The architecture of the Buem Structural Unit: Implications for the tectonic evolution of the Pan-African Dahomeyide Orogen, West Africa.

Abstract

The Buem Structural Unit (BSU) occurs about 50 km west of the high-pressure (HP) rocks of the Pharusian suture zone (PSZ) of the Dahomeyide Orogen, West Africa. Shale and sandstone intercalations form the base of the BSU, followed by serpentinised peridotite, gabbro, massive and pillow basalts (fragment of oceanic crust), jasper/ chert, and limestone/dolostone, with sandstone and diamictite forming the top of the BSU. The package was deformed at lower greenschist-facies conditions during the Pan-African orogeny. The N-S-trending BSU has been affected by three deformational events. The main deformation event D2, which transposed bedding-parallel D1 fabrics, produced a steeply ENE-dipping S2 tectonic foliation. S2 is axial planar to upright or west-verging isoclinal and chevron F2 folds. Top-to-the-west D2 shear zones in the sandstone, shale and serpentinised peri dotite are associated with a down-dip ENE-plunging L2 stretching lineation. The mafic-ultramafic rocks occur as D2 tectonic slices, marking the boundaries between thrust sheets within the BSU. The D2-related N-S trending structural and short-wavelength (< 1 km) magnetic grain of the BSU is superimposed on ENE-trending long wavelength (> 1 km) magnetic anomalies that are continuous along trend with the regional structures of the West African Craton (WAC) and are interpreted as signals from the basement rocks of the BSU. D2 deformation is interpreted to result from overthrusting of the BSU onto the southeastern margin of the WAC during collision of the WAC and Benino-Nigerian Shield (BNS). Meter-scale, steep and open F3 kink and crenulation folds with steeply SE-dipping axial planes are sporadically observed throughout the entire BSU. D3 deformation is inferred to reflect a period of NW-SE-shortening. Deformation history of the BSU shares structural similarities with the one of the PSZ. We, therefore, interpret the BSU as an allochthonous fold and thrust belt with a geodynamic evolution involving subduction of the WAC beneath the BNS and the ensuing collision between the two terranes. The tectonic framework of the BSU suggests a geodynamic evolution of the Dahomeyide Orogen evolving from initial accretion of oceanic crust and deposition of passive margin sedimentary sequences, and their inversion due to continent-continent collision during the Pan-African orogeny.