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The early Proterozoic sedimentary record in the Blouberg area, Limpopo Province, South Africa; implications for the timing of the Limpopo orogenic event
Journal of African Earth Sciences 39 (2004) 123–131 www.elsevier.com/locate/jafrearsci
The early Proterozoic sedimentary record in the Blouberg area, Limpopo Province, South Africa; implications for the timing of the Limpopo orogenic event A.J. Bumby *, P.G. Eriksson, R. Van Der Merwe Department of Geology, University of Pretoria, Pretoria 0002, South Africa
Abstract At present, the timing and geometry of a presumed collision of the Limpopo Belt (southern Africa) remain controversial. Data from tectonic and metamorphic studies, and a number of radiometric dates, have been interpreted to produce widely varying models. In this work, we aim to help constrain these models using data from the sedimentary rocks in the Blouberg area, which lie above and adjacent to the Palala Shear Zone in the southwestern part of the belt. Here, a thick sedimentary package consisting of the syn-tectonic Blouberg Formation, medial parts of the Waterberg Group and the Soutpansberg Group is developed nonconformably above the granulite-grade gneiss of the Limpopo Belt. These three sedimentary units are separated from each other by angular unconformities. The youngest rocks of this succession (the Soutpansberg Group) have an imprecise age of ca. 1.8–1.97 Ga. The inferred long tectono-sedimentary history necessary to produce these three unconformity-bounded sequences is interpreted to support an older, Archaean-age (ca. 2.6–2.7 Ga) for the timing of the Limpopo collision, rather than reflecting a younger Proterozoic (ca. 2.0 Ga) event. It is proposed that syn-sedimentary tectonism recorded in the Blouberg Formation may reflect a southward-vergent 2.0 Ga tectonic event, though this is interpreted as reactivation within the Limpopo Belt, rather than the primary collision itself. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Limpopo; Blouberg; Sediments; Age constraints; Palala Shear Zone
1. Introduction The Limpopo Belt of southern Africa was thought to have formed by a ca. 2.6–2.7 Ga collisional orogen between two Archaean cratons, Kaapvaal to the south and Zimbabwe to the north. It was believed to represent one of the earliest examples of collision between two large-scale cratons, which is more analogous to modern-day plate tectonic collisions (e.g. Light, 1982; Treloar et al., 1992). After collision, the combined Kaapvaal and Zimbabwe cratons formed the larger-
*
Corresponding author. E-mail address: [email protected] (A.J. Bumby).
0899-5362/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2004.07.056
scale Kalahari Craton. Recently, however, the Limpopo Belt has become the focus of controversy. Much work now advocates dextral transpressive tectonics at 2.0 Ga (e.g. Barton et al., 1994; Holzer et al., 1998; Schaller et al., 1999; Kreissig et al., 2001). Both the age and geometry of the Limpopo Belt therefore remain poorly defined and the object of much debate and research (e.g. Bumby, 2000). The Limpopo Belt consists of high-grade metamorphic rocks exposed in a ca. 250 km-wide E.N.E– W.S.W.-trending belt which passes through the northern-most part of South Africa and the southernmost part of Zimbabwe, westwards into eastern Botswana (Fig. 1), with an overall length of about 550 km. In the east, the belt is overlain by Jurassic flood basalts
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Fig. 1. Map and cross-section of the Limpopo Belt showing component zones and bounding shear zones, and the location of the study area (after Kro¨ner et al., 1999; Roering et al., 1992).
related to the break-up of Gondwana, and to the west, in Botswana, it is covered by Tertiary-Recent sands of the Kalahari desert (Fig. 1). The Limpopo Belt comprises a Central Zone (C.Z.), which is flanked to the north and south by a Northern Marginal Zone (N.M.Z.) and Southern Marginal Zone (S.M.Z.), respectively (Fig. 1). The N.M.Z. is bounded by the northwards-verging Northern Marginal Thrust Zone to the north, and in the south is separated from the Central Zone by the generally dextral Triangle Shear Zone. Relationships in the S.M.Z. are almost a mirror-image, to those of the N.M.Z. The S.M.Z. is bound to the south by the southwards-vergent Hout River Shear Zone, and in the north it is separated from the Central Zone by the generally sinistral Palala Shear Zone (Fig. 1) (McCourt and Vearncombe, 1987, 1992). The Palala Shear Zone has, however, been interpreted to have been reactivated at ca. 1.97 Ga, with dextral sense of shear (Schaller et al., 1999). Tectonic controversies surrounding the Limpopo Belt concern the provenance of the Central Zone. The N.M.Z. is composed of granulite-grade rocks, which are regarded as the higher-grade equivalent to greenschist-facies rocks of the Zimbabwe Craton. Similarly the S.M.Z. is considered to comprise rocks of the Kaapvaal Craton at granulite-grade. The Central Zone, how-
ever, appears to be an exotic crustal block, which contains evidence for an entirely separate tectonic and sedimentary history (van Reenen et al., 1992). Sedimentary protoliths (now also at granulite-grade) in the Central Zone are preserved as the Beit Bridge Complex. The dextral and sinistral sense of movement in the Triangle and Palala Shear zones which bound the Central Zone to the north and south, respectively, suggest that the Central Zone may have been thrust westwards as a giant nappe over the top of the previously assembled Kalahari Craton (McCourt and Vearncombe, 1987, 1992), thus leading to granulite-grade metamorphism in the marginal zones of the adjacent cratons beneath the nappe. Accommodation of loading by the nappe is thought to have produced the Northern Marginal and Hout River thrusts (McCourt and Vearncombe, 1992). Alternative models consider the possibility of diachronous collisions in the Limpopo Belt, whereby the C.Z. collided initially with the Zimbabwe Craton, followed by later docking of the Kaapvaal, or initial collision between the C.Z. and Kaapvaal, with subsequent accretion of the Zimbabwe Craton (Watkeys, 1984; Roering et al., 1992; Rollinson, 1993). Controversies surrounding the timing of the Limpopo event focus on the recent spate of dates (derived from metamorphic silicate minerals in the Triangle and
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Palala Shear zones, and from melt patches and anatectic granitoids throughout the CZ) suggesting that peak metamorphism and thus collision occurred as recently as 2.0 Ga (e.g. Holzer et al., 1998; Schaller et al., 1999). Such dates are at odds with the more traditional minimum ages for the Limpopo collision, determined from syn-tectonic granites in the Central Zone, (e.g. McCourt and Armstrong, 1998). Ages for such granites cluster between 2.6 and 2.7 Ga. Assuming a 2.0 Ga collisional event, ca. 2.6–2.7 Ga events can be accounted for by considering the long independent history of the Central Zone which appears to have been entirely separate from that of either of the adjacent cratons; the 2.6–2.7 Ga event within this model is thus envisaged to be restricted to the pre-collisional Central Zone block, and unrelated to the Limpopo Orogen (Holzer et al., 1998). Assuming a 2.7–2.6 Ga Limpopo event, the record of metamorphism at 2.0 Ga is best explained by reactivation tectonics (probably related to the ca. 2.15–1.85 Ga Africa-wide Eburnean event) within the previously assembled Kalahari Craton (McCourt and Armstrong, 1998). The Eburnean-aged southward-vergent 2.0 Ga Magondi Orogen on the northwestern margin of the Zimbabwe Craton is envisaged as a possible cause of similarly-aged reactivation in the Limpopo Belt (McCourt and Arm-
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strong, 1998; Bumby et al., 2001a). However, at present, there is no evidence for the 2.0 Ga event in the S.M.Z. Recent dating of strata adjacent to the Hout River Shear Zone suggests an age of peak metamorphism that supports a 2.6–2.7 Ga Limpopo event (Kreissig et al., 2001). Thus there remains little consensus regarding either the timing or the tectonic history of events involved with the Limpopo Orogen. The aim of this contribution is to consider the enigmatic timing of the Limpopo Orogen from a different standpoint. The present work will examine how the sedimentary rock record, which rests nonconformably on the high-grade rocks of the Limpopo Belt, can help constrain hypotheses regarding timing of the collisional event. This work will focus on the ages and tectono-sedimentary history of the Palaeoproterozoic (2050–1600 Ma Orosirian–Statherian periods; IUGS, 2000) Blouberg Formation, the Waterberg and Soutpansberg Groups which outcrop above and adjacent to the Palala Shear Zone (Fig. 2).
2. Blouberg Formation The Blouberg Formation (Fig. 2) is an entirely clastic sedimentary sequence, which outcrops in areas restricted
Fig. 2. Map of the Blouberg area showing outcrops of the sedimentary sequences of the Blouberg Formation, Waterberg and Soutpansberg Groups. The Palala Shear Zone is inferred to be present beneath these strata (after Bumby et al., 2001a).
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to those above the projected eastward extension of the Palala Shear Zone (Fig. 1). The Blouberg Formation was deposited nonconformably over the granulite-grade gneisses of the Limpopo Belt, and attains a maximum thickness of about 1400 m (Jansen, 1975; Bumby et al., 2001a), though generally only sequences of less than 300 m outcrop. In the thickest preserved section, the Blouberg Formation consists of a 600 m-thick lower member, where facies associations comprise trough cross-bedded coarse arkosic sandstone and channel-fills of feldspathic granulestone (Wentworth, 1922). Generally, the size of sets within the trough cross-bedded facies decreases upwards in the lower member (Bumby et al., 2001a). These lower Blouberg facies associations are interpreted as reflecting deposition within braided streams. In contrast to the lower part of the Blouberg Formation, the upper member consists of coarse, feldspathic sedimentary breccia and conglomerate. Cobbles and boulders within the upper member consist of quartzite and foliated feldspathic gneiss. The strata of the upper member are interpreted as reflecting deposition within alluvial fans (Bumby et al., 2001a). The sedimentary strata of the Blouberg Formation are generally characterized by steeply-dipping bedding planes (Bumby et al., 2001a). In outcrops immediately to the south of Blouberg mountain, where only rocks of the lower member are recorded, strata typically dip southwards, with angles in excess of 45°. Locally, Blouberg strata are overturned and dip northwards at 70° or more. Locally, the Blouberg Formation contains steeply northward-dipping reverse faults and rarely the Limpopo gneiss can be seen to overlie the Blouberg Formation, having being emplaced by southward-vergent thrusts. The rare outcrops of the rudaceous upper member are generally sub-horizontally inclined, and in places may unconformably overlie the steeply-dipping lower member (Bumby et al., 2001a).
3. Waterberg Group Three formations of the Waterberg Group crop out in the Blouberg area, immediately to the south of Blouberg mountain (Fig. 2). The Waterberg strata contrast from the Blouberg Formation in that they are generally sub-horizontally inclined (Callaghan et al., 1991; Bumby et al., 2001a). The lowermost Setlaole Formation is poorly exposed, and consists of coarse arkosic sandstone and granulestone. The Setlaole Formation may tentatively be correlated with the Blouberg Formation (Jansen, 1976; Bumby, 2000), though its sub-horizontal aspect suggests that Setlaole deposition may post-date inversion of the Blouberg basin (Meinster, 1977). The predominantly fluvial Setlaole Formation grades upwards into mature sandstone of the Makgabeng Formation. The latter consists predominantly of aeolianite
facies, with minor facies of playa lakes, interdune deposits and ephemeral rivers becoming increasingly abundant towards the top (Eriksson et al., 2000; Simpson et al., 2002). The Makgabeng Formation is disconformably overlain by the Mogalakwena Formation, and extensive exposures of this disconformity show little palaeorelief on the upper surface of the Makgabeng strata, suggesting that the Makgabeng Formation was peneplaned prior to Mogalakwena deposition. The Mogalakwena Formation consists of interbedded sheets of coarse sandstone and conglomerate, interpreted as braided stream and alluvial fan deposits, respectively (Bumby et al., 2001a). Palaeocurrent directions recorded from trough cross-bedded sandstone in the Mogalakwena Formation suggest unimodal flow towards the southwest. Conglomeratic facies become increasingly abundant to the northeast, with proximity to Blouberg mountain (Bumby, 2000; Bumby et al., 2001a). Waterberg strata in the foothills of Blouberg mountain consist only of the Mogalakwena Formation. The Makgabeng Formation appears to wedge out in a northerly direction about 5 km south of the Blouberg foothills (Bumby, 2000). The sub-horizontal Mogalakwena strata here unconformably overlie the steeply-dipping strata of the Blouberg Formation, and outcrops which show this spectacular unconformity (Fig. 3) suggest that the steeply-dipping Blouberg Formation was almost peneplaned prior to deposition of the Mogalakwena Formation. The unconformity between the Blouberg and Mogalakwena Formations and the disconformity between the Makgabeng and Mogalakwena Formations may represent the same depositional hiatus, though it is most likely that the aeolian deposits preserved in the sub-horizontal Makgabeng Formation post-date the steeplydipping fluvial Blouberg strata. Rare outcrops of the Mogalakwena Formation to the north of Blouberg mountain are dominated by sandstone facies, with only thin basal conglomerates immediately above the unconformity with the Blouberg Formation. In contrast, Mogalakwena rocks in the southern foothills of Blouberg mountain are dominated by conglomerate facies (Bumby et al., 2001a).
4. Soutpansberg Group The foothills of Blouberg mountain are underlain by rocks of the Blouberg Formation, unconformably overlain by strata of the Mogalakwena Formation. The high terrain of Blouberg mountain is underlain by the Soutpansberg Group which succeeds the Mogalakwena Formation on a low-angle unconformity (Fig. 4). At the western edge of the mountain, the basaltic Sibasa Formation of the Soutpansberg Group crops out rarely, while the majority of Blouberg mountain comprises
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Fig. 3. Angular unconformity developed between the steeply-dipping Blouberg Formation and the sub-horizontal Mogalakwena Formation above.
sandstone, quartzite and thin pebble conglomerates of the overlying WyllieÕs Poort Formation. The Soutpans-
berg Group has been interpreted as reflecting deposition within an aulacogen (Jansen, 1976), a post-tectonic
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Fig. 4. Gentle angular unconformity developed between the Mogalakwena Formation and the WyllieÕs Poort Formation above. Cliff section is about 200 m high.
half-graben (Barker, 1983; Bumby et al., 2002) or as representing a remnant of a former craton-wide cover sequence (Cheney et al., 1990). The age of the Sibasa basalt has been estimated at 1769 ± 34 Ma (Rb–Sr whole rock; Barton, 1979), though this estimate was interpreted as reflecting subsequent hydrothermal alteration by Cheney et al. (1990). Cheney et al. (1990) suggest instead that the Soutpansberg strata were deposited earlier, sometime between 1974 Ma and 1800 Ma.
5. Discussion The stratigraphic relationships of the sedimentary rocks at Blouberg suggest that the granulite-grade rocks of the Limpopo Belt are nonconformably overlain by the Blouberg Formation, which are in turn unconformably overlain by strata of the medial part of the Waterberg Group (Mogalakwena Formation), which are in turn unconformably overlain by the rocks of the ca. 1.8– 1.97 Ga Soutpansberg Group (Sibasa and WyllieÕs Poort Formations) (Bumby et al., 2001b). It has been suggested (van Reenen et al., 1992) that up to 35 km of exhumation has been accomplished to account for the presence of granulite-grade rocks of the Limpopo Belt at surface levels. The presence of Blouberg sedimentary strata (which show no evidence of any metamorphism) nonconformably above Limpopo rocks suggest that this exhumation had been accomplished prior to Blouberg deposition. Blouberg strata, which have been estimated to be slightly
younger than ca. 2017 Ma, based on the age of discordant melt patches in C.Z. gneisses (e.g. Jansen, 1976; Barker et al., in press), show evidence for having being deposited in a syn-tectonic environment; the contrast in facies between the granulestone lower and conglomeratic upper members, and the relative immaturity of the upper member suggests that the Blouberg Formation records sedimentation as a response to proximal tectonic activity. The sharp nature of the conformable contact between the upper and lower members does not favour an alternative model of Blouberg deposition as a response to climate change (Bumby, 2000; Bumby et al., 2001a), which is likely to have resulted in a more gradual change in sedimentary facies. Although the nature of such an inferred proximal tectonic event is uncertain, the geometry of the lower member of the Blouberg Formation (steeply dipping to the south and overturned, locally steeply dipping to the north) suggests southwardvergent tectonics. Such structures have not been recorded in the rarely preserved beds of the upper member, which generally have only a shallow dip, suggesting that southward-vergent tectonism is syn-depositional with, rather than post-depositional to, the Blouberg Formation. Further evidence for southward-vergent, syn-Blouberg tectonism is recorded in northwards-dipping reverse faults in the lower member, and the presence locally of Limpopo gneisses thrust southwards over lower member strata (Bumby, 2000). Poor field exposures of the contact between the Blouberg and Makgabeng Formations leads to some
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question regarding age relationships between these two units. However, the Blouberg Formation was deposited on ÔbasementÕ strata of the Limpopo Belt, whilst the Makgabeng Formation was conformably deposited on the Setlaole Formation of the Waterberg Group. Other medial and lower Waterberg Group strata outcrop beneath the Magkabeng Formation further to the south (Callaghan et al., 1991), and thus it seems most likely that the Blouberg Formation must considerably predate the deposition of the aeolianites of the Makgabeng Formation. Such an interpretation is also evidenced by the long record of tectonism present in the Blouberg Formation, that is not present in the adjacent, sub-horizontal outcrops of the Makgabeng Formation. The sharply angular unconformity between the Blouberg and Mogalakwena Formations, and the disconformity between the Makgabeng and Mogalakwena Formations both show evidence for a peneplaned palaeorelief established prior to Mogalakwena deposition. Again, such peneplanation must reflect a considerable hiatus in time. The strata of the Mogalakwena Formation generally appear to become more mature (thinner, less common conglomeratic beds) towards the southwest from Blouberg mountain, and south-westerly palaeocurrent directions suggest this is related to downstream fining. The most immature Mogalakwena sediments (where the conglomerates are best developed) are located in the southern foothills of Blouberg mountain, supporting the presence of a fault-bounded source area immediately to the north, bounding the northern margin of the Waterberg basin. However, in the northern foothills of Blouberg mountain, Mogalakwena sediments are more mature (similar to those found several kilometres to the south-west of the mountain), which suggests that Mogalakwena sediments onlapped northwards across a palaeo-fault scarp, located beneath Blouberg mountain and which bounded the northern margin of the Waterberg basin. It is possible that this fault scarp was developed during southward-vergent, syn-Blouberg tectonism (Bumby, 2000; Bumby et al., 2001a). The angular unconformity between the Mogalakwena Formation and the WyllieÕs Poort Formation is shallow (ca. 5°; Fig. 4), but nevertheless records yet another considerable break in sedimentary deposition and time. If the ca. 1.8–1.97 Ga age for the Soutpansberg Group is correct, the following events have to accommodated in the period of time between the Limpopo event and the eruption of Sibasa lavas in the medial part of the Soutpansberg Group: 1. Exhumation of up to 35 km of Central Zone gneisses. 2. Creation of the Blouberg depository (possibly a pullapart basin: Brandl, 1986; Bumby, 2000). 3. Deposition of the lower member of the Blouberg Formation.
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4. Southward-vergent tectonism, possibly along reactivated strike-slip faults; folding and thrusting of gneiss and lower-Blouberg sedimentary rocks. Syn-tectonic deposition of the upper member of the Blouberg Formation. 5. Deposition of the lower and medial Waterberg Group, including the Setlaole and Makgabeng Formations. 6. Peneplanation of the Blouberg and Makgabeng Formations in the Blouberg area. 7. Deposition of the Mogalakwena Formation, onlapping northwards (possibly over syn-Blouberg fault scarps). 8. Erosion of the Mogalakwena Formation, followed by deposition of the lower Soutpansberg Group. The main focus of this contribution is to consider how the record of sedimentary rocks in the Blouberg area can be used to shed light on the enigma of the timing of the Limpopo collisional event, whether at ca. 2.6– 2.7 Ga or 2.0 Ga. The peak metamorphism of this event is recorded by the granulite-grade gneiss of the Limpopo Belt, and the strata in the Blouberg area indicate a long tectono-sedimentary history between the Limpopo event and the ca. 1.8–1.97 Ga Soutpansberg Group. A younger (2.0 Ga) age for the Limpopo event is clearly less compatible with this evidence, as such a long sequence of events needs to be accomplished prior to 1.8–1.97 Ga. It is likely that the Blouberg Formation itself has an age of about 2.0 Ga. This is based upon the age of other early Waterberg strata in the Loskop Formation, which contain pebbles of Bushveld mafic rocks, and yet are intruded by Bushveld granites (Martini, 1998), thus establishing their age of deposition, and the onset of Waterberg sedimentation, between 2054 and 2057 Ma (Harmer and Armstrong, 2000). The tectono-sedimentary history outlined above is therefore considered here to be more compatible with an older, Archaean-aged Limpopo collision, thus allowing sufficient time for exhumation of Limpopo gneiss prior to deposition of the Blouberg Formation. Models proposed by Holzer et al. (1998) and Schaller et al. (1999), involve transpressive tectonism during a ca. 2.0 Ga orogeny, followed by transtension to create the Soutpansberg rift. Such models do not allow for the deposition of the Waterberg Group strata (including the Blouberg Formation), or the development of unconformities between these two events. The 2.0 Ga dates determined from the Limpopo rocks are therefore interpreted here to be more likely to reflect a reactivation event within the Kalahari Craton at that time. Syn-sedimentary southward-vergent tectonics recorded in the Blouberg Formation at about 2.0 Ga may be a shallow-level manifestation of such a reactivation event, though clearly the lack of metamorphism in the Blouberg Formation argues strongly against such an
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event being the main Limpopo collision itself, as such an event would have led to high-grade metamorphism of Blouberg sedimentary rocks. It may be that enigmas regarding the timing and tectonic history of the Limpopo event can be resolved by invoking a diachronous collision (e.g. van Reenen et al., 1992). The Kaapvaal and Central Zone terrains may have collided initially in the late-Archaean, and were exhumed gradually throughout the early Proterozoic, thus allowing for the deposition of the Blouberg strata nonconformably on granulite-grade gneiss at ca. 2.0 Ga. The Zimbabwe Craton then may have collided with the Central Zone (combined with the Kaapvaal Craton to the south) at 2.0 Ga causing dextral transpressive reactivation along the Palala suture, and leading to syn-tectonic deposition in the Blouberg Formation. Such a model may also account for the general paucity of support for the 2.0 Ga age in the S.M.Z. (e.g. Kreissig et al., 2001). However it remains difficult to explain the sinistral and dextral senses of movement within the Palala and Triangle Shear zones, respectively, using such a model, unless both collisional events were extremely oblique. 6. Conclusions Although the sedimentary record in the Blouberg area cannot be used as a direct measure for the timing of the Limpopo event, it can be used to help constrain previously published models based on dating techniques. Controversies surrounding the timing of the Limpopo event remain under debate, and constraining of alternative models using new data from seemingly unrelated geological disciplines seems appropriate. The long tectono-sedimentary record in the Blouberg area which was accomplished in the interval between the Limpopo collisional event and the deposition of the Soutpansberg strata at ca. 1.97–1.8 Ga is less compatible with a younger (ca. 2.0 Ga) timing for the Limpopo event, and rather favours an older (ca. 2.6–2.7 Ga) age for the collision. Acknowledgments The authors wish to acknowledge the The generous financial support from Gold Fields of South Africa Ltd., the University of Pretoria, and the National Research Foundation. Peter and Janine Snyman of Blouberg Conservation Project are gratefully acknowledged for providing accommodation to A.J. Bumby during extensive field investigations. Wlady Altermann and Gu¨nther Brandl acted as referees for this manuscript, and Bob Thomas acted as guest-editor. Their constructive comments greatly improved this manuscript.
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A.J. Bumby et al. / Journal of African Earth Sciences 39 (2004) 123–131 Hout River Shear Zone and the metamorphism in the Southern Marginal Zone of the Limpopo Belt, Southern Africa. Precambrian Res. 109, 145–173. Kro¨ner, A., Jaeckel, P., Brandl, G., Nechin, A.A., Pidgeon, R.T., 1999. Single zircon ages for granitoid gneisses in the Central Zone of the Limpopo Belt, southern Africa and geodynamic significance. Precambrian Res. 93, 299–337. Light, M.P.R., 1982. The Limpopo Belt: a result of continental collision. Tectonics 1, 325–342. Martini, J.E.J., 1998. The Loskop Formation and its relationship to the Bushveld Complex, South Africa. J. Afr. Earth Sci. 27, 193–222. McCourt, S., Armstrong, R.A., 1998. SHRIMP U–Pb zircon chronology of granites from the Central Zone, Limpopo Belt, southern Africa: implications for the age of the Limpopo Orogeny. S. Afr. J. Geol. 101, 329–337. McCourt, S., Vearncombe, J.R., 1987. Shear zones bounding the Central Zone of the Limpopo belt, and adjacent granitoidgreenstone terranes: implications for late Archaean collision tectonics in southern Africa. Precambrian Res. 55, 553–570. McCourt, S., Vearncombe, J.R., 1992. Shear zones of the Limpopo Belt and adjacent granitoid-greenstone terranes: implications for late Archaean collision tectonics in southern Africa. Precambrian Res. 55, 553–570. Meinster, B., 1977. Discussion on H. Jansen: the Waterberg and Soutpansberg Groups in the Blouberg area, Northern Transvaal. Trans. Geol. Soc. S. Afr. 80, 289–296.
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Roering, C., van Reenen, D.D., Smit, C.A., Barton Jr., J.M., De Beer, J.H., De Wit, M.J., Stettler, E.H., Van Schalkwyk, J.F., Stevens, G., Pretorius, S., 1992. Tectonic model for the evolution of the Limpopo Mobile belt. Precambrian Res. 55, 539–552. Rollinson, H.R., 1993. A terrane interpretation of the Archaean Limpopo belt. Geol. Mag. 130, 755–765. Schaller, M., Steiner, O., Studer, I., Holzer, L., Herwegh, M., Kramers, J.D., 1999. Exhumation of Limpopo Central Zone granulites and dextral continent-scale transcurrent movement at 2.0 Ga along the Palala Shear Zone, Northern Province, South Africa. Precambrian Res. 96, 263–288. Simpson, E.L., Eriksson, K.A., Eriksson, P.G., Bumby, A.J., 2002. Eolian dune degradation and generation of massive sandstones in the Palaeoproterozoic Makgabeng Formation, Waterberg Group, South Africa. J. Sediment. Res. 72, 40–45. Treloar, P.J., Coward, M.P., Harris, N.B.W., 1992. HimalayanTibetan analogies for the evolution of the Zimbabwe craton and Limpopo Belt. Precambrian Res. 55, 571–587. Van Reenen, D.D., Roering, C., Ashwal, L.D., de Wit, M.J., 1992. Regional geological setting of the Limpopo Belt. Precambrian Res. 55, 1–5. Watkeys, M., 1984. The Precambrian geology of the Limpopo Belt north and west of Messina. Ph.D. thesis, University of the Witwatersrand, Johannesburg, unpublished. Wentworth, C.K., 1922. A scale of grade and class terms for clastic sediments. J. Geol. 30, 377–392.
The early Proterozoic sedimentary record in the Blouberg area, Limpopo Province, South Africa; implications for the timing of the Limpopo orogenic event A.J. Bumby *, P.G. Eriksson, R. Van Der Merwe Department of Geology, University of Pretoria, Pretoria 0002, South Africa
Abstract At present, the timing and geometry of a presumed collision of the Limpopo Belt (southern Africa) remain controversial. Data from tectonic and metamorphic studies, and a number of radiometric dates, have been interpreted to produce widely varying models. In this work, we aim to help constrain these models using data from the sedimentary rocks in the Blouberg area, which lie above and adjacent to the Palala Shear Zone in the southwestern part of the belt. Here, a thick sedimentary package consisting of the syn-tectonic Blouberg Formation, medial parts of the Waterberg Group and the Soutpansberg Group is developed nonconformably above the granulite-grade gneiss of the Limpopo Belt. These three sedimentary units are separated from each other by angular unconformities. The youngest rocks of this succession (the Soutpansberg Group) have an imprecise age of ca. 1.8–1.97 Ga. The inferred long tectono-sedimentary history necessary to produce these three unconformity-bounded sequences is interpreted to support an older, Archaean-age (ca. 2.6–2.7 Ga) for the timing of the Limpopo collision, rather than reflecting a younger Proterozoic (ca. 2.0 Ga) event. It is proposed that syn-sedimentary tectonism recorded in the Blouberg Formation may reflect a southward-vergent 2.0 Ga tectonic event, though this is interpreted as reactivation within the Limpopo Belt, rather than the primary collision itself. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Limpopo; Blouberg; Sediments; Age constraints; Palala Shear Zone
1. Introduction The Limpopo Belt of southern Africa was thought to have formed by a ca. 2.6–2.7 Ga collisional orogen between two Archaean cratons, Kaapvaal to the south and Zimbabwe to the north. It was believed to represent one of the earliest examples of collision between two large-scale cratons, which is more analogous to modern-day plate tectonic collisions (e.g. Light, 1982; Treloar et al., 1992). After collision, the combined Kaapvaal and Zimbabwe cratons formed the larger-
*
Corresponding author. E-mail address: [email protected] (A.J. Bumby).
0899-5362/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2004.07.056
scale Kalahari Craton. Recently, however, the Limpopo Belt has become the focus of controversy. Much work now advocates dextral transpressive tectonics at 2.0 Ga (e.g. Barton et al., 1994; Holzer et al., 1998; Schaller et al., 1999; Kreissig et al., 2001). Both the age and geometry of the Limpopo Belt therefore remain poorly defined and the object of much debate and research (e.g. Bumby, 2000). The Limpopo Belt consists of high-grade metamorphic rocks exposed in a ca. 250 km-wide E.N.E– W.S.W.-trending belt which passes through the northern-most part of South Africa and the southernmost part of Zimbabwe, westwards into eastern Botswana (Fig. 1), with an overall length of about 550 km. In the east, the belt is overlain by Jurassic flood basalts
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Fig. 1. Map and cross-section of the Limpopo Belt showing component zones and bounding shear zones, and the location of the study area (after Kro¨ner et al., 1999; Roering et al., 1992).
related to the break-up of Gondwana, and to the west, in Botswana, it is covered by Tertiary-Recent sands of the Kalahari desert (Fig. 1). The Limpopo Belt comprises a Central Zone (C.Z.), which is flanked to the north and south by a Northern Marginal Zone (N.M.Z.) and Southern Marginal Zone (S.M.Z.), respectively (Fig. 1). The N.M.Z. is bounded by the northwards-verging Northern Marginal Thrust Zone to the north, and in the south is separated from the Central Zone by the generally dextral Triangle Shear Zone. Relationships in the S.M.Z. are almost a mirror-image, to those of the N.M.Z. The S.M.Z. is bound to the south by the southwards-vergent Hout River Shear Zone, and in the north it is separated from the Central Zone by the generally sinistral Palala Shear Zone (Fig. 1) (McCourt and Vearncombe, 1987, 1992). The Palala Shear Zone has, however, been interpreted to have been reactivated at ca. 1.97 Ga, with dextral sense of shear (Schaller et al., 1999). Tectonic controversies surrounding the Limpopo Belt concern the provenance of the Central Zone. The N.M.Z. is composed of granulite-grade rocks, which are regarded as the higher-grade equivalent to greenschist-facies rocks of the Zimbabwe Craton. Similarly the S.M.Z. is considered to comprise rocks of the Kaapvaal Craton at granulite-grade. The Central Zone, how-
ever, appears to be an exotic crustal block, which contains evidence for an entirely separate tectonic and sedimentary history (van Reenen et al., 1992). Sedimentary protoliths (now also at granulite-grade) in the Central Zone are preserved as the Beit Bridge Complex. The dextral and sinistral sense of movement in the Triangle and Palala Shear zones which bound the Central Zone to the north and south, respectively, suggest that the Central Zone may have been thrust westwards as a giant nappe over the top of the previously assembled Kalahari Craton (McCourt and Vearncombe, 1987, 1992), thus leading to granulite-grade metamorphism in the marginal zones of the adjacent cratons beneath the nappe. Accommodation of loading by the nappe is thought to have produced the Northern Marginal and Hout River thrusts (McCourt and Vearncombe, 1992). Alternative models consider the possibility of diachronous collisions in the Limpopo Belt, whereby the C.Z. collided initially with the Zimbabwe Craton, followed by later docking of the Kaapvaal, or initial collision between the C.Z. and Kaapvaal, with subsequent accretion of the Zimbabwe Craton (Watkeys, 1984; Roering et al., 1992; Rollinson, 1993). Controversies surrounding the timing of the Limpopo event focus on the recent spate of dates (derived from metamorphic silicate minerals in the Triangle and
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Palala Shear zones, and from melt patches and anatectic granitoids throughout the CZ) suggesting that peak metamorphism and thus collision occurred as recently as 2.0 Ga (e.g. Holzer et al., 1998; Schaller et al., 1999). Such dates are at odds with the more traditional minimum ages for the Limpopo collision, determined from syn-tectonic granites in the Central Zone, (e.g. McCourt and Armstrong, 1998). Ages for such granites cluster between 2.6 and 2.7 Ga. Assuming a 2.0 Ga collisional event, ca. 2.6–2.7 Ga events can be accounted for by considering the long independent history of the Central Zone which appears to have been entirely separate from that of either of the adjacent cratons; the 2.6–2.7 Ga event within this model is thus envisaged to be restricted to the pre-collisional Central Zone block, and unrelated to the Limpopo Orogen (Holzer et al., 1998). Assuming a 2.7–2.6 Ga Limpopo event, the record of metamorphism at 2.0 Ga is best explained by reactivation tectonics (probably related to the ca. 2.15–1.85 Ga Africa-wide Eburnean event) within the previously assembled Kalahari Craton (McCourt and Armstrong, 1998). The Eburnean-aged southward-vergent 2.0 Ga Magondi Orogen on the northwestern margin of the Zimbabwe Craton is envisaged as a possible cause of similarly-aged reactivation in the Limpopo Belt (McCourt and Arm-
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strong, 1998; Bumby et al., 2001a). However, at present, there is no evidence for the 2.0 Ga event in the S.M.Z. Recent dating of strata adjacent to the Hout River Shear Zone suggests an age of peak metamorphism that supports a 2.6–2.7 Ga Limpopo event (Kreissig et al., 2001). Thus there remains little consensus regarding either the timing or the tectonic history of events involved with the Limpopo Orogen. The aim of this contribution is to consider the enigmatic timing of the Limpopo Orogen from a different standpoint. The present work will examine how the sedimentary rock record, which rests nonconformably on the high-grade rocks of the Limpopo Belt, can help constrain hypotheses regarding timing of the collisional event. This work will focus on the ages and tectono-sedimentary history of the Palaeoproterozoic (2050–1600 Ma Orosirian–Statherian periods; IUGS, 2000) Blouberg Formation, the Waterberg and Soutpansberg Groups which outcrop above and adjacent to the Palala Shear Zone (Fig. 2).
2. Blouberg Formation The Blouberg Formation (Fig. 2) is an entirely clastic sedimentary sequence, which outcrops in areas restricted
Fig. 2. Map of the Blouberg area showing outcrops of the sedimentary sequences of the Blouberg Formation, Waterberg and Soutpansberg Groups. The Palala Shear Zone is inferred to be present beneath these strata (after Bumby et al., 2001a).
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to those above the projected eastward extension of the Palala Shear Zone (Fig. 1). The Blouberg Formation was deposited nonconformably over the granulite-grade gneisses of the Limpopo Belt, and attains a maximum thickness of about 1400 m (Jansen, 1975; Bumby et al., 2001a), though generally only sequences of less than 300 m outcrop. In the thickest preserved section, the Blouberg Formation consists of a 600 m-thick lower member, where facies associations comprise trough cross-bedded coarse arkosic sandstone and channel-fills of feldspathic granulestone (Wentworth, 1922). Generally, the size of sets within the trough cross-bedded facies decreases upwards in the lower member (Bumby et al., 2001a). These lower Blouberg facies associations are interpreted as reflecting deposition within braided streams. In contrast to the lower part of the Blouberg Formation, the upper member consists of coarse, feldspathic sedimentary breccia and conglomerate. Cobbles and boulders within the upper member consist of quartzite and foliated feldspathic gneiss. The strata of the upper member are interpreted as reflecting deposition within alluvial fans (Bumby et al., 2001a). The sedimentary strata of the Blouberg Formation are generally characterized by steeply-dipping bedding planes (Bumby et al., 2001a). In outcrops immediately to the south of Blouberg mountain, where only rocks of the lower member are recorded, strata typically dip southwards, with angles in excess of 45°. Locally, Blouberg strata are overturned and dip northwards at 70° or more. Locally, the Blouberg Formation contains steeply northward-dipping reverse faults and rarely the Limpopo gneiss can be seen to overlie the Blouberg Formation, having being emplaced by southward-vergent thrusts. The rare outcrops of the rudaceous upper member are generally sub-horizontally inclined, and in places may unconformably overlie the steeply-dipping lower member (Bumby et al., 2001a).
3. Waterberg Group Three formations of the Waterberg Group crop out in the Blouberg area, immediately to the south of Blouberg mountain (Fig. 2). The Waterberg strata contrast from the Blouberg Formation in that they are generally sub-horizontally inclined (Callaghan et al., 1991; Bumby et al., 2001a). The lowermost Setlaole Formation is poorly exposed, and consists of coarse arkosic sandstone and granulestone. The Setlaole Formation may tentatively be correlated with the Blouberg Formation (Jansen, 1976; Bumby, 2000), though its sub-horizontal aspect suggests that Setlaole deposition may post-date inversion of the Blouberg basin (Meinster, 1977). The predominantly fluvial Setlaole Formation grades upwards into mature sandstone of the Makgabeng Formation. The latter consists predominantly of aeolianite
facies, with minor facies of playa lakes, interdune deposits and ephemeral rivers becoming increasingly abundant towards the top (Eriksson et al., 2000; Simpson et al., 2002). The Makgabeng Formation is disconformably overlain by the Mogalakwena Formation, and extensive exposures of this disconformity show little palaeorelief on the upper surface of the Makgabeng strata, suggesting that the Makgabeng Formation was peneplaned prior to Mogalakwena deposition. The Mogalakwena Formation consists of interbedded sheets of coarse sandstone and conglomerate, interpreted as braided stream and alluvial fan deposits, respectively (Bumby et al., 2001a). Palaeocurrent directions recorded from trough cross-bedded sandstone in the Mogalakwena Formation suggest unimodal flow towards the southwest. Conglomeratic facies become increasingly abundant to the northeast, with proximity to Blouberg mountain (Bumby, 2000; Bumby et al., 2001a). Waterberg strata in the foothills of Blouberg mountain consist only of the Mogalakwena Formation. The Makgabeng Formation appears to wedge out in a northerly direction about 5 km south of the Blouberg foothills (Bumby, 2000). The sub-horizontal Mogalakwena strata here unconformably overlie the steeply-dipping strata of the Blouberg Formation, and outcrops which show this spectacular unconformity (Fig. 3) suggest that the steeply-dipping Blouberg Formation was almost peneplaned prior to deposition of the Mogalakwena Formation. The unconformity between the Blouberg and Mogalakwena Formations and the disconformity between the Makgabeng and Mogalakwena Formations may represent the same depositional hiatus, though it is most likely that the aeolian deposits preserved in the sub-horizontal Makgabeng Formation post-date the steeplydipping fluvial Blouberg strata. Rare outcrops of the Mogalakwena Formation to the north of Blouberg mountain are dominated by sandstone facies, with only thin basal conglomerates immediately above the unconformity with the Blouberg Formation. In contrast, Mogalakwena rocks in the southern foothills of Blouberg mountain are dominated by conglomerate facies (Bumby et al., 2001a).
4. Soutpansberg Group The foothills of Blouberg mountain are underlain by rocks of the Blouberg Formation, unconformably overlain by strata of the Mogalakwena Formation. The high terrain of Blouberg mountain is underlain by the Soutpansberg Group which succeeds the Mogalakwena Formation on a low-angle unconformity (Fig. 4). At the western edge of the mountain, the basaltic Sibasa Formation of the Soutpansberg Group crops out rarely, while the majority of Blouberg mountain comprises
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Fig. 3. Angular unconformity developed between the steeply-dipping Blouberg Formation and the sub-horizontal Mogalakwena Formation above.
sandstone, quartzite and thin pebble conglomerates of the overlying WyllieÕs Poort Formation. The Soutpans-
berg Group has been interpreted as reflecting deposition within an aulacogen (Jansen, 1976), a post-tectonic
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Fig. 4. Gentle angular unconformity developed between the Mogalakwena Formation and the WyllieÕs Poort Formation above. Cliff section is about 200 m high.
half-graben (Barker, 1983; Bumby et al., 2002) or as representing a remnant of a former craton-wide cover sequence (Cheney et al., 1990). The age of the Sibasa basalt has been estimated at 1769 ± 34 Ma (Rb–Sr whole rock; Barton, 1979), though this estimate was interpreted as reflecting subsequent hydrothermal alteration by Cheney et al. (1990). Cheney et al. (1990) suggest instead that the Soutpansberg strata were deposited earlier, sometime between 1974 Ma and 1800 Ma.
5. Discussion The stratigraphic relationships of the sedimentary rocks at Blouberg suggest that the granulite-grade rocks of the Limpopo Belt are nonconformably overlain by the Blouberg Formation, which are in turn unconformably overlain by strata of the medial part of the Waterberg Group (Mogalakwena Formation), which are in turn unconformably overlain by the rocks of the ca. 1.8– 1.97 Ga Soutpansberg Group (Sibasa and WyllieÕs Poort Formations) (Bumby et al., 2001b). It has been suggested (van Reenen et al., 1992) that up to 35 km of exhumation has been accomplished to account for the presence of granulite-grade rocks of the Limpopo Belt at surface levels. The presence of Blouberg sedimentary strata (which show no evidence of any metamorphism) nonconformably above Limpopo rocks suggest that this exhumation had been accomplished prior to Blouberg deposition. Blouberg strata, which have been estimated to be slightly
younger than ca. 2017 Ma, based on the age of discordant melt patches in C.Z. gneisses (e.g. Jansen, 1976; Barker et al., in press), show evidence for having being deposited in a syn-tectonic environment; the contrast in facies between the granulestone lower and conglomeratic upper members, and the relative immaturity of the upper member suggests that the Blouberg Formation records sedimentation as a response to proximal tectonic activity. The sharp nature of the conformable contact between the upper and lower members does not favour an alternative model of Blouberg deposition as a response to climate change (Bumby, 2000; Bumby et al., 2001a), which is likely to have resulted in a more gradual change in sedimentary facies. Although the nature of such an inferred proximal tectonic event is uncertain, the geometry of the lower member of the Blouberg Formation (steeply dipping to the south and overturned, locally steeply dipping to the north) suggests southwardvergent tectonics. Such structures have not been recorded in the rarely preserved beds of the upper member, which generally have only a shallow dip, suggesting that southward-vergent tectonism is syn-depositional with, rather than post-depositional to, the Blouberg Formation. Further evidence for southward-vergent, syn-Blouberg tectonism is recorded in northwards-dipping reverse faults in the lower member, and the presence locally of Limpopo gneisses thrust southwards over lower member strata (Bumby, 2000). Poor field exposures of the contact between the Blouberg and Makgabeng Formations leads to some
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question regarding age relationships between these two units. However, the Blouberg Formation was deposited on ÔbasementÕ strata of the Limpopo Belt, whilst the Makgabeng Formation was conformably deposited on the Setlaole Formation of the Waterberg Group. Other medial and lower Waterberg Group strata outcrop beneath the Magkabeng Formation further to the south (Callaghan et al., 1991), and thus it seems most likely that the Blouberg Formation must considerably predate the deposition of the aeolianites of the Makgabeng Formation. Such an interpretation is also evidenced by the long record of tectonism present in the Blouberg Formation, that is not present in the adjacent, sub-horizontal outcrops of the Makgabeng Formation. The sharply angular unconformity between the Blouberg and Mogalakwena Formations, and the disconformity between the Makgabeng and Mogalakwena Formations both show evidence for a peneplaned palaeorelief established prior to Mogalakwena deposition. Again, such peneplanation must reflect a considerable hiatus in time. The strata of the Mogalakwena Formation generally appear to become more mature (thinner, less common conglomeratic beds) towards the southwest from Blouberg mountain, and south-westerly palaeocurrent directions suggest this is related to downstream fining. The most immature Mogalakwena sediments (where the conglomerates are best developed) are located in the southern foothills of Blouberg mountain, supporting the presence of a fault-bounded source area immediately to the north, bounding the northern margin of the Waterberg basin. However, in the northern foothills of Blouberg mountain, Mogalakwena sediments are more mature (similar to those found several kilometres to the south-west of the mountain), which suggests that Mogalakwena sediments onlapped northwards across a palaeo-fault scarp, located beneath Blouberg mountain and which bounded the northern margin of the Waterberg basin. It is possible that this fault scarp was developed during southward-vergent, syn-Blouberg tectonism (Bumby, 2000; Bumby et al., 2001a). The angular unconformity between the Mogalakwena Formation and the WyllieÕs Poort Formation is shallow (ca. 5°; Fig. 4), but nevertheless records yet another considerable break in sedimentary deposition and time. If the ca. 1.8–1.97 Ga age for the Soutpansberg Group is correct, the following events have to accommodated in the period of time between the Limpopo event and the eruption of Sibasa lavas in the medial part of the Soutpansberg Group: 1. Exhumation of up to 35 km of Central Zone gneisses. 2. Creation of the Blouberg depository (possibly a pullapart basin: Brandl, 1986; Bumby, 2000). 3. Deposition of the lower member of the Blouberg Formation.
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4. Southward-vergent tectonism, possibly along reactivated strike-slip faults; folding and thrusting of gneiss and lower-Blouberg sedimentary rocks. Syn-tectonic deposition of the upper member of the Blouberg Formation. 5. Deposition of the lower and medial Waterberg Group, including the Setlaole and Makgabeng Formations. 6. Peneplanation of the Blouberg and Makgabeng Formations in the Blouberg area. 7. Deposition of the Mogalakwena Formation, onlapping northwards (possibly over syn-Blouberg fault scarps). 8. Erosion of the Mogalakwena Formation, followed by deposition of the lower Soutpansberg Group. The main focus of this contribution is to consider how the record of sedimentary rocks in the Blouberg area can be used to shed light on the enigma of the timing of the Limpopo collisional event, whether at ca. 2.6– 2.7 Ga or 2.0 Ga. The peak metamorphism of this event is recorded by the granulite-grade gneiss of the Limpopo Belt, and the strata in the Blouberg area indicate a long tectono-sedimentary history between the Limpopo event and the ca. 1.8–1.97 Ga Soutpansberg Group. A younger (2.0 Ga) age for the Limpopo event is clearly less compatible with this evidence, as such a long sequence of events needs to be accomplished prior to 1.8–1.97 Ga. It is likely that the Blouberg Formation itself has an age of about 2.0 Ga. This is based upon the age of other early Waterberg strata in the Loskop Formation, which contain pebbles of Bushveld mafic rocks, and yet are intruded by Bushveld granites (Martini, 1998), thus establishing their age of deposition, and the onset of Waterberg sedimentation, between 2054 and 2057 Ma (Harmer and Armstrong, 2000). The tectono-sedimentary history outlined above is therefore considered here to be more compatible with an older, Archaean-aged Limpopo collision, thus allowing sufficient time for exhumation of Limpopo gneiss prior to deposition of the Blouberg Formation. Models proposed by Holzer et al. (1998) and Schaller et al. (1999), involve transpressive tectonism during a ca. 2.0 Ga orogeny, followed by transtension to create the Soutpansberg rift. Such models do not allow for the deposition of the Waterberg Group strata (including the Blouberg Formation), or the development of unconformities between these two events. The 2.0 Ga dates determined from the Limpopo rocks are therefore interpreted here to be more likely to reflect a reactivation event within the Kalahari Craton at that time. Syn-sedimentary southward-vergent tectonics recorded in the Blouberg Formation at about 2.0 Ga may be a shallow-level manifestation of such a reactivation event, though clearly the lack of metamorphism in the Blouberg Formation argues strongly against such an
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event being the main Limpopo collision itself, as such an event would have led to high-grade metamorphism of Blouberg sedimentary rocks. It may be that enigmas regarding the timing and tectonic history of the Limpopo event can be resolved by invoking a diachronous collision (e.g. van Reenen et al., 1992). The Kaapvaal and Central Zone terrains may have collided initially in the late-Archaean, and were exhumed gradually throughout the early Proterozoic, thus allowing for the deposition of the Blouberg strata nonconformably on granulite-grade gneiss at ca. 2.0 Ga. The Zimbabwe Craton then may have collided with the Central Zone (combined with the Kaapvaal Craton to the south) at 2.0 Ga causing dextral transpressive reactivation along the Palala suture, and leading to syn-tectonic deposition in the Blouberg Formation. Such a model may also account for the general paucity of support for the 2.0 Ga age in the S.M.Z. (e.g. Kreissig et al., 2001). However it remains difficult to explain the sinistral and dextral senses of movement within the Palala and Triangle Shear zones, respectively, using such a model, unless both collisional events were extremely oblique. 6. Conclusions Although the sedimentary record in the Blouberg area cannot be used as a direct measure for the timing of the Limpopo event, it can be used to help constrain previously published models based on dating techniques. Controversies surrounding the timing of the Limpopo event remain under debate, and constraining of alternative models using new data from seemingly unrelated geological disciplines seems appropriate. The long tectono-sedimentary record in the Blouberg area which was accomplished in the interval between the Limpopo collisional event and the deposition of the Soutpansberg strata at ca. 1.97–1.8 Ga is less compatible with a younger (ca. 2.0 Ga) timing for the Limpopo event, and rather favours an older (ca. 2.6–2.7 Ga) age for the collision. Acknowledgments The authors wish to acknowledge the The generous financial support from Gold Fields of South Africa Ltd., the University of Pretoria, and the National Research Foundation. Peter and Janine Snyman of Blouberg Conservation Project are gratefully acknowledged for providing accommodation to A.J. Bumby during extensive field investigations. Wlady Altermann and Gu¨nther Brandl acted as referees for this manuscript, and Bob Thomas acted as guest-editor. Their constructive comments greatly improved this manuscript.
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