Structural and metamorphic evolution of Neoproterozoic rocks in Bensa-Girja area, southern Ethiopia

Abstract

The Bensa-Girja area is part of the Adola Neoproterozoic collisional belt in southern Ethiopia. Lithological, structural, and metamorphic characteristics of the area are presented here based on the interpretation of field, microstructural and metamorphic mineral data. The area comprises poly-deformed and poly-metamorphosed basal gneissic units, accretionary wedge metasediments, mafic-ultramafic rocks and associated upper metasediments. These units, bounded by reverse shear zones, were assembled during the Neoproterozoic East African orogeny. Two subsequent episodes of deformation are recognized. A progressive D₁ deformation consisting of an early-stage event (D_1A) forming regional N-oriented S_1A foliation, WNW-plunging stretching lineations (L_1A), N- to NNE-plunging recumbent isoclinal F_1A folds and N-S oriented reverse shear zones, followed by a later stage (D_1B) developing upright F_1B folds with N-S axial trace. The D₂ deformation phase overprinted earlier fabrics and resulted in N- to NNW-oriented subvertical, sinistral shear zones and associated F₂ folding. Mineral assemblages and microprobe data indicate peak M₁ metamorphism (epidote-amphibolite to amphibolite facies) at ∼489–729 ^oC and 4.3–8.7 kbar during the D₁ deformation, whereas greenschist-facies retrogression (M₂) was coeval with D₂ shearing. Structural data suggest a top-to-ESE sense of movement related to broadly westward subduction zone, possibly coincident with the north-west direction of subduction reported in Eritrea and Kenya. The D₂ shear zones are considered to share similar structural trends and shear sense with the ductile shear zones developed during the Loldaikan–Kipsingian event (<570 to ∼530 Ma) in northern Kenya, Eritrea, Sudan, and Egypt.