African Orogeny Research Articles

Terrane rotation during the East African Orogeny: Evidence from the Bulbul Shear Zone, south Ethiopia

The 100 km long, N-trending, Neoproterozoic–Early Paleozoic Bulbul Shear Zone in southern Ethiopia is marked by sheared ophiolites at the interface between the Arabian–Nubian Shield in the north and the Mozambique Belt in the south. This shear zone separates the low-grade meta-volcanic and meta-sedimentary rocks of the Bulbul Terrane in the east from the medium- to high-grade gneissic, migmatites and granulites of the Alghe Terrane in the west. Stretching lineations along the Bulbul Shear Zone vary from NE-plunging in the northern part, shallowly N- and S-plunging in the central part, to SE-plunging in the south. These lineations are developed along N-trending mylonitic foliation that is moderately to steeply E-dipping. The northern part of the Bulbul Shear Zone is dominated by SW-verging fold and thrust belt indicating top-to-the southwest tectonic transport. The central part is characterized by dextral strike-slip displacement. The southern part is dominated by E-dipping oblique normal-slip planes associated with top-to-the southeast tectonic transport. Down dip stretching lineations along E-dipping slip planes are well-developed in the eastern part of the Alghe terrane and the western part of the Bulbul terrane. We interpret the along-strike variation of stretching lineations and kinematic indicators as due to NE–SW directed oblique collision between the Bulbul Terrane and the Alghe Terrane accompanied by anti-clockwise rotation of the Bulbul Terrane. Such collision and rotation are manifested by SW-verging fold and thrust belt in the north, N-trending dextral strike-slip shear zone in the center, and SE-directed normal-slip displacement in the south. This tectonic event might have occurred between 820 and 580 Ma. This was followed by E-ward slipping of the Bulbul Terrane relative to the Alghe Terrane, probably between 580 and 500 Ma.

Tectono sedimentary evolution of the Umm Ghaddah Formation (late Ediacaran-early Cambrian) in Jordan

The terrestrial Umm Ghaddah Formation of late Ediacaran-early Cambrian age was deposited in NE–SW elongated intracontinental rift system basins and sub-basins bounded by active listric half-graben faults. Basin fill consists of conglomerate facies association A, deposited in a fault-controlled transverse alluvial fan system that drained northwestward and graded laterally into sandstone facies association B, deposited by a braided river system flowing northeastward axial to the rift basin. The alluvial fan facies association was deposited by rock falls and non-cohesive debris flows of sediment gravity flow origin, and by sheetflood processes. The Umm Ghaddah Formation is dominated by a large-scale fining upward succession interpreted to reflect a gradual cessation of the Pan African Orogeny. Within this large-scale trend there are also minor fining and coarsening upward cycles that are attributed to repeated minor tectonic pulses and autocyclic shifting of the system. The distribution pattern of the Umm Ghaddah Formation and the underlying Ediacaran Sarmuj Conglomerates, Hiyala Volcaniclastics and Aheimir Volcanics in Jordan and adjacent countries in isolated extensional half-grabens and grabens formed during the extensional collapse phase of Arabia associated with the Najd Fault System seems to be unrelated to the present day Wadi Araba-Dead Sea transform fault system.

Biota in the terminal Proterozoic successions on the Indian subcontinent: a review

Abstract On the Indian subcontinent, Late Proterozoic rocks form part of a continuous sequence grading into Cambrian, postdating the Sturtian glaciation (Cryogenian) and terminated by the Xingkaian/Pan African Orogeny (Late Cambrian). These sequences are restricted to the northwestern part of the Indian Shield, are overlain by Cenozoic sediments of the Indo-Gangetic Plain, and rest unconformably on the oldest platform sequences (Meso—Cryogenian) in many parts of the Lesser and Tethys/Higher Himalaya. Of these, the succession in the Krol Belt, Lesser Himalaya, divided into Baliana, Krol and Tal Groups, is fossiliferous. Changes in faunal composition of acritarchs and cyanobacteria are recorded in the upper part of the Baliana Group, as is the appearance and extinction of forms assigned to the Ediacara fauna in the overlying Krol. Such biotic change is accompanied by a significant depletion in δ 13 C values in the ‘cap carbonates’. Another δ 13 C depletion is recorded in the upper part of the Krol Group along with appearance of spiny acanthomorphic acritarchs, scaphomorphs and hercomorphs, small shelly fossils, a variety of Early Cambrian trace fossils and trilobites in the overlying Tal Group. A review of biota reported from the oldest platform sediments (Vindhyan Supergroup, Karnool, Bhima Groups) suggests a Meso-Cryogenian (pre-Sturtian glaciation) age.

Evolution of the Central Iranian basement

Most of the Infracambrian–Lower Paleozoic sedimentary rocks occurring in Iran cover a crystalline basement. In Central Iran, and the Saghand region along the Chapedony and Poshte Badam faults, the basement complexes show structural vestiges of Precambrian deformational, depositional, erosional, metamorphic and magmatic events. In this region, the basement complexes contain the Chapedony, Poshte Badam, Boneh Shurow and Tashk formations. The prevailing metamorphism, from low grade to high grade, is pre-Pan African Orogeny (i.e. from 2400 to 570 Ma). The presence of greenstone belts, a paleo-suture zone and ophiolitic rocks (i.e. from 2100 to 1500 Ma) around the high-grade metamorphic rocks of the Chapedony Formation, provide evidence that cratonization forming the Iranian basement occurred during the Paleo- and Mesoproterozoic. After formation of the basement complexes, metamorphism of the Precambrian formations and establishment of the Arabo–Iranian coherent platform at the end of Pan African Orogeny, Central Iran broke up during a Neoproterozoic–Early Cambrian extensional tectonic regime. Infracambrian–Phanerozoic sedimentary rocks were then deposited on the basement complexes. The Iranian basement complexes are extensively overprinted by Pan African Orogeny and younger igneous events.

Geology and geochemistry of the Neoproterozoic Tuludimtu Ophiolite suite, western Ethiopia

The Kemashi Domain, a lithotectonic subdivision of the Neoproterozoic Tuludimtu Orogenic Belt of western Ethiopia, consists of a suite of mafic–ultramafic volcanic and plutonic rocks, and interbedded deep marine sediments, mainly graphite-bearing pelitic schists and phyllites, and graphitic quartzites and cherts. Pillow structures indicate submarine extrusion of the volcanics, whilst partings within some of the basalts may represent sheeted dykes. An associated mélange unit, composed of blocks of the same rock types as above, set in a fine schistose matrix, also occurs. This assemblage is interpreted as a dismembered ophiolite—the Tuludimtu Ophiolite—formed in a deep oceanic environment. A turbiditic sequence is also present in the domain. The Tuludimtu Ophiolite underwent intense compression during the Neoproterozoic Pan African Orogeny, resulting in early recumbent folding and westwards-directed thrusting, followed by reactivation of steeper zones of the thrusts as N–S orogen-parallel strike-slip shear zones, accompanied by refolding of early folds into upright horizontal folds. This was followed by development of deep crustal NNW–SSE orogen-transecting shear zones, which were reactivated as brittle faults during orogenic collapse of the Tuludimtu Belt. Metamorphism to lower greenschist facies grade accompanied orogenesis. Major, trace and REE geochemistry of volcanic and some plutonic igneous rocks, has been employed to define the tectonic setting of the terrane. Tectonic discrimination diagrams, utilising REE and HFSE, indicate a wide distribution spectrum but with the majority of samples plotting in arc basalt and MORB fields, suggesting derivation from sources similar to N-MORB and depleted MORB (typical of many arc basalts). Most of the samples exhibit a slight depletion of immobile elements, relative to N-MORB values and also show depletion of Zr, Ti, Nb and Y, implying that their source had been depleted by an earlier melting episode. Overall, the geochemistry typifies spreading centre basalts with some compositional features transitional to those of arc basalts, a characteristic of back-arc basalts. Lithological association, structural style and geochemistry of the rock assemblage in the Kemashi Domain thus define an ophiolite interpreted to have formed within a deep marine environment. This is thought to have been due to rifting of continental crust within a back-arc basin regime in a continental margin type extensional setting. Comparison with other ophiolitic terranes within the Arabian Nubian Shield, suggests that many of these terranes may represent back-arc basin type tectonic settings, similar to the Kemashi Domain. This supports the multi-stage accretion model for closure of the Mozambique Ocean, for which the Pacific Ocean may be a present day analogue.

Constraints on the tectonometamorphic evolution of the Western Ethiopian Shield

The Western Ethiopian Shield comprises three lithotectonic units: the Birbir domain, an assemblage of deformed and metamorphosed volcano-sedimentary rocks and associated ultramafic to felsic magmatic rocks, is bounded by the dominantly orthogneissic Baro and Geba domains. Geochemical considerations suggest the igneous rocks of the Birbir domain are predominantly calc-alkaline and similar to those generated by subduction in modern oceanic island-arcs, broadly consistent with Sm–Nd data that indicate an early crust-forming event at ca. 0.95–1.1 Ga. This complex was accreted during the Neoproterozoic East African Orogeny and formed the core and source area for subsequent plutonism and metamorphism. The earliest recorded deformation resulted in a sub-horizontal gneissosity within the gneissic terrains, synchronous with an early (ca. 770–800 Ma) metamorphic event (M 1), which records amphibolite facies conditions (600–800 °C and ca. 6–8 kbar) in the gneissic terrains and greenschist to amphibolite facies conditions in the Birbir domain. The igneous protoliths to the orthogneisses (i.e., pre-dating M 1) are dated at 830–785 Ma. All terrains were subsequently deformed in the D 2 event, the result of oblique westerly directed transpression, with consequent folding and thrusting concentrated at domain boundaries. Subsequent D 3 deformation was focussed within mylonitized zones that likely represent reactivated D 2 thrusts and record both dextral (early) and sinistral (late) transcurrent movement. A second protracted metamorphic event, M 2, at around 650–550 Ma, resulted in rehydration of M 1 parageneses and was fluid-driven. M 2 metamorphism was synchronous with D 3 shearing. Fluid incursion was concentrated within sheared domain boundaries and caused isotopic re-equilibration. The WES records a long-lived but episodic tectonothermal event that can be explained in terms of arc–continent collision. Melting of a subducting slab and associated intrusion caused metamorphism and the formation of an oceanic island-arc complex. Continued plate convergence caused severe E–W shortening and basin closure. Further attenuation gave rise to transcurrent shearing, fluid influx and a second thermal event during accretion and orogenic closure.

A review of the evolution of the Mozambique Belt and implications for the amalgamation and dispersal of Rodinia and Gondwana

Abstract Geochronological, isotopic, lithological and structural data from the Mozambique Belt, and its extensions in Antarctica, Sri Lanka, India and Madagascar, are summarized and reviewed within a Gondwana framework. Much of the southern Mozambique Belt is dominated by Rodinian-Grenvillian-age juvenile magmatism and crustal genesis, with a strong metamorphic overprint during the Pan-African. Magmatism at c. 800 Ma, possibly related to Rodinian fragmentation, is restricted to the Zambesi Belt, a few examples in northern Mozambique, southern Malawi, Tanzania, Madagascar and the Rayner Complex in Antarctica. Significantly, no crust of this age is recognized in western Dronning Maud Land. Amalgamation of Gondwana initially involved the closure of the Mozambique Ocean between the Tanzanian-Congo Cratons and the Madagascar-India-Enderby Cratons forming the East African Orogeny between c. 580 and c. 800 Ma. This was followed by the c. 500–580 Ma transpressional sinistral collision of the combined Kalahari-East Antarctic Cratons and associated Grenville-age belts in the south, with the northern block comprising the combined Tanzanian-Congo-Madagascar-India-Enderby Block.

Tectonic evolution of the Allaqi shear zone and implications for Pan-African terrane amalgamation in the south Eastern Desert, Egypt

The complex major Allaqi shear zone occurs within the meta-volcanic and meta-sedimentary Neoproterozoic rocks of the central Wadi Allaqi and is important for our understanding of the development of the Arabian-Nubian Shield within the evolution of the East African (Pan-African) Orogeny (950—550 Ma). Early-formed northerly-facing complex thrust duplex systems, with attendant west-northwest—east-southeast orientated recumbent folds, dominate the area. Early ductile structural phases are superseded by more brittle structures, and the intrusion of four granitoid plutons punctuates the development history. The Allaqi shear zone is interpreted as the most northerly major dislocation of a fault-duplex zone of regional significance but an interpretation of the zone as a palæo-suture is not proven despite the presence of an allochthonous ophiolitic unit (the Gebel Taylor Wedge), which has undergone blueschist-facies metamorphism.

Neoproterozoic Crustal Evolution of Northwestern Indian Shield: Implications on Break up and Assembly of Supercontinents

The Neoproterozoic geological history in western Rajasthan, northwest Indian Shield began with the intrusion of anorogenic bodies of diorites at ca. 1000 Ma. Recently available single zircon dates indicate possible continuity of the “Grenville belt” beyond Eastern Ghats through the Satpura Orogenic Belt into the Aravalli Mountains. Closely following this tectono-thermal event at the Meso-Neoproterozoic boundary, some narrow basins opened west of the Aravalli Mountains. The basin closing related to the tectonic inversion and associated magmatism at ca. 835 Ma completed the cratonisation process of the Precambrian Aravalli crust. Subsequent geological events witnessed over a wide region to the southwest of the Aravalli Mountains, were in the form of “plume-related” magmatism of the Malani Group, which comprises bimodal volcanics (dominantly felsic and minor mafic), minor sediments, and peraluminous and peralkaline granitoids. An unconformity indicating a hiatus is noticed at the base of the Malani Group. The final phase of the Neoproterozoic cratonic history is associated with thick platformal deposits of the Marwar Supergroup. The Marwar basins show a clear sedimentological affiliation with the sub-Himalayan basin of “Saline Series” in Pakistan. The beginning of the Neoproterozoic history in the northwestern Indian Shield is correlated with the events related to the possible break up of the Rodinia Supercontinent. Much of the later phases of the Neoproterozoic geological events witnessed in the Indian Shield are traditionally described as the “Pan-African”. However, the geological events recorded in the northwestern part of Indian Shield are neither strictly coeval nor are tectonically correlatable with the ‘orogeny and fabric-forming contemporary events’ of the East African Orogeny (EAO), which is undoubtedly the type terrane of the Pan-African Tectono-thermal Belt. The evolution of the northwestern Indian Shield during the Neoproterozoic does not appear to be related in any way with the Pan-African events observed in EAO. Further, the most talked about ‘Pan-African’ dates at ca. 500±50 Ma, are manifestations of anorogenic thermal event, which possibly marks an aborted attempt to fragment the ‘Greater Gondwana’ during the early Palaeozoic.

The Tectonic Architecture of Central Madagascar: Implication on the Evolution of the East African Orogeny

The Tectonic Architecture of Central Madagascar: Implication on the Evolution of the East African Orogeny

Neoproterozoic extensional detachment in central Madagascar: implications for the collapse of the East African Orogen

A laterally extensive, Neoproterozoic extensional detachment (the Betsileo shear zone) is recognized in central Madagascar separating the Itremo sheet (consisting of Palaeoproterozoic to Mesoproterozoic sediments and underlying basement rocks) from the Antananarivo block (Archaean/Palaeoproterozoic crust re-metamorphosed in the Neoproterozoic). Non-coaxial deformation gradually increases to a maximum at a lithological contrast between the granitoids and gneisses of the footwall and the metasedimentary rocks of the hangingwall. Ultramylonites at this highest-strained zone show mineral-elongation lineations that plunge to the southwest.σ-, δ- and C/S-type fabrics imply top-to-the-southwest extensional shear sense. Contrasting metamorphic grades are found either side of the shear zone. In the north, where this contrast is greatest, amphibolite-grade footwall rocks are juxtaposed with lower-greenschist-grade hangingwall rocks. The metamorphic grade in the hangingwall increases to the south, suggesting that a crustal section is preserved.The Betsileo shear zone facilitated crustal-scale extensional collapse of the East African Orogeny, and thus represents a previously poorly recognized structural phase in the story of Gondwanan amalgamation. Granitic magmatism and granulite/amphibolite-grade metamorphism in the footwall are all associated with formation of the Betsileo shear zone, making recognition of this detachment important in any attempt to understand the tectonic evolution of central Gondwana.

True polar wander, a supercontinental legacy : Evans D.A., Earth and Planetary Science Letters, 1998, 157/1–2 (1–8)

A palaeomagnetic study is reported of reddened facies (Fengtai Formation) of Neoproterozoic glacial rocks which underlie Early Cambrian rocks with disconformity in the Huabei (North China/Sino-Korean) Block. The diamictite (preferred age 620-600 Ma) carries a dual polarity remanence residing in hematite of the red matrix. The mean direction derived from 62 samples is DI = 205.9− 32.4° (α95 = 3.9°) yielding a pole position at 233°E, 62°N. Tests on the matrix deformed beneath dropstones suggest that remanence was fixed before full compaction, although clasts near the base of the formation are largely overprinted. Overlying Early Cambrian sediments of the Houjiashan and Yutaishan formations have a similar remanence also of dual polarity (DI = 205.5− 32.1°, α95 = 3.9°, 32 samples). The magnetisation in the diamictite is therefore interpreted to have been acquired during loading by the ice sheet and/or the overlying Cambrian succession. Red shales from the Liulaobei Formation (∼890-840 Ma) near the base of the Neoproterozoic succession in the Huabei Block yield a contrasting remanence of DI = 59.475.3° (α95 = 7.1°, 19 samples) equivalent to a pole position at 150°E, 43°N. Pole positions from this study accord with Lower-Middle Cambrian poles from the Australian and South China Blocks with North China sited adjacent to northeastern Australia in accordance with recent biogeographic and palaeogeographic models. Although the palaeolatitude derived from the Fengtai diamictite (17°) is Cambrian in age and probably later than the glaciation, the correlation with Australia implies that glaciation in North China took place in low palaeolatitudes (∼20°) and reinforces the view that late Neoproterozoic glaciation, at least at this perimeter of Gondwana, occurred at low latitudes. Neoproterozoic-Cambrian poles from North China, South China and Australia fail to conform to the popular Rodinia reconstruction with the latter blocks adjacent to western North America and also fail to support the view that Gondwana formed by the fusion of widely separated eastern and western segments during Pan African orogeny. Instead they accord to poles from Africa and South America in a precursor of Gondwana (Protogondwana). The subsequent Palaeozoic derivation of this supercontinent was probably achieved by intracontinental strike-slip motion during Pan African deformation following extinction of the Afro-Arabian arc.

Palaeomagnetism of Neoproterozoic glacial rocks of the Huabei Shield: the North China Block in Gondwana

A palaeomagnetic study is reported of reddened facies (Fengtai Formation) of Neoproterozoic glacial rocks which underlie Early Cambrian rocks with disconformity in the Huabei (North China/Sino-Korean) Block. The diamictite (preferred age 620-600 Ma) carries a dual polarity remanence residing in hematite of the red matrix. The mean direction derived from 62 samples is D I = 205.9 − 32.4° ( α 95 = 3.9°) yielding a pole position at 233°E, 62°N. Tests on the matrix deformed beneath dropstones suggest that remanence was fixed before full compaction, although clasts near the base of the formation are largely overprinted. Overlying Early Cambrian sediments of the Houjiashan and Yutaishan formations have a similar remanence also of dual polarity ( D I = 205.5 − 32.1° , α 95 = 3.9°, 32 samples). The magnetisation in the diamictite is therefore interpreted to have been acquired during loading by the ice sheet and/or the overlying Cambrian succession. Red shales from the Liulaobei Formation (∼890-840 Ma) near the base of the Neoproterozoic succession in the Huabei Block yield a contrasting remanence of D I = 59.4 75.3° ( α 95 = 7.1°, 19 samples) equivalent to a pole position at 150°E, 43°N. Pole positions from this study accord with Lower-Middle Cambrian poles from the Australian and South China Blocks with North China sited adjacent to northeastern Australia in accordance with recent biogeographic and palaeogeographic models. Although the palaeolatitude derived from the Fengtai diamictite (17°) is Cambrian in age and probably later than the glaciation, the correlation with Australia implies that glaciation in North China took place in low palaeolatitudes (∼20°) and reinforces the view that late Neoproterozoic glaciation, at least at this perimeter of Gondwana, occurred at low latitudes. Neoproterozoic-Cambrian poles from North China, South China and Australia fail to conform to the popular Rodinia reconstruction with the latter blocks adjacent to western North America and also fail to support the view that Gondwana formed by the fusion of widely separated eastern and western segments during Pan African orogeny. Instead they accord to poles from Africa and South America in a precursor of Gondwana (Protogondwana). The subsequent Palaeozoic derivation of this supercontinent was probably achieved by intracontinental strike-slip motion during Pan African deformation following extinction of the Afro-Arabian arc.

Litholog and structure of the Grenville-aged (≈1.1 Ga) basement of heimefrontfjella (East Antarctica)

The Heimefrontfjella mountains, Western Dronning Maud Land (East Antarctica), are dominantly composed of Grenville-aged (≈ 1.1 Ga) rocks, which were reworked during the Pan -African orogeny at ≈500 Ma. Three discontinuity-bounded Grenville-aged terranes have been recognized namely (from north to south) the Kottas, Sivorg and Vardeklettane terranes. The terranes contain their own characteristic lithological assemblages, although each is made up of an early supracrustal sequence of metavolcanic and/or metasedimentary gneisses, intruded by various (predominantly granitoid) suites. No older basement upon which the protoliths of these older gneisses were deposited has been recognized. In each terrane the older layered gneisses were intruded by various plutonic suites ranging in age from ≈ 1150 to ≈1000 Ma. The Vardeklettane terrane is characterized by abundant charnockites and two-pyroxene granulite facies parageneses in metabasites, whereas the Sivorg and Kottas terranes were metamorphosed to amphibolite facies grade. P-T estimates show that peak metamorphic conditions changed from ≈600°C at 8 kbar in the south, to ≈700 °C at 4 kbar in the northern Sivorg terrane. Regional greenschist retrogression of high-grade assemblages may be of Pan-African age. The Heimefrontfjella terranes were juxtaposed and pervasively deformed during a complex and protracted period of E-W collision orogenesis in a transpressive regime at ≈ 1.1 Ga. This is manifest as early, gently dipping thrust-related shear fabrics (D1), succeeded by the initiation of an important (D2) steep dextral shear zone (Heimefront shear zone, HSZ), during which the early fabrics and structures were steepened and rotated in an anticlockwise sense. The HSZ is a curvilinear structure which changes from a dextral oblique strike-slip lateral ramp in the north to a steep dip-slip frontal ramp in the south, where it forms the boundary between the Sivorg and Vardeklettane terranes. The Pan-African event is manifested as discrete, low- to medium-temperature ductile to brittle shears (D3) and numerous K/Ar cooling ages.

A Pan African origin and uplift for the gneisses and peridotites of Zabargad Island, Red Sea: A Nd, Sr, Pb, and Os isotope study

A Sr, Nd, Pb, and Os isotopic study of peridotites and granulite‐facies gneisses from Zabargad Island in the Red Sea suggests that the tectonothermal, petrogenetic, and geochemical evolution of these rocks occurred largely during the Pan African Orogeny rather than the recent opening of the Red Sea. Sm‐Nd model ages and whole rock errorchrons indicate that spinel Iherzolites and gneisses differentiated from a common depleted mantle source about 700 Ma. The Iherzolites were mylonitized, metasomatized, and amphibolitized during a structural event that juxtaposed the peridotites with the gneiss complex and uplifted the gneiss/peridotite complex to relatively shallow crustal levels. Most radiometric dating schemes suggest a Pan African age for this event. The gneisses generally have lower 143Nd/144Nd, 87Sr/86Sr, 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios than the peridotites. They extend linear trends defined by the spinel and amphibole peridotites on Sr‐Nd, Sm‐Nd, and Pb‐Pb diagrams, suggesting the gneisses were either the source or buffering medium for the Pan African metasomatism. Only one post‐Pan African event had a significant effect on the geochemistry of the gneiss/peridotite complex: shallow level metasomatism by ultrahot (750–900°C) hypersaline solutions with high 87Sr/86Sr (≈0.710) ratio led to the development of gem‐quality olivine crystals as well as low‐pressure mineral assemblages in the peridotites, gneisses and younger rocks. Plagioclase‐rich assemblages with apparent igneous textures (“troctolites”) that are most common in the southern peridotite body may have formed by interaction of these fluids with peridotite (i.e., are “pseudo‐troctolites”). Metasomatism changed the 87Sr/86Sr, Sm/Nd, and Re/Os ratios of the plagioclase peridotites making them unsuitable representatives of the Pan African mantle.

Regional framework, structural and petroleum aspects of rift basins in Niger, Chad and the Central African Republic (C.A.R.)

This paper overviews the regional framework, tectonic, structural and petroleum aspects of rifts in Niger, Chad and the C.A.R. The data base is from mainly proprietary exploration work consisting of some 50,000 kilometres of seismic profiles, 50 exploration wells, one million square kilometres of aeromagnetics coverage and extensive gravity surveys. There have been 13 oil and two oil and gas discoveries. A five phased tectonic history dating from the Pan African orogeny (750−550 MY B.P.) to the present suggests that the Western Central African Rift System (WCAS) with its component West African Rift Subsystem (WAS) and Central African Subsystem (CAS) formed mainly by the mechanical separation of African crustal blocks during the Early Cretaceous. Among the resulting rift basins in Niger, Chad and the C.A.R., seven are in the WAS—Grein, Kafra, Tenere. Tefidet, Termit, Bongor, and N'Dgel Edgi and three, Doba, Doseo, and Salamat are in the CAS. The WAS basins in Niger and Chad are all extensional and contain more than 14,000 m of continental to marine Early Cretaceous to Recent clastic sediments and minor amounts of volcanics. Medium to light oil (20° API–46° API) and gas have been discovered in the Termit basin in reservoir, source and seal beds of Late Cretaceous and Palaeogene age. The most common structural styles are extensional normal fault blocks and transtensional synthetic and antithetic normal fault blocks. The CAS Doba, Doseo and Salamat are extensional to transtensional rift basins containing up to 7500 m of terrestrial mainly Early Cretaceous clastics. Heavy to light oil (15°–39° API) and gas have been discovered in Doba and Doseo basins. Source rocks are Early Cretaceous lacustrine shales, whereas reservoirs and seals are both Early and Late Cretaceous. Dominant structural styles are extensional and transtensional fault blocks, transpressional anticlines and flower structures. The existence of a total rift basin sediment volume of more than one million cubic kilometres with structured reservoir, source and seal rocks favours the generation, migration and entrapment of additional significant volumes of hydrocarbons in many of these basins.

The Northeast Brazilian Rift System

The northeast Brazilian rift basins provide important data critical to the understanding of continental rifting processes associated with the opening of the South Atlantic. These basins represent the locus of intersection of the Southern and Equatorial branches and some basins yield substantial chronostratigraphic data that constrain the temporal and spatial interaction of the rift phases. Similar data are not found in its counterpart in Africa, especially for the Neocomian. These Early Cretaceous rift basins of northeast Brazil illustrate key three‐dimensional geometries of intracontinental rift systems, mainly controlled by the basement structural framework. During the main rift phase (Syn‐rift II, Neocomian‐early Barremian) extensional deformation was distributed over three main rift axes: (1) the Gabon‐Sergipe Alagoas (GSA) trend, (2) the Recôncavo‐Tucano‐Jatobá (RTJ) trend and (3) the Cariri‐Potiguar (CP) trend. During this phase, extensional deformation jumped west from the easternmost basins (GSA trend) to a series of NE trending intracratonic basins (RTJ and CP trends), characterized by a set of asymmetric half grabens separated by basement highs, transfer faults, and/or accommodation zones. These basins are typically a few tens of kilometers wide and trend NE‐SW, roughly perpendicular to the main extension direction during the Neocomian. Preexisting upper crustal weakness zones, like the dominantly NE‐SW trending shear zones of the Brazilian/Pan‐African orogeny, controlled the development of intracrustal listric normal faults. Internal transverse structures such as transfer faults and accommodation zones were also controlled by the local basement structural framework. The megashear zones of Pernambuco (Brazil) and Ngaundere (Africa) seem to have behaved like a huge accommodation zone, accommodating extensional deformation along the RTJ/GSA trends with simultaneous extension along the CP trend. During the late Barremian (Syn‐rift phase III), a significant change in rifting kinematics occurred, when the CP trend was aborted and major rifting initiated at the Equatorial branch. During the Aptian, while the Equatorial branch and Benue trough (Africa) experienced the main rift phase, the RTJ trend was aborted and the GSA trend developed a transitional phase between the rift and drift stage. The GSA trend and the offshore Potiguar basin represent the site of continued evolution into passive margin basins following the main rift episode.

The evolution of the Pan-African granitoid rocks: geochemical evidences from SW Sinai massif, Egypt

Wadi Seih area is covered by “Old Granites”, porphyritic granites, mafic-ultramafic cumulates, rhyolite porphyries, Younger Granites and Post-Granite Dykes which represent ≥200 Ma of Pan African orogeny. G-I “Old Granites” are given mainly by tonalites, monzodiorites and qz-monzodiorites. They are derived from calc-alkaline metaluminous to peraluminous I-type magma and were emplaced under prevailing compressional regime which are associated with subduction and arc granitic rocks formation. Porphyritic granites are given by monzogranites and were emplaced due to compressional forces during the late orogenic stage. “Younger Granites” are distinguished into two types: G-II and G-III granites. They have distinct petrologic, chemical and tectonic setting differences which supporting two independent magma sources. G-II granites are given by monzogranites and syenogranites and were derived from magma rich in Ba and Sr and depleted in Nb, Ce, Zr, Y, Rb and alkalies and were intruded under compressional regime prevailing in the granitic arc environment, at late- to post-orogenic stage. Whereas G-III granites are given by syenogranites and were derived from maga rich in Nb, Ce, Zr, Y, Rb and alkalies and were intruded under extensional forces prevailing within the plate frame which correspond to anorogenic regime. Rhyolite porphyries are enriched in alkalies and are related to the porphyritic and G-II granites. They were extruded under an environment transitional between volcanic arc and within plate regime.

A Pan African age for the HP-HT granulite gneisses of Zabargad island: implications for the early stages of the Red Sea rifting

Up to now the age of granulite gneisses intruded by the Zabargad mantle diapir has been an unsolved problem. These gneisses may represent either a part of the adjacent continental crust primarily differentiated during the Pan African orogeny, or new crust composed of Miocene clastic sediments deposited in a developing rift, crosscut by a diabase dike swarm and gabbroic intrusions, and finally metamorphosed and deformed by the mantle diapir. Previous geochronological results obtained on Zabargad island and Al Lith and Tihama-Asir complexes (Saudi Arabia) suggest an Early Miocene age of emplacement for the Zabargad mantle diapir during the early opening of the Red Sea rift. In contrast, Sm Nd and Rb Sr internal isochrons yield Pan African dates for felsic and basic granulites collected 500–600 m from the contact zone with the peridotites. Devoid of evidence for retrograde metamorphic, minerals from a felsic granulite provide well-defined Rb Sr and Sm Nd dates of 655 ± 8 and 699 ± 34 Ma for the HP-HT metamorphic event (10 kbar, 850°C). The thermal event related to the diapir emplacement is not recorded in the Sm Nd and Rb Sr systems of the studied gneisses; in contrast, the development of a retrograde amphibolite metamorphic paragenesis strongly disturbed the Rb Sr isotopic system of the mafic granulite. The initial 143Nd/ 144Nd ratio of the felsic granulite is higher than the contemporaneous value for CHUR and is in agreement with other Nd isotopic data for samples of upper crust from the Arabian shield. This result suggests that source rocks of the felsic granulite were derived at 1.0 to 1.2 Ga from either an average MORB-type mantle or a local 2.2 Ga LREE-depleted mantle. Zabargad gneisses represent a part of the disrupted lower continental crust of the Pan African Afro-Arabian shield. During early stages of the Red Sea rifting in the Miocene, these Precambrian granulites were intruded and dragged upwards by a rising peridotite diapir.

Late Proterozoic paleomagnetism and tectonic models: a critical appraisal

Interpretations of Proterozoic orogenic belts in terms of plate tectonic processes have been widely divergent. Published models range between the extremes of no relative motions between continental nuclei (implying ensialic orogenic processes) on the one hand, to large-scale relative motions with oceans opening and closing (resulting in continent-continent collisions) on the other hand. Paleomagnetic data can, in theory, contribute significantly to this debate; however, as shown in this paper, several tectonic interpretations on the basis of paleomagnetic data have been premature. A critical continent in many of the previous models is Africa. In order to test hypotheses, for instance, for the late Proterozoic-Cambrian Pan African orogeny, a compilation of paleopoles has been made for Africa, with age ranges falling fully or partially within the interval of 1150 to 500 Ma. A quantitative comparison of the quality of this African dataset with the Phanerozoic poles for North American and Europe shows that the late Proterozoic paleopoles of Africa generally have very low reliability. It appears that the data from other Gondwana continents are equally unreliable and even less abundant. This means that currently the dataset of Gondwanaland cannot be used with confidence for the testing of tectonic models such as the Precambrian supercontinent, at least for the time after 1150 Ma. Well-dated late Proterozoic paleopoles from the three cratonic nuclei within Africa (Congo, Kalahari, West Africa) define relatively short apparent polar wander path segments, but each with different age ranges. This implies that they cannot be compared with each other to test relative motions between the cratonic nuclei and that a choice between ensialic and ensimatic models for the Pan African orogenic belts cannot yet be based on paleomagnetic data. While this does not imply that the tectonic models (e.g. those of Piper and McWilliams) are wrong, it does mean that substantial paleomagnetic support for them will have to wait more and higher-quality paleopole determinations with better dating precision.