Geophysical evidence of Cretaceous volcanics in Logone Birni Basin (Northern Cameroon), Central Africa, and consequences for the West and Central African Rift System

Tectonophysics 583 (2013) 88–100

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Geophysical evidence of Cretaceous volcanics in Logone Birni Basin (Northern Cameroon), Central Africa, and consequences for the West and Central African Rift System Jean-Pierre Loule a,⁎, Lubomil Pospisil b, 1 a b

National Hydrocarbons Corporation (SNH), P.O. Box 955, Yaounde, Cameroon Institute of Geodesy, Faculty of Civil Engineering, Brno University of Technology, Veveri 95, 60 200, Brno, Czech Republic

a r t i c l e

i n f o

Article history: Received 27 February 2012 Received in revised form 17 October 2012 Accepted 22 October 2012 Available online 31 October 2012 Keywords: Logone Birni Basin Northern Cameroon West and Central African Rift System Depth migrated versions Buried volcanic bodies Syndepositional faults

a b s t r a c t Detailed analyses and interpretation realized by combining existing 2D reflection seismic and Gravity/Magnetic data of the Logone Birni Basin (LBB) in the West and Central African Rift System (WCAS) have revealed the distribution of the main buried volcanic bodies as well as their relationships with the structural and tectonic evolution of this basin. The volcanic activity in the LBB is restricted to the Cretaceous period. Three main volcanic episodes are identified and are associated to the Neocomian, Late Albian and Cenomanian–Turonian rifting phases respectively. The volcanic bodies within the Lower Cretaceous are either lying directly on basement or are mainly interbedded with the contemporaneous sediments whereas the Upper Cretaceous bodies are morphologically expressed in the forms of dykes and sills. The volcanic activity was more intense in the western region of the central LBB (Zina sub-basin) along the Cameroon–Nigeria border whereas it was scanty and scattered in the other parts of the basin. The main volcanic dykes are found on the flanks of the major faults bounding basement horsts or in crestal positions in association with syndepositional faults. Although WCAS is associated with large amount of crustal extension and minor volcanism, the intense volcanic activity observed in LBB during the Cretaceous suggests that the intrusive zone during this period was confined to the basement beneath the study area flanked respectively to the north, south and southwest by the Lake Chad, Poli and Chum triple junctions. Tensional stresses generated by this localized domal uplift accounts for most of the observed tectonic structures where major faults transected the entire lithosphere, thus providing conduits for magma migration. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Widespread igneous activity is documented in West and Central Africa during the Ear1y Mesozoic (Triassic–Jurassic) prior to the development in these regions of major intra-continental rift systems during the Cretaceous (Bertrand and Villeneuve, 1989; Fitton, 1983; Guiraud, 1990; Mascle et al., 1988; Ngako et al., 2006). This activity is translated in the field by doleritic intrusives related to mantle plume upwelling connected with the St. Helena mantle plume beneath the Niger delta– Mt. Cameroon region of West Africa during the Ear1y Cretaceous (Coulon et al., 1996; Guiraud et al., 1992; Wilson and Guiraud, 1992). Previous findings from Cretaceous outcrops in the Upper Benue (Guiraud, 1990, 1991; Popoff et al., 1982, 1983) documented two magmatic episodes. The initial magmatic episode, bimodal in nature, consists of alkaline rhyolites and transitional basalts from Late Jurassic to Neocomian (147 Ma ± 7 to 127 Ma ±6). The volcanism took place

⁎ Corresponding author. Fax: +237 22204651. E-mail addresses: [email protected] (J.-P. Loule), [email protected] (L. Pospisil). 1 Fax: +420 543251584. 0040-1951/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2012.10.021

along N60°E trending Bima synsedimentary fractures suggesting that it took place during the early activation of the lower Bima synsedimentary faults. The second magmatic episode is dated from Albian to Turonian times and is made up of transitional alkaline and tholeitic basalts (104 Ma± 5.2 to 90 Ma ±4.5) which in the contrary erupted along N120°E to N135°E striking lower Bima synsedimentary faults. In the south of Chad there is a series of troughs including the Bongor, Doba, Doseo, Salamat and Birao basins (Fig. 1), aligned along the Central African Shear Zone (CASZ). These half grabens trending N70°E to E–W were also active from Neocomian to Early Aptian, similar to those of north Cameroon and the Upper Benue (Guiraud and Maurin, 1992; Guiraud et al., 1992). Genik (1992) indicated minor evidence of magmatic activity in the form of sills of altered basalt and basaltic andesite, radiometrically dated Late Albian (97–101 Ma) in Doseo basin. In the Cenozoic, alkaline magmatic activity occurred widely within West and Central Africa, both within the Cretaceous rift system and outside (Fig. 1). Much of this magmatism is alkali basaltic in composition and is related, at least in part, to the reactivation of major deep seated lithospheric fracture zones (Ngako et al., 2006; Wilson and Guiraud, 1992). Several of the volcanic complexes are associated with broad domal basement uplifts, with diameters which suggest

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Fig. 1. West and Central African Rift System (WCAS) and East African Rift System (EARS). 1: Exposed Quaternary–Tertiary volcanics, 2: Mesozoic rifts, 3: main fault zones and rift margins, 4: documented triple junctions (Lake Chad, Poli, Chum) in the West African Rift sub-System, 5: location of Logone Birni Basin (LBB), 6: Central African Shear Zone (CASZ), 7: West African Strike-Slip Fault and Chad Shear Zone, 8: average direction of extension in Cretaceous, 9: average direction of shortening in Cretaceous. Interpreted schematic cross-section through LBB below shows the lateral distribution of Cretaceous buried volcanics. Adapted from Genik (1992).

that uplift may have been initiated by diapiric upwelling within the upper mantle above 650 km depth (Nnange et al., 2000). The timing of uplift in these areas is, in general, poorly constrained and this remains an area for further research. The relationship between the activity of mantle plumes and the geodynamics of rifting is complex, with some mafic magmas being derived from plume-modified mantle sources more than 100–150 Ma after the actual activity of the plume ceased beneath a particular part of the African plate (Guiraud et al., 1992; Ngako et al., 2006; Wilson and Guiraud, 1992). In the north of Cameroon there are many small basins of Neocomian– Early Aptian age (ca. 130–118 Ma) which have a general E–W trend.

They are older than the large Albo-Aptian basins in the adjacent Benue Trough, Eastern Niger and southwestern Chad (Guiraud and Maurin, 1992). These small basins contain interstratified alkali basalt flows (Fig. 2), associated with dolerite dykes and sills as well as post sedimentary intrusive igneous bodies which cut through the sedimendary sequence (Brunet et al., 1988; Loule et al., 1997). The dykes are numerous, often up to 50 km long, and strike N70°E to E–W parallel to the basin bounding faults. Some of these dykes were deformed during the Santonian compressional tectonic event, thus constraining their minimum age to Coniacian (>85 Ma) (Guiraud, 1993; Guiraud and Maurin, 1992; Maurin and Guiraud, 1990).

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Fig. 2. Alkali basalt pillows (Arrow N 95° E) interbedded in Neocomian sediments in the Hamakoussou basin in northern Cameroon (Loule et al., 1997).

sedimentation and non-conformities in both seismic sections and outcrops. These basins and particularly the LBB are the product of a complex period of continental disruption, as Africa responded to plate tectonic fragmentation of the Gondwana. From literature, three main mechanisms have been described to account for such continental disruption. These are: up-doming of the crust over a hot-spot, asthenospheric mantle rise to high levels beneath existing fractures, and convective processes in the underlying asthenosphere. Three main episodes of basin development have been documented after the PanAfrican crustal consolidation (750–550 Ma) and the Paleozoic-Jurassic platform development (550–130 Ma). Rifting initiated during Neocomian to Aptian (130–98 Ma) resulted in the communication between the WCAS and the West African Salt Basins (WASB). The grabens formed were re-opened during Aptian–Albian in a regime of oblique extension (Avbovbo et al., 1986; Benkhelil, 1988; Genik, 1992). These rifting events were followed by thermal tectonic subsidence within the rifts (80 Ma–75 Ma). The thermal subsidence was accompanied by a major marine transgression which reached northern Cameroon from the Tethys, to the north, through Mali and Algeria and from the South Atlantic, to the South, via the Benue Trough into western Chad and Niger. This transgression reached its maximum eastern extent in the western most end of the Doba basin at 85–80 Ma (Genik, 1992). It was followed by a regression of the sea due to an epeirogenic uplift. By 75 Ma, the shallow seas had regressed leaving behind a regional unconformity which is documented in many basins in central and northeast Africa as well as in South Atlantic and in northeast Brazil. The last episode of development (30 Ma–Present), is a post rift stage during which most of the area was uplifted. This emergent stage was accompanied by some uplifting in the east of the Cameroon Volcanic Line in the Adamawa region (Fitton, 1980) giving way to the erosion of high lands, where up to 2500 m of sediments were eroded in the western parts of the Doba and Bongor basins and up to 500 m of continental sediments were deposited in the present location of Lake Chad (Genik, 1992; Mothersill, 1975). Volcanism is recorded in the WCAS from Early Cretaceous through Tertiary (Fig. 3). 3. Stratigraphy

Tectonically, three plumes generated Mesozoic triple junctions: Chum, Poli and Lake Chad (Fig. 1) have been documented in the WCAS from outcrop and gravity studies (Burke and Dewey, 1973; Burke and Whiteman, 1973; Cratchley and Jones, 1965; Cratchley et al., 1984; Fairhead, 1986; Grove, 1986; Louis, 1970). Burke and Whiteman (1973) noted a striking similarity between the Bouguer anomalies in the Kenya and Ethiopian rifts and the 1500 km-long line of positive Bouguer anomalies mapped from Poli, situated some 300 km south of the LBB (Fig. 1), northward to the Tibesti and which was interpreted by Burke and Dewey (1973) to represent axial dikes emplaced in a rift branch that did not spread. Using all the above knowledge and the SNH existing geophysical database, this paper investigates the volcano-tectonic activity of the Logone Birni Basin (LBB) situated in northern Cameroon. This basin lies south of the Chad Lake (Fig. 1) and is limited in the north by the Termit basin, in the west by the Bornu basin and in the south and southeast by the Bongor and Doba basins. It is tectonically situated northeast of the Chum triple junction, south of the Lake Chad triple junction, and east of the branch of the Poli triple junction that did not spread. 2. Geological setting The LBB which is the focus of this study is blanketed by Quaternary sediments and belongs to the West and Central African Rift System (WCAS) (Fig. 1). The genesis of the WCAS sedimentary basins is characterised by polyphase rifting (Fig. 3) separated by tectonic events that can be recognised as regional deformation (folding), hiatus of

The general stratigraphy of the LBB was seismically derived from that of the surrounding basins (Avbovbo et al., 1986; Genik, 1993; Nwachukwu, 1985; Petters and Ekweozor, 1982). Sedimentation in this basin commenced during the Neocomian–Albian rifting period. A comparative chronostratigraphy is presented in Fig. 3. From seismic and gravity evidences, more than 6000 m of sediments overlie the basement in the deepest parts of the basin. From the tectonic evolution outlined by Manga et al. (2001), the oldest sediments in the LBB are of Neocomian–Barremian age. These are fluviolacustrine shale sands characterised by possible piedmont alluvial fans attributable to the Pre-Bima Formation. This sequence is succeeded upward by a reflection free (seismically transparent) monolithic Upper Albian unit attributable to the Bima Formation. Overlying this Upper Albian unit are high amplitude, discontinuous and sub-parallel reflectors that correspond to the Cenomanian– Turonian, alternating sandstone and shale with limestone unit of the Gongila Formation of NE Nigeria. This in turn is overlain by another reflection free unit similar to the one attributed by Avbovbo et al. (1986) to the Turonian–Senonian Fika Formation. Overlying the Fika Formation is the Upper Senonian–Maastrichtian Gombe Formation characterised by parallel to sub-parallel, discontinuous to chaotic reflectors (inter-bedded sand-shale). The Eocene– Oligocene Kerri Kerri Formation unconformably overlies the Gombe Formation. It comprises continental shale and sand series. This unit is essentially made up of sands to shale sands exhibiting low to high amplitude continuous and parallel reflectors.

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Fig. 3. Comparative stratigraphic columns between Chad Basin of NE Nigeria and the Logone Birni Basin in northern Cameroon. 1: Extension, 2: compression, 3: CASZ, 4: thermo-tectonic sag, 5: eustatic and thermo-tectonic sag, 6: uplift, 7: volcanism in WAS, 8: volcanism in CAS. Seismic packages L, M and T are from Manga et al. (2001).

4. Database and methodology In 1997, SNH in a joint venture partnership with a Czech based corporation Geofyzika a.s. acquired about 1000 km of 2D seismic in the Logone Birni Basin (Fig. 4). Along measured seismic lines gravity measurements were also taken. For the present study, the seismic and gravity data at measured profiles are complemented by aero gravity and aero magnetic data acquired over the LBB by Carson Services on behalf of SNH in 1998. This data has been used for the analyses and interpretation of sources of magnetic anomalies in the LBB. 4.1. Seismic data Migrated seismic sections were processed and converted to depth versions in order to better determine the depths of the various anomalies as well as their likely morphology. Even though this conversion was realized without a detailed interval velocity analysis, but solely on the basis of smoothed stacking velocities, the final image of the seismic sections arrived at enable us to use this data in the interpretation of the gravity and magnetic data and to effect some structural evaluation. The methodology consisted of first checking and reviewing raw migration data, then applying a time variant filter, random noise attenuation in F/X domain and dynamic equalization in two windows. Migration velocities (95% of smooth stack velocities for North and South Logone Birni, 98% for Major or Central Logone Birni) were checked and adjusted, especially at intersections in appropriate scale using average velocities field displays. Depth conversions

were performed from datum plane +250 m to 5000 m for Northern Logone Birni, 6000 m for Central and 12,000 m for Southern LBB. Fig. 5 displays a comparison between a depth version section generated from the parent migrated section. 4.2. Gravity data The Centesimal Gravity Meter CG-2, No. 295 was used for the field measurements. The standard deviation of the supporting network amounted to 0.014 mGal. The spacing interval of the gravity sampling along seismic lines was 500 m. The gravity observation at selected positions was repeated every two hours to eliminate daily drift and tidal effects. The gravity data was pre-processed daily to eliminate the above mentioned effects. As a whole 803 gravity data points and 124 test points were measured. Daily gravity profiles were realized and the standard deviation of detailed gravity survey was determined at m = 0.030 mGal. This field data was used to adjust the aero gravity data. The gravity data was gridded at 2 km grid spacing. The map generated for the LBB is presented in Fig. 6 (left). 4.3. Magnetic Data The magnetic data processed by Carson Services for SNH (1999) was used for the analyses of volcanic complexes in LBB. The Data is gridded at an interval of 2 km and corrected for the International Geomagnetic Reference Field (IGRF) at the time of acquisition. Given that the magnetic equator passes through the middle of the

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Makari Sub-basin (Northern LBB)

Zina Sub-basin (Central LBB)

Yagoua Sub-basin (Southern LBB)

Fig. 4. The LBB database for the study: 9608 km of aeromagnetic data and 8166 km of aerogravity data acquired by Carson Services for SNH (1999). 1000 km of 2D seismic and gravity data acquired by Geofyzika s.a. for SNH (1998).

LBB, hence the RTP map of the Total Magnetic Field contains unacceptable north–south dominated edge effects, only the magnetic sources located within the sedimentary basin were considered. Fig. 6 (right) is the magnetic map generated for the LBB. 5. Interpretation of volcanic bodies 5.1. Geophysical profile analysis Selective 2-D modelling perpendicular to strike from the PC program Model Vision Pro was used to check whether an interpreted

geological cross-section produces the anomalous gravimetric or magnetic response measured at the surface. Four profiles are presented in this study to illustrate the results obtained from the modelling. These are profiles A in the northern LBB (Makari sub-basin), B and C in the central or major LBB (Zina sub-basin) and D in the southern LBB (Fig. 6). The seismic two-way times sections were converted to depth migrated versions and major horizons were delineated. The initial densities assigned to each layer are those from a study of the well data by Fairhead and Green (1989). The models were refined by increasing the densities of the sediments, and adjusting the position of the basement, until a more

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acceptable fit between the observed and the calculated curves was achieved. The densities used in the modelling are given in the table below.

Pliocene Tertiary Upper-Cretaceous (+volcanics) Lower Cretaceous (+interbeded volcanics) Basement

Former density model (g/cm3)

Final density model (g/cm3)

2.2 2.3 2.4 2.55

2.1–2.2 2.25–2.3 2.35–(2.8) 2.55–2.7

2.7

2.7

Although the 3-D inversion of the gravity and magnetic data was not realized, the 2D modelling was sufficient to complement the seismic interpretation. Given that depth estimates are crucially dependent on the curvature of the anomalies which get significantly smoothed in the gridding process, the initial approach for each anomaly analysed was based on the choice of the type of model (e.g. ribbon, block, dyke or step), together with a simple starting model. The horizontal prism model was also tested with continuous changes of depth of magnetic horizon spread over the basin (interbedded volcanics) to discard the effects of N–S orientation of the profiles (Fig. 7). The results obtained in this study corroborate the depth estimates for the anomalies interpreted over the study area by Bennett et al. (1995).

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has been recognized (Turonian–Senonian). It is associated in the eastern region with a flower structure. 5.3. Central or Major LBB (Zina sub-basin) Two perpendicular depth versions of migrated seismic sections (Profiles B and C) are illustrated along with the corresponding gravity and magnetic data. Profile B oriented NW–SE shows intense volcanic activity in the northwestern area on the flank of a half graben (Fig. 9). To the southeast, although basement is shallow, there is no evidence of volcanism probably due to the absence of shear in this area. Along the seismic profile C (Fig. 10), the largest negative magmatic anomaly in LBB is observed. It coincides with strong and chaotic reflections on the seismic interpreted as volcanic complexes. These complexes occur on the opposing flanks of basement horsts separating adjacent grabens. They often intertongue with the adjacent sediments. In the depocenters of the grabens volcanic activity is not observed. The volcanic bodies in the central LBB are found at depths ranging from 3000 to 1000 m either overlying basement in the western parts of the basin or within the Upper Albian Bima formation and Turonian to Lower Senonian Fika formation. At least three volcanic episodes can be recognized. Fig. 12 shows the distribution of buried volcanic complexes or bodies in the central LBB. They exhibit three buried volcanic trends which are E–W, NE–SW and NNW–SSE. Volcanic activity was more intense in the western region of the sub-basin. 5.4. Southern LBB

5.2. Northern LBB (Makari sub-basin) Depth migrated seismic profile A highlights some strong reflectors associated with negative magnetic anomalies interpreted to be volcanic bodies at depths between 900 and 600 m (Fig. 8). They are found on crestal positions associated with major thrust faults within the Turonian–Lower Senonian sedimentary sequence (Fika Formation). The gravity and seismic data indicate that this sub-basin deepens in the NE direction reaching depths up to 3500 m. The buried volcanic complexes identified in the northern LBB are aligned roughly in the NNW–SSE and NE–SW directions (Fig. 12). Only one volcanic episode

The interpretation of the seismic data and its comparison to the gravity data, indicates the existence of an elongated narrow graben with a sedimentary package up to 1500 m thick (Figs. 11 and 12). The interpreted basement is highly faulted and uplifted. The Lower Cretaceous sedimentary package has been gently folded and some faults show compressional offset indicating inversion of this portion of the basin in late Lower Cretaceous. The magnetic data consist of many smaller anomalies with wavelengths less than 3 km within the confines of the large gravity lows. This situation infers the existence of numerous intrusions. The combination of these results

Fig. 5. Depth version of migrated section from Central LBB (left) compared to its migrated section (right).

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Fig. 6. The gravity (left) and magnetic (right) maps of the LBB. Major faults are presented on magnetic map. Pink lines are from North to South: Profiles A (Northern LBB), B and C (Central LBB) and D (Southern LBB).

conclude to a Lower Cretaceous sedimentary package interbedded with basaltoid volcanics. 6. Discussion 6.1. Implications of the volcanic activity observed in the LBB Three main volcanic episodes have been identified in the central or major LBB (Zina sub-basin) during the Cretaceous respectively during the Rift Phases I and II of Genik (1993, 1992). Two episodes occurred in Lower Cretaceous (Rift Phase I) and the last episode took place during Upper Cretaceous (Rift Phase II). The first episode is represented by volcanic bodies lying directly on basement or interstratified in early Cretaceous sediments similar to the initial magmatic activity recorded in the WAS from Neocomian to Aptian and which consists of outcrops of alkaline rhyolites and basalts lying directly on Pre-Cambrian or Pan African basement or occurring along syndepositional faults oriented N60°E in the Upper Benue trough (Baudin, 1991; Guiraud, 1990, 1991; Popoff et al., 1982) almost in the same NE–SW to E–W volcanic alignments observed in the northern and central LBB. The equivalent volcanic activity in northern Cameroon half grabens is expressed at outcrops in the

Hamakoussou as fissural basaltic flows interbedded in BarremoAptian sediments (Loule et al., 1997; Maurin and Guiraud, 1990); whereas in the Mayo Oulo-Lere and Babouri-Figuil it is expressed as dykes and sills of basalts, microgabbros, olivine dolerites (continental tholeiites), and camptonites and benmoreites (alkaline rocks) cutting or underbedded into the Lower Cretaceous sediments (Ngounouno et al., 2001; Maurin and Guiraud, 1990). This volcanic episode is related to the onset of Rift Phase I. The second volcanic episode occurred during late Rift Phase I in the central and southern LBB where magnetic and gravity data allude to an intense volcanic activity. This volcanic episode is associated with the resumption of rifting during Aptian–Albian times in a regime of oblique extension. The associated volcanic bodies are in the southeastern continuation of the NNW–SSE alignment of the East Niger rift basins in the Lake Chad area. Volcanism during this period has only so far been documented at outcrops in some CAS basins in the form of sills of altered basalt and basaltic andesite (Genik, 1992, 1993). The last volcanic episode is observed both in the northern and central LBB and is related to Rift Phase II (Cenomanian–Maastrichtian). The corresponding volcanic bodies are morphologically expressed in the forms of dykes and sills along synsedimentary strike-slip faults

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S

95

N

Fig. 7. Modelling scheme based on prismatic and non-regular form of magnetic sources used in the Northern LBB to discard the effects of North–south orientation of the profiles.

similar to those described at outcrops in some WAS basins (Genik, 1992; McHargue et al., 1992; Wilson and Guiraud, 1992, 1998) along N120°E–N135°E synsedimentary sinistral faults (Guiraud, 1990, 1991). The volcanic activity highlighted in the LBB during the Cretaceous is more intense than the one so far known from other WCAS basins. This high level of volcanism is related to wrench-induced pull-apart basins and oblique-slip zones (Garfunkel and Ben-Avraham, 2001; Laville and Petit, 1984; Manspeizer, 1982) and is often enhanced by the fact that the major wrench faults cut through the entire lithosphere, thus providing conduits for magma migration to the surface (Wilson and Guiraud, 1992). Moreover, Ziegler and Cloetingh (2004) have highlighted that volcanic activity is often associated with more or less doming of the rift zone, therefore rifts totally devoid of volcanic rocks or that show only low level of volcanism are generally not associated with large radius crustal doming. The intense volcanism observed in the LBB could therefore be linked to mantle processes that are known to cause both linear and radial zones of high heat flow, low density upper mantle, volcanic activity and topographic uplift as documented in the Red Sea, Gulf of Aden and the East African Rift System (Reading, 1986). 6.2. Tectonic Evolution of the LBB and its relationship to the WCAS The presence of two separate volcanic episodes during the Lower Cretaceous and their almost orthogonal orientations concur with the change in the stress regime in the LBB during Rift Phase I (Neocomian to Albian). This change has also been illustrated by Warren (2009) who identifies two extensional cycles during this period in the central part of the LBB through the existence of two opposing sin-rift growth packages. The older interval thickens to the southern basin-bounding fault, and a younger interval that thickens to the northern basinbounding fault. He further noted that the top sin-rift package has been gently folded and some faults show compressional offset indicating inversion of this portion of the basin in late Lower Cretaceous. The presence of sibilant flower structuring to the east of the Lower Cretaceous section in the southern LBB (Fig. 11) which could have been attributed to the late Albian transgression is in this context related to inversion as indicated by uplift and erosion truncation of the Lower Cretaceous sediments. Close examination of the LBB seismic data indicates that the volcanic bodies associated with the Upper Cretaceous episode occur within a sedimentary sequence eroded by a strongly truncation unconformity

that cuts through inversion anticlines and transgression related flower structures attributable to the Santonian transpersonal event. These transgression structures are often well developed to the East and south of the LBB perhaps due to closer proximity to the Central Africa Shear Zone (CASZ). Therefore, strike-slip tectonics associated with the CASZ reactivated deep seated fractures which serve as conduits for the deep-seated magma during Upper Cretaceous. The intense volcanic activity observed in the western region of the central LBB (Zina sub-basin) along the Cameroon–Nigeria border is in the contrary attributable to the proximity to the WAS tectonics. From the preceding, the rifting phase I in the LBB comprises two sub rifts phases: Ia) from Neocomian to Early Aptian and Ib) in Aptian– Albian, with a renewed tectonic intensity during the Early Aptian, accompanied by volcanism. Unlike in other WCAS basins, the rift phase I terminated with the inversion of the LBB in late Albian. Phase II of rifting in LBB, similar to other WCAS basins, began with a short lived rifting period in Cenomanian followed by sag and tectonic subsidence during Turonian and Coniacian. This sag period was interrupted in Santonian by a short lived compressional event reflected in the WAS by folds parallel to the ENE–WSW axis of the Benue Trough and NE–SW transpersonal anticlines in Niger and Chad, whereas in the CAS widespread dextral strike-slip movements created reverse faulting and inversion features as well as a counter clockwise rotation of the Iola, Bongor, Doba and Doseo basins (Castro, 1987; Genik, 1992; Guiraud and Bosworth, 1997; Guiraud et al., 1992; Manga et al., 2001; McHargue et al., 1992; Warren, 2009); features also observed in LBB. Volcanism is associated with this rift phase in the LBB as also known from other WAS basins. As already indicated by Ngako et al. (2006) and Maurin and Guiraud (1990) for the northern Cameroon basins of Figuil, Mayo Oulo and Koum, it is difficult to prove that the magmas were emplaced during the progressive opening of the LBB although, in the Hama-Koussou basin nearby basaltic lava flows are interbedded with shales whose age is constrained in the Neocomian shales (Loule et al., 1997). 7. Conclusions The interpretation of the existing seismic, gravity and magnetic data of the LBB indicates an intense volcanic activity in this region during the Cretaceous compared to the neighbouring WCAS basins. The three main volcanic episodes identified are associated respectively to the onset of

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Fig. 8. Profile A (Northern LBB). Bottom: uninterpreted depth migrated seismic section. Middle: structural interpretation of the migrated section showing a flower structure in the East and the interpreted volcanic bodies. Top: modelled gravity and magnetic.

crustal extension following the break-up of Gondwana in Neocomian, then resumption of rifting in Aptian-Albian in a regime of oblique extension and finally changes in the drift pattern of Africa under a renewed regional stress regime in Cenomanian–Turonian. The fact that the central sub-basin presents the three volcanic episodes unlike the northern and southern LBB testifies to the known variability of the

level and timing of volcanic activity in rift systems. This intense volcanic activity in LBB during Cretaceous is attributed to the peculiar tectonic evolution of this transitional basin which is related to its location between the WAS constituted by a series of juxtaposed pull-apart basins generated along sinistral N 60° E strike-slip faults in the west and the CAS dominated by a major, ENE–WSW oriented, dextral, strike-slip

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Fig. 9. Profile B (Central LBB). Bottom: uninterpreted depth migrated seismic section. Middle: structural interpretation of the depth migrated section showing interpreted volcanic bodies. Top: modelled gravity and magnetic.

CASZ along which NW–SE and WSW–ENE narrow pull-apart basins were developed (Djerem Mbere, Doba, Doseo and Bongor) in the east. Although some may argue for partial melt beneath the rifts during periods of high stress or significant stress change, the high level of volcanic activity observed in the LBB suggests crustal doming due to thermal changes in the upper mantle in the study area flanked respectively to the north, south and southwest by the Lake Chad, Poli and Chum triple junctions and where major wrench faults transected the entire lithosphere, thus providing conduits for magma migration to the surface (Wilson and Guiraud, 1992).

Acknowledgements We would like to thank the Executive General Manager of the National Hydrocarbons Corporation of the Republic of Cameroon (SNH) for his continued support and vision without which the database used

in this study would not have been set up. Our thanks also go to the Institute of Geodesy of the Brno University of Technology, Czech Republic for allowing the use of their geophysical infrastructures to perform this work. References Avbovbo, A.A., Eyoola, E.O., Osahon, G.A., 1986. Depositional and structural styles in Chad basin of NE Nigeria. American Association of Petroleum Geologists 70 (12), 1787–1798. Baudin, P., 1991. Le magmatisme mésozoique à cénozoique du fossé de la Bénoué (Nigeria) : Géochronologie, pétrogenèse, cadre géodynamique. Ph.D. Thesis, Université Aix-Marseille III. Benkhelil, J., 1988. Structure et évolution géodynamique du basin intracontinental de la Benoué, Nigeria. Bulletin des Centres de Recherches Exploration-Production ElfAquitaine 12 (1), 29–128. Bennett, K.J., Fairhead, J.D., Somerton, I.W., Stuart, G.W., 1995. Seismic, gravity and magnetic interpretation of the Logone-Birni Basin, Cameroon. Report No. G9508, Contract No. GT3607. Geophysical Exploration Technology (GETECH) a division of University of Leeds Innovations Ltd (ULIS) — SNH, Yaoundé (33 pp.).

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Fig. 10. Profile C (Central LBB) perpendicular to Profile B. Bottom: uninterpreted depth migrated seismic section. Middle: structural interpretation of the depth migrated section showing interpreted volcanic complexes or bodies. Top: modelled gravity and magnetic.

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Fig. 11. Eastern part of interpreted profile D in Southern LBB where the narrow graben is combined with activity of marginal faults (“Flower” structure). A: Quaternary–Tertiary shallow cover, B: Upper Cretaceous sediments with interbedded volcanic, H1: Base Upper Cretaceous unconformity, C: Lower Cretaceous sediments, D: interpreted faulted Paleozoic complex?, Cr: crystalline basement. Location of profile in Figs. 6 and 12.

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Fig. 12. Distribution of buried volcanic bodies in LBB based on combined interpretation of seismic, gravity and magnetic data. Profile A is in northern LBB, profiles B and C in Central LBB, profile D in southern LBB. Numbers in white squares in map are depths of interpreted magnetic sources (supposed volcanic bodies).