9.1 Introduction

The Neoproterozoic-Cambrian period witnessed a significant phase of orogenesis, basement reactivation, and granitization that had a pervasive impact on the African continent. The magmatism and metamorphism associated with the deformation and strike-slip activity during the late Neoproterozoic and Cambrian in Gondwana-Land is widely known as the Pan-African event (Kennedy 1965; Kröner 1980). That term originally referred only to the latest episode of the orogeny (Kennedy 1964; Gass 1977; Kröner and Stern 2004). In Africa, the term Pan-African is widely used to emphasize the non-cratonic regions in the African crystalline basement and designates the mobile belts such as the Mozambique, Lufilian, West Congo belts around the Congo Craton, Damaran, Zambezi, Saldania belts around the Kalahari, and Ahaggar Massif and Mauretanides around the West African Craton (Fig. 9.1). Although this term is invented for the African continent it is now also applied for the Neoproterozoic events during the final assembly of the Gondwana-Land, including Antarctica, South America, and Australia (Kröner 1980; Kröner and Stern 2004).

Fig. 9.1
A map of Africa and Arabia for Pre-Neoproterozoic rock distribution. Most of the northern section is Mesoproterozoic followed by Archean. Most of the southern section is under Archean followed by Palaeoproterozoic.

Pre-Neoproterozoic rock distribution in Africa and Arabia. The dark gray color indicates the entire craton, i.e., the shield(s) plus the surrounding platforms. The light gray color indicates areas listed as cratons in the literature but do not meet the criteria (see Şengör et al. 2022). 1–Ahaggar Massif, 2–Reguibat Shield, 3–Taodenni Basin, 4–Man Shield, 5–Nigerian Massif, 6– Ntem Block, 7–Cuvette Centrale, 8–Angolan Shield, 9–Kalahari Shield, 10–Kaapval Shield, 11–Madagascar, 12–Tanzanian Block, 13–Kasai Block, 14–Mbomou Block, 15–Arabian Shield, 16–Jebel Uweinat, 17–Tibesti Massif

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The West African Craton and the Sub-Saharan Craton (including Congo, Tanzania, Zimbabwe, Kaapval) are the main elements of Africa with their Archaean and Palaeoproterozoic crystalline basements (Fig. 9.1). The vast crustal region, which extends from Ahaggar Mountains in the west to the Arabian Shield in the east, and from Congo Craton in the south to the Mediterranean in the north exhibits geological complexity and has been known with various names in the literature such as, ‘Craton nilotique’ (Rocci 1966) or ‘paléocraton du Sahara central’ (Caby et al. 1985), ‘East Saharan Craton’ (Bowen and Jux 1987; Schandelmeier et al. 1988; Petters 1991; Vail 1991; Stern et al. 1994) ‘Saharan Metacraton’ (Abdelsalam et al. 2002), ‘East African Orogen’ (Stern 1994; Fritz et al. 2013), Saharan Craton (Schmitt et al. 2018). In this paper, we review and discuss the concept of ‘Saharan cratons’ or ‘metacraton’.

The West African craton and Congo craton are remnants of an ancient continental crust that served as a stable foreland during the Neoproterozoic era. The West African Craton hosts the east–west trending Toudani Basin in the middle that is bounded by the Man Shield in the south and the Reguibat Shield in the north (Ennih and Liégeois 2008). The Anti-Atlas Belt forms its northern limit where the Palaeoproterozoic basement crops out. The craton was consolidated during the Mesoproterozoic (1.7–1.0 Ga) and experienced low-grade metamorphism during the Pan-African orogeny (Black and Liégeois 1993). Almost 2 km thick volcanoclastic sediments were deposited on its northern margin (∼0.8 Ga Tizi n-Taghatine Group; Bouougri and Saquaque 2004). The magmatic activity along its margin is marked in Cryogenian by the calc-alkaline intrusive rocks (677–645 Ma) and trondjhemites (∼760 Ma) that were intruded in the Tasriwine ophiolite (Samson et al. 2003). The metasediments and granitoids of the Cryogenian are unconformably overlain by the Ediacaran volcanics and volcanoclastics (Schiavo 2007). The kimberlites are found within the Archaean basement of Reguibat shield containing diamonds (Kahoui et al. 2008).

The Archaean Congo Craton of the Sub-Saharan Craton consists of Archaean formations on its rim, around the Congo Basin and early to mid-Proterozoic fold belts between them with overlying late Proterozoic cover (Goodwin 1991). Within the Congo Craton, Archaean rocks are exposed in three main blocks: the Ntem block to the west, the Mbomou block to the northeast, and the Kasai block to the south (Thiéblemont et al. 2018). The Tanzanian Block is separated from the Kasai block by the Mezoproterozoic Kibaran Belt. The Ntem Complex forms the northwestern margin of the Archaean Congo Craton and consists of gneisses that were intruded by high-K calc-alkalic syntectonic magmatics during the Neoproterozoic in its northern domain (Owona et al. 2011). The southern domain includes strongly deformed metasedimentary rocks of the Yaounde Group that were intruded by diorites (Pb-Pb zircon age of 620 ± 10 Ma, Penaye et al. (1993) as cited in Owona et al. 2011). The Yaounde Group overthrusts to the SW, onto the Archean and Palaeoproterozoic rocks of the Ntem Complex (Mvonda et al. 2007). Paragneisses yielded 613 ± 33, 605 ± 12 Ma (Th–U–Pb dating of metamorphic monazite) Ediacaran ages indicating crustal stacking in a subduction setting associated with the Pan-African Orogeny (Toteu et al. 2006; Owona et al. 2011).

The western margin of the Congo Craton, south of the Ntem block, is characterized by a Palaeoproterozoic basement that is invaded by the Neoproterozoic granites and volcanic equivalents (Thiéblemont et al. 2018). A thick basaltic member which is associated with ophiolites was emplaced during the Tonian (915 ± 8 Ma, Callec and Fullgraf, 2015 as cited in Thiéblemont et al. 2018). The tuffaceous rocks dated at 713 ± 49 Ma (Thiéblemont et al. 2009 as cited in Thiéblemont et al. 2018) indicate volcanism continued in the Cryogenian.

9.2 The Tectonic Units of the Saharides

The Saharides (Suess 1909; Şengör et al. 2020, 2021) form the Pan-African orogen east of the West African craton and north of the sub-Saharan craton. It extends eastward into the Arabian Shield and abuts against the Archaean and Palaeoproterozoic Al-Mahfid rocks (Windley et al. 1996; Whitehouse et al. 1998) and their subsurface equivalents extending from Yemen into Iraq (Johnson and Stewart 1995). Apart from the cratonic regions, Archaean and Palaeoproterozoic rocks are present as separated inliers in the Sahara Desert.

The first step in our approach to the Saharides was to identify the first order tectonic environments) to establish their essential units and to reconstruct the primary orogenic architecture (Şengör and Okuroğulları, 1991; Lom et al. 2018). These are the basic elements of an orogenic setting such as the magmatic arc, the fore-arc and back-arc basins, the fore-arc accretionary complex, the fore- and hinterland fold-and-thrust belts, and molasse basins. They may not develop fully in every orogen, but usually most of them exist in any well-developed orogenic belt. After identifying the first order or essential units we described the secondary or accidental units which occur during the ongoing deformation by dismembering the essential units such as the fore-arc slivers, block uplifts, and subsidences as seen in the US Rockies, for example. Defining both of these units ideally requires well-mapped regions which were available for Saudi Arabia and Egypt, the best-exposed part of the Saharides. The Ahaggar Massif is also similarly accessible, but the geology in the Tibesti Massif and Jebel Uweinat is not known in sufficient detail. Although we had access to the geological maps of Chad, Nigeria, Sudan, and Ethiopia the ages of the magmatics and even the sedimentary rocks are questionable. Figure 9.2 is a tectonic map showing the present distribution of the Pan-African tectonic environments in the Saharides.

Fig. 9.2
A map of Saharides with classification of the main accidental units based on the identification of their tectonic environments. The units are from 1 to 9 starting from the right bottom corner towards the left. Unit 4 has parts a and b. Unit 9 has parts a and b.

Map of the Saharides showing the main accidental units based on the identification of their tectonic environments. Black lines indicate strike-lines of structures that are mapped using Google Earth® checked against the available maps and data in the literature. The map was made on a reconstructed North Africa/Arabia ensemble and GPlates freeware (Boyden et al. 2011) was used for the reconstruction. I. Bayuda-Atbay Unit, II. Mecca-Jizan Unit, III. Nufud al- ‘Urayq unit, IV. Baranis Uweinat Unit, IVa-b. Tibesti Unit, V. Midian-Liya unit, VI. Tadrart Unit, VII. Aïr-Biu Unit, VIII. Pharusian Unit, IX. The Iforas Unit, IXa. The Tassendjanet Unit, IXb. Tilemsi Unit

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The Ahaggar Massif (Hoggar Massif) in Algeria constitutes the largest outcrop in the Sahara with almost 550,000 km2 surface area (Goodwin 1991; Liégeois 2021). The massif is also called the Tuareg Shield because its Precambrian crystalline body rises from under a Phanerozoic cover, mostly flat-lying to gently dipping sedimentary rocks. Liégeois et al. (2003) has grouped the Ahaggar Massif as a part of a metacraton called LATEA (Laouni–Azrou n’Fad–Tefedest–Egéré-Aleksod). According to their interpretation this metacraton represents a Paleoproterozoic granulite–amphibolite facies basement with Archaean relics and has been reactivated during the Pan-African Orogeny. They have identified twenty-three terranes separated by either thrusts or shear zones with a north–south trend accompanied by greenschist to amphibolite Orosirian (~2.05–1.8 Ga) or Ediacaran metamorphism and followed by granitoid intrusions during the Ediacaran. Oldest rocks are dated as Neo-Archaean are only exposed on the margin of West African Craton in the Tilemsi Unit (Fig. 9.2, IXb) (Bosch et al. 2016). The Tilemsi unit represents an accretionary complex that has been invaded by quartz-diorite and orthogneisses of Tonian age and plagiogranite, orthogneiss, granodiorite, and diorite (~726 Ma U–Pb dating on zircon, Caby et al. 1989) of Cryogenian age.

The Tassendjanet Unit (IXa) is mainly composed of a huge monotonous formation of turbiditic greywackes and conglomerates formed by the erosion of calc-alkalic lavas and plutons ranging in composition from basaltic andesites to dacites (Caby et al. 1977; Chikhaoui et al. 1978; Berger et al. 2014) and overlying volcanic and volcanoclastics (<680 Ma, (Caby and Monié, 2003). It is invaded by gabbros and granites of Lower Ediacaran (Hadj-Kaddour et al. 1998). The Iforas Unit (IX) includes the Tassandijanet, Ahnet, Ouzzal, Kidal, Tirek, and Iforas granulitic units of Black et al. (1994). It is composed of Tonian quartz-diorite and orthogneisses and Cryogenian greenschist facies volcano-sedimentary rocks representing an accretionary complex with an arc developing on top of it. Lower Ediacaran granodiorite, alkalic granite, rhyolitic lavas, monzogranite, and tonalites indicate the presence of a magmatic arc. This composite group comprises the late post-collisional alkali-calcic and alkalic plutons, dykes, and volcanic rocks (570–520 Ma) of the Adrar des Iforas, the Tapurirt province in LATEA and in adjacent ‘terranes’ to the west and additional more isolated bodies such as the Tisselliline pluton in NE LATEA or the Tin Bedjane pluton and associated Tin Amali dyke swarm in Eastern Ahaggar (Liégeois et al. 2003; Liégeois 2019). Unit includes eclogite lenses (~623 Ma, Berger et al. 2014).

The Pharusian Unit (VIII) includes the Tin Zaouaten, the In Tedeini, the Iskel, the Laouni, the Tefedest, and the Azrou n’Fad units of Black et al. (1994). The Tonian is represented by the Maru Schist Belt which contains pelitic rocks, mainly as phyllites and slates interlaminated with siltstones. Banded iron formation containing magnetite, hematite, and garnet is also present indicating deep-water deposition. Impure micaceous quartzites occur near the eastern margin of the belt. Mafic rocks represented by amphibolites are present in several localities. Tonian magmatism is recorded by granites and quartz-diorites (870–830 Ma, Caby et al. 1982). The whole thing is intruded by granites, granodiorites, and syenites. Cryogenian greenschist facies volcano-sedimentary rocks are found with Cryogenian quartz–syenite. The Upper Ediacaran Anka Schist Belt represents an accretionary complex with metaconglomerates, sandstones, slates, phyllites, and felsic volcanic rocks. The conglomerates contain pebbles of granite, quartzite, phyllite, and volcanic rocks. In the western part of the belt phyllites dominate with some metasiltstones and metasandstones with rhyolitic to dacitic volcanic rocks. The metamorphic mafic and ultramafic rocks are considered older than 1 Ga which is entirely probable representing ocean floor (see Ogezi 1977). The associated amphibolites are tholeiitic in composition. The unmetamorphosed volcanic and sedimentary rocks are dated at 516 ± 20 Ma. There are eclogite lenses of Lower Ediacaran age (Berger et al. 2014). This unit continues in Nigeria, south of the Ahaggar Massif and mapped as ‘Older Granites’. They range widely in age and composition. But many geochronological constraints suggest early Ediacaran for the emplacement of the Older Granites. Their composition ranges from tonalites and diorites through granodiorites to true granites and syenites (Obaje 2009).

The Aïr-Biu Unit (VII) covers the Egéré-Aleksod, Sérouénout, the Tazat, the Tchilit, the Assodé-Issalane, the Barghot, and the Aouzegueur units of Black et al. (1994). It is represented by Cryogenian shelf and volcano-sedimentary rocks metamorphosed in greenschist grade, implying an accretionary complex setting. The unit is intruded by tonalite-trondhjemite-granodiorites (730 Ma), high-calc-alkalic granitoids (715–665 Ma), alkalic-gneiss and potassic granite (670 Ma) high-K calc-alkalic granitoids (645–580 Ma) indicating a continuous magmatism during the Cryogenian and the Ediacaran (Black et al. 1994; Liégeois et al. 1994).

The Tadrart Unit (VI) corresponds to the Edembo and the Djanet units of Black et al. (1994). In the Djanet ‘Terrane’ it is in greenschists-facies sedimentary sequence (the Djanet Group), whereas it is made of amphibolite facies in Edembo ‘Terrane’ (Fezaa et al. 2010). In Edembo, we observe lithologies including biotite mica schists, metagreywacke with pebbles, phlogopite marble, hornblende metabasalt, and migmatitic gneiss; garnet is abundant in many lithologies (Fezaa et al. 2010). The Djanet Group is mainly a clastic series, comprising slates, quartzites, and conglomerates interleaved with decimeter to meter thick sills of pyroxene-amphibole andesite and minor amounts of more acid compositions, up to rhyolite (Fezaa et al. 2010). These sills are often boudinaged within the sedimentary sequence. Zircon, Pb ages suggest ~ 590 Ma for the deposition and metamorphism of the Djanet Group (Fezaa et al. 2010).

The Tibesti Massif (IVa) is separated from the Ahaggar Massif by the Murzuq Basin and it is composed of two metamorphosed supracrustal units. The Tonian (?) Lower Tibestian Metamorphic Series is exposed along the eastern part of the Tibesti Massif. It contains highly metamorphosed sedimentary rocks intercalated with basic volcanics in upper greenschist to lower amphibolite facies (El Makkrouf 1988). Major rock types include gneisses, amphibolites, mica schists, staurolite-garnet-mica schists, and hornblende-mica schists, graphite schists, and calc-silicates and marbles. Serpentinites occur partly as small pods as veins within epidote boudins in metavolcanics, which may represent oceanic crust (El Makkrouf 1988). Carbonate rocks occur to the west and are interbanded with mica schist and pyroclastics. Metaconglomerates at the top of the Lower Tibestian Series contain cobbles of granite, amphibolite, quartzite, gneiss, and rhyolite (El Makkrouf 1988). The Upper Tibestian Series consists of low greenschist facies siliceous schists and quartzites with conglomerates, interbedded greywackes and arkoses, and interbeds of rhyolitic lavas with schists. The Upper Tibestian Series extends west across the entire Tibesti Massif to the Bin Ghanimah batholith (549 ± 22 Ma, 586 ± 24 Ma K–Ar ages from biotites Ghuma 1975; El Makkrouf and Fullagar 2000).

The Baranis Uweinat (IV) is located at the intersection of Libya–Egypt–Sudan and the Precambrian rocks crop out from Jebel Uweinat in the west to the Jebel Kamil in the east. The massif contains a deformed suite of tonalite–trondhjemite–granite and gabbro-diorite gneisses yielding zircon ages between 3.1 Ga and 3.3 Ga (Bea et al. 2011). The gneisses are intruded by Ediacaran granites and diorites (607–580 Ma, U–Pb zircon ages Zhang et al. 2019). The Tonian is represented by accretionary complexes and magmatics, volcanic rocks invading them (Bea et al. 2009). The oldest Tonian (?) Nafirdeib and Odi series are made up of sedimentary volcanic complex (Yassin et al. 1984). The metasedimentary volcanic rocks are of the greenschist facies with basic and ultrabasic intrusions, and calc-alkali batholith granitoids. In south Sinai there is a Metagabbro-Metadiorite Complex, K–Ar age dating of the hornblendes and biotites yield 794 ± 30–667 ± 25 Ma age (Abdel-Karim 1995). The Tonian is described with increasing magmatic activity, the arc-related magmatism is either found as clusters of granitoid intrusions, including, monzogranite, syenogranite, granodiorite, and subordinate tonalite and alkali-feldspar granite or as small intrusions within the accretionary complexes.

The geology of the Arabian-Nubian Shield is divided into five groups in this study according to their similarities in their lithologies, ages, and the tectonic environments which they represent. They are separated by major strike-slip boundaries (Fig. 9.3).

Fig. 9.3
A map of generalized accidental units with magmatic arc activity and magnetic lineations. The units are from 1 to 9. Unit 1 and 2 are mostly Tonian, unit 3, 4, and 5 are Cryogenian, sixth is Ediacaran, and 7, 8, and 9 are pre-Saharides.

The map showing generalized accidental units with magmatic arc activity and magnetic lineations (red lines). Small, colored circles indicate the localities and the age distribution where isotopic ages reported in detail in Şengör et al. (2020). High-quality isotopic ages and subsurface data are used to identify magmatic fronts and their younging directions to correlate accidental units. Black dashed lines represent plate boundaries

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The Bayuda-Atbay Unit (I) is composed predominantly of Tonian accretionary complex that is invaded by Neo-Proterozoic magmatic arcs. The Tonian units showing accretionary complex character are made of series mainly of metasediments such as quartzites, quartzitic schists, paragneisses, mics schists, graphite schists, calc-mica schists, calc-silicate rocks, and marbles with intrusions of mafic to felsic metavolcanic rocks (the Absol Series, the Nafirdeib and Odi series, the Kurmut Series, the Rahaba Series, the Tsaliet Group, the Bidah Belt, the Al Lith, the Hali Group, the Zibarah Group, the Samran Group, Johnson 2006) (Yassin et al. 1984; Reischmann et al. 1992; Küster et al. 2008; Evuk et al. 2014). The Didikama Formation and the Tambien Group represent fore-arc or intra-arc basin setting and include slates, chlorite and graphite phyllite, limestone, dolomite (Swanson-Hysell et al. 2015). Arc magmatism is recorded from Tonian to Ediacaran by the mafic, intermediate, and felsic plutonic rocks of calc-alkalic (Tonian: the Kamil suite, the Qiya Complex, the Hufayriyah Complex, the Buwwah suite, the At Ta'if Group; Cryogenian: the Homogar Series, the Nu'man Complex, the Samd tonalite; Lower Ediacaran: the Al Hawiyah granite suite, see Johnson 2006 for detailed description of the units).

Within the Mecca-Jizan Unit (II) we identified four different tectonic settings based on the lithologies and their evolution through time (Şengör et al. 2020, 2021). The Tonian fore-arc or intra-arc basins are represented by the Matheos and the Didikama formations which are made up of slates and limestone with stromatolite dolomitic limestone, dolomites (MacLennan et al. 2018). The magmatic activity was in progress from the Tonian to the late Ediacaran and is widespread in the Mecca-Jizan Unit. The Tonian magmatism and volcanism is recorded by the An Nimas Complex, the Hufayriyah Complex, the Ram Ram Complex, the Bari Bari granodiorite, the Shwas Belt, the Buwwah suite, the Sarjuj Formation. The Nabitah gneiss, Ruwayhah, Ibn Hasbal, Kawr suites, the Alawah tonalite, the Hishash granite represent the Cryogenian magmatic activity. Ediacaran magmatics, volcanics, and volcanoclastics are found in the Thurayban granodiorite, the Abbasiyah granodiorite, the Khaniq Formation, the Hadb ash Sharar suite, and the Shayma Nasir Group (see Johnson 2006 for detailed description of the units).

The accretionary complexes of Tonian age that are invaded by arc magmatics or volcanics are represented by granodiorites, granites, tonalites, gabbro and diorites, basaltic and andesitic flows (the Tsaliet Group, the An Nimas Complex, the Hufayriyah Complex, the Ram Ram Complex, the Bari granodiorite, the Malahah Belt, the Tayyah Belt, the Khadra Belt, the Arj and Mahd Groups, and the Samran Group: Johnson 2006). The Tonian Sumayir Formation shows ophiolitic character with its mafic volcanic and fine-grained sedimentary rocks intercalated with the Bi’r Umq mafic–ultramafic complex in the northeastern part of the Jiddah terrane (Johnson 2006). 40 km west-northwest of Al Muwayh interbedded basaltic, andesitic, dacitic, and rhyolitic flows and tuffs, volcaniclastic conglomerate, sandstone, quartzite, calc-silicate rock, and ironstone metamorphosed to the greenschist facies are found and is interpreted as a magmatic arc evolving on a mélange belt (Johnson 2006).

The Nufud al- ‘Urayq unit (III) makes up with the northeastern part of the Arabian Shield. It is dominated by the Cryogenian rocks. A continuous magmatic activity from the Tonian to the Ediacaran is also present in this unit and represented by granites, granodiorites, monzogranites, syenogranite, diorite, tonalite, rhyolitic to rhyodacitic ash-flow tuffs (Şengör et al. 2020, 2021) (Tonian magmatism: the Hulayfah intrusives, the Al-Mukalla island arc, the Sarjuj Formation, the Fuwayliq granodiorite, the Tamran Formation, the Silham Group, the Jidh Suite, the Isamah formation, the Dhukhr Complex, the Banana and Sufran Formations; Cryogenian magmatism: the Hamls suite, the Ar Rika Formation, the Hadhaq Complex, the Al Amar intrusives, the Rubayq Complex, the Khishaybi Suite, the Laban and Kilab complexes; Ediacaran magmatism: the Shammar Group, the Abbasiyah granodiorite, the Al Khushaymiyah suite, the Ar Ruwaydah suite, the Gharamil monzogranite, the Badwah granite, the Shammar intrusives, the Jurdawiyah Group, the Idah Suite, the Hadn Formation, the Bani Ghayy Group, the Najirah granite, the Malik granite, the Jibalah Group, the Abanat suite, for detailed descriptions, see Johnson 2006). Tonian accretionary complexes are identified in a few locations. They are usually formed of metavolcanics ranging from basalts to dacites and rhyolites with predominance of andesites, and associated with greywackes, agglomerates, and slates (the Tsaliet Group, the Tays Formation and Kabid Paragneiss, the Dhiran, Nafi, and Hillit formations, Ajal Group, and Dukhnah gneiss, the Mughah Complex, see Johnson 2006 for detailed description of the units).

The Midian-Liya Unit (V) is mostly composed of Tonian and Cryogenian accretionary complexes (the Silasia Formation, the Hegaf Formation) and fore-arc basins (Zaytah Formation) that are invaded by Tonian to Upper Ediacaran magmatic and volcanic rocks (the Najah granodiorite, the Ghawjah Formation, the Al Bad granite suite, the Bayda Group, the Muwaylih Suite, the Qazaz granite super suite, the Katherine Ring Complex, the Dokhan volcanics, the Jibalah Group; Holail and Moghazi 1998; Noweir et al. 1990; Eliwa et al. 2006; Johnson 2006; Moussa et al. 2008; Azer et al. 2014). The accretionary complexes contain rocks that are folded and faulted, locally strongly, and are metamorphosed to the greenschist, and locally higher, facies represented by amphibolite, mafic schist, quartz-feldspathic mica schist, and calc-silicate rock. They usually have a basal section of oceanic crust including ultramafics, gabbros, and pillowed basalts. Fore-arc basins are filled with conglomerate and sedimentary breccia, immature sandstone, greywacke, siltstone, shale, and minor amounts of quartzite.

9.3 Discussion

The evolution of the Northeast Africa and the Arabian Peninsula is usually explained by amalgamation of multiple cratons and accretion of crustal material during the Pan-African Orogeny (Black and Liégeois 1993). However, cratons and island arc terranes involved in intercontinental collisions to form the ‘ghost craton’ (Black and Liégeois 1993) or ‘metacraton’ (Abdelsalam et al. 2002) are impossibly smaller compared to their present-day representatives (Şengör et al. 2020, 2021).

According to Liégeois et al. (2013) cratons can be reactivated due to the orogenic processes around their margins and this reactivation can damage the lithospheric rigidity which eventually leads to transformation of cratonic margins into orogenic belts with formation of intracontinental orogenic belts. According to Abdelsalam et al. (2002) these damaged cratons can be recognized through their ‘rheological, geochronological and isotopic characteristics’.

As it is first introduced, the term ‘kratogen’ (Kober 1921) or its present-day anglicized equivalent ‘craton’ (Stille 1936; Kay 1947) drives from two ancient Greek words: κράτος (kratos) meaning might, strength and γένεσις (genesis) meaning origin, creation. So that the term cratogen means ‘born strong’. Thus, the definition of a craton is solely based on its strength and resistance to the deformation which may occur along its periphery. Neither its age nor its isotopic characteristic is considered integral components of the craton’s definition (see Şengör et al. 2020).

The metacraton hypothesis requires a craton to be a part of the subducted lower plate (Liégeois et al. 2013) which is highly unlikely considering its neutral or even positive buoyancy (Jordan 1975, 1978). The magmatism is expected to be confined to post-collisional episode although throughout the presumed Saharan ‘metacraton’, subduction-related magmatism seems to have persisted throughout the entire evolution of the Saharides, spanning from approximately 900–500 Ma (Figs. 9.2 and 9.3; Şengör et al. 2020, 2021). This observation challenges the fundamental notion of a craton, or even a metacraton.

Moreover, the two prominent African Archaean cratons in the vicinity of the Sahara, namely the Congo and the West African cratons, have experienced very limited internal deformation despite their involvement in significant orogenic events, such as the Pan-African orogeny (Stern, 2004). It has been suggested that mobile belts can form a shield around the cratons which may protect them from the plate-tectonic processes and therefore it has a potential to significantly enhance its stability (Lenardic et al. 2000, 2003; Yoshida 2012).

Seismic tomography images by McKenzie (2020) and Celli et al. (2020) indicating the unusual thinness of the lithosphere under the Saharide realm contradict the interpretation of Sobh et al. (2020) claiming lithosphere thickness reaching 200 km beneath the Al Khufra area, southeast Libya. Liégeois (2019) assumes that a proper Saharan Craton existed until about 580 Ma ago and the extensive occurrence of high-K calc-alkaline granitoids and migmatization around 600 million years ago may have been a result of mantle delamination or convective removal during the Neoproterozoic era (Abdelsalam et al. 2011). However, cratons do not spontaneously delaminate neither by convection nor by any other processes as we know from the other major cratons of the world. Only mantle plumes can destroy a craton in a time scale of 400–500 million years if the craton is to be destroyed by thermal processes or in a few million years if it is to be destroyed mechanically as shown in Şengör (2001).

Abdelsalam et al. (2011) interpreted differential S-wave velocity anomalies in the upper 100 km as an indicator of a cratonic lithosphere. However, S-waves are not characteristics of cratons and the primary determinant of shear wave velocities in the mantle is temperature, rather than composition (McKenzie 2020). The real indicator of the lithosphere is accepted to be the surface waves as shown by Priestly et al. (2018).

Figure 9.2 is a tectonic map showing the distribution of the Pan-African tectonic environments in a reconstructed map of Africa generated by the GPlates software. For this map, we used Torsvik and Cocks (2017) as proxy to the Ediacaran reconstruction of Africa. This way we could eliminate the recent extension in the Red Sea and the internal deformation in Phanerozoic in North Africa and Arabia. The map includes data from the trend lines which enables one to follow the average tectonic trend in a region and is highly useful where the other markers are missing or covered. These data including strike of bedding planes, fold axial planes, or fault planes, trend of morphological units or intrusions, has been collected from the geological and structural maps. Where the outcrop is not available the additional insight has been gained by the magnetic lineations. We gathered the available magnetic anomaly maps from the Saharides and simplified to use them as trend lines where the outcrops are not visible. The magnetic anomaly maps are best to use in regions where accretionary complexes and magmatic arcs are present due to their iron-rich mineral composition. Figure 9.3 shows the magnetic lineations deduced from the magnetic anomaly maps and the locations of the ages we cited in Şengör et al. (2020, electronic supplement) on a reconstructed map of Africa. The dataset shows the age distribution of arc-related igneous rocks especially from Tonian to Lower Devonian for the Saharides. We compiled 936 high-quality isotopic ages from the literature. The fundamental assumption is the continuity of the subduction zones both in time and space, therefore the continuity of the most prominent feature, the magmatic arc. To identify the magmatic arcs, the intermediate and felsic magmatic rocks, namely granodiorites, diorites, andesites, granites, and rhyolites are used. When we see a train of calc-alkalic mafic to felsic intrusives and volcanics we provisionally label them as products of a magmatic arc. For this purpose, geological maps of Algeria, Libya, Egypt, Sudan, Mali, Nigeria, Chad, Sudan, Eritrea, Ethiopia, Somalia, Kenya, Yemen, Saudi Arabia were digitized and mapped to identify magmatic arcs. All our accidental units, seem to have been parts of one magmatic arc here named the Tuareg Magmatic Arc (Şengör et al. 2020, 2021).

Figure 9.4 shows the temporal and spatial evolution of the Saharides. All the units discussed above constitute what Şengör et al. (2020, 2021) called the Tuareg arc that was originally a continental margin arc hugging the Congo Craton. During the Tonian this arc began separating from the Congo Craton. This separation was mainly by strike-slip faulting in the east of the craton (present geographic orientation) and by rifting in its north opening what is called the Tshiluba back-arc basin. In the Cryogenian, the northeastern end (present geographic orientation) of the Tuareg arc collided with the West African Craton causing the onset of the Pharusian orogeny.

Fig. 9.4
3 maps of the Congo Craton with Saharides during the Neoproterozoic. A is Tonian with distance between Tuareg magmatic arc and W A C. B is Cryogenian with Tuareg Magmatic arc touching the W A C. C is for Ediacaran with the arc collapsing into W A C.

Three time-lapse frames showing the palaeotectonic situation and evolution of the Saharides during the Neoproterozoic. The Roman numerals correspond to the accidental unit numbers. a The reconstruction of the Saharides during the Tonian. b The Saharides during the Cryogenian. c The Saharides during the Ediacaran. The reconstruction of cratons is adopted from Li et al. (2008)

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The continuing approach of the Congo and the West African cratons led to the oroclinal bending of the Tuareg arc creating the subsurface geology of the central and eastern Sahara. There is no indication whatever of the presence of a craton in the central Sahara now or in the past. The lithospheric thickness now underlying the Saharides is uniformly less than that of a mature oceanic floor based on the seismic surface wave observations (Priestley et al. in press). Throughout the evolution of the Saharides, subduction magmatism and intense deformation characterized their geological development. Apart from the Pharusian collision and the tightening of the central Saharan orocline there were no continental collisions in North Africa during the Pan-African times. The same was true for the Arabian shield, except that the Al-Mahfid continental sliver slid into place to delimit the Arabian magmatic arc activity. When the geology of Iran is considered, it seems very likely that the Al-Mahfid sliver was a thin piece of continent inserted into the Saharide collage and east of it Saharide-type evolution continued into the Cambrian.

When the Saharide evolution was completed, a magmatic arc with a subduction zone dipping under Gondwana-Land came into being. Lom et al. (2017) called this arc Protogonos, meaning ‘the first born’. The evolution of this arc controlled the geological development of Europe until the Hercynian collisions, but it continued its activity until the Jurassic in Asia, until the closure of the Palaeo-Tethys.

Even in Phanerozoic orogenic belts, of Palaeozoic and early Mesozoic age direction of convergence is extremely difficult to establish especially if they are disentangled in the Pan-African system. The Damaran Belt as a cross section is very much like that of the Alps. This indicates a possible head on convergence and this is consistent with the orogenic belts east of the Congo craton that thus must have a strong left lateral component (Martin and Porada 1977a, b). This is consistent with our model. In the Saharides the convergence direction variable because of the double orocline but the last convergence across the Saharides was east–west which tightened the orocline (Fig. 9.5).

Fig. 9.5
2 maps. A is for previous interpretation of the North Africa and the Arabian Peninsula. Saharan Craton is above Congo Craton in the north with Mozambique belt under areas affected by Pan-African Alpinotype orogeny. B is for interpretation of the Saharides in the northern section and Mozambique belt.

Map summarizing the conventional interpretation of the Pan-African areas in North Africa and Arabia. a the previous interpretation of the North Africa and the Arabian Peninsula (Black and Liégeois 1993; Abdelsalam et al. 2002) and b The interpretation of the Saharides as part of the Pan-African realm of deformation as presented in this paper (see also Şengör et al. 2020, 2021). The yellow dashed lines correspond to the trend lines of the orogen

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Figure 9.5 summarizes the main conclusions of this study and brings into focus the difference of our interpretation from most of the earlier ones. In a nutshell, our study has shown the presence of a major orogenic system under the Sahara and the Arabian shield characterized by two oroclines in its trend lines during the Neoproterozoic to the earliest Cambrian times. There is no evidence of a craton of any sort, meta- or pristine-, under the central and eastern Sahara as maintained by Abdelsalam et al. (2002) and Liégeois et al. (2013).

9.4 Conclusion

Where the geological record has not been completely erased, the kinematic reconstructions are relatively easy because it is possible to get information from GPS, marine magnetic anomalies, by using the fracture zone orientations reconstruction of major plates can be accomplished with a satisfying accuracy. These methods lose resolution with increasing time and the oceanic record is not available prior to the Jurassic. The Precambrian era is extra challenging where the biostratigraphy is not applicable and therefore the ability to correlate different layered rocks in different areas. This is a vital obstacle for ancient orogenic belts because most of them have experienced multiple phases of deformation, erosion throughout their lifespan that destroy their geological record, or they are mostly covered by younger strata. By using reliable geological field data, high-quality isotopic ages, and subsurface geophyscial data as supplementary data, it is possible to reconstruct complex ancient orogenic collages and trace their evolution. We describe a major Turkic-type orogenic complex, the Saharides, as a part of the larger Pan-African orogeny. The Saharides has formed by subduction and strike-slip stacking of arc material mainly during the Neoproterozoic by pre-collisional coast-wise transport of arc fragments along the southern part of Congo craton.