Relationships between structural lineaments and Cenozoic volcanism, Tibesti swell, Saharan metacraton

Accepted Manuscript Relationships between structural lineaments and Cenozoic volcanism, Tibesti swell, Saharan metacraton Collin Nkono, Jean-Paul Liégeois, Daniel Demaiffe PII:

S1464-343X(18)30153-5

DOI:

10.1016/j.jafrearsci.2018.05.022

Reference:

AES 3225

To appear in:

Journal of African Earth Sciences

Received Date: 20 February 2018 Revised Date:

29 May 2018

Accepted Date: 30 May 2018

Please cite this article as: Nkono, C., Liégeois, J.-P., Demaiffe, D., Relationships between structural lineaments and Cenozoic volcanism, Tibesti swell, Saharan metacraton, Journal of African Earth Sciences (2018), doi: 10.1016/j.jafrearsci.2018.05.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Relationships between structural lineaments and Cenozoic

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volcanism, Tibesti swell, Saharan metacraton.

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Collin Nkono1, 2, Jean-Paul Liégeois3*, Daniel Demaiffe1

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1. Laboratoire de Géochimie, DGSE (CP 160/02), Université Libre de Bruxelles (ULB) 50, Avenue

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Roosevelt, B-1050 Bruxelles, Belgique.

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2. Present address: Intercommunale du Brabant Wallon, 10 Rue de la Religion 1400 Nivelles,

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Belgique.

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Africa, B-3080 Tervuren, Belgium

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3. Geodynamics and Mineral Resources, Earth Sciences Department, Royal Museum for Central

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(*) corresponding author: [email protected].

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Abstract. This work reports an analysis of the relationships existing between the structural

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lineaments and the Cenozoic volcanism of the Tibesti area (northern Chad). Shield volcanoes,

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cinder cones, structural lineaments, intersection points of lineaments and faults are mapped

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using the combination of Shuttle Radar Topography Mission (SRTM), Digital Elevation

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Models (DEMs) and Landsat satellite images of the Tibesti Volcanic Province.

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The interpretation of the distribution of these structural and morphological features

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allows constraining the structural/tectonic setting of the Tibesti. We show that the

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relationships between the lineaments and the volcanic centres of the Tibesti province can

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locally be explained as the result of the combination of two Riedel dextral tectonic systems,

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respectively oriented at N120°E and N30-35°E. Taking into account the geological features of

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the area, a geodynamical model is proposed: the emplacement of the Tibesti Volcanic

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Province results from the reactivation of inherited structures of the Saharan metacraton,

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characterized by relict rigid cratonic nuclei and metacratonic areas reworked during the Pan-

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African orogeny, among which is located the Tibesti. The contrasted behaviour of these

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rheologically different zones can explain the location and the evolution of the Tibesti swell

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and volcanism. The new data presented in this paper and their interpretation in terms of the

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emplacement of the Tibesti volcanic province in the Saharan metacraton bring a new and

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major information about the behaviour of the African plate within its collisional context with

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Europe.

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Key words: Cenozoic volcanism; lineaments; metacraton; Tibesti; Africa-Europe

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convergence; SRTM DEM images

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1. Introduction

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The Cenozoic Tibesti Volcanic Province (TVP), located in northern Chad (Sahara, Africa;

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Fig. 1), extends over a surface area of c. 30,000 km² and comprises numerous cinder cones

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and several shield volcanoes, some of which higher than 3000 m, as the huge (60 x 80 km)

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Emi Koussi (3415 m) (Fig. 2, 3). These volcanoes are located on the Tibesti swell basement

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made of Precambrian and Paleozoic rocks that often reach an altitude of 1500 m. This

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geological setting is similar to that observed 1000 km to the northwest in the Tuareg Shield in

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Algeria (Rougier et al., 2013; Fig. 1, 2). This raises the question of the origin of this

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swell/volcanism association and two contrasting models have been proposed. 1. The

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association is a manifestation of a deep mantle plume (Burke & Wilson, 1976; Sicilia et al.,

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2008), possibly linked to the postulated Afar Plume (Sebai et al., 2006), although the typical

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geological features of plumes are poorly represented (Zhao, 2007). 2. Alternatively, the swell

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and volcanism are the result of the reactivation of lithospheric structures due to the intraplate

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stress generated by the Africa-Europe collision (Liégeois et al., 2005; Pik et al., 2006;

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Radivojevic et al 2015). The TVP has been poorly studied geologically because of the

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remoteness and lengthy, ongoing political instability of the area. A recent review of available

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fieldwork observations has nevertheless been published by Deniel et al (2015 and references

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therein).

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Establishing the possible genetic links between the volcanic edifices and the regional

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structural trends within the Saharan metacraton structure (which comprises both more rigid

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cratonic nuclei and more mobile metacratonic regions; Liégeois et al., 2013) would be a new

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and solid contribution to this debate. Indeed, a metacraton has been defined as “a craton that

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has been remobilized during an orogenic event but is still recognizable dominantly through its

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rheological, geochronological and isotopic characteristics” (Abdelsalam et al., 2002).

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Metacratonization can occur at the margins of cratons when the latter are subducted during a

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continental collision and can attain the craton interior when the metacratonization process is

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very intense (Liégeois et al., 2013). This was the case of the Saharan metacraton at the end of

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the Pan-African orogeny, during the second half of the Ediacaran period (Fezaa et al., 2010).

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Such a pervasive metacratonization occurs through the reactivation of pre-existing zones of

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weakness and is heterogeneous, leaving unaffected cratonic nucleus among reactivated

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metacratonic areas. This is the case of the Murzuq, Al Kufrah and Chad cratons included

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within the Saharan metacraton (Liégeois et al., 2013) and located below the large subcircular

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basins with which they share the name (Fig. 1). Subsequent intracontinental reactivations of

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metacratonic area due to stress applied at plate margins can produce uplift or doming that can

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be accompanied by asthenospheric volcanism (Liégeois et al., 2005; 2013).

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Actually, in such a vegetation-poor desert environment, the Cenozoic TVP volcanic features

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(large shield volcanoes and smaller cinder cones) and structural lineaments can be mapped

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with high confidence combining Shuttle Radar Topography Mission (SRTM) Digital

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Elevation Models (DEM) and Landsat satellite images. The close spatial association of these

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features (faults, lineaments, volcanic centres) and their statistical distributions can be

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modelled in terms of regional deformation patterns. Indeed, the linear distribution of

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volcanoes related to tension fractures can be used as indicators of the tectonic stress regime,

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which is a key problem in the study of large volcanic provinces. These methods, shortly

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described below, have been successfully applied to the distribution of volcanic centres and

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lineaments of the huge Cameroon Volcanic Line province of Central Africa (Nkono, 2008;

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Nkono et al, 2009).

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2. Location and geological setting The Tibesti Volcanic Province (TVP) extends from approximately 19 to 23° N latitude

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and 16 to 19° E longitude in the northern part of the Republic of Chad, in the Borkou–

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Ennedi–Tibesti province, in the Sahara desert (Fig. 1, 2). The Tibesti volcanism has been

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sporadically studied mostly in the 20th century (Tilho, 1920; Gèze et al., 1959; Grove, 1960;

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Vincent, 1970; Malin, 1977), with very few recent publications: Gourgaud and Vincent

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(2004) studied the petrology of the Emi Koussi volcanic series (Fig. 4) while Permenter and

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Oppenheimer (2007) interpreted remote sensing data sourced from the ASTER

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instrument/mission. A thorough geological review of all the available data has been provided

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recently by Deniel et al (2015).

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The TVP is located in the centre of the Saharan metacraton (Abdelsalam et al., 2002),

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between the two large subcircular cratonic sedimentary Murzuq and Al Kufrah basins (e.g.

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Ghienne et al., 2013; Le Heron and Howard, 2012) that began to subside during the Cambrian

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(Fig. 1). The TVP itself overlies a late Neoproterozoic metamorphic basement intruded by

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granitoids and covered by Paleozoic (Cambrian to Devonian) sandstones, limited

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Carboniferous limestones and marls and Cretaceous sediments (Nubian sandstones) (Fig. 1,

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2). The Neoproterozoic basement comprises two parts: 1) to the west, low- to upper-

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greenschist facies sandstones, shales and carbonates intruded by c. 550 Ma granitoids and 2)

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to the east, metasedimentary and metavolcanic rocks mostly in upper greenschist to

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almandine-amphibolite facies, also intruded by granitoids of unknown age (Pegram et al.,

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1976; Suayah et al., 2006). These two parts, initially considered to be separated by an

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unconformity (Suayah et al., 2006), were referred to as Upper Tibestian (to the west) and

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Lower Tibestian (to the east). The boundary between the two units is now recognized to be a

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major fault (Yébbigué Fault; Fig. 1, 2) reactivated several times during the Cenozoic. The

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Cenozoic Tibesti volcanic province occurs astride this fault (Deniel et al., 2015). These two

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units could be the equivalents of the Djanet and Edembo terranes in Algeria, which have the

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same contrasted metamorphic signature and also comprise c. 570-540 Ma magmatic rocks

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(Fezaa et al., 2010). In that case, the Yébbigué fault could correspond to a terrane boundary.

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More to the west, an elongated basement elevation determine the Tibesti-Sirt high (Woller

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and Fediuk, 1980), which is parallel to the Yébbigué fault (Fig. 1). Between these two

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structures, a parallel fault, here named the Tibesti-Nuqay fault, is of major importance and has

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been reactivated several times, allowing the preservation of elongated NE-SW Cretaceous

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basins in the Tibesti, determined uplift of Paleozoic series to the SW and bounds to the west

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the Nuqay volcanic field (Fig. 1). The boundary of the Tibestian 1 and 2 curving to the SE in

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southern Tibesti (Fig. 1), we privilege the Tibesti-Nuqay fault over the Yébbiggé fault as the

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main NE-SW fault but it is likely that they converge at depth. It constitutes also likely the

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border fault of the Tibesti-Sirt high. Whatever, there is a strong NE-SW trend that affects the

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Tibesti region.

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To the south-west, the Tibesti Precambrian basement is bounded by a linear NW-SE

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oriented cliff of Paleozoic sediments corresponding to the great flexure of Tibesti and named

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Abo trend (Deniel et al., 2015; Fig. 1). The Abo trend is parallel with the Djanet and Edembo

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terranes in eastern Hoggar formed during the late Ediacaran Murzukian orogenic episode

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(Fezaa et al., 2010) and with the buried Grein and Ténéré Cretaceous grabens (Genik, 1992),

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which belong to the West and Central rift system of Northern Africa (Ye et al., 2017) (Fig. 1).

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There is thus also an important NW-SE trend that affects the Tibesti region, meaning that the

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Tibesti volcanic field is located at the intersection of two conjugated main reactivation trends

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(Deniel et al., 2015).

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Emplacement age of the Tibesti volcanism extends from the Miocene (minimum age

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of 17 Ma; Deniel et al., 2015) to the Present. In nearby Libya, the earliest basalts overlie

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Lutetian (48 – 41 Ma) sediments (Lelubre, 1946), which gives a maximum age for that

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volcanic activity. Minor present-day activity has been reported (Soborom solfatara field,

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Tarso Voon and Ehi Toussidé fumaroles; Vincent, 1970; Deniel et al., 2015; Fig. 4). ). (U-

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Th)/He thermochronological data obtained on Hoggar apatites, provided a range of ages from

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78 ± 22 Ma to 13 ± 3 Ma for the doming, demonstrating the existence of a widespread Eocene

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exhumation of the shield before the beginning of the volcanic activity (Rougier et al., 2013).

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This concurred to relate the doming to the coupled interaction of edge-driven convection

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along thick cratons with the intraplate stress induced by the Alpine collision (Liégeois et al.,

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2005; Rougier et al., 2013). The Tibesti volcanism emplaced during the uplift of the

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Precambrian basement that is currently forming a swell, with a mean altitude over 1,000 m,

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reaching locally 2,000 m, in a similar manner as the Tuareg Shield (Rougier et al., 2013),

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could be related to the same mechanism.

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The Tibesti volcanism is mostly bimodal, with alkaline basaltic plateau lavas and

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ignimbritic sheets and with volcanoes being either felsic (rhyolites, trachytes and minor

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phonolites) or mafic (tholeiitic or alkali basalts) (Vincent, 1970; Deniel et al., 2015). Regional

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basement faults striking NNE–SSW follow Precambrian structural directions throughout the

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entire Tibesti region (El Makhrouf 1988), but are generally obscured within the TVP by the

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volcanic products (Fig. 1, 2).

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Some authors associate the Tibesti volcanism to deep mantle plumes (e.g. Dautria and

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Lesquer, 1989; Wilson and Guiraud, 1992; Burke, 1996; Aït-Hamou et al., 2000) while more

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recent studies envisage the reactivation of Pan-African shear zones allowing asthenosphere

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uprise along them as a consequence of the intraplate stress generated by the Africa-Europe

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convergence (Liégeois et al., 2005; Azzouni-Sekkal et al., 2007; Beccaluva et al., 2007, 2008;

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Bardintzeff et al., 2012), with, in the case of Tibesti, a main role played by the NW-SE and

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NE-SW tectonic trends (Deniel et al., 2015).

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3. Methodology Lineaments (joints, faults) on one hand and cinder cones and large volcanic centres on

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the other hand can be recognized as linear features and circular features (with high slopes),

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respectively, on high resolution topographic data set (SRTM – DEMs and Landsat satellite

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images; Fig. 3). We use the SPO2003 and the INTERCEPTS2003 softwares (Launeau and

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Robin, 1996, 2003) to analyse the spatial distribution and the mean orientation of the cinder

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cones and of the lineaments. This method has already been used in geology (i.e. Leymarie,

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1968; Moreau et al, 1987). It has been extensively developed by Nkono (2008) and applied

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with success to the distribution of volcanic centres of the Cameroon Volcanic Line of Central

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Africa (Nkono et al, 2009) where a detailed description of the methodology can be found.

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4. Results

From the original topographic dataset of the Tibesti volcanic province, an altimetry

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database has been computed, from which a 3D view (Fig 3a) and a slope shade view (Fig 3b)

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have been extracted.

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The distribution of lineaments in the studied area is shown in Fig. 4a. Length

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resolution for the SRTM images is 3-arc second, which corresponds to 270 m. The basement

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lineaments are largely hidden by the lava flows and/or the large volcanic edifices. The

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distribution of the large volcanic centres (shield volcanoes) and cinder cones of the TVP is

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represented in Fig. 4b.

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The distribution of lineaments and its interpretation in terms of structural features

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(density, directions …) are shown in Fig. 5. The rose diagram (Fig. 5b) drawn from the

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lineament distribution shows four main directions (N27°E, N67°E, N112°E, and N153°E).

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The intersection points of these lineaments were extracted and plotted in Fig. 5c which gives a

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global view of their distribution. This figure also provides an overview of the weakness zones

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in terms of fractures and intersection points assuming that they can be considered as indicator

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of the lithosphere weakness. The contrast and colours of the covariogram (Fig. 5d) indicate the general directions of

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the distribution of a given feature: colour variation from green and orange to white

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corresponds to decreasing feature density associated with a given direction. Here, the analysis

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shows that the maximum of intersections occur around two directions, N35°E and N120°E

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(Fig. 5e), which approximate to the two principal regionall lineament orientations (N27°E and

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N112°E). The summary of the results in terms of directions and density obtained from the

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analysis of lineaments and intersection points is given in Fig. 5f.

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The magmatic entities of the TVP (Fig. 6) are subdivided in two groups: (1) 16 large

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volcanic edifices (up to 80 km in diameter), which are actually shield volcanoes, they often

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have calderas (Permenter and Oppenheimer, 2007) (Fig. 6a, b and c) and (2) small cinder

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cones (~500m wide) (Fig. 6d, e and f). The spatial distributions of these 2 types of volcanic

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objects were also analysed using the SPO2003 software (Launeau and Robin, 1996) to extract

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the mean directions of their distributions. These distributions provide two covariogram maps,

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one for the shield volcanoes (Figs 6b and 6c) and one for the cinder cones (Figs 6e and 6f).

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This analysis indicates that the shield volcanoes are distributed around two directions, N30°E

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and N125°E (Fig. 6c) and that the cinder cones are also distributed around two directions

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N35°E and N°175°E (Fig. 6f). The summary of the principal directions deduced from the

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analysis of the distribution of the volcanic units is given in Fig. 6g.

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5. Discussion

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In the studied area (~100,000 km2), 597 cinder-cones, 16 shield volcanoes and 747

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lineaments were observed. Cinder-cones and intersection points of lineaments were

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considered as point features and analysed through the centre-to-centre method using the

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SPO2003 software. Lineaments were globally analysed as a network through the intercept

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method (INTERCEPTS2003 software). All features mapped using SRTM-Landsat derived images have been analysed to

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identify their main orientations that were further used for geodynamic interpretations. The

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different orientations and geometric relations of the structural features were examined

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altogether in terms of the Riedel system (Riedel, 1929) for the geodynamic interpretation of

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the mode(s) of emplacement of the volcanic edifices. This approach has already been used for

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volcanic provinces in general (van Wyk de Vries and Merle, 1998 ) and especially for the

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Cameroon Volcanic Line (or Cameroon Hot Line, sensu Deruelle et al., 2007) (e.g. Moreau et

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al., 1987; Nkono, 2008; Nkono et al., 2009, 2014). The most conspicuous structural element

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of this idealized geometry is the Riedel conjugate set, comprising synthetic Riedel fractures

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(R) and conjugate antithetical Riedel fractures (R'), oriented at 45°±θ/2, where θ is the internal

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angle of friction of the rock. Other important structural elements are the synthetical P shear

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fractures (at −45°+θ/2) and the purely tensional (T) fractures (at 45° in simple shear).

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There are two main directions along which the structural features of the Tibesti

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province are distributed: (1) the NW-SE direction, which is represented by the N125°E

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direction of the cinder cones, the N120°E direction of the intersection points and the N112°E

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for the lineaments; (2) the NE-SW direction represented by the N30°E direction for the shield

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volcanoes, the N35°E for cinder cones, and the N27°E for lineaments (Fig. 7a).

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In the Riedel model used in this study, the emplacements of the numerous cinder

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cones and of the fewer shield volcanoes, as well as the intersection points of the lineaments,

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can be related to tension fractures either around the NW-SE or the NE-SW directions. For

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going further in this interpretation, it was assumed that the distribution of the intersection

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points can be an indicator of the lithosphere weakness (Burke, 2001; Takeshi et al., 2004;

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Nkono et al., 2014).

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The NW-SE direction corresponds to large-scale flexures along which the feeding dike

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swarms of the volcanoes are concentrated and the NE-SW direction corresponds to the

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decreasing age of the volcanic activity (Deniel et al., 2015). Guiraud and Maurin (1992)

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suggested that the magmatic activity in West, Central and North Africa increased during the

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Neogene, many volcanic fields in this large portion of Africa being concentrated in the Pan-

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African mobile belts; by contrast, on cratons and cratonic nuclei, Cenozoic volcanism is

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completely lacking (Fezaa et al., 2010; Liégeois et al., 2013). The uplift of the large domal

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structures characterized by widespread Neogene volcanic activity (e.g. Adamawa, Darfur,

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Tibesti, Hoggar and Air) is variably interpreted and related to yet poorly understood

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geodynamic processes affecting the sub-lithospheric mantle of Africa (Guiraud and Bellion,

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1995). Two main kinds of models have been proposed: (1) those giving the active role to the

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sublithospheric mantle either as a mantle plume impingement (Aït-Hamou et al., 2000 and

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references therein) or as an isostatic response of the crust to the emplacement of mantle-

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derived magmas at its base (Wilson and Guiraud, 1998) and (2) those giving the active role to

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the behaviour of the lithospheric mantle in response to the Africa-Europe collision (Liégeois

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et al., 2005; Azzouni-Sekkal et al., 2007; Beccaluva et al., 2007, 2008; Bardintzeff et al.,

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2012)

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According to the second model (behaviour of the lithospheric mantle in response to

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plate collision), the complex distribution of lineaments, cinder cones and shield volcanoes of

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Tibesti can be locally related to two main directions present in the area, the direction of the

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Tibesti-Nuqay fault and the direction of the Abo trend (Fig. 1).

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Application of the Riedel model to the distribution of volcanic features (both cinder

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cones and shield volcanoes) and of lineaments (Fig. 7a) suggests that two successive

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structural systems/events were at work for explaining these distributions, and that they can be

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related to available geochronological data and field relative ages.

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The first structural event (Fig. 7b) can be related to the NW–SE striking regional fault

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swarm (Abo trend, Fig. 1) affecting the Tibesti Precambrian basement (El Makhrouf, 1988).

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Indeed, the Cenozoic cinder-cones are mainly distributed along N125°E stress fractures (T)

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and secondarily along N175°E fractures, while the lineaments are mostly oriented N112°E

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(R) and N153°E (R') with an extension direction (E) at N27°E (Fig. 7b and c). In this

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transtensive system, the extensional direction at c. N30°E corresponds to the spatial

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decreasing age pattern of the volcanic activity, which started before 17 Ma, being

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contemporaneous with the Tibesti plateau volcanism, and whose end is not precisely known

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but could be as young as 8 Ma (Deniel et al., 2015; Fig. 7b). This early part of this stage

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corresponds to the end-Aquitanian-Burdigalian (Guiraud et al., 2005) or mid-Miocene

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(Swezey, 2009) unconformity of regional extent. This event can be considered as the

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consequence of a change of direction towards the NE of the African plate during the

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Aquitanian (Guiraud et al., 2005). In turn, this plate motion change could be at the origin of

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the south-westwards Miocene Tibesti volcanic age migration, which is perpendicular to the

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feeding NW-SE flexures (Deniel et al., 2015).

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The second structural event (Fig. 7d and e) is related to the NE–SW striking regional

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faults (Tibesti-Nuqay fault, Fig. 1), which have a general N30°E direction throughout the

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entire province and correspond to the Precambrian Tibesti-Sirt uplift (e.g. Woller and Fediuk,

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1980; El Makhrouf, 1988) and to the Tibesti-Nuqay fault (Fig. 1). The compressional

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direction of this second transpressive event is at N112°E and corresponds to the orientation of

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lineament intersection points, to the main orientation of shield volcanoes and to the Abo trend

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(Deniel et al., 2015). This second system can be related to the subsequent uplift of the

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regional basement along a N-NW axis, which was followed, at a regional scale, by its overall

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tilting to the N-NE (Malin, 1977) and by the reactivation of the Tibesti-Nuqay fault. We relate

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this second structural event to the second stage of volcanic activity in the central TVP ranging

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from 8 to 7-5 Ma and associated with the reactivation of pre-existing NNE faults during the

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late Miocene (Deniel et al., 2015). This period is also that of a second major unconformity in

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Sahara called Upper Miocene or Tortonian-Messinian unconformity (Guiraud et al., 2005;

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Swezey, 2009). It is linked to another change in motion of the African plate that moved to the

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NW from 8.5 Ma (Guiraud et al., 2005). This period corresponds also to a major inversion

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phase in the Atlas system (late Miocene-Pliocene; Frizon de la Motte et al., 2009)

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The relation between the orientations of lineaments and cinder-cones can be explained

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as the superposition of a first transtensional (strike-slip + extension) event corresponding to a

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dextral N120°E strike-slip regime (Fig. 7b and 6d), and a second transpressional (strike-slip +

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compression) event corresponding to a dextral N30-35°E strike-slip regime (Fig. 6b and 6e).

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Permenter and Oppenheimer (2007), based on satellite image study, and Deniel et al (2015)

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based on Vincent (1970) field work, give different chronological histories of the volcanic

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activity in the TVP. In this paper, we used satellite images as the former but our results are in

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agreement with the field work data.

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Recent surface geology and tomography at a broad regional scale (Liégeois et al.,

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2013) have highlighted the existence of several preserved cratonic nuclei with thick

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lithosphere (Murzuq, Chad and Al Kufrah; Fig. 8), within the Saharan metacraton (SmC);

316

they correspond to remnants of the pre-Neoproterozoic Saharan craton. The juxtaposition of

317

partly reworked metacratonic areas and pristine cratonic blocks that escaped the regional

318

metacratonization of the SmC during the Pan-African orogeny, gives rise to rheological

319

contrasts and lithospheric structural discontinuities explaining why the internal zones of the

320

SmC can be reactivated by the far-distant stress of the Africa-Europe convergence and mid-

321

ocean ridge push (Liégeois et al., 2013).

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These rheological and structural discontinuities result in uplifting of metacratonic

323

areas that can be accompanied by intraplate volcanism, especially along the boundaries of the

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cratonic nuclei (Liégeois et al., 2005). This is nicely illustrated in the Tibesti region, which is

325

located in between the three cratonic blocks of Murzuq, Chad and Al Kufrah (Fig. 8). Indeed,

326

one can observe that the different directions obtained in this study from the complex

327

distribution of lineaments (coined L), cinder cones and shield volcanoes (coined C) are

328

aligned parallel to the boundaries of these cratonic blocks: the large N27°E (L) / N30°E (C)

329

directions, corresponding to the Tibesti-Nuqay fault, extend along the north-western margin

330

of the Al Kufrah craton. The other large N112°E (L) / N125°E (C) directions extend along the

331

northern margin of the Chad craton (Fig. 8). The less intense N153E (L) direction extends

332

along the south-western boundary of the Murzuq craton, which is also the direction of the

333

major Niger-Chad and Tripoli Mesozoic rift systems as well as that of structures present in the

334

northern part of the Murzuq craton (Fig. 8). Finally, the secondary N67°E (L) / N75° (C)

335

directions are parallel to the eastern boundary of the Murzuq craton. The Tibesti major uplift

336

with its Cenozoic intraplate volcanism is thus located at the intersection of these reactivated

337

pre-existing structures inherited from the Neoproterozoic metacratonization (Liégeois et al.,

338

2013).

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The volcanism in Tibesti thus appears to be controlled by the existing lithospheric

340

architecture, as for nearby Libyan, Hoggar, or Aïr volcanic fields (e.g., Goudarzi, 1980;

341

Woller and Fediuk, 1980; Schäfer et al., 1980; Liégeois et al., 2005; Azzouni-Sekkal et al.,

342

2007) and especially by the internal rheologically contrasted structure of the Saharan

343

metacraton (Liégeois et al., 2013). The reactivation of these lithospheric megastructures in

344

relation to the Africa-Europe convergence and collision certainly played a major role in the

345

formation of the Tibestian, Libyan and Hoggar volcanic fields. This sheds a new light on the

346

behaviour of the internal portions of the African plate during its long collisional history with

347

the European plate.

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6. Conclusions The distribution of lineaments/faults and volcanic features (cinder cones and shield

352

volcanoes) in the ~100.000 km2 Tibesti volcanic province (TVP, Northern Chad) has been

353

studied combining Shuttle Radar Topography Mission (SRTM), Digital Elevation Models

354

(DEMs) and Landsat satellite images. These methods are not expensive and not

355

computationally voracious.

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The main purpose of this paper was to decipher the structural relationships between the

357

lineaments and the volcanic features in this rather poorly known province. The distribution of

358

faults and of volcanic features has been studied to infer their main orientations. The

359

distribution data were interpreted locally in a general Riedel model that appears to be

360

controlled by the reactivation of pre-existing Pan-African faults affecting the basement. The

361

tectonic setting of emplacement of the Tibesti volcanic province has been explained as

362

resulting from two successive structural events, a first (lower Miocene) with a dextral

363

transtensional N125°E strike-slip regime and a second (upper Miocene), with a dextral

364

transpressional N30-35°E strike-slip regime. Regionally, the main directions deduced from

365

the lineaments and volcanic centres alignments can be tightly correlated with the general

366

orientations of the boundaries of the relictual cratonic nuclei (Murzuq, El Kufrah and Chad

367

cratons) identified in the huge Saharan metacraton. These rigid nuclei have indeed driven the

368

orientation of these structures during the Pan-African metacratonization and also during the

369

reactivations induced by the Alpine Africa-Europe collision. The spatial/geographic position

370

of the Tibesti province at the triple point between these cratonic nuclei is probably a major

371

key for understanding the localization and the development of this large swell and its

372

impressive intraplate volcanism.

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Acknowledgements

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We warmly thank Andy Moore and Bernard Bonin for their thorough reviews that

376

significantly improved our paper.

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Figure Captions

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Fig. 1. Geological map of the region considered in the paper, covering northern Chad,

558

southern Libya, north-eastern Niger and south-eastern Algeria. Modified from Meinhold et

559

al., 2011 and references therein, Genik, 1992, Fezaa et al., 2010).

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Fig. 2. a) Location of the studied area, the Tibesti Volcanic Province (Chad, northern

561

Africa) (dark grey box) and of the Tuareg Shield (red box); b) Detailed regional lithology

562

after the geological map of Africa at 1:10M (Thiéblemont et al., 2016)

563

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Fig. 3. a) Topographic map showing the main shield volcanoes of the Tibesti Volcanic

564

Province with orange >2500m, yellow >2000m, green > 1500 m, light blue >1000 m, deep

565

blue > 500m. b) The slope map has been obtained by applying a Fourier transform analysis to

566

the topography.

24

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Fig. 4. a) Map of lineaments (straight continuous lines). b) Map of objects with strong

568

geological slopes (cinder-cones) drawn from the SRTM images. The names of the main

569

volcanic centres of the Tibesti Volcanic Province are taken from Permenter & Oppenheimer

570

(2007).

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Fig. 5. Analysis of the distribution of lineaments and their intersection points

572

identified on SRTM images for the studied area. a) Lineaments map. b) Rose diagrams of

573

lineaments by the INTERCEPTS2003 software (Launeau and Cruden, 1998). Analyses were

574

made every 10°. c) Map of the Lineament intersection points. d) and e) Mean directions under

575

which lineament intersections point are distributed. These directions were obtained by the

576

centre-to-centre method using SPO2033 (Launeau and Robin, 2003). The green colour

577

corresponds to the highest concentration of cinder-cones, the white zone marks the absence of

578

cinder-cones; yellow and orange correspond to intermediate concentrations. f) Synthetic

579

representation of the distribution of lineaments (mean directions and density)..

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Fig. 6. Analysis of the distribution of large shield volcanoes and cinder cones as

581

identified on SRTM images for the studied area. a) Map of shield volcanoes (after Permenter

582

& Oppenheimer, 2007). b) and c) Mean directions under which volcanoes are distributed.

583

These directions were obtained by the centre-to-centre method using SPO2033 (Launeau and

584

Robin, 2003). The green colour corresponds to the highest concentration of volcanoes, the

585

white zone marks the absence of volcanoes; yellow and orange correspond to intermediate

586

concentrations. d) Distribution map of cinder cones. e) and f) Mean directions under which

587

cinder cones are distributed. These directions were obtained by the centre-to-centre method

588

using SPO2033 (Launeau and Robin, 2003). Same colour code as in Figs 5b and c. g)

589

Synthetic representation of distribution of shield volcanoes and cinder cones (mean directions

590

and density).

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Fig. 7 Proposed geodynamic interpretations for the Tibesti volcanic features and

592

lineaments. a) Synthetic representation of the distribution results of volcanoes, cinder cones,

593

lineaments and their intersections points. b) First tectonic event deduced from the distribution

594

of shield volcanoes and cinder cones; the dashed black line indicate the volcanic age

595

migration, wich occurred southwestwards and perpendicular to the feeding NW-SE flexures

596

(first phase of volcanic activity that started >17 Ma; Deniel et al., 2015); the black arrows

597

represent the orientation of the extensional constraint; the salmon strain ellipse represents the

598

non-coaxial deformations; c) Second tectonic event deduced from the distribution of

599

lineaments and their intersection points; the black arrows represent the orientation of the

600

compressional constraint; the green strain ellipse represents the non-coaxial deformations.d)

601

Superimposition of the structural directions of the first tectonic event on feature map. e)

602

Superimposition of the structural directions of the second tectonic event on feature map.

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Fig. 8. Main rheological domains of Saharan Africa centered on the Saharan

607

metacraton (slightly modified from Liégeois et al., 2013). Main lineament orientations

608

(ellipses, orientation values marked L) and shield volcanoes/cinder cones orientations (lines,

609

orientation values marked C) from this study. The green orientations are parallel to the

610

western Al Kufrah craton boundary and eastern Murzuq craton boundary, both marked by

611

green bands. The deep blue orientations are parallel to the northern Chad craton boundary,

612

marked by a deep blue band. The light blue orientations are parallel to the south-eastern

613

Murzuq boundary (marked by light blue band) and to the Mesozoic Niger-Chad rifts and

614

parallel structures in the northern part of the Murzuq craton, both marked by hatched light

615

blue bands.

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ACCEPTED MANUSCRIPT Ghadamès basin

Cenozoic volcanism

Sirt basin

Quaternary (out/in bassin)

fa u lt

As Sawda

st i-

Paleozoic sediments

Nuqay

Precambrian basement

dem

E et-

Basin boundary

es

bbi gué

ra n

fau

ter

lt

bo

nd

e tr

Aïr

sti

o Ab

e Tib

h ug tr o

h troug

15°E

Chad

20°E

AC C

EP

TE D

M AN U

10°E

Al Kufrah basin (cratonic)

Yé

ein

Niger

SC

Gr

ré Tené

Hoggar

Mesozoic sediments (out/in bassin)

RI PT

n Dja

Tib es ti-

ia

20°N

Cenozoic sediments (out/in bassin)

Mesozoic rift (buried)

Ti

er

Murzuq basin (cratonic)

Libya

be

lg

Si rt hi gh

A

25°N

Nu q

ay

Al Haruj

Tibesti lineament

Political boundary

Latitude/longitude 250 km

ACCEPTED MANUSCRIPT 19°E

Study Area

N so

500 km

Zouar

Misk

Ab

Recent basalt

o tre

Rhyolitic dome Bimodal volcanism

one D oh

M AN U

nd

Recent Ignimbrite

Country boundary

Fault

16°E

Paleozoic

EP

TE D

Nubian and Bardaï sandstone (Cretaceous)

SC

i

Final volcanism trachyandesite

AC C

b

Zoumri

r Ta

Plateau volcanism

YA

Bardaï

d si

s

u To

LIB

RI PT

é

Aozou

Yeb big ué fa

Ti be st i-

Tuareg Shield

ult

Nu qa y

fa ul t

a

22°N

1

2

Precambrian: Lower (1) and Upper (2) Tibestian

100 km

19°N 19°E

ACCEPTED MANUSCRIPT

16°E 22°N

19°E 22°N

a

N

lineame

cinder c

one

shield v

22°N/16°E

M AN U

SC

olcano

RI PT

nt

N

TE D

b

100 km

22°N/19°E

lineament

shield volcano

AC C

EP

cinder cone

100 km

19°N 16°E

19°E

ACCEPTED MANUSCRIPT 16°E 22°N

19°E 22°N

ura

oO

s Tar

Tar s

Tn Bn

Vn

Ey

RI PT

ide

Na

Ab

Ss

i

ss

Mg

Dk

Ts

To u

Ch mi

so

oE

Ti

Ta r

s Tar

ri

oT oh

Tk

Yg

SC

hon oA

s Tar

Ks

M AN U

a N

100 km

22°N/19°E

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AC C

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TE D

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b

N

100 km

19°N 16°E Lineaments !

Cinder cones Shield volcanoes

Ab - Abeki Bn - Bounai Dk - Doon Kidimi Ks - Emi Koussi Mg - Ehi Mousgou Na - Trou au natron Ss - Ehi Sosso

Ti - Timi Tk - Tarso Tieroko Tn - Tarso Toon Ts - Tousside Vn - Tarso Voon Yg - Tarso Yega Ey - Ehi Yey

19°E

ACCEPTED MANUSCRIPT

a

N2

7°

E

b

°E

RI PT

N67

N1 12

5 N1

°E

SC

e

M AN U

d

TE D

3°E

c

N2 5°E

20

°E

°E

°E

N1

53

20

N3

N3

N1

E

° N67

N1

AC C

5°E

EP

7°E

f

N11

2°E

ACCEPTED MANUSCRIPT a

i

^

Ey

Vn

Ab

12

^

s Tar

Tk

oA

Yg

5°

0°E

^

E

N3

^

N

Bn

^

Mg

Ss Ts Na

Ch

Tn

^ _^

mi

ri

Dk

^

oE

ura oO

oT oh

Ti

c

b

s Tar

s Tar

Tar s

hon

d

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!

!

E N175°

! ! !! ! ! !! ! !! ! ! ! ! ! !! ! ! ! ! ! ! !!!!! !! ! !! ! ! !!!! ! ! !! ! ! !! ! ! ! !! !! ! ! ! !! ! !!! ! ! ! !! ! !!! ! ! ! ! !! ! !! ! !!! ! ! !! !! ! !! ! ! ! ! ! !

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0°E

!

12

5°

E

Cinder cones Shield Volcanoes

Ab - Abeki Bn - Bounai Dk - Doon Kidimi Ks - Emi Koussi Mg - Ehi Mousgou

AC C

EP

TE D

E

N175°

N

M AN U

!!

g

N3

RI PT

Ks

Na - Trou au natron Ss - Ehi Sosso Ti - Timi Tk - Tarso Tieroko Tn - Tarso Toon Ts - Tousside Vn - Tarso Voon Yg - Tarso Yega Ey - Ehi Yey

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Highlights - The Tibesti volcanic province (TVP) is one of the major swells of Africa. - We use SRTM, DEMs and Landsat images combined with geological structures. - Lineaments and volcanic centres result from two Riedel dextral tectonic systems.

RI PT

- The TVP rose from the reactivation of Saharan metacraton inherited structures.

AC C

EP

TE D

M AN U

SC

- The TVP developed as a result of the Cenozoic Africa-Europe collision.