A Proterozoic-rift origin for the structure and the evolution of the cratonic Congo basin (original) (raw)
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Instantaneous dynamics of the cratonic Congo basin, central Africa
2009
The rheological and compositional structure of the continental lithosphere is not well understood. In particular, it has been difficult to quantify the viscous, compositional and density structure of cratonic lithosphere. An understanding of cratonic lithosphere structure can be inferred from study of the epirogenic motions within cratons that result in the formation of intracratonic basins. Unfortunately, intracratonic basin formation is itself a poorly understood process because these basins exhibit a variety of subsidence rates and styles, often have multiple subsidence episodes and sediment deposition is subject to the varying influences of tectonics and eustasy. Our poor understanding of intracratonic basin formation also stems from the rarity of currently active intracratonic basins. One mechanism of intracratonic basin formation is lithospheric instability resulting in mantle downwelling and depression of the cratonic surface. Admittance modeling indicates the presence of such a downwelling beneath the intracratonic Congo basin, providing the opportunity to test the lithospheric instability hypothesis at a geodynamically active basin, and use the results to constrain the structure of the lithosphere. The Congo basin is approximately 1000 km across, has had several subsidence events since the early Paleozoic and overlies crustal rift structures. The Congo basin is unique because it is coincident with a 70 mGal long-wavelength free-air gravity low, overlies a high shear wave velocity structure in the uppermost mantle, as imaged in seismic tomography, and its most recent subsidence deposited continental sediments by a previously unknown mechanism. We created dynamic models of the Congo downwelling that are tightly constrained by the gravity and topography of the basin and are consistent with the results of seismic imaging. The long wavelength of the basin simplifies model formulation because we can ignore lithospheric flexure. We parameterize several different models of lithospheric density and viscosity structure and do a search over these parameters to find the instantaneous models that best fit geophysical observations. We find that the observations at the Congo basin can be explained by a high-density anomaly viscously supported within the uppermost mantle. The best fitting models predict a 30-60 kg/m3 positive density anomaly situated at a depth of 100 km. The large magnitude of this anomaly suggests it has a compositional rather than a thermal origin.
Instantaneous dynamics of the cratonic Congo basin
Journal of Geophysical Research, 2009
1] Understanding the formation mechanisms of cratonic basins provides an examination of the rheological, compositional and thermal properties of continental cratons. However, these mechanisms are poorly understood because there are few currently active cratonic basins. One cratonic basin thought to be active is the Congo basin located in equatorial Africa. The Congo basin is coincident with a large negative free-air gravity anomaly, an anomalous topographic depression, and a large positive upper mantle shear wave velocity anomaly. Localized admittance models show that the gravity anomaly cannot be explained by a flexural support of the topographic depression at the Congo. We analyze these data and show that they can be explained by the depression of the Congo basin by the action of a downward dynamic force on the lithosphere resulting from a high-density object within the lithosphere. We formulate instantaneous dynamic models describing the action of this force on the lithosphere. These models show that the gravity and topography of the Congo basin are explained by viscous support of an anomalously dense region located at 100 km depth within the lithosphere. The density anomaly has a magnitude within the range of 27-60 kg m À3 and is most likely compositional in origin. Our models do not provide a constraint on the lithospheric viscosity of the Congo craton because the shallow location of the anomaly ensures strong coupling of the anomaly to the surface regardless of viscosity structure.
New ion microprobe U-Pb zircon ages, as well as some geochemical and isotopic analyses, for key igneous units within the central part of the West Congo belt are integrated with geological information to provide an updated geological map (1:1000000 scale) and a synthetic type cross-section of the belt, as well as an updated lithostratigraphic chart of the 'West Congo Supergroup'. Three Neoproterozoic units are recognised, from oldest to youngest, the Zadinian, Mayumbian and West Congolian 'Groups'. Emplacement of early Zadinian peralkaline granites (Noqui massif, 999 97 Ma) and rhyolites (Palabala) was accompanied by incipient rift sedimentation, corresponding to the onset of transtensional rifting, preferentially in a transverse mega-shear setting along the margin of the Congo craton. Subsequent upper Zadinian magmatism produced a thick (1600-2400 m) basaltic sequence (Gangila), which has geochemical characteristics typical of continental flood basalts (CFBs). The Gangila basalts, associated with major pull-apart rifting, were followed rapidly by the 3000-4000 m thick Mayumbian rhyolitic lavas, dated at 920 9 8 Ma at the base and 912 97 Ma at the top. The felsic lavas are intruded by coeval high-level (micro)granites, whose emplacement is dated at 924 925 Ma (Mativa body) and at 917 914 Ma (Bata Kimenga body) in the Lufu massif. This voluminous bimodal magmatic province is similar to the Paraná and Deccan provinces, and shares similar lithospheric sources. It corresponds to the initial, transtensional rifting stage along the western edge of the Congo craton before Rodinia breakup. The early Neoproterozoic rocks of the West Congo Supergroup rest unconformably on a ca. 2.1 Ga Palaeoproterozoic polycyclic basement (Kimezian Supergroup). No Mesoproterozoic events are recorded in the area. Following the initial, transtensional early Neoproterozoic (ca. 1000-910 Ma) rifting stage, Bas-Congo behaved as a passive margin of the Congo craton, as indicated by deposition of ca. 4000 m of Neoproterozoic (pre-Pan-African) platform sediments (lower part of West Congolian Group) preceding ca. 2000 m of Pan-African molasse-type sediments (upper part of West Congolian Group). In the late Neoproterozoic, during Pan-African assembly of Gondwanaland, the Bas-Congo passive margin, which was largely protected by thick lithosphere of the Congo craton, collided with a western active margin to form the Brasiliano-Araçuaí belt, now : S 0 3 0 1 -9 2 6 8 ( 0 1 ) 0 0 1 9 2 -9 L. Tack et al. / Precambrian Research 110 (2001) 277-306 278 preserved adjacent to the São Francisco craton of Brazil. This collision, which ended in Bas-Congo at ca. 566 Ma, induced relatively limited effects in the West Congo belt, which experienced no late Neoproterozoic magmatic activity.
Could the mantle have caused subsidence of the Congo Basin?
2011
The Congo Basin is characterised by a near-circular shape, a pronounced negative free-air gravity anomaly, and a subsidence history that is slow and long-lived. The basin is often considered as an intracratonic basin, implying an unknown formation mechanism. However, the Congo Basin probably initiated by Precambrian rifting and the larger part of its older subsidence history could be explained by post-rift thermal relaxation. The uppermost layer of Mesozoic to Cenozoic sedimentary rocks in the basin appears discontinuous in its evolution and several studies have proposed that these rocks were deposited in response to a process in the mantle. We have examined gravity data and seismic tomographic models to evaluate the role of the sub-crustal
Tectonics, 1991
these areas, basin depth, rift flank elevation, and free air gravity anomaly may be modified by magmatic underplating of the crust. Estimates of effective elastic thickness Te obtained from our forward models vary between 17 and 38 km. These Te estimates are consistent with previous values obtained through the wavenumber domain correlation of Bouguer gravity and topography data. We find that the lithosphere beneath East Africa maintains significant flexural strength in extension despite localized, but intense, fracturing of the crust by normal faults that penetrate to lower crustal levels. Therefore the assumption of local isostasy ( Te ~ 0 ) appears invalid within the youthful Western rift system.
Journal of African Earth Sciences, 2019
During the Cryogenian-Ediacaran, tectonically-and climatically-driven perturbations created sea level changes. Snowball Earth-type glaciations suggest~500-800 m sea level fall or at least 1,000-1,500 m in eustatic sea-level change as a result of severe climate changes. However, in Central Africa, geologic evidence of such processes is lacking. In the Lower Congo region (Democratic Republic of the Congo), detailed facies analysis and sequence stratigraphy of the Neoproterozoic uppermost Haut Shiloango-Lukala carbonate ramp system allowed to reconstruct the multiple order relative sea-level changes. Two hundred twenty-three fifth-order elementary parasequences, grouped into eleven fourth-order depositional sequences or parasequence sets, recorded a severe sea-level fall up to 15 m for slope-outer-to-outer ramp facies of the uppermost Haut Shiloango Subgroup-Upper Diamictite Formation followed by two distinct sea-level rises for the slope-outer-to-inner ramp facies of the Lukala Subgroup. First marine transgression shows a cumulative~60 m of sea-level rise throughout C1 to C3 (Lukala Subgroup) sediment accumulations. Second marine transgression shows a cumulative~50 m sea-level rise throughout C4 to C5 sediment accumulations. The transition from uppermost Haut Shiloango to Upper Diamictite ramp system points to an isostatic rebound and uplift of the rift flanks of the Congo Craton, creating paleoreliefs, potentially allowing local glaciation and periglacial sedimentation. This rebound can be ascribed to a diachronous far-field effect of~660 Ma Macaúbas Basin opening in Brazil. First marine transgression is interpreted as a consequence of syn-to post-rifting related to sea-floor spreading, which initiated the subsidence of the basin. A~30 m of sea-level rise drowned the C1 formation carbonate ramp, which turned on the oversupply and progradation of offshore to nearshore marine deposits on the wedges of the basin. An abrupt change from both marine transgressions points to~25 m of sea-level rise that is interpreted as the consequence of the development of~585-560 Ma Araçuaì-West Congo Orogen, which significantly increased the siliciclastic supply in the basin. Our results show that no anomalous climatic or eustatic events such as those proposed in the Snowball Earth model are recorded in DRC. On the contrary, relative sea-level changes result from long-term overriding regional tectonic processes controlling diachronous sedimentation along the western passive margin of the Congo Craton. Hoffman and Schrag, 2002). However, the link between tectonicallyand climatically-driven perturbations has not sufficiently been highlighted throughout Neoproterozoic times, particularly during the Ediacaran period, which conditioned carbonate factories. Other alternative hypotheses (Slushball-Waterbelt Earth, Zipper-rift Earth, High-Obliquity Earth) for the Snowball Earth model, which have been
Sedimentology, 2018
The Kamoa sub-basin, in the southeastern part of the Democratic Republic of Congo, is a rift basin that hosts a world-class stratiform copper deposit at the base of a very thick (1.8 km) succession of matrix-supported conglomerates (diamictite) (Grand Conglomérat Formation) that has been interpreted by some as the product of deposition in the aftermath of a planet-wide glaciation. Newly This article is protected by copyright. All rights reserved. available subsurface data consisting of more than 300 km of drill core throws new light on the origin of diamictite and associated facies types, and their tectonic, basinal and palaeoclimatic setting. Initiation of rifting is recorded by a lowermost subaqueous succession of fault-related mass flow conglomerates and breccias (the 'Poudingue') with interdigitating coeval and succeeding sandstone turbidites (Mwashya Subgroup). Overlying diamictites of the Grand Conglomérat were deposited as subaqueous debrites produced by mixing and homogenization of antecedent breccias and gravel from the Poudingue and Mwashya sediments with basinal muds. Failure of over-steepened basin margins and debris flow was likely to be triggered by faulting and seismic activity and was accompanied by syn-depositional subaqueous basaltic magmatism recorded by peperites and pillow lavas within diamictites. The thickness of diamictites reflects recurring phases of faulting, volcanism and rapid subsidence allowing continued accommodation of rapidly deposited resedimented facies well below wave base. A distal or glacial influence in the form of rare dropstones and striated clasts is evident, but tectonically-driven mass flow destroyed any primary record of glacial climate originally present in basin margin sediments. Such basin-margin settings were common during Rodinia rifting and their stratigraphy and facies record a dominant tectonic, rather than climatic, control on sedimentation. Deposition occurred on tectonic time scales inconsistent with a Snowball Earth model for Neoproterozoic diamictites involving a direct glacial contribution to deposition.