Paleoproterozoic structural evolution of the Man-Leo Shield (West Africa). Key structures for vertical to transcurrent tectonics

Abstract

In the Man-Leo Shield, Paleoproterozoic (Birimian) belts crop out in nine countries of West Africa. Dominant domains include: (i) greenstone belts composed of plutono-volcanic, volcano-clastic and sedimentary rocks, deformed and weakly metamorphosed under regional greenschist facies conditions; (ii) widespread granitoid batholiths. The domains display a basin and dome-like architecture, and are overprinted by partitioned structures from successively shallower crustal depth. Analyses of key ductile and brittle structures has shown that the structural evolution of Man-Leo Shield is characterized by early vertical magmato-tectonics and subsequently, horizontal transcurrent tectonics with progression from ductile to brittle behavior. Basin and dome-like architectures, and the formation of an ubiquitous vertical foliation (MF) formed during emplacement of early amphibole-bearing (PAG) granite plutons at ca. 2.2 Ga by diapirism during NW–SE crustal shortening. Subsequent to a late stage of predominantly NW–SE shortening that created steeply-dipping mylonite zones (Mz1), transcurrent faults became predominant. The formation of transcurrent faults began transpressively, with development of N–S trending regional-scale mylonite zones (Mz1), and a steeply-plunging stretching lineation that probably formed during emplacement of PAG-type granitoids ca. 2.15 Ga. NNE–SSW transpressive sinistral horsetail faults and many NW–SE trending tension veins are interpreted to have formed at this stage. After cooling of the upper crust ca. 2.1 Ga, transcurrent faults became strike-slip in character with formation of dominantly NE–SW dextral faults (Mz2) and the passive emplacement of biotite (PBG) granitoids. Clockwise rotation of the extensional stress axis ( σ 3) from NNE–SSW trending to ENE–SSW trending assisted the propagation of dextral NE–SW and sinistral NW–SE extensional en echelon horsetail faults. WNW–ESE trending extension jogs (Egz) are interpreted to have been initiated under the same stress conditions. Displacements on strike-slip/transcurrent faults are interpreted as the product of rotation of rigid nuclei blocks producing faults’ re-activation. On the Man-Leo Shield Paleoproterozoic rocks are poorly exposed, but the tectonic model proposed in this study can help to shed light on the structural setting in areas of the shield which are poorly exposed, and in particular, why regional-scale structures do not display significant horizontal displacements. For practical use, key structural criteria can help to identify mylonite zones and transcurrent faults at different scales of investigation.