Could the mantle have caused subsidence of the Congo Basin? (original) (raw)
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A Proterozoic-rift origin for the structure and the evolution of the cratonic Congo basin
Earth and Planetary Science Letters, 2011
Keywords: intracratonic basins "Cuvette Centrale" of Congo continental rifting sediment backstripping We interpret the cratonic Congo basin, a large circular "Cuvette Centrale" filled with up to 9 km of Proterozoic to Neogene sediments, as the consequence of a Neo-Proterozoic rift. Firstly, the magnitude and the long-term subsidence are consistent with the thermal time-constant of a 200-250 km thick lithosphere inferred from several tomographic studies. Secondly, the surface accumulation of sediments is compensated at depth by crustal thinning, whose magnitude can be estimated from the analysis of the surface gravity: after backstripping the effect of the sediments, a residual NW-SE positive and narrow gravity anomaly is observed across the "Cuvette Centrale" and is interpreted as the remaining crustal thinning associated with this rift. Assuming that isostasy is governed by a necking level and a flexural response to sediment loads, we have estimated the combination of the depth of necking and the equivalent elastic thickness of the lithosphere that provide the best fit with the residual gravity, i.e. 10 km and 100 km respectively. The corresponding uplift of the upper mantle is in the continuity of the Mbuji-Mayi Supergroup to the SE and the Liki-Bembian Group to the NW. These two groups represent older stages of rifting in the Congo craton, which shows that rifting has periodically affected and weakened the "Cuvette Centrale" during a long period of time.
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.
2017
Cretaceous to Recent evolution of the Congo Basin in Central Africa is still poorly documented although its history over the last 75 Myr has potentially recorded global and major regional events, including the Paleocene-Eocene Thermal Maximum at ∼56 Ma and the Miocene aperture of the Western branch of the East African Rift System along its eastern border at ∼25 Ma. Available data for associated off-shore deposits show that in parallel, the Congo River delta experienced a starvation period during the Midto Late Cretaceous and Paleogene, with endorheic lacustrine to desert environments in the upstream basin, followed by a period marked by high rates of drainage and sediment supply in the Neogene.
Zenodo (CERN European Organization for Nuclear Research), 2022
The Kasai region is an area historically known for its wealth in minerals such as diamonds and gold. This study aims to study the structural geology of the northern part of the Kasai province in the D.R. Congo in order to guide future mining and oil prospecting. The analysis of the gravity data using the regional-residual separation method, the vertical and horizontal derivatives as well as the estimation of the depth of the sources of the anomalies by the Euler deconvolution method allowed us to identify and study several gravity signatures that can be associated with geological structures of economic interest. Thanks to the results obtained, we have produced structural maps in which all the identified geological structures have been well defined. The northern part of this zone is occupied by a gigantic high gravity while the southern part, for its part, is characterized by several gravity depressions. By analyzing the gravity maps of this area, we found that the structure of our study area is arranged in Horst (uplifted part in the north) and grabens (subsided part in the south). This interpretation is all the more supported by the fact that the anomaly contrast between these two structures is very clear, which could indicate the presence of a series of faults oriented in the NW-SE direction. Several gravimetric signatures representing structures that can promote the migration and trapping of hydrocarbons such as faults, antiform folds and salt domes have also been identified in this part. The southern part of the zone is located around the Archean rocks of the Kasaï craton which is very rich in minerals. Negative anomalies could be associated with diamondiferous kimberlite intrusions that are abundant in the region while positive anomalies could reveal the concentration of economically important metal ores.
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
Geophysical Journal International, 2004
The deep structure of the West African continental margin between 5°S and 8°S was investigated using vertical reflection and wide-angle reflection/refraction techniques, during the ZaïAngo project, a joint programme conducted in 2000 April by Ifremer and TotalFinaElf. To penetrate below the salt layer, a non-conventional, low-frequency seismic source was used in the ‘single-bubble’ mode, together with ocean bottom instruments (hydrophones and seismometers) and a 4.5 km long streamer that recorded multichannel seismic reflection (MCS). The data show that the continental crust thins abruptly over a lateral distance of less than 50 km, from 30 km thick below the continental platform (based on gravity data), to less than 4 km thick below the Lower Congo Basin that formed prior to the Aptian salt deposition. This subsalt sedimentary basin (180 km wide, 4 km thick, with velocities varying from 4.7 km s−1 to 5.8 km s−1 at the bottom) is located between the foot of the continental slope and the oceanic domain. It is underlain by crust of an intermediary or transitional type, between continental crust and what can be recognized as oceanic crust. In the transitional zone, a crustal upper layer is present below the pre-salt sedimentary basin, 3 to 7 km thick, with velocities increasing from 5.8 km s−1 at the top to 6.8 km s−1 at the bottom of the layer. This layer appears to thin regularly, from 6–7 km thick below the depocentre of the pre-salt sedimentary basin to 3–4 km thick below the western termination of the basin. Below this upper crustal layer, an anomalous velocity layer (7.2 to 7.8 km s−1), is documented, below the eastern side of the basin, where the crustal thinning is at a maximum. The origin of this layer is unknown. Several arguments, like rifting duration (between 15 Ma and 30 Ma) or the absence of seaward-dipping reflectors, precludes the hypothesis of underplated mantle material, but other hypotheses (such as serpentinized material or high-grade metamorphic crustal rocks or a mixture of mafic and ultramafic crustal rocks) are plausible. Near the ocean termination of the basin, the transitional zone is bounded to the west by a basement ridge that is clearly documented on two profiles (‘7+11’ and 14) having a dense ocean bottom seismometer/hydrophone (OBS/OBH) spacing. On these profiles, an anomalous velocity layer is present in the westernmost part of the transitional zone (below the basement ridge) and in the oceanic domain. This layer, absent on profile 3, may be related either to oceanization and slow seafloor spreading processes or to a consequence of the rifting process.