377) claimed that the lower parts of the greenstone belts are older than the adjacent granite units, and he made the bold statement, "nowhere can it be demonstrated that they [the lower vol-canic assemblages] overlie granite crust." Indisputable evidence, however, that the sedimentary rocks of the greenstone belts were deposited on an older basement comes from south of Shabani (loc. A, Fig. 3; Mac-gregor, 1951; Oldham, 1970; Bickle and others, 1975) where tilted but otherwise undeformed sedimentary rocks rest with clear unconformity upon an erosion surface cut into previously foliated granite (Fig. 2). The granite is part of a basement migmatite suite that crops out south and east of Sha-bani (Oldham, 1970) and that shows a long history of deformation. Within the migma-


NORTHERN MARGIN OF THE LIMPOPO MOBILE BELT, SOUTHERN AFRICA 6 0 3

171 ripple marks 17^ 1 s / ja/e [77\ siltstone G 3 [2] conglomerate |. greenstone [73 weathered granite P\1 granite with quartz-

leaf foliation

>//


ment containing granite, gneiss, and green-stone belts all of pre-Bulawayan age and hence there must be at least two ages of greenstone belts in Rhodesia. Recent geo-chronologic work by Hawksworth and others (1975) and Hickman (1974) has yielded dates of approximately 3,500 m.y. B.P. for these basement granite and gneiss units and approximately 2,600 m.y. B.P. for the Bulawayan greenstone belts. All the granite and gneiss units, however, were in-truded into or derived from sedimentary rocks, as evidenced by the inclusions of supracrustal material that are found in many of the early-formed granite units. In this respect, therefore, Anhaeusser (1973) appears to be correct in that, so far, no evi-dence has been found for a fundamental granite crust.

Early Deformation of the Greenstone Belts: The F, Phase

Most of the greenstone belts show evi-dence of deformation that occurred before the cleavage-producing F2 phase. The Shabani-Belingwe belt, for example, was folded into a major syncline before the F2 deformation (Fig. 4C); the main F2 cleavage cuts across the syncline with a similar

bedding-cleavage relationship on each limb. The tightness of the fold may be due partly to the superimposed F2 deformation, but the F2 deformation here is very weak. On the east side of the belt, although the beds are vertical, most of the rocks show no cleavage, the basal unconformity is pre-served intact, and sedimentary ripple marks are exceptionally well preserved in pelitic sedimentary rocks.

Similarly, in the Tati belt in northeast Botswana (Fig. 4B), the greenstone belts were deformed into an inclined syncline whose southern limb was overturned before the F2 deformation (Mason, 1970); Coward and James (1974) detected evidence of some slight flattening of the volcanic rocks that occurred during this early deforma-tion. The axial plane of the early fold trended east-northeast. In addition, the Bulawayan greenstone belt was folded into a large-scale, northeast-trending fold before F2 deformation (Fig. 4A). F! deformation is also present in the Gwanda (Fig. 8) and Fort Victoria greenstone belts (Coward and James, 1974).

The rocks of the Selukwe greenstone belt was nearly flat lying but fully overturned before the main phase of the Limpopo de-formation (Stowe, 1968; Cotterill, 1969).

Thus, conglomerate beds contain chrome-bearing fragments derived from the overly-ing but older chrome-bearing ultramafic rock (Cotterill, 1969). Thrust faults and zones of silicification were produced during the overturning (Stowe, 1968), but large areas show no penetrative cleavage, and much of the conglomerate shows no evi-dence of distortion. Stowe (1968) consid-ered that the nappe originated in a mylonite zone immediately south of the greenstone belt.

Although the F[ folding was locally tight, no penetrative fabric was produced, and the deformation may have taken place at a high crustal level or before the rocks were fully compacted. Deformation took place by bulk translation and rotation of the green-stone belts, and there may have been some shortening within the gneiss units. How much of this early deformation occurred is masked by intrusive granite and the later, more intense, cleavage-producing deforma-tion, and it has not therefore been possible to place quantitative estimates on the amount of shortening during this Fi phase.

Migmatization and Granite Intrusion

Within the greenstone belts, the F2 de-formation was preceded, accompanied, and followed by emplacement of many quartzi-tic and quartzofeldspathic veins and intru-sion of large bodies of diapiric "granite." The diapirs, dominantly tonalitic in com-

Jenya shear

Figure 4. Maps of the (A) Bulawayan, (B) Tati, and (C) Shabani greenstone belts. The F2 cleavage is shown by faint lines; the bedding, by heavy lines. The arrow in the map of the Bulawayan belt indicates the sheared and mylonitized granite contact with the greenstone belt. The double arrows in the map of the Tati belt indicate directions along which the ages of the rock units decrease. The stippled area at Shabani represents a large body of intrusive ultramafic rock.



Figure 5. Map of the northern margin of the Limpopo belt and southern part of the Rhodesian craton. The trend of the F2 cleavage is shown. Localities: B, Bangala Dam; T, Tod's Hotel.

position, intruded the earlier-formed base-ment gneiss units and cut across the F! structures in the greenstone belts in such a way that the resulting boundaries are ar-cuate and the whole greenstone belt may be star-shaped in plan (Anhaeusser, 1973). The mechanism of intrusion was locally that of stoping; the batholiths on the north-ern edge of the Tati greenstone belt, for ex-ample, produced no fabric in the adjacent greenstone but contain numerous large in-clusions "megaxenoliths" of green-stone material. Other granite intrusions were more forceful. Thus, a rim syncline surrounds granite that intruded the Mont D'Or area of Selukwe (Cotterill, 1969), and the greenstone in the southern part of the Tati belt was deformed to a distance of 1 km away from the contact of a large tona-lite batholith before the main F2 deforma-tion (Coward and James, 1974).

Within the Limpopo belt, the F2 deforma-tion was preceded by the segregation of quartzofeldspathic material, the emplace-ment of granite and pegmatite veins, and the intrusion of large sheets of granite, often porphyritic. Many of the migmatite units and the large sheets of granite may well be the deeper level equivalents the "roots" of the granite diapirs seen on the craton. Mason (1973) and Bahnemann (1973) con-sidered this granite to be basement to the Messina Formation. Much of the granite may indeed be remobilized basement, but

much also intrudes and contains inclusions of the sedimentary rocks of the Messina Formation.

In southern Rhodesia, granulite-facies metamorphism predated the F2 deforma-tion; south of the Buchwa Range, pyroxene granulite is intruded by porphyritic granite units (Robertson, 1968, 1973), which are themselves deformed by the main F2 phase. In northern Rhodesia, these porphyritic granite units remain in the amphibolite facies, but to the southwest near Tod's Hotel (Figs. 1, 5) granite units that invade granulite have been through a subsequent granulite-facies metamorphism. South of Fort Victoria, porphyritic granite units cut the F! folds but are heterogeneously de-formed by F2 and remain in the amphibolite facies. Farther south, near Bangala, large, locally porphyritic granite units cut the Fi folds but show granulite-facies mineral as-semblages. Hence, there are two or more phases of granite intrusion separated by the granulite-facies metamorphism. Through-out the area, however, where the F2 defor-mation was intensified within shear zones, the granulite-facies minerals were broken down to amphibolite- or greenschist-facies minerals.

The granulite units in the southern part of Rhodesia either overlie an area that once had a high thermal gradient, or they repre-sent material tectonically uplifted from lower in the crust.

Early Mafic and Ultramafic Intrusions

At Mashaba (west of Fort Victoria) and at Shabani, two large ultramafic igneous complexes intrude the early-formed migma-tite suites and parts of the greenstone belts and yet are deformed by F2 structures (Wil-son, 1973). According to Wilson, the possi-bly time-equivalent rocks on the Kaapvaal craton are the Usushwana complex of Swaziland, dated at 2,874 30 m.y. B.P. (Davies and others, 1970).

Along the eastern edge of the Matsitama belt, there is a similar large ultramafic in-trusion with serpentinite, locally showing cumulate textures of magnetite and altered olivine grains. This intrusion, however, has been extensively elongated by the F2 defor-mation and now outcrops as a string of pod-shaped bodies (boudinage).

In the central part of the Limpopo belt, there are widespread intrusive sheets of a pyroxenite-gabbro-anorthosite suite that may possibly be related to the intrusions described above. These mafic rocks intrude and contain inclusions of sedimentary rocks of the Messina Formation; in southeast Botswana and east of Messina, the mafic rocks locally show well-preserved cumulate textures. These rocks are useful for deter-mining original orientations (see Fig. 11). A suite of mafic dikes intrudes the gneiss and early-formed granite units of southwest Rhodesia and Botswana. Although often



V-1

LOWER GWANDA

Figure 6. Natural strain plots showing the intensity of deformation measured at localities within the Bulawayan, Shabani, Fort Victoria, Tati, and Lower Gwanda greenstone belts. The natural strain plots are of intensity of deviatoric strain (e,) against Lodes unit V, where V = + 1 for uniaxial oblate strain and V = 1 for uniaxial prolate strain. This type of plot has been ex-plained in detail by Hossack (1968). Percentage elonga-tion in the X direction (X 3s Y 5s Z) and percentage shortening in the Z direction are shown. The measure-ments in the Fort Victoria and Tati belts are after Cow-ard and James (1974).

tightly folded with the gneiss units, these dikes still locally retain their discordant re-lationship with the gneissic foliation. The cores of the large dikes retain igneous tex-tures and minerals although the edges of the dikes may be strongly deformed and they may be cut by ductile shear zones.

F2 Phase of Deformation

The penetrative fabric that can be traced south from Fort Victoria to South Africa and from Shabani west to Francistown in Botswana (Fig. 5) across greenstone belts and granite units has been designated F2. In the greenstone belts, this fabric is shown by the preferred or ientat ion of chlorite, hornblende, and mica; by pressure-solution stripes; and by the shape of deformed ob-jects such as pillows, spherulites, ocellae, and pebbles and grit-clasts in conglomerate (Coward and James, 1974). In the granite and gneiss units, F2 is shown by the pre-ferred orientation of minerals and by the shape of deformed grains and inclusions. In the coarse-grained granite and granulite units, the shape of the quartz grains may reflect the intensity of deformation: with increase in intensity of deformation, the grains become ellipsoidal and are eventu-ally drawn out into leaves. Ratios of greater than 30 to 1 in the X-Z plane have been re-corded from some of the more deformed quartz leaves in the Shashi area.

The intensity of F2 deformation varies. There are zones of high-intensity deforma-tion that surround enclaves where F2 de-formation is much less intense. These zones of high-intensity deformation increase in frequency and intensity away from the granulite units (toward the Rhodesian greenstone belts). Strain ratios have been measured from certain of the greenstone belts and are shown in Figure 6. Apart from a local intensification within shear zones, the F2 deformation is most intense in the Francistown-Shashi-Gwanda area in the west and south of Fort Victoria in the east. A more detailed investigation of this varia-tion in the F2 fabric is in progress in the east, south of Fort Victoria (P. R. James, in prep.).

In the area south of Shabani, the early-formed migmat i te and the Shabani-Belingwe greenstone belts show very little F2 strain (Fig. 6). There is only a weak cleavage; in more competent horizons, ocel-lae and variolites in pillow lavas remain spherical, and sedimentary structures re-main undeformed. Similarly, the Mashaba igneous complex northwest of Fort Victoria is strongly deformed only in the south; the main part of the complex is only gently folded and thrust over gneiss (compare, Wilson, 1968). This area east of Shabani can therefore be considered as part of a small craton, the older equivalent of the much larger cratons that dominated the

V FORT VICTORIA

crustal evolution of Africa throughout the remainder of Precambrian time.

Between the greenstone belts, the granite and gneiss units also show the F2 fabric and evidence of F2 ductile deformation. How-ever, the bodies of diapiric granite re-sponded to F2 deformation in different ways. Many deformed homogeneously with the gneissic matrix or greenstone; they show a pervasive fabric throughout and have been changed from circular to ellipti-cal in plan section. Other granite intrusions remained as massive competent bodies; in-ternally, they deformed heterogeneously, and localized mylonite zones were pro-duced. The term "billiard-ball tectonics" has been applied to this type of granite de-formation, in which the granite behaves as a solid, competent body within a less compe-tent matrix (Coward and James, 1974).

South of the main greenstone belts and granite units, the F2 fabric can be traced into granulite, the northern edge of which was thought by Worst (1962) to be a thrust because of the sharp transition between granulite- and amphibolite-facies rocks. In the area south of Fort Victoria, the north-ern edge of the granulite is marked by the intensification of the fabric within a mylo-nite zone with a downdip lineation and a south side upward sense of movement. This zone may represent such a thrust. In the western part of the granulite outcrop north of Tod's Hotel (Figs. 1, 5), however, the granulite contact is gradational (compare, Robertson, 1968), and in this area the granulite-facies rocks lie near the core of a large antiform. Thus, in the east, the granulite-facies rocks appear to be tectoni-cally emplaced by shear zones with a thrust sense, whereas in the west, these rocks have been uplifted in the core of a large fold.

Many of the granulite-facies rocks show



Figure 7. Map of the northern margin of the Limpopo belt and adjacent part of the Rhodesian craton showing the major shear zones. These shear zones accentuate the F2 foliation shown in Figure 5.

only weak F2 deformation, and many of the inclusions within them remain undeformed and show no fabric. However, steep zones of intense deformation cut across the granulite-facies rocks (Fig. 7) and locally produce a mylonite fabric; most of the de-formation within the area of granulite-facies rocks is concentrated within these narrow zones.

There is a regional swing of F2 fabric around the main area of low-intensity de-formation in central Rhodesia (Fig. 5). The fabric orientation changes from N60E south and west of Fort Victoria to N30W around Bulawayo. Much of this change is gradual, and there is no evidence that the two orientations relate to different defor-mation events, although locally the fabric may sharply change orientation and be intensified within a later shear zone or fold (Fig. 7).

Much of the F2 deformation throughout southern Rhodesia appears to have been by sliding along block boundaries. Deforma-tion was concentrated in zones between relatively undeformed material, which may be more competent granite, areas of granulite-facies rock, or the small craton east of Shabani. In the southwest, away from the zones of intense deformation, maximum shortening occurred across steeply dipping principal planes, and the maximum elongation direction was verti-cal. Within the zones of more intensive de-

format ion where the foliation wraps around the less deformed blocks, however, the maximum elongation direction varies in orientation and may be horizontal. The most prominent zones of intense deforma-tion with their direction of differential movement (compare, Bridgewater and others, 1973) are shown in Figure 7.


In southwest Rhodesia and northern Botswana, the F2 fabric is folded and locally intensified by later structures that have a similar northwest trend but which locally produce a new fabric. This new fabric (F3) is generally a steeply dipping crenulation cleavage, but at the edges of the Matsitama schist belt and within the Gwanda belt (Fig. 8), F3 folds are isoclinal and carry an in-tense, locally mylonitic axial planar fabric. F3 folds are also important in the central part of the Limpopo belt, but their geometry is obscured by later deformation.

Many localized variations in plunge of the F2 folds and stretching direction affect the plunge of the F3 folds, and many of the intensifications of the F2 fabric may be con-sidered as F3 in age.


The F2 and F3 fabrics and folds are de-formed by a series of structures loosely

termed F4. The F4 structures in central Rhodesia are a series of shear zones and folds that are cut by the Great Dyke and are therefore older than 2,580 m.y. In the southern part of Rhodesia and in the north-ern Transvaal and Botswana, the F4 struc-tures are more widespread. The F2 fabric is intensified in a major shear zone and strongly folded. Within central Botswana, on the basis of an Rb-Sr isochron, Hickman and Wakefield (1975) recorded a geo-chronologic event at around 2,100 m.y. B.P. that they correlated with the F4 defor-mation and metamorphism (this is F3 in the terminology of Hickman and Wakefield). It is possible, therefore, that the F4 structures in central Rhodesia and in southern Rhodesia and Botswana are very different in age, and hence, they will be described separately.

F4 Deformation in Central Rhodesia. Northwest of Shabani (Figs. 4, 9) the F2 fabric is tightly folded by north-north-east-trending structures that locally pro-duce a new axial planar fabric. This fabric is locally mylonitic; the Surprise shear zone (a 0.5-km-wide zone of mylonite west of Selukwe; compare, Stowe, 1974) and the Wanderer shear zone (east of Selukwe) may be of this age. These F4 structures are re-stricted to a narrow zone about 20 km wide, and it may be more than coincidence that this zone of F4 deformation is followed almost exactly by the Great Dyke.


608 COWARD AND OTHERS

0 10 20 30 Km

Figure 8. Map of the Gwanda belt showing the form of the F2 and F3 structures and fabrics in several subareas (outlines of the subareas shown on inset map).

Fabric of F2 age

Fabric of F3 age

Zones of F3 shearing

The F2 fabric and F4 structures north of Shabani are deformed by an east-trending shear zone, the Jenya-Mushandike shear (Fig. 9). From the curvature of the foliation into the shear and the displacement of simi-lar granite units across the shear (A. Mar-tin, 1973, oral commun.), the movement appears to have been sinistral and of 8- to 10-km magnitude. The Mushandike end of the shear, however, deforms older shear zones in the Mushandike granite and all structures in the Fort Victoria belt and ap-pears to have a dextral sense of movement (compare, Coward and James, 1974). In the west, the Jenya-Mushandike shear zone is cut by the Great Dyke but has undergone later movement that resulted in a 1-km dex-tral offset to the Great Dyke itself. This shear zone is therefore a locus of complex movement.

F4 Deformation in Southern Rhodesia, Northern Transvaal , and Central Bots-wana. In the southern part of Rhodesia, the F2 fabric, folds, and older structures are cut by a large-scale, gently southward-dipping shear zone (Fig. 9). Where this zone cuts rel-atively undeformed granite and granulite, it forms a new, nearly horizontal fabric, but

elsewhere, the older fabrics are rotated to produce an intense finite strain fabric that locally may be mylonitic or blastomyloni-tic. At the northern edge of the zone, de-formation is heterogenous, and lozenge-shaped blocks of undeformed material are separated by intensely sheared gneiss (Fig. 10); within the zone proper, however, the foliation has a nearly constant dip of be-tween 5 and 15SSE. On the mylonitic fo-liation surface, there is a well-developed lineation shown by elongate quartz leaves, feldspar augen, and so forth. The move-ment direction within the shear zone is pre-sumably marked by the trend of this linea-tion, which remains due east no matter what the orientation of the shear zone (Fig. 10). From the orientation of lineation, the sense of rotation of porphyroblasts, and the curvature of the foliation into the more de-formed gneiss, the movement can be seen to be dextral, with the upper gneiss units of the southern block moving westward over the northern block.

Near the Bulawayo-Messina road, the zone of sheared gneiss is about 2 km thick; in the east, near Chiredzi, the zone reaches 10 km in true thickness. Strained aggre-

gates, quartz leaves, and flattened folds within the sheared gneiss units show strain ratios in t heX-Z plane in excess of 15 to 1 which, if a dominantly simple shear process is assumed, indicates a shear strain greater than 4. The amount of movement must have been at least 8 km near the Beit Bridge road (Fig. 9) and possibly 30 to 50 km in the east.

The sheared and locally mylonitic gneiss grades upward and southward into less in-tensely sheared but still dominantly flat-lying gneiss units interbanded with sedi-mentary rocks belonging to the Messina Formation (compare, Mackie and Oos-thuizen, 1973). Within these gneiss units and sedimentary rocks there is evidence t h a t the fo l i a t ion was once s teeper ; gabbro-anorthosite within the sedimentary rocks east of Messina contains less de-formed cores in which the tectonic foliation is steep and nearly perpendicular to that in the more deformed edges. Many of the flat, sheetlike metagabbro-anorthosite bodies of the Messina area are intensely sheared; their present form may be due to this phase of horizontal shear deformation (compare, Graham, 1974).



Figure 9. Map of the northern margin of the Limpopo mobile belt and the Rhodesian craton. The form of the F4 structures and mylonite zones is shown. The F2 foliation is folded, but in the zone west of Chiredzi, it is intensified by F4 mylonitization. Localities: M, Messina; BB, Beit Bridge; BD, Baines Drift; P, Pikwe. Shear zones: Ai, Mushandike; J, Jenya; S, Surprise; W, Wanderer; L, Lethakane.

flat-lying gneiss units, the granulite-facies rocks of Rhodesia show no F4 deformation. The shear zone, therefore, has acted as a plane of dcollement between folded rocks above and the undeformed granulite-facies rocks below. The difference in amount of movement along the shear zone is presum-ably taken up in the folds, and the Messina sedimentary rocks and gneiss units can be considered to have been crumpled over the granulite-facies rocks while sliding along a basal mylonite zone.

In Botswana the F4 folds are more wide-spread. They can be traced from Francis-town in the north to Baines Drift in the southeast where they appear to be part of the same group of F4 folds as in the Messina area (Mason, 1973). However, F4 struc-tures are only weakly developed across a zone of massive granite about 20 km south of Shashi, where F2 deformation is also very weak. Immediately south of this zone of granite, the F4 folds are very tight. Locally, they have a mylonitic axial planar fabric and often become obliterated by intensely developed mylonite. This mylonitic axial planar fabric and the tight folds have been

Figure 10. Detailed map of the edge of the main mylonite zone along the Msane River and tributaries near Towla, southern Rhodesia, and stereographic plot showing the variation in orienta-tion of the mylonite at the edge of this zone (poles to foliation shown by triangles) and the orientation of the stretching direction within the east-trending plane (lineations shown by dots).

The tectonically flattened and sheared gneiss units and sedimentary rocks south of the main shear zone are folded into a series of periclinal F4 folds, with axial planes trending approximately N10-30E. These F4 folds become more intense to the south and east; east of Messina, they are isoclinal, and flattening strains measured from ptyg-

matic quartzofeldspathic veins within the gneiss units indicate a shortening of ap-proximately 80 percent. These F4 folds are further deformed in steep east-trending dextral shear zones and are folded around later (F5?) structures that trend northwest (Fig. 11).

Beneath and to the north of the zone of



dragged by a younger ductile shear, the Lethakane shear, which shows a 25-km dextral displacement (Coward and others, 1973). Such shearing and mylonite forma-tion may be localized along the southern edge of the massive granite in order to maintain compatibility between the rocks deformed in the F4 phase in the south and those far less deformed in the north.

To the south of the Lethakane shear, near Pikwe, the F4 structures (F3 of Hickman and Wakefield, 1975) are more open and can be resolved into two sets, one set trending northeast and the other, a younger set, trending northwest. These may correspond to the F4 and F5 sets at Messina.

MOVEMENT PATTERN ACROSS THE LIMPOPO MOBILE BELT

The deformation of the Limpopo mobile belt and the adjacent part of the Rhodesian craton can be considered as intracratonic, similar to the large intracratonic deforma-tion zones described from Canada (Watson, 1973). The zones can be analyzed in terms of intracratonic block tectonics, in which small blocks of crust move relative to each other and produce local shear zones and zones of flattening deformation.

During the F2 and F3 phases, the defor-mation pattern can be considered as one of a Rhodesian block moving south-south-westward (Fig. 12a) and producing hori-zontal shortening in southwestern Rhodesia and some shortening, but also considerable simple shear, in southeastern Rhodesia. During these phases, the granulite-facies rocks and many of the granite units moved against each other by smaller scale sliding along block boundaries, and the granulite-facies rocks were locally uplifted by a large fold and a major shear zone with a thrust sense.

Deformation was not homogeneous, and it is difficult to make any reliable estimate of the amount of movement. Within the greenstone belts, shortening strains of 50 to 60 percent have been recorded (Fig. 6; Coward and James, 1974), but in the inter-vening gneiss units, the deformation may be much less. Strain measurements were taken in areas where simple shear deformation was assumed to be insignificant. If these measurements are representative of the whole region, the shortening between Shashi and Shabani during the F2 phase may be estimated at 200 km. As there is probably more rotational deformation than we have assumed, this estimate is likely to be too high.

The deformation increases to the south-west away from the center of the craton. It may be significant that sedimentary rocks in the Francistown region both in the Tati schist belt and to the south yield kyanite instead of sillimanite (Key, 1973, oral, commun.); this possibly indicates higher pressure conditions.

Figure 11. Sketch map of a folded gabbro-anorthosite east of the Sand River (S), about 8 km east of Messina. The form of the main fold structure is shown. Within the gabbro-anorthosite (stippled areas), solid lines indicate the lithologic layering; fine dashed lines indicate the F2 cleavage; and arrows point toward rocks of increasingly younger age. The inset shows the amount of F4 shortening as mea-sured from flattened, ptygmatically folded veins in the southwest portion of the map. F indicates the number of strain measurements.

Figure 12. Directions of relative displacement in the Limpopo belt during (a) the F2 and F3 phases and (b) the F4 phase.



The F4 deformation can be considered in terms of a Messina block moving west against a Rhodesian block (Fig. 12b). The junction between the Messina block and the east Rhodesian block was a zone of gently dipping mylonite, whereas that with the west Rhodesian block is a steep shear zone. Given the intensity of deformation within the mylonite zone, estimates have been made of between 30 and 50 km of dis-placement across the main mylonite units in southeast Rhodesia. This estimate is likely to be low because much of the tectonic flattening of the gneiss units south of this mylonite zone may also have been accom-panied by simple shear.

The F4 folds in the Messina block may indicate regional horizontal shortening. This shortening may be more widespread throughout Botswana where the mylonite zone is more diffuse and indicates less dis-placement.

The location of the southern edge of the Messina block is uncertain. The F4 struc-tures may die out against the Soutpansberg fault zone (Fig. 1), which Mason (1969) considered to represent another major cataclastic zone, or they may tighten across this fault zone and represent the main de-formation in the northern Transvaal.

Thus, the tectonic pattern of the Lim-popo belt was formed from two intra-cratonic movements the first during the F2 and F3 phases and the second during the F4 phase and these two periods of movement may be widely separated in time. During both periods, movement took place obliquely to, not perpendicularly to, the general trend of the Limpopo belt.

ACKNOWLEDGMENTS

P. R. James and L. I. Wright gratefully acknowledge Natural Environment Re-search Council postgraduate studentship grants. The project was financed by a NERC research grant and by funds from the Research Institute of African Geology, University of Leeds. The authors are in-debted to Professor R. M. Shackleton, who initially conceived the project and provided inspiration and ideas in the field.

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M A N U S C R I P T RECEIVED BY THE SOCIETY M A R C H 10, 1975

REVISED M A N U S C R I P T RECEIVED J U N E 1 6 , 1 9 7 5 M A N U S C R I P T ACCEPTED J U N E 2 5 , 1 9 7 5

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