METAMORPHIC AND STRUCTURAL EVOLUTION OF THE GORE-GAMBELLA AREA, WESTERN ETHIOPIA

Abstract

The Gore-Gambella area comprises of three lithotectonic units: the Birbir domain, an assemblage of mafic to felsic intrusive and extrusive rocks and mainly volcanogenic sedimentary rocks, is metamorphosed to low amphibolite facies. It is enclosed between the Baro and Geba domains, that consist of upper amphibolite facies ortho- and paragneisses and migmatites. The Birbir domain is a major transcurrent shear belt, in which all rock units have been mylonitized to varying degrees. Foliations are steep and primarily sub-parallel to layering; extension lineations are sub-horizontal. The boundaries of the domain
are tectonic, and deformation is concentrated near the western contact, in the 5 kID-wide Birbir shear zone. Gneissic layering in the high grade domains is openly folded, demonstrating at least two periods of deformation; lineations are shallow like those of the Birbir domain. There are steep gradients in metamorphic conditions at the domain boundaries: diagnostic metamorphic assemblages in the western Birbir domain indicate ca. 5200 C and 4 kbar, whereas those in the eastern Baro domain denote ca. 7000 C and 7 khar.