Pan-African Nappe Research Articles

Petrology of ophiolites of Memel, Nsimè-Kellé and Mapan (Yaoundé group): Evidence of the geodynamic evolution of the Pan-African orogeny in South Cameroon

Ophiolites are the remnants of ancient oceanic crust that can serve as tools to reconstruct the evolutionary history of fossil oceans. At Boumnyebel, the western part of the Yaoundé group, the sole of the lower unit (magnesian mica-schists and gneiss) of the Pan-African nappes is made up of ophiolitic rocks (talc schists, chlorite schists, serpentinites, amphibolites, pyroxenites, hornblendites). Structurally, the lower unit has a flat-lying schistosity with some recumbent folds while the underlying TTG gneisses are marked by a high-dipping foliation at Mapan. The contact between the Pan-African mica schists and the basement formations (Congo craton) is therefore discordant. In ophiolitic rocks, amphibole is actinolite, tremolite or magnesio-riebeckite. Chlorite compositions correspond to clinochlore. Talc shows notable substitution of Mg by Fe whereas substitution of Mg by Fe and Al prevails in serpentine. Spinel compositions are either Cr-rich with high FeO/Fe2O3 values (peridotitic spinels) or Cr-poor with low FeO/Fe2O3 values (melt-derived or magmatic spinels). Main mineral features indicate a metamorphic retromorphosis of the original lherzolites, harzburgites, dunites and pyroxenites to hornblendites and serpentinites. This is followed by hydrothermal alteration (at various temperatures: 250–570 °C and low pressure: < 2 kbar) that yielded chlorite schists and talc schists. Chemically, talc schists are made up of SiO2, MgO and minor FeO and Al2O3, while chlorite schists and hornblendites contain notable amounts of Al2O3 and CaO, respectively. All the ultramafic rocks display the same range of high Ni, Cr, Co contents with various LREE enrichments but flat HREE patterns. Multi-element patterns show that most talc schist are depleted in almost all elements. All the ultramafic rocks are marked by deep Nb, Ta, K, Pb, Sr, Ti, Zr and Hf troughs suggesting that the ultramafic rocks possibly derived from heterogeneous mantle sources varying from fertile to refractory peridotites. Amphibolites are generated from N-MORB, IAT or CA geotectonic settings. The compositions of spinel and ultramafic whole rocks reveal an arc setting that has evolved from divergent plate setting (oceanisation, N-MORB) to convergent plate (IAT, CA, SSZ D-MORB) settings. In summary, ophiolites emplaced in various geotectonic settings and were amalgamated during the Pan-African collision.

Pan African Nappe system: evidence of thrust structures from Okemesi, southwestern Nigeria

The Okemesi area, southwestern Nigeria, is part of the Ilesha schist belt and play host to the Okemesi fold belt and the Ifewara Shear Zone. Nappe structures had been proposed for some parts of Southwestern Nigeria without very clear meso or micro structural evidence. This study provides a lot of new structural evidences to substantiate the inferred Nappe structures within the Okemesi fold belt. Mapping of the Okemesi fold has been hampered by thick vegetation in the past. However, a major deformation zone was exposed by the recent landslide activities in the area, structures and lithologies exposed by the landslide were studied and structural features that are typical of transtensional and transpressional settings were observed. This paper presents the result of the systematic filed mapping which revealed the complete, fold transposition, elements of internal strain such as cleavages and mineral lineation, intrafolial folds, flexure slip, sheath, recumbent and conjugate kink folds, winged porphyroblast, faults (low angle reverse faults, dextral faults, normal faults), ramp and flat structures, etc. The characteristics and the geometry of the observed thrust structures make it an important piece of evidence, not only to infer the tectonic events in the area, but also to constrain the timing and nature of movement associated with thrusting in the area.

Magnetic fabrics and Pan-African structural evolution in the Najd Fault corridor in the Eastern Desert of Egypt

In order to assess the Pan-African structural evolution from early orogenic fabrics through Najd wrenching to the latest orogenic collapse/extension, the authors used field work, aided by aerial photographs and satellite images. This work is complemented by the study of the anisotropy of the magnetic susceptibility (AMS, or magnetic fabric). The Pan-African rock associations of the Um Gheig-Kadabora area can be divided into a lower tier composed mainly of amphibolite–migmatite and granitoid gneisses, and an upper tier of ophiolitic rocks, metavolcanics and their related volcaniclastics, and molasse-type Hammamat sediments. Both these units are intruded by late orogenic granitoid plutons and dykes. The lower tier is exposed in a domal structure in the El Sibai area, the upper tier forms a series of weakly to highly deformed thrust units, called Pan-African Nappes here, which are dissected by high strain shear zones. According to their age, these rock units are divided here into early and late-orogenic. The early orogenic rock association is characterized by medium–high metamorphic grades. The late orogenic rock association is characterized by low metamorphic grade. The rocks in the upper tier form a series of low angle thrust sheets, which are bounded by NW-striking high angle shear zones related to the Najd Fault System. The early orogenic rocks show a polyphase structural evolution with early folds, thrusts, and strike-slip shear zones. The late orogenic rocks show a relatively weaker deformation. The latest intrusives studied here are the dykes dissecting the late orogenic Kadabora granite. In the present work magnetic fabric data document the deformational features in detail and assess the role of the Najd Fault System in the deformational evolution. A strong variation in volume susceptibility of various rocks, due to their variations in mineral composition, is observed. Lower values are in the range of 10−6 SI units for late-orogenic alkaline granite and the dykes dissecting it, the highest susceptibilities exceed 7×10−2 SI units in magnetite-bearing serpentinite. Early orogenic rocks are characterized by relatively high anisotropies (P′ up to 1.7) and are deformed in numerous shear zones. Most of these shear zones can be related to the Najd Fault System. In contrast, late orogenic sediments and intrusives show mostly low anisotropies. However, magnetic lineations are still distinctly oriented parallel with the Najd Fault trend. The very latest Pan-African intrusives, the broadly N–S trending dykes crosscutting the Kadabora pluton, imply c. E–W directed extension. Such an extension is consistent with the magnetic fabric in some of the dykes. Therefore, the Kadabora dykes mark the end of Najd wrenching and a late stage of extension in this part of the Eastern Desert of Egypt. The other dykes display mostly primary fabrics, related to magma flow during their intrusion and are thus post-deformational with regard to the Pan-African orogeny.

Geological context of the Boumnyebel talcschists (Cameroun): Inferences on the Pan-African Belt of Central Africa

The talcschists of the Boumnyebel area (southern Cameroon) form ≤ 30 m thick discontinuous layers within a Pan-African nappe unit (Yaoundé group), which includes, at the base, muscovite + biotite ± garnet micaschists associated with amphibolites and pyroxenites, and, at the top, muscovite + biotite + garnet + kyanite micaschists locally associated with marble and amphibolites. The metamorphic peak (∼650 °C/9.5 kbar; ca. 620 Ma) postdates nappe emplacement. Isograds are in normal position, micaschists passing downwards to migmatites in the northwestern part of the area studied. The rock types in the lower part of this nappe suggest active margin environments with detrital input from a nearby continental crust (arc or back-arc context).

The northern dome of Wadi Hafafit culmination, Eastern Desert, Egypt: Structural setting in tectonic framework of a scissor-like wrench corridor

Although the gneissic domes in Eastern Desert of Egypt have been studied recently in a considerable detail; their origin remains controversial. Basically four schools of thought exist: one argues for an origin parallel crustal extension, a second suggests emplacement within antiformal stacks, a third envisages young emplacement within a core of a sheath fold and finally some authors believe that the emplacement is due to overlap of regional folds and extension parallel to the fold axes. The Wadi Hafafit Culmination is one of these domes and occupies the southern part of the Central Eastern Desert of Egypt. The culmination is cored with five separated gneissic domes ranging in composition from orthogneiss to paragneiss. They are overthrust by a low-grade, volcano-sedimentary association constituting the Pan-African cover nappes. Detailed structural mapping of the northern dome reveals that, the gneisses are vertically emplaced through the cover rock units. This is based on field evidence which shows that the gneisses experienced vertical flattening associated with exhumation corresponding to coaxial deformation. It is suggested that the emplacement of gneissic core occurred during accretion of the Pan-African nappes. Later, strike-slip shear zones of Najd Fault System and the associated subsidiary shear arrays postdate emplacement of the dome. The gneisses contiguously underlie the Pan-African nappe assemblages through discrete low-angle left-lateral thrust-dominated shear zones from the east. Ongoing accretion of nappe assemblages on the gneisses increases the density contrast between the overlying denser nappe and the underlying lighter quartz-rich gneisses, leading to squeezing the gneissic materials in oblique convergence regime. As a consequence, the gneisses are suggested to have up-domed vertically through the nappe rock units.

A late Neoproterozoic paleomagnetic pole for the Congo craton: Tectonic setting, paleomagnetism and geochronology of the Nola dike swarm (Central African Republic)

New structural, geochronological and paleomagnetic data were obtained on dolerite dikes of the Nola region (Central African Republic) at the northern border of the Congo craton. In this region, metavolcanic successions were thrust southward onto the craton during the Panafrican orogenic events. Our structural data reveal at least two structural klippes south of the present-day limits of the Panafrican nappe suggesting that it has once covered the whole Nola region, promoting the pervasive hydrothermal greenschist metamorphism observed in the underlying cratonic basement and also in the intrusive dolerite dikes. Paleomagnetic measurements revealed a stable dual-polarity low-inclination magnetization component in nine dikes (47 samples), carried by pyrrhotite and magnetite. This component corresponds to a paleopole at 304.8°E and 61.8°S (d p = 5.4, d m = 10.7) graded at Q = 6. Both metamorphism and magnetic resetting were dated by the 40Ar/ 39Ar method on amphibole grains separated from the dikes at 571 ± 6 Ma. The Nola pole is the first well-dated paleomagnetic pole for the Congo craton between 580 and 550 Ma. It marks a sudden change in direction of the Congo craton apparent polar wander path at the waning stages of the Panafrican orogenic events.

Sr–Nd isotopes and geochemistry of the infrastructural rocks in the Meatiq and Hafafit core complexes, Eastern Desert, Egypt: Evidence for involvement of pre‐Neoproterozoic crust in the growth of Arabian–Nubian Shield

Abstract Meatiq and Hafafit core complexes are large swells in the Eastern Desert of Egypt, comprising two major tectono‐stratigraphic units or tiers. The lower (infrastructure) unit is composed of variably cataclased gneissose granites and high‐grade gneisses and schists. It is structurally overlain by Pan–African ophiolitic mélange nappes (the higher unit). The two units are separated by a low‐angle sole thrust, along which mylonites are developed. Major and trace element data indicate formation of the gneissose granites in both volcanic arc and within‐plate settings. Nevertheless, all analyzed gneissose granites and other infrastructural rocks, exhibit low initial ratios (Sri) (&lt;0.7027), positive εNd(t) (+4.9 to +10.3) and Neoproterozoic Nd model age (TDM) (592–831 Ma for the gneissose granite samples). Although these values are compatible with other parts of the Arabian– Nubian Shield considered to be juvenile, the εNd(t) values and several incompatible element ratios of the gneissose granites are too low to be derived from a mantle source without contribution from an older continental crust. Our geological, Sr–Nd isotopic and chemical data combined with the published zircon ages indicate the existence of a pre‐Neoproterozoic continent in the Eastern Desert that started to break up at ca 800 Ma. Rifting and subsequent events caused the formation of oceanic crust and emplacement within‐plate alkali basalts in the hinterland domains of the old continent. The emplacement of basaltic magma might have triggered melting of lower crust in the old continent and resulted in emplacement of the within‐plate granite masses between 700 Ma and 626 Ma. The granite masses and other rocks in the old continent have been subjected to deformation during the over‐thrusting of Pan–African nappes, probably because of the oblique convergence between East and West Gondwanaland. Rb–Sr isotopes of the gneissose granites in both Meatiq and Hafafit core complexes defines an isochron age of 619 ± 25 Ma with Sri of 0.7009 ± 0.0017 and mean squares of weighted deviates = 2.0. We interpret this age as the date of thrusting of the Pan–African nappes in the Eastern Desert. Continued oblique convergence between East and West Gondwanaland could have resulted in the formation northwest–southeast‐trending Meatiq and Hafafit anticlinoriums.

Isotope chemostratigraphy of marbles in northeastern Mozambique: Apparent depositional ages and tectonostratigraphic implications

Marbles are minor but characteristic components of metasedimentary units within nappes in the Pan-African Mozambique Belt in NE Mozambique. Metasedimentary units remain largely undated, and carbon and strontium isotope stratigraphy of marbles has been used for indirect dating of the depositional history in this part of the Mozambique Belt. Sixty-nine samples from nine occurrences of dolomite, calcite and magnesite marbles in the Montepuez, Xixano, Lalamo, Ocua and Nampula metamorphic complexes were analysed for major and trace elements, and a subset of 39 samples for C, O and Sr isotopes. The least altered δ 13C values range from −3.5 to +7.1‰ (V-PDB) and 87Sr/ 86Sr ratios from 0.70504 to 0.70671. These values are considered as the best proxy to seawater composition at the time of deposition. The apparent deposition ages, derived from available seawater evolution curves, range from c. 1250 to c. 660 Ma. An age of 1250–910 Ma is obtained from a tripartite marble unit in the Montepuez Complex which is exposed in the Montepuez quarries. Five other age-groups are represented by marble units with apparent depositional ages of 800–750 Ma (Xixano North), 800–660 Ma (Montepuez West), c. 750 Ma (Nampula), c. 740 Ma (Xixano South and Lalamo), and 740–670 Ma (Montepuez East). The data suggest that: ( i) Pan-African nappes in NE Mozambique include Neoproterozoic and probable Mesoproterozoic sediments; ( ii) Neoproterozoic rocks of the Xixano and Nampula complexes might have different ancestry and were tectonically juxtaposed during the Neoproterozoic Pan-African orogeny.

U–Pb TIMS age constraints on the evolution of the Neoproterozoic Meatiq Gneiss Dome, Eastern Desert, Egypt

Ages are used to constrain the temporal evolution of the Meatiq Gneiss Dome, Eastern Desert, Egypt, by dating (ID-TIMS) pre-, syn-, and post-tectonic igneous rocks in and around the dome. The Um Ba’anib Orthogneiss, comprising the deepest exposed structural levels of the dome, has a crystallization age of 630.8 ± 2 Ma. The overlying mylonites are interpreted to be a thrust sheet/complex (Abu Fannani Thrust Sheet) of highly mylonitized metasediments (?), migmatitic amphibolites, and orthogneisses with large and small tectonic lenses of less-deformed intrusives. Two syn-tectonic diorite lenses in this complex have crystallization ages of 609.0 ± 1.0 and 605.8 ± 0.9 Ma, respectively. The syn-tectonic Abu Ziran diorite, cutting across the tectonic contact between mylonite gneisses of the Abu Fannani Thrust Sheet and a structurally overlying thrust sheet of eugeoclinal rocks (“Pan-African nappe”), has a magmatic emplacement age of 606.4 ± 1.0 Ma. Zircons from a gabbro (Fawakhir ophiolite) within the eugeoclinal thrust sheet yielded a crystallization age of 736.5 ± 1.2 Ma. The post-tectonic Fawakhir monzodiorite intrudes the ophiolitic rocks and has an emplacement age of 597.8 ± 2.9 Ma. Two other post-tectonic granites, the Arieki granite that intrudes the foliated Um Ba’anib Orthogneiss, and the Um Had granite that cuts the deformed Hammamat sediments, have emplacement ages of 590 ± 3.1 and 596.3 ± 1.7 Ma, respectively. We consider formation of the Meatiq Gneiss Dome to be a young structural feature (<631 Ma), and our preferred tectonic interpretation is that it formed as a result of NE–SW shortening contemporaneous with folding of the nearby Hammamat sediments around 605–600 Ma, during oblique collision of East and West Gondwana.

Thrusting and transpressional shearing in the Pan-African nappe southwest El-Sibai core complex, Central Eastern Desert, Egypt

The Wadi El-Shush area in the Central Eastern Desert (CED) of Egypt is occupied by the Sibai core complex and its surrounding Pan-African nappe complex. The sequence of metamorphic and structural events in the Sibai core complex and the enveloping Pan-African nappe can be summarized as follows: (1) high temperature metamorphism associated with partial melting of amphibolites and development of gneissic and migmatitic rocks, (2) between 740 and 660 Ma, oblique island arc accretion resulted in Pan-African nappe emplacement and the intrusion of syn-tectonic gneissic tonalite at about 680 ± 10 Ma. The NNW–SSE shortening associated with oblique island arc accretion produced low angle NNW-directed thrusts and open folds in volcaniclastic metasediments, schists and isolated serpentinite masses (Pan-African nappe) and created NNE-trending recumbent folds in syn-tectonic granites. The NNW–SSE shortening has produced imbricate structures and thrust duplexes in the Pan-African nappe, (3) NE-ward thrusting which deformed the Pan-African nappe into SW-dipping imbricate slices. The ENE–WSW compression event has created NE-directed thrusts, folded the NNW-directed thrusts and produced NW-trending major and minor folds in the Pan-African nappe. Prograde metamorphism (480–525 °C at 2–4.5 kbar) was synchronous with thrusting events, (4) retrograde metamorphism during sinistral shearing along NNW- to NW-striking strike-slip shear zones (660–580 Ma), marking the external boundaries of the Sibai core complex and related to the Najd Fault System. Sinistral shearing has produced steeply dipping mylonitic foliation and open plunging folds in the NNW- and NE-ward thrust planes. Presence of retrograde metamorphism supports the slow exhumation of Sibai core complex under brittle–ductile low temperature conditions. Arc-accretion caused thrusting, imbrication and crustal thickening, whereas gravitational collapse of a compressed and thickened lithosphere initiated the sinistral movement along transcurrent shear zones and low angle normal ductile shear zones and consequently, development and exhumation of Sibai core complex.

U–Pb dating of plutonic rocks involved in the nappe tectonic in southern Cameroon: consequence for the Pan-African orogenic evolution of the central African fold belt

TIMS-ID and SIMS U–Pb dating on zircons from metaplutonic rocks involved in the Pan-African nappe of southern Cameroon allow definition of three groups of subduction-related intrusions: group-I intrusions represented by the Masins metagabbro in the Lomie region yielded 666 ± 26 Ma; group-II intrusions represented by the Mamb metasyenogabbro and the Yaoundé pyriclasite yielded ca. 620 Ma and are broadly coeval with the deposition of the Yaoundé metasediments; group-III intrusions represented by the Elon augen metagranite and the Ngaa Mbappe metamonzodiorite yielded ca. 600 Ma. The onset of the nappe tectonics occurred under high-grade conditions in the range 616 to 610 Ma and continued around 600 Ma with the emplacement of the shallowest nappes. Finally, the construction of southern Cameroon proceeded by a multi-stage evolution characterized by a long-lived development of magmatic arcs associated with rapid opening and closure of sedimentary marginal basins in relation to a northward subduction.

Phénomènes karstiques fossiles et actuels au sein des formations métamorphiques silico-alumineuses de la nappe pan-africaine de Yaoundé (Sud-Cameroun)

Numerous karstic features have been recognised in the non-carbonaceous micaschists and gneisses of the Yaounde Pan-African nappe, south of Cameroon. It is shown that their formation was controlled by the structural features of the rocks. The wells and the pipes in the bedrock outcrops point out a current karstification process, resulting from the plagioclase dissolution by the acid rain waters. Hill wall alveoli and caves, of pre-Miocene age, are exhumed features that were done by dissolution in the aquiferous underground.

Pan-African nappe system in southwest Nigeria: the Ife-Ilesha schist belt

This paper presents new petrostructural data and a detailed geological map of the Ife-Ilesha area, a classical schist belt in southwest Nigeria. The overall syn-metamorphic flat-lying structures, recumbent folding and associated strong boudinage of the mafic and ultramafic rocks are the result of Pan-African syn-metamorphic stacking of thrust units above the Archæan basement. Supracrustal units are considered to be part of a former late Palæoproterozoic pericratonic cover, whereas mafic/ultramafic rocks may derive from a dismembered Proterozoic sill complex. It is proposed that this area is part of a Pan-African nappe system observed at mid-crustal levels. The northeastward tectonic vergence of units reconstructed from kinematic criteria is at variance with the southwestward motion of the nappes near the Pan-African suture 400 km to the west. Such stacking of units resulting from the same northeast-southwest compression at the scale of that part of Gondwana may correspond to the closure of a former basin floored by thinned Archæan crust with abundant magmatism of late Neoproterozoic age. Post-nappe upward doming of the slightly anatectic Archæan basement and upright folding of the already formed recumbent structures were linked to the initiation of the dextral strike-slip motion along the steep north-northeast-trending Ifewara mylonitic zone that compares well with similar shear zones of the Trans-Saharan Belt in the Tuareg Shield.

The Permo-Jurassic alkaline province of Tadhak, Mali: Geology, geochronology and tectonic significance

The Tadhak alkaline complexes, located along the eastern edge of the West African craton in northeastern Mali, intrude the 2 Ga old basement or the overlying Pan-African nappes and are covered by Cretaceous sediments. This magmatic activity was accompanied by doming and rifting. Northnortheast trending normal faults define two crustal segments exhibiting variously sized ( <1 km to 17 km) intrusive complexes. These comprise eroded undersaturated ring-complexes with occasional carbonatites in the west and high-level phonolitic plugs with shallow intrusive nepheline syenites in the east. New RbSr isotopic data indicate 100 Ma of within-plate magmatic activity in the area without any obvious age progression. In addition to the Adrar Tadhak complex (262±7 Ma), the following intrusions have been dated: Tirkine (215±11 Ma), Anezrouf (184±14 Ma) and Tidjerazraze-In Imanal (161±5 Ma). A clear break thus exists with the Cambrian alkaline granite province of the Iforas located on the opposite side of the Pan-African suture. 87Sr/ 86Sr initial ratios are similar throughout (between 0.7044 and 0.7049) which indicates a bulk Earth isotopic composition. The conclusion of exclusively mantle origin (“Dupal composition”) previously demonstrated for the Adrar Tadhak complex can now be extended to the other complexes. This implies repeated tapping of a single mantle source throughout the life of the province. The described magmatic and tectonic Permo-Jurassic activity has been triggered by intraplate stresses focussed along the pre-existing Pan-African suture zone. Together with similar features in Europe and Africa, the Permo-Jurassic Tadhak alkaline province heralds the progressive break-up of Pangaea and the opening of the Atlantic Ocean.

Tectonics of Hank sequence (upper proterozoic) in the eastern part of Eglab massif, reguibat shield (West African craton)

The Eglab massif is widely considered to have been stabilized by the Eburnean orogeny (> 1.7 G.a.). However, sedimentary rocks of the Hank sequence (< 1 G.a.) are locally folded and detached from their Lower Proterozoic (> 1.9 G.a.) crystalline substratum in the eastern part of the massif. Moreover, these rocks and their substratum are: (1) thrusted westward along NNE trending, high angle, reverse faults, and (2) shifted by two sets of strike-slip faults, a dextral NE one, and a sinistral SE one. The displacements were accompanied by reactivation of older, NE trending, dextral, strike-slip faults in the substratum. The deformation diminishes westward. It was due to horizontal compression oriented perpendicularly to the Pan-African suture situated 300 km further to the east. The time of deformation is constrained by an angular unconformity between the rocks of the Hank formation and the uppermost Proterozoic (> 595 M.a.) sedimentary rocks which overlie them horizontally. The deformation is referred to the Pan-African orogeny (500–700 M.a.). The discussed region was situated in the far foreland of Pan-African nappes.