geophysical signature of a late middle Proterozoic rift system. ... lithologically correlated by many authors, radiometric ages partly contradict these interpretations.
Precambrian Research, 38 (1988) 75-90
75
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
THE KORAS-SINCLAIR-GHANZI RIFT IN SOUTHERN AFRICA. VOLCANISM, SEDIMENTATION, AGE RELATIONSHIPS AND GEOPHYSICAL SIGNATURE OF A LATE MIDDLE PROTEROZOIC RIFT SYSTEM GREGOR BORG
Department of Geology, University of the Witwatersrand, Johannesburg 2001 (South Africa) (Received December 1, 1986; revision accepted August 14, 1987)
Abstract Borg, G., 1988. The Koras-Sinclair-Ghanzi Rift in southern Africa. Volcanism, sedimentation, age relationships and geophysical signature of a late middle Proterozoic rift system. Precambrian Res., 38: 75-90. A number of relatively undeformed late middle Proterozoic volcano-sedimentary basins are aligned along the western and northern margins of the Kalahari Craton. Although the assemblages of these basins have been compared and lithologically correlated by many authors, radiometric ages partly contradict these interpretations. Attempts to incorporate all of these basins into a regional structural interpretation have been rare. Watters' magmatic arc interpretation is contradicted by the overall geochemistry of the volcanic rocks, the coarse continental red-bed sediments and the structural style of the basins. The depositional troughs were narrow, fault-bounded continental rift grabens with a structural style and sedimentation pattern controlled by strong vertical tectonism. The development of the basins along two branches of a propagating continental rift system, the Koras-Sinclair-Ghanzi Rift (KSG-Rift), is proposed. Radiometric ages reveal a distinct younging along the rift from the southern toward the northeastern branch. The geophysical signature of the rift is dominated by positive Bouguer gravity anomalies, flanked by gravity lows which follow a linear trend. The basins contain distinctly bimodal volcanic rocks with some intermediate volcanics in the Sinclair basin. Contemporaneous with the extrusion of the volcanics, granitic high-level intrusions were emplaced along the rift. The possible existence of a northern, coastal branch of the KSG-Rift and the location of a triple junction is discussed. The rift process possibly continued and led to the development of the early Damara rift in SWA/Namibia. The African plate ( Kalahari and Congo Cratons) migrated southward between 1050 and 950 Ma ago. A stationary mantle plume probably caused the development of both the KSG and Damara Rift. A comparison with rift migration in Kenya is drawn. The regional trends and structures of the KSG-Rift were reactivated several times, undergoing both extension and compression. The most prominent of these events was the development of the Damara Orogen, which followed the regional trend of the late middle Proterozoic rift, but fault structures have been reactivated up to recent times.
Introduction A n u m b e r o f middle P r o t e r o z o i c b a s i n s are aligned a p p r o x i m a t e l y parallel to t h e s o u t h western, w e s t e r n , n o r t h w e s t e r n a n d n o r t h e r n m a r g i n s of t h e K a a p v a a l C r a t o n (Fig. 1 ). T h e y
0301-9268/88/$03.50
were filled b y t h i c k s e q u e n c e s o f v o l c a n i c a n d s e d i m e n t a r y rocks, w h i c h since t h e n r e m a i n e d relatively u n d e f o r m e d a n d u n m e t a m o r p h o s e d (Fig. 2 ). W h e r e a s c r a t o n i c regions a n d mobile belts, b o u n d i n g t h e s e basins, h a v e b e e n i n t e n sively studied, t h e n a r r o w belt w h i c h divides
© 1988 Elsevier Science Publishers B.V.
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both terranes has received relatively little attention. Lithological aspects of each basin have been studied, and most authors have pointed out similarities in lithology and structure of the various basins and made attempts to correlate them (Handley, 1965; Martin, 1965; Schalk, 1970; Toens, 1975; SACS, 1980; Ruxton, 1981 ). Radiometric dating seemed to create considerable confusion by yielding differing ages for units which had previously been correlated. Mason (1981) and Cahen and Snelling (1984) gave brief descriptions of the late middle Proterozoic basins and discussed them within the regional framework of southern Africa. These authors pointed out the narrow, fault-bounded nature of the basins and concluded that they formed in an extensional crustal environment without mentioning how the individual basins relate to each other or their geometric distri-
bution pattern. Watters (1974) described the geology of the Sinclair basin in great detail and proposed t h e existence of a 'Rehoboth Magmatic Arc'. In his model, he included the other basins lying to the northeast in SWA/Namibia and Botswana (Fig. 1), again based only on lithological similarities. His interpretation of the arcuate belt, formed in response to subduction of ocean floor beneath the Kalahari Craton (Watters, 1974, 1976), has been questioned by other authors (KrSner, 1977; Mason, 1981; Cahen and Snelling, 1984). The magmatic arc theory is opposed by field evidence, which indicates a cratonic setting of the narrow, faultbounded troughs (KrSner, 1977; Mason, 1981; Cahen and Snelling, 1984; Borg and Maiden, 1986a,b, 1987a,b) and by the composition of the volcanics and high level intrusions. The possibility of continental rifting prior to Damaran
77 I
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rifting is briefly mentioned in Porada (1985). This paper describes the sedimentological, stratigraphic, structural, magmatic and geophysical characteristics of each basin, summarising previous work and incorporating new work carried out by the author. A comparison of the basins is followed by a regional tectonic synthesis. The Koras basin, Cape P r o v i n c e , South Africa A volcano-sedimentary sequence that accumulated in the Koras basin (new name), occurs along the Orange River, east of Upington (Fig. 2 ). The rocks of the Koras Group ( SACS,
1980) have been described in detail by Grobler et al. (1977). The outcrop of the Koras rocks is bordered by older rocks and covered by younger sediments to the north. The rocks dip at low to moderate angles and are unmetamorphosed. Grobler et al. (1977) reported gentle synclinal structures which Du Toit (1965) had previously interpreted as sedimentary basins formed by sediment tranport from different directions into fault-bounded grabens with syndepositional activity along these faults. The rocks of the Koras basin rest on ~ 2100-Ma-old schists and gneisses of the Kheis Group (Grobler et al., 1977; Botha et al., 1979 ). The stratigraphy and lithology of the Koras Group is illustrated in Fig. 2.
78 The volcano-sedimentary infill of the Koras basin represents a succession of immature, coarse clastic continental red-bed sediments and distinctly bimodal lavas. Sediments were deposited in narrow yoked and graben-like basins where they accumulated as alluvial fans or alluvial aprons along the flanks of the fault scarps (Grobler et al., 1977). The faults, bounding the grabens and horst blocks, controlled the extrusion and the distribution of the basic, acid and minor intermediate lavas. Continued syndepositional activity of the faults, together with erosion and reworking of uplifted early Koras rocks, is indicated by the erosional unconformities.
The Sinclair basin, southern SWA/Namibia The Sinclair basin in southern SWA/Namibia occupies the largest area of exposed late middle Proterozoic rocks along the entire belt (Fig. 2 ). Because of the good exposure, the Sinclair basin has been relatively well studied as summarized in Watters (1974, 1977, 1982), SACS (1980), Hoal (1985), Hoal etal. (1986) and Brown and Wilson (1986). The sediments and volcanics of the Sinclair Sequence ( SACS, 1980) crop out between the sand cover of the Namib Desert to the west and rocks of the early Palaeozoic Nama Group to the east. To the north they are overlain by rocks of the Nama Group and the Naukluft Nappe Complex. In the south the Sinclair Sequence borders rocks of the Namaqualand Metamorphic Complex. The major portion of the Sinclair rocks has been affected by metamorphism of lower greenschist facies and has not been deformed by folding. The succession has been block faulted both during and after deposition, and shear zones developed during several deformational events (Hoal, 1985). Higher metamorphic grades occur, together with ductile deformation, in some marginal regions of the basin (Hoal, 1985). The stratigraphy and lithology of the Sinclair basin are shown in Fig. 2.
The Klein Aub basin, central SWA/Namibia The Klein Aub basin, situated west-southwest of Rehoboth (Fig. 1 ), contains a volcanosedimentary succession with a total thickness of ~ 11 000 m. The late middle Proterozoic formations are exposed between the igneous and metamorphic complex of the Rehoboth Inlier (SACS, 1980) to the north and Damara Sequence metasediments and Nama Group sediments to the south. To the east the units are covered by Cainozoic Kalahari sand and calcrete, and to the west they are bounded by high angle faults and the overlying Naukluft Nappe Complex. The rocks are well exposed and have been studied by numerous authors summarized in Borg and Maiden (1986a,b, 1987a,b) and Borg et al. (1987). The stratigraphy and lithology are illustrated in Fig. 2. The rocks have undergone metamorphism under greenschist facies conditions with temperatures not exceeding 350 °C (Ahrendt et al., 1978), but sedimentary structures are commonly preserved. The peak of metamorphism and deformation has been dated at 530 + 10 Ma (K/Ar dating of muscovite) which can be attributed to the main deformational event of the Damara Orogeny (Ahrendt et al., 1978). Deformation has caused the development of large open folds and a distinct slaty cleavage, dipping steeply to the north. Strike slip faults and zones of strong shearing developed under a transpressional regime, probably during a late phase of the Damara Orogeny (Borg et al., 1987). The general direction of strike is from east to west or northeast to southwest with the beds dipping at moderate angles to the south.
The Dordabis/Witvlei basin, central SWA/Namibia The late middle Proterozoic rocks which are exposed between Dordabis, some 80 km southeast, and Witvlei, 150 km east of Windhoek, occur as narrow thrust slices (Fig. 1 ). Outcrop is
79 generally poor, and the sand cover of the Kalahari inhibits field observations. The region has been investigated by numerous authors summarized in Borg and Maiden (1987b). Stratigraphic relationships are obscured by deformation of Damara age. In contrast to the Klein Aub basin, which appears to have been protected from more intense deformation (especially southward thrusting) by the rigid block of the Rehoboth Inlier to the north, the Dordabis/Witvlei region underwent intense folding, faulting and thrusting. This produced northeast trending fold axes within the thrust slices and caused the development of at least one strong cleavage. Primary sedimentary structures were generally destroyed or obscured in the fine-clastic rocks. Metamorphism of greenschist facies is the same as that which affected the Klein Aub basin. A much stronger schistosity is developed in the rocks of the Dordabis/Witvlei basin, complicating the lithological comparison with rocks from the Klein Aub region. The stratigraphic units in the Dordabis area are shown in Fig. 2.
steeply inclined to an interpreted depth of 15 000 m (Reeves, 1979; Meixner, 1983 ), where it forms the basement of the Passarge basin. The late middle Proterozoic rocks have been deformed during the Damara Orogeny which produced large open folds with fold axes trending southwest to northeast and during Karoo times when block faulting produced deep graben structures. Metamorphism reached lower greenschist facies grade and the contemporaneous deformation caused the development of a weak cleavage which generally has not destroyed primary sedimentary structures.
The Ghanzi/Lake N'Gami basin, Botswana
A small window through the Nama and Karoo cover in southern SWA/Namibia (Farm Khorrobees 65; 27 ° 09' S, 18 ° 06' E ) exposes underlying rocks which are possible correlatives of the Sinclair and Koras Sequences (Fig. 1). A fault-bounded horst block consists mainly of rocks of the Namaqua Metamorphic Complex flanked by younger Karoo sediments and lavas to the east and the rocks of the Nama Group to the west. Between the highly deformed and metamorphosed Namaqua rocks to the south and the younger cover rocks to the west and east an area of ~ 5 km of undeformed and unmetamorphosed rocks of pre-Nama age is exposed (H.J. Blignault, personal communication, 1986). The rocks consist of poorly sorted conglomerate with angular and subangular clasts and arkose and subaerially extruded, amygdaloidal basalt. The rocks have not been affected by the deformation which produced the very distinct schistosity observed in rocks of the Namaqua
The correlatives of the formations described from South Africa and SWA/Namibia occur in a 30-40 km wide belt stretching from the village of Ghanzi in Botswana, close to the border with SWA/Namibia, towards Lake N'Gami (Fig. 2 ). In this belt the rocks crop out only sporadically and are otherwise covered by Kalahari sand and calcrete. Very limited work on these successions has been carried out by some authors which has been summarized in Key and Rundle (1981) and Ruxton (1981). Geological mapping, supported by geophysical studies, has shown that the late middle Proterozoic rocks form a prominent northeast trending structural ridge, referred to as the Ghanzi-Chobe Foldbelt (Meixner, 1983). To the northwest, the ridge is flanked by the Damara Orogenic Belt. The southeastern flank of the ridge is
Possible links between, and extensions to, related basins Small geological units of uncertain age, which could represent late middle Proterozoic strata, are preserved between some of the basins. Their locality and lithology are described below.
The Koras/Sinclair link
80 Metamorphic Complex exposed immediately to the south. Both terranes are separated by a steeply dipping west-northwest trending fault with the down-thrown block to the northnortheast. Since this undeformed and unmetamorphosed volcano-sedimentary sequence underlies the sediments of the Nama Group and is distinctly different from the rocks of the Namaqualand Metamorphic Complex, it is proposed that it might represent an equivalent of the Sinclair and Koras basins. The Nauzerus/Naukluft link On the Farm Nauzerus 11, some 100 km west of Rehoboth, thick purple conglomerates are exposed underneath the sole-dolomite of the Naukluft Nappe Complex (23 ° 50' S, 16 ° 20'E). The clasts consist of red quartz- and quartzfeldspar porphyry and minor granitic pebbles. The source can be found some 5-10 km to the north. The conglomerate is undeformed and is probably an equivalent of the Doornpoort Formation in the Klein Aub basin. The Oorlogsende link Some 80 km northwest of Rietfontein, close to the border between SWA/Namibia and Botswana ( see Fig. 4 ), a thin tectonic sliver of late middle Proterozoic rocks is exposed in a shallow valley, known as the Epukiro Omuramba (21°25'S, 20°15'E). With the exception of these small outcrops the area is covered by thick layers of calcrete and other surficial deposits. As yet, geological relations with adjacent regions have not been established. In this area quartz porphyry, rhyolitic, tuffaceous ignimbrite and flow banded rhyolite are exposed ( Hegenberger and Burger, 1985). The rocks are unmetamorphosed but show a subvertical fracture cleavage. The Shinamba Hills extension The Shinamba Hills are situated within the Chobe Game Reserve (Fig. 1) where surface
expression is limited to two small hills on which clastic metasediments are exposed. Except for these hills the area is covered by densely vegetated soil and Kalahari sand. Geological information is limited to data from exploration boreholes. The beds have been deformed into south verging open folds which have been truncated by later subvertical faults. Whether deformation was limited to a single event, the Damara Orogeny, has not been established. A thick succession of rhyolitic lavas, conglomerate, sandstone, minor shale, lenses of silicified argillite and chert, and stromatolitic limestone is regarded as an equivalent of the Kgwebe and Ghanzi Formations in the Lake N'Gami basin. Although no basic volcanic rocks have been found within the stratigraphy, the sediments were probably deposited in the vicinity of basic volcanic centres. This is suggested by the high portion of basic volcanic and pyroclastic clasts both in the conglomerate and the grit. Intrusive and e x t r u s i v e magmatic rocks As shown in the detailed description of the geology of the individual areas a major portion of the basin fill consists of volcanic rocks with a wide range of chemical compositions. Additionally, in each of the areas, or its surrounding terranes, contemporaneous intrusive magmatic bodies have been emplaced. Each basin hosts a suite of distinctly bimodal (basaltic and rhyolitic) volcanic rocks (Fig. 3), and in the Koras and Sinclair basins intermediate volcanic suites are present. The basic volcanics in the different basins are represented mainly by tholeiitic, continental basalts and subordinate calc-alkaline and alkaline basalt. In the Koras and Sinclair basins andesite, shoshonite and dacite have also been described (Watters, 1974; Grobler et al., 1977; Brown and Wilson, 1986). The acid volcanics consist of rhyolite, rhyolitic ignimbrite and pyroclastics with only minor occurrences of volcanics of rhyodacitic or rhyotrachytic com-
81 %
FREQUENCY
30[~--m
20-
10-
0 30
~'o
so
6'o
io
e'o O/o sio~
Fig. 3. Si02-content of the late middle Proterozoic volcanics from all basins along the KSG-Rift. Data compiled from Watters (1974), Grobler et al. (1977), Sanderson-Damstra ( 1982 ), Williams-Jones (1984), Borg and Maiden (1986b), Brown and Wilson (1986), and own data.
position. A portion of the acid volcanics in each basin has peralkaline character and can possibly be compared to silicic peralkaline volcanic rocks of the Afar Depression, Ethiopia (Barberi et al., 1974, 1982). High-level granitic intrusions occur in the vicinity of the basins or intrude the lower part of the stratigraphy. Their contemporaneous relationship with the volcano-sedimentary basin filling has been demonstrated by different authors (Botha et al., 1979; Watters, 1977; SACS, 1980). No contemporaneous plutonic intrusions have been found related to the Dordabis/Witvlei or Lake N'Gami basins but a prominent negative gravity anomaly in the Witvlei area might be interpreted as an underlying granitic body (Fig. 4).
Geophysical signature of the basins The basins described above are associated with a number of elongated gravity highs which produce a distinct linear feature in the southern part of SWA/Namibia (Fig. 4). The Bouguer gravity map ( after Kleywegt, 1967) shows that these gravity highs are flanked by negative gravity anomalies. The positive gravity anomalies are caused by dense basaltic lava which is part of the infill of the rift grabens. In the Dor-
dabis area a distinct gravity high is bordered to the east by a prominent negative anomaly ( Fig. 4 ). This can be explained by either a thick sedimentary pile in this trough or an underlying (probably granitic) body of low density which has not been uncovered by erosion. Between the Koras and the Sinclair basins with their well-defined gravity highs, several smaller highs occur along an east-south-easterly trend. This region is covered by rocks of the Nama Group and Karoo Sequence which are unlikely to have produced this gravity signature. It is more likely that the basins are linked by other elongated troughs which contain rock sequences equivalent to those in the Koras and Sinclair basins. The idea that basic volcanic rocks of late middle Proterozoic age are the cause of the gravity highs is supported by the rocks exposed in the inlier between the Koras and Sinclair basins. Unpublished exploration results have shown that these undeformed basalts and intercalated red-bed sediments are more widely distributed underneath the cover rocks than is indicated from surface exposure.
Discussion Spatial correlations versus age relationships
Numerous attempts have been made to correlate the stratigraphic successions of the individual late middle Proterozoic basins as summarized in Ruxton (1981). Nevertheless these attempts have not overcome the basic problem that lithological correlations are at least partly contradicted by results from radiometric dating, e.g. the felsic volcanic units from the Sinclair and Klein Aub basins have been correlated, based on lithological grounds, with the Kgwebe and Goha Hills Porphyry of Botswana (e.g. Toens, 1975). Radiometric ages of these rocks yield a difference of nearly 400 M a , or 40% of the total age (Cahen and Snelling, 1984). In this specific case the poor quality of some of the data must be taken into account,
82
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Fig. 4. Bouguer gravity map of SWA/Namibia south of the Damara Orogen (after Kleywegt, 1967).
but age differences of some 200 Ma remain and have to be explained. The basic misunderstanding arises from the attempt to regard the development of all basins as strictly contemporaneous. Between Dordabis and Ghanzi, strata which have been correlated exhibit an unconformable relationship to each other. The Aubures Formation within the Sinclair basin has been correlated with the Doornpoort Formation within the Klein Aub basin ( SACS, 1980), but the former is intruded by the Gamsberg Granite Suite, whereas the latter contains clasts of eroded Gamsberg Granite. The basins and their similar volcano-sedimentary infill formed diachronously. This ex-
plains both the similarity in stratigraphic evolution of each basin and their marked differences in age. The radiometric age data available from the different basins were obtained by various dating methods, and some of the results must be regarded with caution (Cahen and Snelling, 1984). Nevertheless, the data appear to cluster in three discrete periods which correlate positively with the Koras-Sinclair, Klein Aub-Dordabis/Witvlei-Oorlogsende and Ghanzi-Lake N'Gami-Goha Hills areas. The age data shown in Fig. 5 reveal a marked younging trend of the dated rocks from the Koras-Sinclair region via the Klein AubOorlogsende region towards the Ghanzi-Lake N'Gami region.
83
DATING TECHNIQUE •
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Fig. 5. Radiometric age data from the different basins of the KSG-Rift displaying a younging trend from south to north. All error bars indicate 2-sigma. Data from du Toit (1965), de Waal (1966), van Niekerk and Burger ( 1967, 1969), von Brunn and Dodson (1967), Harding and Snelling (1972), Burger and Coertze (1973-74), Hugo and Schalk (1974), KrSner (1975, 1977), Geringer and Botha (1976), Kr6ner et al. (1977), Botha et al. (1979), Key and Rundle (1981), Watters (1982), Hegenberger and Burger (1985), Hoal et al. (1986) and Malling and Reid (in prep.).
Structural aspects and rift geometry The late middle Proterozoic basins are aligned along two approximately linear zones, generally parallel to the margin of the Kaapvaal Craton {Fig. 1 ). The rocks occur in elongated, fault-bounded troughs which can be interpreted as grabens and yoked basins. During deposition of the volcano-sedimentary successions the structural style was dominated by strong block faulting. In each area the intensity of block faulting appears to have decreased during the evolution of the grabens (Borg and Maiden, 1986a). Sedimentary facies distribution was commonly controlled by syndepositional faulting and ceased in the basins of the northern rift branch only during the late phase of basin widening and the associated marine transgression. In some basins normal faults, associated with the rift grabens, were reactivated at least during the Damara Orogeny. The reac-
tivation of these faults was strongest in the basins of Dordabis/Witvlei and Klein Aub where intensive thrusting as well as reverse and strike slip faulting occurred (Borg et al., 1987). The structural style, together with the coarse clastic, continental nature of the red-bed sediments, is characteristic of an intracratonic rift setting. The field evidence opposes Watters' (1974) hypothetical 'magmatic arc', that should have formed as a consequence of plate subduction, and was based on the chemical composition of a portion of the volcanics in the Sinclair area. The development of the KSG-Rift probably followed older sutures between the Kalahari Craton (Clifford, 1970) and adjacent cratonic regions to the north and west. The orientation of the two linear trending branches of the KSGRift, at an angle of -~ 80 °, supports the possibility that a triple junction with a third, northern branch might exist. The orientation and
84
A
I /
I \
C
Fig. 6. Sketch diagram of schematic tiff-evolution, demonstrated by the example of the Klein Aub basin.
development of the early Damara rifts (Kasch, 1983; Porada and Wittig, 1983, Porada, 1985) followed older zones of crustal weakness ( Martin and Porada, 1977) which were the structural trends of the KSG-Rift.
Thermotectonic evolution of the KSG-Rift Rift evolution The evolution of the rift basins is illustrated by the Klein Aub basin which has been studied by the author in more detail than the other basins {Borg and Maiden, 1986a). This evolution, which generally applies to the basins, is summarised in three simplified sketches in Fig. 6. Phase A: Crustal extension, crustal thinning and a high heat flow regime caused partial melting of the lower continental crust and supplied magma for felsic, high-level intrusions and extrusions. Intensive block faulting in response to crustal extension formed grabens and yoke basins and provided zones of weakness along which felsic volcanics extruded into narrow troughs. Erosion of the uplifted horst blocks was limited due to the rapid extrusion of large quan-
tities of felsic volcanics which filled the tectonic depressions and covered the region. Phase B: Still under tension, the deeply penetrating normal faults tapped mantle sources and basic lavas were extruded in the vicinity of the faults. The distribution of the basaltic and minor andesitic lava flows was controlled by the block faulted terrane where vertical tectonism was still active. Increasing denudation of the uplifted rift flanks and more central horst blocks provided coarse, immature sediments which were fed into the narrow fault-bounded depressions. The facies distribution was controlled by basement highs and syndepositional faulting. In some basins phase B consists of a number of volcano-sedimentary cycles, but in most cases an initial basic volcanic phase was followed by the deposition of continental clastic red-bed sediments. Phase C-Thermal subsidence: Relaxation of the continental crust caused cessation of block faulting and volcanism. At the same time the basins widened due to erosion of the rift flanks and some basins were covered by a marine transgression (e.g. Klein Aub, Ghanzi, Lake N'Gami, Shinamba Hills). In other basins lakes formed in the depressions (Ruxton and Clemmey, 1986). Clastic, low energy sediments were deposited in shallow aqueous environments, where sulphate reducing bacteria produced dark pyritic sediments. Such rock types have not been found in the Koras and Sinclair basins because they were either not deposited or might have been eroded.
Lateral rift-progression The linear trend of the radiometric age data shows an overall younging of the volcanic rocks, and therefore of the intercalated sediments, along two rift branches. Based on this correlation (Fig. 5) it is possible to reconstruct the regional evolution of the KSG-Rift approximately parallel to the margin of the Kaapvaal Craton (Fig. 7 ). Although the absolute ages of the different periods might be questioned, the relative younging of the phases of rift develop-
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change in the direction of rift propagation. The Congo Craton caused the development of a triple junction and a change in direction towards the northeast, probably following zones of weakness between the Kalahari and Congo Cratons (cratonic regions as defined by Clifford, 1970). Thus, the northeastern branch of the KSG-Rift might be regarded as the failed proto-rift which separated the Kalahari Craton from the Congo Craton.
Relationship to early Damara rifting.
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Fig. 7. Sketch maps showing the lateral evolution of the KSG-Rift in time and the position of a possible triple junction with a northern, 'coastal branch' of the rift. CC = Congo Craton, KC = Kalahari Craton.
ment remains unaffected by this problem. The rift development started with crustal extension, intensive block faulting and bimodal volcanism along the southern branch of the KSGRift (Fig. 7 ). Crustal extension and, as a result, the development of rift grabens prograded towards the north-northwest, where the rigid block of the Congo Craton might have caused a
No significant deformational event marks the transition from the late phase of the KSG-Rift (1000-950 Ma) to the early phase of Damara rifting (950-850 Ma). It appears that the rifting-process was almost continuous, producing domains with a very similar structural style. A possible explanation is the migration of the African plate over an elongated mantle plume which acted like a hot spot, causing intermittent crustal extension. If this assumption is correct, the northeastern (inland) branch of the KSG-Rift and the inland branch of the early Damara Rift formed in response to the same mantle upwelling. The polar wander path for the Proterozoic shows that the African plate drifted towards the northwest between 1050 and 950 Ma ago (McElhinny and Williams, 1977). Such a 'rift jump', as defined by Wood (1983), would also explain the lack of a prominent compressional phase between the development of the two rift systems. The inactive KSG-Rift became compressed, only by Damara crustal extension which occurred to the north, leading to the consolidation of the KSG-Rift with the Kalahari Craton. The formation of both rifts in response to a southward migration of the African plate is illustrated in Fig. 8. During the last phase shown in the diagram, compression caused intense shortening, folding and thrusting within the Damara Orogen while the KSGRift was less affected. Continental rift jumps or the migration of plates over hot- spots or '-lines', have been described by Wood (1983). Probably the most
86 NNW
KORAS- SINCLAIR GHANZI-RIFT
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