Sébastien Owona

Eruptive history of the Barombi Mbo Maar, Cameroon Volcanic Line, Central Africa: Constraints from volcanic facies analysis

Abstracthis study presents the first and detail field investigations of exposed deposits at proximal sections of the Barombi Mbo Maar (BMM), NE Mt Cameroon, with the aim of documenting its past activity, providing insight on the stratigraphic distribution, depositional process, and evolution of the eruptive sequences during its formation. Field evidence reveals that the BMM deposit is about 126m thick, of which about 20m is buried lowermost under the lake level and covered by vegetation. Based on variation in pyroclastic facies within the deposit, it can be divided into three main stratigraphic units: U1, U2 and U3. Interpretation of these features indicates that U1 consists of alternating lapilli-ash-lapilli beds series, in which fallout derived individual lapilli-rich beds are demarcated by surges deposits made up of thin, fine-grained and consolidated ash-beds that are well-defined, well-sorted and laterally continuous in outcrop scale. U2, a pyroclastic fall-derived unit, shows crudely lenticular stratified scoriaceous layers, in which many fluidal and spindle bombs-rich lapilli-beds are separated by very thin, coarse-vesiculatedash-beds, overlain by a mantle xenolith- and accidental lithic-rich explosive breccia, and massive lapilli tuff and lapillistone. U3 displays a series of surges and pyroclastic fall layers. Emplacement processes were largely controlled by fallout deposition and turbulent diluted pyroclastic density currents under “dry” and “wet” conditions. The eruptive activity evolved in a series of initial phreatic eruptions, which gradually became phreatomagmatic, followed by a phreato-Strombolian and a violent phreatomagmatic fragmentation. A relatively long-time break, demonstrated by a paleosol between U2 and U3, would have permitted the feeding of the root zone or the prominent crater by the water that sustained the next eruptive episode, dominated by subsequent phreatomagmatic eruptions. These preliminary results require complementary studies, such as geochemistry, for a better understanding of the changes in the eruptive styles, and to develop more constraints on the maar’s polygenetic origin.

Quantification of Strain of the Pan-African in Mvog-Betsi Area (Yaoundé Group, Cameroon)

The ductile deformation in the paragneissic bed of Mvog-Betsi in the north-eastern part of Yaounde (Cameroon) appears to be intensive and may be traduced by a high shear rate (more than 10%). Some marker subjects that may quantify this strain are observed. Those are elliptical quartz and feldspar, and folds. The study of elliptical markers shows their preferential orientation. The initial rate R i of the markers before the strain approaches 3.76, and the harmonic value of R f is between 1.51 and 1.71. Main orientation O f of strain’s ellipse from the direction of stretching in actual position is situated between -10 and -19. The strain’s rate R S is comprised between 1.1 and 1.7. The orientation O S of the strain’s ellipse is situated between N10E and N20E. The rate of shortening varies between 20% and 75%.

Evidence of the ~NE-SW extension in the Sa’a - Monatélé Region as in the Bafia and Yaounde groups within the Central African Fold belt (Cameroon): Implication for the Southern Cameroon Neoproterozoic extension

1 Abstract The Sa’a-Monatélé Region within the Neoproterozoic North Equatorial Fold Belt located between the Bafia and Yaounde groups, highlights a polyphase D1D4 Pan-African deformation that emplaced the Yaounde tectonic nappe transported top-to the SSW onto the Congo Craton and responsible for its actual geometry. D1 is a compression tectonic phase dominated by the simple shear type, represented by S0/1 foliation and F1 folds. D2 is a general flattening under a pure shear regime associated to the exhumation of D1 nappe due to the NE-SW extension; responsible for L2 lineations, B2 boudins, F2 folds, S0/1/2 and S2 foliations in metapelites and metagranitoids, respectively and C2 shear planes. D3 mainly an ~E-W shortening and a ~NE-SW extension, emplaced F3 mesoand large-scale folds that form tectonic units of the Yaounde nappe. D4 was an N-S to NE-SW shortening, perpendicular to the D3 once and represented by F4 large-scale folds. C3 shear zones as the NE-SW Central Cameroon shear zone was emplaced under the same regime. The Sa’a-Monatélé Region may correspond mostly accordingly to a thrusting zone than a real tectonic suture between the Bafia and Yaounde groups. In comparison, D1-D4 stages show similar geometry and kinematics in Bafia and Yaounde groups. Both groups are composed of similar rock types made of lowto high grade metapelites, amphibolites and metagranitoids with comparable volcano-sedimentary origin for metapelites and amphibolites; Archean to Proterozoic (Nd model ages 3500-2100 Ma) in metapelites, crustal rocks with minor mantle contributions origin for metagranitoids; the Neoproterozoic as the same maximum deposit age fixed at ~1600 Ma (U/Pb-, Pb/Pb-detrital zircons) as well as a comparable syntectonic granitoids emplaced between ~625 – 600 Ma (U/Pb-, Pb/Pbzircons); an analogous Pan-African orogeny (650-542 Ma) with the retrograde phase from the granulite to amphibolite facies that ranges 616 – 540 Ma (Sm/Nd-garnet, EMP-monazite, Rb/Srwhole rock biotite ± muscovite). The admitted Bafia and Yaounde groups that outline the similar litho-chemistry nature and origin, a comparable Pan-African thermotectonic evolution can therefore be considered as a single and a same litho-chrono-thermo-structural unit: the Yaounde Group. ISSN 1029–2225©2014 Sciences, Technologies et Développementos