Margin Of Africa Research Articles

For Africans in World History: Extended Forms of Democratic Pluralism

Abstract Few scholars have been successful at articulating with as much clarity as Patrick Manning does, the relevance and centrality of African history to world history. The historical experiences of the peoples of Africa, within, above, and beyond the Anthropocene, had not been synthetized with a view of globalizing certain Forms of the African Past as integral pieces of the mosaic of the Human Adventure. This essay presents the extent of Manning’s contributions to the debates regarding the general concept of Afrocentricity in practice, namely in relation to the construction of functional global institutions where learned citizens congregate to boost humanity’s intellectual capital. Pat Manning stands out for deconstructing in engaging arrangements—that is, in bravura and substance—the marginalization of Africa and Africans in the academic deliberations about the emergence of cosmopolitan Modernity over the past six or seven centuries at a global scale. Manning-Senseï reverberates in global academia the influences of Black peoples on “the Human System in Movement.” On the other hand, Manning arguably evades the moralization of the discourse that participates in the travails for the restoration of historical consciousness in Black Africa, inducing thereby the ubiquitous question of contemporary world historians’ political responsibility.

Tectonic reconstruction of Cyprus reveals Late Miocene continental collision of Africa and Anatolia

The extended northern continental margin of Africa is currently colliding with and under-thrusting northward below Cyprus. The age of onset of this collision is poorly constrained, but is critical if we are to quantify continental subduction and evaluate its role during spectacular recent uplift of southern Turkey from ~7 Ma to the present. Here, we reevaluate the evolution of northern Cyprus and document the modern structure of the Kyrenia fold-thrust belt in a balanced cross section for the first time, to determine the timing and amount of shortening. The belt deformed an Upper Cretaceous to Miocene stratigraphy, which was deposited onto a metamorphic basement. The fold-thrust belt was previously proposed to have formed during two stages: in Eocene and late Miocene time, based on stratigraphic and limited structural evidence. We revaluate evidence for an Eocene phase of thrusting in Kyrenia and find that 1) repetitions of Eocene and older rocks are explained by Miocene thrusting, and newly documented olistoliths were misinterpreted as pre-Miocene thrust slices 2) uplift in Eocene time was not isolated to Kyrenia but also affected northern Arabia and results from regional dynamic topography or forebulge formation. Based on kinematic constraints from a plate reconstruction, we interpret that metamorphic rocks in Kyrenia were metamorphosed by Cretaceous burial below the Troodos ophiolite and were subsequently exhumed by upper plate extension during latest Cretaceous obduction onto the African margin. The latest Cretaceous-Miocene plate boundary between Africa and the Taurides was located to the north of Kyrenia. The Kyrenia fold-thrust belt accommodated a minimum of 17.5 km of shortening as a result of ~9–6 Ma thrusting as a result of initial Africa-Taurides collision. The northern edge of African continental crust arrived below the Taurides, where it has been seismologically imaged, in late Pleistocene time and may have caused or contributed to recently recognised major Tauride uplift since ~0.5 Ma.

Mental Health and the Maintenance of Kinship in South Africa

ABSTRACTAt the turn of the millennium, people with mental disturbance often lived in circumstances of economic marginalization in South Africa. The historical material of one low-income urban area reveals the place of kin relations and reciprocity in enabling negotiation of a more fluid set of responses to mental illness. In this sociocultural context, “stigma” was not an inevitable reaction to mental illness, and a more complex set of social dynamics could mitigate marginalization. Research on how changing informal care practices relate to state-based community care continues to be important to inform contemporary health reforms.

Continental Slopes of the West Africa Region: A Unique Treasure of Hydrocarbons

The West African region embraces a number of coastal sedimentary basins, which continued in deep-water areas of the Atlantic Ocean. It includes the following oil-and-gas-bearing basins: the Gulf of Guinea, the Kwanza–Cameroonian, and the Namibian. The sedimentary cover of the basins of this passive margin is represented by Mesozoic–Cenozoic deposits. The composition of sediments accumulated in them is quite specific and surprisingly units over the vast areas. The tectonic structure of the majority of the continental margins of West Africa makes possible to refer them to the margins of epiplatform orogenic belts. The existence of two systems of linear troughs—internal and external—on the passive margins at the early stages of continent–ocean transition zones relates deep-water hydrocarbon deposits to internal troughs filled by younger sediments: the alluvial fans of submarine rivers and landslide fronts with prograde formations (turbidites, debris flows, etc.). Late Cretaceous and Middle Paleogene clay formations played the role of source beds in the region, so-called “black clays.” An analysis of over 200 hydrocarbon fields, mainly petroleum, discovered in the past 10–15 years in the region revealed a clear tendency of these fields occurring in a productive zone of oil pools extending in a sea depth interval of 400–3000 m on the continental slope and possibly to 4000 m at the continental rise. Moreover, all discovered fields have been estimated in terms of reserves from large to giant. It is also noteworthy that within the shallow of this region, which includes the shelf and the coastal plain, only a number of small, insignificant oil and gas pays have been discovered. The main of oil and gas bearing potential prospects are related to deposits in the middle and lower parts of the continental slope and possibly adjacent areas of the continental rise. In the long term, the drilling objectives will be both postsalt and presalt deep-water oil-and-gas fields.

Liassic age of an oceanic gabbro of the External Rif (Morocco): Implications for the Jurassic continent–ocean boundary of Northwest Africa

This work concerns the northernmost limit of the West African Craton (WAC) and Variscan WAC-related terranes of NW Africa. Based on newly obtained radiometric age of an oceanic gabbro from the “Mesorif Suture Zone” of the External Rif Belt, we propose a revised interpretation of this puzzling lineament. We report on a 190±2 Ma LA–ICP–MS U–Pb zircon age of a trondhjemite vein cross-cutting the Bou Adel gabbro, which is one of the largest oceanic units of the quoted suture zone. We previously interpreted the arcuate MSZ in terms of transported, hyper-extended margin of the Alpine Tethys, based on a K–Ar 166±3 Ma age ascribed to the Bou Adel gabbro in the literature. The new, Early/Middle Liassic age coincides instead with the onset of oceanic floor formation in the Central Atlantic. We hypothesize that the Mesorif suture zone corresponds to the transported trace of the West African Atlantic margin surrounding the northwestern Moroccan Meseta promontory and connecting with the ENE-trending North African Transform North African transform. The latter zone sharply bounded the North Africa margin and connected the Central Atlantic with the Alpine Tethys. We propose that transported elements from the North African transform constitute the “Mesorif Basalt–Breccias” lineament parallel to and more external than the Mesorif suture zone. If correct, this new interpretation provides an opportunity to develop detailed field and laboratory studies of an exhumed segment of the up-to-now conceptual Jurassic North African transform.

On the motion of lithospheric plates in the oceans and transition zones

The formation; horizontal displacement and immersion of lithospheric plates; corresponding to the plate tectonics hypothesis; are considered. It is shown that the magmatism of the mid-oceanic ridges does not correspond to the volume of the crust of plates and the magmatism of the transition zones from the continents to the oceans. When plates move to the continents 70 % do not submerge. Another 7 % of the length of the transition zones is the depth of hypocenters of earthquakes of 20 ± 20 km (transitional zones of the Colombian type). Seismicity on the continents near the “passive” eastern margins of North America and Africa; the western and southern margins of Australia; the western edge of Eurasia is less intense; but in principle not different from the “ñolombian”. Approximately 2 % of the length of the transition zones are earthquakes with depths of foci up to 70—100 km. About 14 % occupy zones with depths of hypocenters to 250—450 km; 7 % of the length of the boundaries of the continents and oceans are represented by the deepest foci (450—700 km). Thus; the total focal regions (and; consequently; the subduction zones) are not the rule; but the exception. The geological and geophysical data do not correspond to the scheme by which the edge of the slab is compacted before the trench due to cooling. Plate dipping is unrealistic due to its low density and large mantle resistance under the trough. The location of earthquakes’ hypocenters under a trough and an island arc does not agree with subduction. It is impossible to ex-plain the immersion of a plate by compacting its crustal part during the polymorphic transformation of pyroxenes into garnets. The analysis of the energy of subduction process was carried out using the data on the parameters of the earthquake foci. It is showed that for a process according to the plate tectonics scheme; a power source is 1000 times more powerful than the actual ones. The obtained results allow us to consider the hypothesis of subduction of lithospheric plates unrealistic. An alternative scheme for the formation of focal zones is given.

Post-rift stratigraphic evolution of the Atlantic margin of Namibia and South Africa: Implications for the vertical movements of the margin and the uplift history of the South African Plateau

Abstract The aim of this study is to constrain the post-rift deformations of the Atlantic passive margin of Namibia and South Africa using an extensive industrial 2D reflection seismic dataset calibrated by wells and onshore outcrops that have been revaluated in age (biostratigraphy) in order to discuss the evolution of the South African Plateau uplift. The first-order evolution of the margin is tectonically driven and can be divided into three principal phases. The first (131–93.5 Ma) comprises an overall retrogradational trend that results from a rate of accommodation, created by the thermal flexure of the margin, that is higher than the sediment supply. The second (93.5–66 Ma) comprises an overall aggradational-progradational trend that results from a relative increase in sediment supply due to an uplift and subsequent erosion of the margin and the inland domain. The third (66–0 Ma) is retrogradational again in response to a decrease in sediment supply induced by a relative stability of the margin and climate conditions. This study demonstrates that the present-day configuration of the margin is acquired during the Late Cretaceous (from 81 to 66 Ma) with a strike, long-wavelength and seaward tilt of the margin which could be related to the growth of the onshore main escarpment. We also characterize a regional Oligocene unconformity marked by a significant downward shift of the shoreline which suggests a moderate uplift of the inland domain and could be associated with a reactivation of the relic late Cretaceous relief.

The near steady state landscape of western Namibia

Quantitative geomorphic field studies and modeling efforts have focused on the margins of southwestern Africa as an example for landscape evolution in prolonged aridity conditions and tectonic quiescence of passive margins. These efforts concluded that this region is a prime example of a steady state landscape, in which relief changes extremely slowly. Using cosmogenic isotopes, these studies suggested overall landscape exhumation rates of 5–10 m Ma−1 over the past 105–106 yrs. Slightly slower rates on flat-lying exposed bedrock surfaces and faster exhumation rates along the Namibian Great Escarpment as well as on steep slopes of granitic inselbergs, such as the Gross Spitzkoppe are also documented. Here we explore the state of “steady state” in central Namibia. Concentrations of 10Be were measured in bedrock and sediment samples collected throughout the watershed of the Ugab River (~29,000 km2), which drains the highlands of central Namibia and flows to the Atlantic Ocean. Samples were collected from the main stem of the ephemeral Ugab River, from the slopes and streams draining the Brandberg, which is the largest inselberg in the Namib, and from smaller inselbergs around it. We also sampled several other formerly large, but currently subdued, inselbergs such as the Messum Crater. 10Be concentrations in sediment transported along the axial Ugab River indicate that its drainage basin erodes uniformly at 5–6 m Ma−1 and sediment transport from its headwaters source to the ocean is rapid. 10Be concentrations measured in sediment transported in ephemeral streams draining the Brandberg indicate its erosion at ~4 m Ma−1. However, slower rates of 1–3 m Ma−1 were measured for bedrock samples collected from (a) flat lying bedrock surfaces within the Brandberg, (b) top of small tors that rise only a few meters above their surroundings, and (c) exhumed and denuded large magmatic complexes such as the Messum Crater. Furthermore, we found that bedrock buried under grus in the hyperarid zone of Namib (<100 mm yr−1) erodes at similar rates as the exposed bedrock. This difference between the rate of bedrock erosion and the overall average erosion rate of drainage basins has been previously attributed to the contribution of sediment weathered from underneath transported sediment and soil on the pediments. Our results do not fully support this explanation. Results from this and earlier studies point to two possible sources of relatively low dosed (i.e. more rapidly eroding) sediment: (a) the steep slopes and cliffs of the large inselbergs and the Great Escarpment, and (b) rock buried under soil in the upper, semi-arid, parts of the drainage systems, where soil and vegetation can promote weathering of plagioclase and biotite and the disintegration of granitic bedrock.We therefore suggest that the “steady state” landscape along the Namibian passive margin be viewed as follows: The entire landscape erodes slowly, generally at ~5 m Ma−1 and this maintains the view of steady state. Small differences in erosion rates between the landscape elements result in very slow and only small changes in relief over time scales ≥106 yrs. We find that the large inselbergs and the Great Escarpment erode primarily by retreat of steep slopes and cliffs within the drainage basins while preserving relief over considerable timescales. In the wetter upper reaches of the Namibian drainage systems, erosion of buried rock is most likely increased by the vegetation-covered soil.

New constraints on the age of the opening of the South Atlantic basin

Abstract Detailed, high quality, marine total field magnetic data recently acquired over parts of the South Atlantic Ocean off the southwestern margin of South Africa display a pattern of well-defined, NW-SE striking linear magnetic anomalies that can be traced with confidence over distances >150 km. The magnetic anomalies are interpreted to be M-series seafloor spreading anomalies M9 to M11, which are consistent with the initiation of seafloor spreading at approximately 135 Ma (Late Valanginian/Early Hauterivian). Seafloor spreading models indicate a more rapid (44 mm/yr) initial spreading phase between M11 and M4/M5 followed by slower (29 mm/yr) spreading from M4/M5 to M0. This two rate spreading model also matches M-series anomalies previously reported over the conjugate South American margin offshore Argentina where the rates are slightly ( Breaks in the continuity of the linear anomaly pattern, observed in map view, are oriented approximately NE-SW and are considered sites of possible fracture zones. One such discontinuity, which we have termed the “Cape Lineament” (CL), marks a significant change in crustal character and Cretaceous depositional history, as revealed by gravity data and seismic reflection data, respectively. Crust NW of the CL, in the Orange Basin, is characterized by greater thickness and the presence of seismically-imaged seaward dipping reflectors (SDRs) whereas SE of the CL the crust has a more “normal” oceanic thickness and SDRs are either absent or more limited in areal extent. Magnetic, gravity and seismic data were combined to develop a crustal model of the South African margin north of the CL that includes: (a) a region of rifted/attenuated continental crust landward of magnetic anomaly G, (b) a 60–80 km wide zone of substantially intruded, underplated thin continental crust overlain by SDRs between anomalies G and M11, and (c) a ∼90 km wide zone of thick oceanic crust associated with a wide intrusion zone and smoother seafloor spreading magnetic anomalies that thins progressively to normal oceanic crustal thickness at the seaward edge of the overlying SDRs. South of the CL, the change in crustal character from attenuated continental crust to normal thickness oceanic crust occurs over a much shorter distance of approximately 35–40 km, possibly indicating the diminished influence of magmatic material. Although linear magnetic anomalies are observed both NW and SE of CL, anomalies to the SE display a better correlation with those predicted by our seafloor spreading model. We have successfully reconstructed the positions of the African and South American margins south of 32°S at M11 time (i.e., ∼135 Ma) using a new rotation pole at 38.86°N, 31.46°W. This pole applies only to the southernmost margins of the Austral segment where M11 is observed. Further north at this time the margins are undergoing non-rigid deformation that includes both crustal extension and magmatic underplating.

Implications of horsts and grabens on the development of canyons and seismicity on the west africa coast

Subsurface basement topography in the Nigerian portion of the Benin Basin has been studied using borehole data of wells drilled to the basement and one strike line of seismic section. Two areas of a sharp drop in topography with a horst in between were observed in the study area. These features were projected to a seismic section in the offshore area of the Benin basin. The result depicts the structural features as horst and grabens coinciding with the Avon platform bounded on the right side by Ise graben, and the Orimedu graben to the left. The observed relationship of the grabens with the present day location of Avon Canyon on the seismic section also suggests an active subsidence along fractured zones. The subsidence, which probably is occurring along similar fracture zones in the Gulf of Guinea, could be responsible for the occasionally reported seismicity on the margin of West Africa. A detailed seismographic study of the fracture zones is recommended.

Intraplate Deformation of Oceanic Crust in the West Somali Basin: Insights From Long‐offset Reflection Seismic Data

AbstractInterpretation of new high‐resolution seismic reflection sections reveals intraplate deformation of oceanic crust in the West Somali Basin, between the East Africa margin and the Davie Ridge and its northern extension. The deformation of the oceanic crust takes the form of 1–2 km amplitude buckle folds with a 20–30 km wavelength associated with thrust faults. Reverse faulting is localized along deformation corridors, about 15–30 km wide that we interpret as inherited oceanic fracture zones of the extinct oceanic spreading center. Deformation of the oceanic crust in the westernmost segment started before Hauterivian time (~132 Ma), while oceanic accretion was ongoing to the east of the Davie Ridge, and stopped at Aptian time (~120 Ma), when spreading ceased in the whole Somali Basin. Prior to the deformation of the oceanic crust, large volcanic edifices have been emplaced on top of the oceanic crust along preexisting oceanic fracture zones. Well‐defined groups of reflectors, within the mantle (&lt;~20 km below top basement), are interpreted as large shear zones that tapped a connection to deep lithospheric melt channels leading to magmatic intrusions within the crust and emplacement of postseafloor‐spreading volcanic edifices within deformation corridors.

The development of the East African margin during Jurassic and Lower Cretaceous times: a perspective from global tectonics

The eastern margin of Africa resulted from the first successful trans-Gondwana rupture which retraced, in part, the earlier unsuccessful Karoo rift system. Widespread volcanism in southern Africa (182 Ma, Toarcian) presaged NW–SE-directed extension between East Gondwana and West Gondwana (Africa). Rifting turned progressively north–south in orientation, leading quickly to ocean growth off Somalia and off central Mozambique while, elsewhere, strike-slip within the stretched margin came to predominate. East Gondwana, including Madagascar, was demonstrably still intact at 151.4 Ma (M22, Kimmeridgian) but, as the two large continental fragments disengaged from each other, pure north–south movement became possible. After about 140 Ma (Berriasian), East Gondwana itself started to fragment off Western Australia but little separation occurred as far west as Madagascar before Aptian times (126.1 Ma). Nevertheless, the geometry of the Australia–India opening required that, in the interval 140 – 120 Ma, Madagascar–India pursued a path against Africa different from that of Antarctica. The arcuate Davie Fracture Zone, 1800 km in length, functioned as a pure strike-slip transform off the Tanzania–Mozambique coast for this fragment until the early Aptian demise of the Somali mid-ocean ridge. The active transform east of the Lebombo in southern Africa, meanwhile, relocated progressively eastwards, finally to outboard of the Mozambique Ridge at 136 Ma (Valanginian), leaving most if not all of the stretched continental crust and its volcanosedimentary load attached to Precambrian Africa.

The Northern Gulf of Mexico During OAE2 and the Relationship Between Water Depth and Black Shale Development

AbstractDespite their name, Oceanic Anoxic Events (OAEs) are not periods of uniform anoxia and black shale deposition in ancient oceans. Shelf environments account for the majority of productivity and organic carbon burial in the modern ocean, and this was likely true in the Cretaceous as well. However, it is unlikely that the mechanisms for such an increase were uniform across all shelf environments. Some, like the northwest margin of Africa, were characterized by strong upwelling, but what might drive enhanced productivity on shelves not geographically suited for upwelling? To address this, we use micropaleontology, carbon isotopes, and sedimentology to present the first record of Oceanic Anoxic Event 2 (OAE2) from the northern Gulf of Mexico shelf. Here OAE2 occurred during the deposition of the well‐oxygenated, inner neritic/lower estuarine Lower Tuscaloosa Sandstone. The overlying organic‐rich oxygen‐poor Marine Tuscaloosa Shale is entirely Turonian in age. We trace organic matter enrichment from the Spinks Core into the deepwater Gulf of Mexico, where wireline log calculations and public geochemical data indicate organic enrichment and anoxia throughout the Cenomanian‐Turonian boundary interval. Redox change and organic matter preservation across the Gulf of Mexico shelf were driven by sea level rise prior to the early Turonian highstand, which caused the advection of nutrient‐rich, oxygen‐poor waters onto the shelf. This results in organic matter mass accumulation rates 1–2 orders of magnitude lower than upwelling sites like the NW African margin, but it likely occurred over a much larger geographic area, suggesting that sea level rise was an important component of the overall increase in carbon burial during OAE2.

Lithospheric thickness jumps at the S-Atlantic continental margins from satellite gravity data and modelled isostatic anomalies

Isostatic equilibrium is a good approximation for passive continental margins. In these regions, geoid anomalies are proportional to the local dipole moment of density-depth distributions, which can be used to constrain the amount of oceanic to continental lithospheric thickening (lithospheric jumps). We consider a five- or three-layer 1D model for the oceanic and continental lithosphere, respectively, composed of water, a sediment layer (both for the oceanic case), the crust, the mantle lithosphere and the asthenosphere. The mantle lithosphere is defined by a mantle density, which is a function of temperature and composition, due to melt depletion. In addition, a depth-dependent sediment density associated with compaction and ocean floor variation is adopted.We analyzed satellite derived geoid data and, after filtering, extracted typical averaged profiles across the Western and Eastern passive margins of the South Atlantic. They show geoid jumps of 8.1m and 7.0m for the Argentinian and African sides, respectively. Together with topography data and an averaged crustal density at the conjugate margins these jumps are interpreted as isostatic geoid anomalies and yield best-fitting crustal and lithospheric thicknesses. In a grid search approach five parameters are systematically varied, namely the thicknesses of the sediment layer, the oceanic and continental crusts and the oceanic and the continental mantle lithosphere. The set of successful models reveals a clear asymmetry between the South Africa and Argentine lithospheres by ~15km. Preferred models predict a sediment layer at the Argentine margin of 3–6km and at the South Africa margin of 1–2.5km. Moreover, we derived a linear relationship between, oceanic lithosphere, sediment thickness and lithospheric jumps at the South Atlantic margins. It suggests that the continental lithospheres on the western and eastern South Atlantic are thicker by 45–70 and 60–80km than the oceanic lithospheres, respectively.

FRONTIER SECURITY IN NORTH EAST AFRICA: CONFLICT AND COLONIAL DEVELOPMENT ON THE MARGINS c. 1930–60

AbstractThis article explores colonial development policy on the margins of British East Africa. It argues that much like current development practice in the region, increased colonial interventionism in the years after 1940 was motivated by security interests as well as environmental and economic considerations. Rural interventions were used as a mechanism to ‘rein in’ what were perceived to be subversive populations, as well as contain potential security threats. The article therefore throws new light on the nature of colonial rural development, as well as the connections between past and present development practice.

Earliest Oligocene hystricognathous rodents from the Atlantic margin of northwestern Saharan Africa (Dakhla, Morocco): systematic, paleobiogeographical, and paleoenvironmental implications

ABSTRACTHystricognathous rodents are among the most common members of African mammal faunas of the mid-Paleogene, but their record has so far remained limited to North and northeast Africa. Here we report the first Paleogene record of hystricognaths from the Atlantic margin of North Africa. The fossils come from the westernmost part of the Sahara, east of the Dakhla peninsula, Morocco, from estuarine deposits dating to the earliest Oligocene (Dakhla level C2 [DAK C2]). Several tens of isolated teeth plus three jaw fragments document seven species of hystricognaths (Gaudeamus cf. aslius, G. cf. hylaeus, Phenacophiomys occidentalis, gen. et sp. nov., Birkamys aff. korai, Mubhammys atlanticus, sp. nov., Neophiomys minutus, sp. nov., and ?Phiocricetomys sp.). Despite the extensive east-west geographic distance, the majority of hystricognath taxa recorded in DAK C2 document primarily close relatives of taxa that are known from a latest Eocene Egyptian locality (L-41) and from early Oligocene localities of both Egypt and Libya. This highlights the widespread east-west distribution of hystricognaths across North Africa, a distribution that reflects the existence of roughly similar tropical environmental conditions in northern latitudes of Africa at that time. The presence of seven hystricognath species plus five anomaluroid species in sympatry during the earliest Oligocene demonstrates that rodents were particularly diverse near the global cooling recorded at the Eocene-Oligocene transition. We describe and compare the species and new species of hystricognaths with their sub-coeval counterparts from northern and northeastern Africa and then discuss the paleobiogeographic and paleoenvironmental implications of that discovery.http://zoobank.org/urn:lsid:zoobank.org:pub:4041680F-65A0-41D5-AF97-D6115A656F15Citation for this article: Marivaux, L., S. Adnet, M. Benammi, R. Tabuce, J. Yans, and M. Benammi. 2017. Earliest Oligocene hystricognathous rodents from the Atlantic margin of northwestern Saharan Africa (Dakhla, Morocco): systematic, paleobiogeographical, and paleoenvironmental implications. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1357567.

A ~565 Ma old glaciation in the Ediacaran of peri-Gondwanan West Africa

In the Cadomian orogen of the NE Bohemian Massif and of SW Iberia, a post-Gaskiers glacial event dated at c. 565 Ma has been detected. Such Ediacaran-aged glaciomarine deposits occur in the Weesenstein and Clanzschwitz groups of the Saxo-Thuringian zone (Bohemia) and in the Lower Alcudian group of the southern Central Iberian zone (Iberia). Both areas are parts of Cadomia situated in the Western and Central European Variscides. Glaciomarine sedimentary rocks are characterized by such features as dropstones, flat iron-shaped pebbles (“Bugeleisen-Geschiebe”), facetted pebbles, dreikanters, and zircon grains affected by ice abrasion. For age and provenance determination, LA–ICP–MS U–Pb ages (n = 1124) and Hf isotope (n = 446) analyses were performed. The maximum age of the glaciomarine deposits within a Cadomian back-arc basin based on U–Pb analytics resulted in the youngest detrital zircon populations showing ages of 562–565 Ma and of c. 566–576 Ma old zircon derived from granitoid pebbles within the diamictites. The youngest age recorded was 538–540 Ma based on zircon from the plutons which had intruded the previously deformed Ediacaran metasedimentary rocks. Previously described glaciomarine diamictites of Cadomia (Weesenstein, Clanzschwitz, and Orellana diamictites) are most definitely younger than the c. 579–581 Ma Gaskiers glaciation in Newfoundland (Gaskiers) and in SE New England (Squantum). We propose the term Weesenstein–Orellana glaciation for this new Ediacaran glacial event, named after the most relevant regions of exposure. Palaeogeographically, these glaciomarine diamictites and related sedimentary deposits lie on the periphery of the West African Craton (western peri-Gondwana), and evidence has been provided by detrital zircon U–Pb ages and their Hf isotope composition. Correlation with similar glaciomarine deposits in the Anti-Atlas (Bou Azzer) and Saudi Arabia suggests a continued distribution of post-Gaskiers glacial deposits along the Gondwana margin of Northern Africa. The Weesenstein–Orellana glaciation correlates in part with the Shuram–Wonoka δ13C anomaly.

Climatic zonation and weathering control on sediment composition (Angola)

Complementary mineralogical and geochemical datasets on fluvial, beach and dune samples collected along the Atlantic margin of subequatorial southwestern Africa are used to investigate the relationships between provenance and climatic controls on sediment composition and to test the reliability of different geochemical and mineralogical weathering proxies as climatic indicators. The studied N/S-trending coastal region is characterized by strong latitudinal and inland climatic gradients, and thus represents an excellent natural laboratory in which to study the effects of climatic-induced weathering on sediment composition. Although the mineralogy and geochemistry of suspended-load muds closely reflects the different weathering intensities over both latitudinal and inland climatic gradients, the composition of mud and sand samples are strongly affected by sediment provenance. Consequently, weathering parameters such as the αAlE values (estimating the degree of depletion in element E relative to the UCC standard), display complex patterns of variation especially for sand samples. By assuming a typical order of bulk-sediment mobility Na>Ca>Sr>Mg>K>Ba≈Rb, anomalously high or low αAl values placing a specific element off the expected mobility order are considered as an indicator of source-rock control on sediment composition. The composition of detritus recycled from Meso-Cenozoic strata reflects the cumulative effect of successive sediment cycles, with recycling processes affecting to a different extent the diverse weathering proxies. In particular, αAlNa appears to be more strongly affected by recycling in muds than in sands. Among all mineralogical and chemical parameters, those that correlate best with rainfall in the drainage areas are αAlNa for sands, αAlMg for muds and smectite content (only in areas of low rainfall). In the geological and geomorphological setting of SW Africa these proxies turn out to be better climate estimators than the classical weathering indices CIA or WIP. This case study reminds us to carefully consider source-rock control and mixing with recycled detritus when drawing inferences on climatic conditions based on weathering indices.

Age of UHP metamorphism in the Western Mediterranean: Insight from rutile and minute zircon inclusions in a diamond-bearing garnet megacryst (Edough Massif, NE Algeria)

Diamond-bearing UHP metamorphic rocks witness for subduction of lithospheric slabs into the mantle and their return to shallow levels. In this study we present U–Pb and trace elements analyses of zircon and rutile inclusions from a diamond-bearing garnet megacryst collected in a mélange unit exposed on the northern margin of Africa (Edough Massif, NE Algeria). Large rutile crystals (up to 300 μm in size) analyzed in situ provide a U–Pb age of 32.4±3.3 Ma interpreted as dating the prograde to peak subduction stage of the mafic protolith. Trace element analyses of minute zircons (≤30 μm) indicate that they formed in equilibrium with the garnet megacryst at a temperature of 740–810 °C, most likely during HP retrograde metamorphism. U–Pb analyses provide a significantly younger age of 20.7±2.3 Ma attributed to exhumation of the UHP units. This study allows bracketing the age of UHP metamorphism in the Western Mediterranean Orogen to the Oligocene/early Miocene, thus unambiguously relating UHP metamorphism to the Alpine history. Exhumation of these UHP units is coeval with the counterclockwise rotation of the Corsica–Sardinia block and most likely resulted from subduction rollback that was driven by slab pull.

Cenozoic sediment budget of West Africa and the Niger delta

AbstractLong‐term (106–7 yr) clastic sedimentary fluxes to the ocean provide first‐order constraints on the response of continental surfaces to both tectonic and climatic forcing as well as the supply that builds the stratigraphic record. Here, we use the dated and regionally correlated relict lateritic landforms preserved over Sub‐Saharan West Africa to map and quantify regional denudation as well as the export of main catchments for three time intervals (45–24, 24–11 and 11–0 Ma). At the scale of West Africa, denudation rates are low (ca. 7 m Myr−1) and total clastic export rate represents 18.5 × 103 km3 Myr−1. Export rate variations among the different drainage groups depend on the drainage area and, more importantly, rock uplift. Denuded volumes and offshore accumulations are of the same magnitude, with a noticeably balanced budget between the Niger River delta and its catchment. This supports the establishment of the modern Niger catchment before 29 Ma, which then provided sufficient clastic material to the Niger delta by mainly collecting the erosion products of the Hoggar hotspot swell. Accumulations on the remaining Equatorial Atlantic margin of Africa suggest an apparent export deficit but the sediment budget is complicated by the low resolution of the offshore data and potential lateral sediment supply from the Niger delta. Further distortion of the depositional record by intracontinental transient storage and lateral input or destabilization of sediments along the margin may be identified in several locations, prompting caution when deducing continental denudation rates from accumulation only.