Pascal Affaton

Thermal and geodynamic setting of the Buem volcanic rocks near Tiélé, Northwest Bénin, West Africa

Abstract The mafic volcanic rocks of Tiele occur as intercalations within very low-grade metasediments, mainly metapelites and meta-arenites. These Proterozoic supracrustal rocks are considered as part of the Buem Structural Unit (BSU) which is the most external tectonic unit of the Pan-African Dahomeyide fold belt. The geodynamic significance of the BSU is still debated: it could represent either a portion of the West-African passive margin during the rifting and the drifting of the Pan-African ocean (Affaton, 1990), or a continental rift-zone (Jones, 1990) or the Pan-African oceanic crust itself (Burke and Dewey, 1972 and Burke and Dewey, 1973; Shackleton, 1976). The mafic lavas derived from tholeiitic magmas, which originated by partial melting of the lithospheric mantle with a possible asthenospheric contribution. They are quite similar to MORBs and free of any crustal contamination. They are related to a strongly attenuated continental lithosphere, and a passive margin would be the most likely tectonic environment for the coeval volcanic and sedimentary activities. On the other hand, these rocks demonstrate effects of early static recrystallizations related to a very low grade metamorphism of the prehnite-pumpellyite facies developed under temperatures of 200 to 300°C. Such a thermal imprint clearly marks an extensional tectonic stage. Later, the early parageneses were re-equilibrated under slightly higher pressures during the Pan-African collision. Both magmatic features and early metamorphic characters of these metavolcanic rocks are in good agreement with a context of lithospheric thinning characterized by mantle-derived magmatism and high thermal-gradient. These results fit with previous interpretations mainly based on sedimentary and structural features: the BSU represents a portion of a Proterozoic passive margin which suffered strong lithospheric stretching, with deep seated normal faults and mantle derived magmatism, and a subsequent severe subsidence evidenced by the prominent detrital sedimentation. Such a tectonic setting developed during the Pan-African oceanic drifting

Pan-African Paleostresses and Reactivation of the Eburnean Basement Complex in Southeast Ghana (West Africa)

This faulting tectonics analysis concerns the southernmost segment of the Dahomeyide Orogen and the West-African craton eastern margin in southeast Ghana. The analysis of strike-slip faults in the frontal units of the Dahomeyide Belt indicates that four distinct compressive events (NE-SW, ENE-WSW to E-W, ESE-WNW to SE-NW and SE-NW to SSE-NNW) originated the juxtaposition of the Pan-African Mobile Zone and the West-African craton. These paleostress systems define a clockwise rotation of the compressional axis during the structuring of the Dahomeyide Orogen (650–550 Ma). The SE-NW and SSE-NNW to N-S compressional axes in the cratonic domain and its cover (Volta Basin) suggest that the reactivation of the eastern edge of the West African craton is coeval with the last stages of the Pan-African tectogenesis in southeast Ghana. An extensional episode expressed as late normal faulting is also recorded in this study. This E-W to SE-NW extension, which is particular to the southernmost part of the Dahomeyide Belt, appears to be post-Pan-African. This extension probably contributed to the formation of a major Jurassic rifting zone that originated the Central Atlantic and the Benue Trough.