Hubertus Porada

Pan-African rifting and orogenesis in southern to equatorial Africa and eastern Brazil

Abstract The Upper Proterozoic Pan-African belts of Africa and Brasiliano belts of South America are assumed to have evolved from an elaborate system of continental rifts which formed on the West Gondwana continent 1100-1000 Ma ago. Reviews of the Damara belt, Kaoko belt, Gariep belt, Saldania belt, West Congolian belt, Lufilian Arc and Zambezi belt of southern and equatorial Africa and the Ribeira-Mantiqueira belt of Brazil show the applicability of the continental rifting model to all these belts. Strict application of the rifting model describing the formation and further evolution of rift structures in the continental crust leads to the conclusion that the West Congolian belt, as traditionally described, is incomplete and has to be supplemented by the Mayumbian belt and a rift structure situated farther to the west. The Lufilian Arc is assumed to have evolved from two rift structures separated from each other by the later ‘Domes region’. The ‘Golfe du Katanga”, which branches off northeastwards from the Lufilian Arc is interpreted as an aulacogen (‘Shaba aulacogen’). The Pan-African-Brasiliano rift system is comparable to the Mesozoic Arctic-North Atlantic rift system with regard to size and distribution of rifts. Ocean floor spreading and opening of a proto-South Atlantic Ocean most probably occurred along a main line of rifting situated between the west coast of Africa and eastern Brazil. The wedge-like ocean may have terminated at the ‘Sao Francisco-Congo cratonic bridge’ between northeastern Brazil and Gabon-Cameroon. There is no evidence of ocean-floor spreading in the Zambezi belt, the Lufilian Arc and the traditional West Congolian belt, which are arranged along another major line of rifting. Closure of the Pan-African-Brasiliano rift system took place during two successive orogenic episodes. The Zambezi belt, Lufilian Arc and West Congolian belt underwent their main orogenic deformation with prevailing ENE- and WSW-oriented tectonic transportation directions during the Katangan episode, at ∼900−750 Ma. The transcontinental sinistral Mwembeshi Shear Zone adjusted opposite movement directions in the Zambezi belt and Lufilian Arc and, at its western prolongation, concurrently supported opening of the Khomas Trough in the southern Damara belt. The proto-South Atlantic Ocean, still opening during the Katangan episode, was gradually closed during the Damaran episode, at ∼750−500 Ma. Closure proceeded from north to south and was accompanied by northwest-directed subduction underneath the Brazilian plate and southeast-directed tectonic transportation in the Kaoko, Damara and Gariep belts. In the previously formed West Congolian belt, Lufilian Arc and Zambezi belt a second deformation phase characterized by strike-slip faulting and local emplacement of nappes occurred during this episode.

Towards a new understanding of the Neoproterozoic-early palæozoic Lufilian and northern Zambezi belts in Zambia and the Democratic Republic of Congo

Abstract The Lufilian Belt is of geological significance and economic importance due to rich CuCo mineralisation in the Katanga Province of the Democratic Republic of Congo and the Copperbelt of Zambia. Though thorough exploration has yielded much information on the mines districts, the understanding of the belt as a whole appears, to some extent, historically charged and confused. In the first part of this article, basic knowledge and assumptions are reviewed and existing models critically assessed. Results include recognition of standard lithostratigraphies of the Katanga Supergroup comprising the Roan, Mwashia, Lower and Upper Kudelungu Groups in the Copperbelt and Katanga, a lower limit for the onset of deposition at about 880 Ma, and a major orogenetic event involving northeast directed thrusting (Lufilian Orogeny) at 560-550 Ma. The depositional history of the Lufilian Belt was controlled by continental rifting leading to formation of a passive continental margin. Continental rifting related to the dispersal of Rodinia began ca 880 Ma ago and was accompanied by magmatism (Kafue rhyolites: 879 Ma; Nchanga Granite: 877 Ma; Lusaka Granite: 865 Ma). Differential subsidence of the northwestward propagating rift soon allowed invasion by the sea advancing from the southeast, and subsequent development of marine rift-basin and platform domains. The standard stratigraphies for the Roan Group are restricted to the platform domain that bordered the rift-basin on its northeastern side. This domain included the Domes region of the Lufilian Belt and extended southeastwards into the northern Zambezi Belt. The platform was differentiated into a carbonate platform (barrier) represented by the Bancroft Subgroup (previously ‘Upper Roan’) in Zambia and Kambove Dolomite Formation in Katanga and a lagoon-basin (lower Kitwe Subgroup/Zambia; Dolomitic Shale Formation/Katanga) with mudflats (R.A.T. Subgroup/Katanga) and a siliciclastic margin towards the hinterland. The mineralised horizons of the ‘Ore Formation’ in Zambia and ‘Series des Mines’ in Katanga are related to temporarily anoxic conditions prevailing in the Roan Lagoon-Basin which had a southwest-northeast extent of ca 400 km. The lagoon-basin was subsequently filled by clastics derived from mainly northeastern sources (upper Kitwe Subgroup/Zambia; Dipeta Subgroup/Katanga). Possibly due to continental rupture in the southeastern, more advanced, segment of the rift and concomitant differential movement in the rupturing plate, the Kundelungu Basin started to open during deposition of the Mwashia Group. Opening of the extensional basin was accompanied by rifting, rapid subsidence of the affected platform segment and widespread mafic magmatism, which lasted until deposition of the Lower Kundelungu Group. The elevated margins of the rapidly subsiding Kundelungu Basin offered favourable conditions for inland glaciation during the Sturtian-Rapitan global glaciation epoch. The diamictites of the Grand Conglomat are thus dated at ca 750 Ma. Tectonogenesis in the Lufilian and Zambezi Belts is related to ca 560-550 Ma collision of the ‘Angola-Kalahari Plate’ (comprising the Kalahari Craton and southwestern part of the Congo Craton) and the ‘Congo-Tanzania Plate’ (comprising the remaining part of the Congo Craton) along a southeast-northwest trending suture linking up the southern Mozambique Belt with the West Congo Belt. Collision was accompanied by northeast directed thrusting involving deep crustal detachments and forward-propagating thrust faults that developed in platform and slope deposits below a high level thrust. In the Domes region, the platform sequence was detached from its basement and displaced for ca 150 km into the External Fold-Thrust Belt of Katanga. The large displacement was enhanced by fluids liberated from evaporite-rich mudflat deposits of the R.A.T. Subgroup. In the Zambezi Belt, northeast directed thrusting was succeeded by southwest directed backfolding and backthrusting, due to greater shortening or thickening of the thrust wedge. The Mwembeshi Shear Zone accommodated greater shortening in the Zambezi Belt relative to the Lufilian Belt by sinistral transcurrent movement. The Mwembeshi Shear Zone is a reactivated pre-existing zone of weakness in the lithosphere of possibly Palaeoproterozoic age. There is no evidence of Neoproterozoic collision along this zone in the Lufilian Belt/Zambezi Belt domain.

The intracratonic branch of the Damara Orogen in South West Africa I. Discussion of geodynamic models

Abstract The late Precambrian Damara Supergroup was deposited in a geosyncline. There is good evidence that the intracratonic branch of this geosyncline began its development with a stage of rifting which produced three widely spaced grabens in sialic crust. During a second stage of subsidence the grabens merged and formed a geosyncline. A third stage produced intense deformation associated with thrusting and nappe transport, high-grade metamorphism, anatexis and large-scale granodioritic to granitic plutonism. A discussion of lifely geodynamic interpretations leads to the conclusion that the development of the intracratonic geosyncline can be best explained by a multiple aulacogen model. The dynamics of the orogeny cannot be readily interpreted with the help of a plate tectonic subduction-collision model. Concepts based on Rambergs (1972) gravitational instability models are considered applicable. The hypothesis is advanced that grabens, aulacogens and mobile belts may represent diverse responses of the crust to astenoliths of different sizes.

Mat-related sedimentary structures in Neoproterozoic peritidal passive margin deposits of the West African Craton (Anti-Atlas, Morocco)

Abstract Proterozoic inliers in the central Anti-Atlas mountains expose predominantly siliciclastic sedimentary successions deposited in peritidal zones along the Neoproterozoic continental margin of the West African Craton (WAC). The low-grade metamorphic and modestly deformed sediments contain a wealth of sedimentary structures related to the former presence and activities of microbial mats and respective physicobiological processes. The well-preserved structures include wrinkle structures, erosion marks, microbial sand chips, spindle-shaped and subcircular microbial shrinkage cracks, and possibly gas domes and cabbage-head structures. Thin sections exhibit mat fragments and dispersed grains of hematite/limonite after pyrite in fine-grained quartzitic storm deposits. Post-storm layers frequently consist of matrix-supported sand-sized to silt-sized grains and are overlain by argillaceous veneers including isolated silt-sized grains and black carbonaceous laminae. The muddy veneers are considered to represent compacted stacks of microbial mats (biolaminites), which colonized and biostabilized storm and post-storm layers, and thus prevented them from eroding. In the absence of grazing and burrowing organisms and at suitable depositional and hydrodynamic conditions, it may be expected that Proterozoic microbial mats extensively grew from the supratidal to the intertidal zones and occasionally, e.g. behind protective barriers, in the subtidal zone and beyond. Mat-related structures, however, need specific conditions for their formation and preservation: Wrinkle structures, erosion marks, and microbial sand chips require tractional currents and soon deposition and burial, respectively. Such conditions are preferably met in intertidal and supratidal zones. Spindle-shaped and subcircular cracks require mat shrinkage due to either desiccation or “syneresis”. Crack propagation implies progressive shrinkage, while superposition of crack generations indicates repeated alternation between mat exposure and flooding. Respective conditions prevail in the upper intertidal zone. Gas domes and cabbage-head structures are related to the production of gas from decaying organic material beneath a sealing cohesive mat. They may be some of the first features formed during genesis of petee structures in the intertidal to lower supratidal zones. Mat-related structures may serve as sensitive facies indicators once their modes of formation are revealed.

The intracratonic branch of the Damara Orogen in South West Africa II. Discussion of relationships with the Pan-African mobile belt system☆

Abstract In Part I the conclusion was reached that the intracratonic branch of the Damara orogen cannot be explained by a plate-tectonics continent/continent collision model. The intracratonic branch is connected with the coastal branch and also with the Zambezi belt. There is a direct stratigraphic and structural connection with the coastal branch which shows some features suggestive of a continental margin geosyncline, and, by inference, an orogeny caused by subduction. Such an interpretation would make it possible to regard the intracratonic branch as a failed arm of a hypothetical triple junction situated to the west of the present-day coast. The evidence for this attractive hypothesis must, however, remain inconclusive because the greater part of the coastal belt is hidden underneath the Atlantic Ocean. The hypothetical ocean arm cannot have been broad and did probably not extent northwards beyond Gabon. The connection with the Zambezi belt and the Lufilian arc is covered by younger deposits. The Zambezi belt is composed of tectonically and thermally reconstituted Archaean basement. It contains no structure which could be interpreted as a continent/continent collision suture. This is also true for the Mozambique belt which is structurally connected with the Zambezi belt. The join between the two can therefore not be regarded as a triple junction in the plate tectonic sense. The Lufilian arc contains a “miogeosynclinal” sequence which is stratigraphically very similar to the Damara Supergroup, but it is still uncertain whether the two are strictly contemporaneous. In these belts vertical movements and transcurrent faults seem to have played a greater role than compressive shortening. The evolution of these belts was to a considerable degree guided by the older Irumide structures which seem to be connected with the approximately contemporaneous Rehoboth-Sinclair igneous province delimiting the southern margin of the Damara belt. A geodynamic model is proposed for the evolution of grabens, aulacogens, ensialic geosynclines and regions with tectono-thermal overprinting interpreting these structures as different reactions of the heterogeneous crust to mantle diapirs of different sizes.

Tholeiitic magmatism associated with continental rifting in the Lufilian Fold Belt of Zambia

Abstract Metabasic rocks form a small but geologically important component of the geology of the base metal-rich Neoproterozoic Copperbelt of central Africa. The disposition of the metabasic rocks follows the 150 km long, arcuate, structural trend of the thrust belt. Despite their structural disposition, the metabasic rocks played a passive role during the thrusting due to their rigidity and lithological contrast with the host sedimentary rocks. The main thrust horizons are located along evaporite layers below the position of the metabasic rocks. The metabasic rocks form part of an allochtonous unit overlying para-autochtonous rocks of the Upper and Lower Katangan sequences. A petrological and geochemical study of the metabasic rocks indicates that they crystallised from genetically related, tholeiitic magmas. Fractionation of olivine, pyroxene and plagioclase played an important role in the generation of the range of mafic compositions. The rocks are enriched in incompatible trace elements with chondrite-normalised ( La Yb ) N values raning from 4 to 7. Incompatible trace element ratios (e.g. La Nb 1.5; La Ta = 16 ) indicate little interaction between the protolith to the metabasic rocks and continental crust. The trace element data instead suggest that melting of an enriched asthenospheric source produced the magmas. A comparison of the studied metabasic rocks with those from other parts of the Lufilian Belt shows an overall geochemical similarity of the magmas. All the metabasic rocks from the Katangan succession show a close chemical affinity with intraplate magmas.

Kinneyia-Type Wrinkle Structures—Critical Review And Model Of Formation

Abstract Kinneyia structures are among the most typical wrinkle structures observed on ancient siliciclastic sediment surfaces since the Archean. Recently, Kinneyia structures have been grouped together with other microbially induced, crinkly decorations on ancient bedding surfaces as wrinkle structures. They are mainly preserved on upper surfaces of ancient siliciclastic-event deposits and are characterized by millimeter-scale, winding, flat-topped crests separated by equally sized round-bottomed troughs and pits. The structure resembles small-scale interference ripples including crest-dominated linear and pit-dominated honeycomb-like patterns. The steep slopes of the crests, however, exclude their formation at the air or water-sediment interface. Thin sections across Kinneyia structures reveal their formation beneath microbial mats. They formed at an early stage and do not arise from loading and other processes related to burial. Based on the close relationship to event deposits, a genetic model considering the specific hydraulic conditions on siliciclastic tidal flats after storms or floods is proposed. Numerical calculations show that, after microbial mats have been reestablished on the new sediment surface and groundwater is still flowing downslope, the top portion of the sediment confined beneath mats may be liquefied, thus allowing grains to move with the groundwater. Oscillations of groundwater flow owing to periodic reversals of flow direction at rising tides, and a tidal signal of oscillating pore pressure may enhance formation of ripplelike structures along the boundary with the overlying mat. The model applies primarily to Kinneyia structures presumed to be formed beneath cohesive microbial mats in peritidal zones.

Mixtite deposits of the Damara sequence, Namibia, problems of interpretation

Abstract Mixtite deposits form extensive outcrops in the Pan-African Damara orogenic belt of Namibia. They occur at two different stratigraphic levels in the lower part of the Upper Proterozoic Damara Sequence (South African Committee for Stratigraphy 1980; Kroner, 1981) . The older Varianto mixtite is confined to the northern margin of the fold belt. No evidence indicating a glacigenic origin has been found, but on the southern foreland of the fold belt two, possibly coeval, mixtite deposits (Court and Blaubeker Formation) show fair evidence of such an origin. The younger Chuos mixtitesoccur in three separate zones of the belt. All these mixtites have been interpreted as glacigenic sediments, and the Chuos Formation has been used as a stratigraphic marker for the correlation between different parts of the fold belt. The present investigation shows that: (1) The mixtite deposits show no features indicating a glacial origin; (2) The sedimentary features indicate deposition by gravity-flow processes; (3) turbidites are in several areas closely associated with and even interbedded in the mixtite deposits; (4) mixtite deposition is in some areas not confined to the Chuos Formation proper, but began at lower stratigraphic levels or persisted to a higher level; (5) in some areas calcareous or dolomitic sediments are interbedded in mixtite, or mixtites are interbedded in such formations. The mixtites of the Chuos Formation are interpreted as various kinds of submarine gravity-flows (slump breccias, mass-flow, slurry, grain-flows, turbidites) which were probably triggered by tectonic activity during a stage of differential subsidence that affected the whole geosyncline, but was concentrated in three rift zones. Under this assumption the majority of the mixtites may have been deposited during a limited time spin. A different model assuming synorogenic deposition of the “pebbly schist” mixtites of the Southern Margin Zone is briefly discussed. It is concluded that the mixtite deposits are not of glacial origin and can therefore not be regarded as reliable chronostratigraphic markers for correlations between the different facies domains of the Damara sequence nor for correlation with other Upper Proterozoic sequences (e.g. Gariep belt, Katanga belt); such a correlation might, nevertheless, exist, if the extensive gravity-flows should have been caused by eustatic changes of the sealevel during a time of widespread glaciation on other parts of the globe. In this case local mountain glaciers might have contributed material to some of the Chuos mixtites.

The metamorphic history of the Damara Orogen based on K/Ar data of detrital white micas from the Nama Group, Namibia

Abstract Detrital white micas from molasse sediments of the Nama Group were dated by the K/Ar method. The Kuibis Subgroup and the underlying pre-Damara Sinclair Sequence contain detrital muscovite with ages of 1100-1000 Ma and thus cannot be derived from the Damara Orogen, but prove to have originated from a basement probably affected by the Kibaran orogeny. Muscovite size-fractions of 250-100 μm from the upper Nama Group (upper Schwarzrand and Fish River Subgroups) yielded K/Ar ages 670-570 Ma. Because these sediments are regarded as molasse series of the Damara Orogen, these K/Ar ages can be interpreted as cooling ages of a first, early tectono-thermal event in the Damara Orogen. A later Damaran metamorphic overprint, which peaked at about 530 Ma, is not documented in the detrital content of the Nama Group at the present level of erosion, because detrital micas with K/Ar ages over 600 Ma, have been found up to the uppermost Fish River Subgroup. It can be concluded that, at least, two tectono-metamorphic events followed by uplift and erosion, have taken place during the development of the Damara Orogen. Thermal alterations of the Nama sediments were detected by means of illite crystallinity and K/Ar dating of clay size-fractions. These data, as well as the almost exclusive occurrence of the 2M mica polytype in these size-fractions point to a diagenetic to very low grade metamorphic alteration of the area investigated at about 530-500 Ma and younger. The deposition of the upper Nama Group is younger than the 570 Ma detrital white micas but occurred before the 530-500 Ma thermal alteration of the Nama deposits. The deposition of the lower Nama Group probably took place between 635 and 570 Ma.

Setting and sedimentary facies of late Proterozoic alkali lake (playa) deposits in the southern Damara belt of Namibia

The late Proterozoic Damara Belt of Namibia has evolved from an elaborate system of continental rifts in which the basal portion (Nosib Group) of the Damara Sequence was deposited. In the southern rift, situated at the southern margin of the Damara Belt, the Nosib Group is represented by coarse elastic sediments (Kamtsas Formation) and fine-grained partly dolomitic deposits (Duruchaus Formation). Both formations occur, with interfingering relationships over nearly the total length of the rift. In the Geelkop Dome area the pelitic-dolomitic Duruchaus Formation includes, in its upper part, a sequence of sediments that are characterized by cyclical deposition, high sodium contents, abundant albite pseudomorphs after primary evaporite minerals, and concordant solution and collapse breccias. This 300 m thick sequence has been interpreted as deposits of an alkali lake or playa complex. Based on the playa lake models of Eugster and Hardie (1975) and Rowlands et al. (1980) four distinct facies have been distinguished in the upper part of the evaporitic Duruchaus Formation: (1) the submergent lake facies, represented by laminated shales containing layers of laminated siltstone; (2) the submergent/emergent mud flat facies, represented by laminated siltstone and calcareous shale with layers of dolomitic mudstone, calcareous shaly siltstone with scapolite and laminated sandy albitic dolomite with abundant pseudomorphs of albite after primary evaporite minerals (e.g. shortite, thermonatrite, borax); (3) the exposed saline crust facies, characterized by solution breccias, “albitolite” and “albitolite” breccias, all originating from former salt crusts; and (4) the elastic marginal facies, represented by quartzites of fluviatile and partly aeolian origin. From identified primary and diagenetic to metamorphic minerals, and from the composition of several generations of fluid inclusions, it is concluded that the brines of the alkali lake were of a NaHCO3(KBClS) type. Material creating the extremely alkaline environment was derived from eroded acid-to-basic magmatic basement rocks and from coeval alkali-rich volcanism. During tectogenesis the evaporite sequence was overridden by nappes approaching from the closing Damara geosyncline. Saline crust horizons were thereby partly mobilized and the resulting mush of dolomite and crystal fragments was squeezed into the thrust planes of the nappes where they acted as lubricants which enhanced late-orogenic thrusting along the southern margin of the Damara Belt.