Constraints on the tectonometamorphic evolution of the Western Ethiopian Shield

The Western Ethiopian Shield comprises three lithotectonic units: the Birbir domain, an assemblage of deformed and metamorphosed volcano-sedimentary rocks and associated ultramafic to felsic magmatic rocks, is bounded by the dominantly orthogneissic Baro and Geba domains. Geochemical considerations suggest the igneous rocks of the Birbir domain are predominantly calc-alkaline and similar to those generated by subduction in modern oceanic island-arcs, broadly consistent with Sm–Nd data that indicate an early crust-forming event at ca. 0.95–1.1 Ga. This complex was accreted during the Neoproterozoic East African Orogeny and formed the core and source area for subsequent plutonism and metamorphism. The earliest recorded deformation resulted in a sub-horizontal gneissosity within the gneissic terrains, synchronous with an early (ca. 770–800 Ma) metamorphic event (M1), which records amphibolite facies conditions (600–800 °C and ca. 6–8 kbar) in the gneissic terrains and greenschist to amphibolite facies conditions in the Birbir domain. The igneous protoliths to the orthogneisses (i.e., pre-dating M1) are dated at 830–785 Ma. All terrains were subsequently deformed in the D2 event, the result of oblique westerly directed transpression, with consequent folding and thrusting concentrated at domain boundaries. Subsequent D3 deformation was focussed within mylonitized zones that likely represent reactivated D2 thrusts and record both dextral (early) and sinistral (late) transcurrent movement. A second protracted metamorphic event, M2, at around 650–550 Ma, resulted in rehydration of M1 parageneses and was fluid-driven. M2 metamorphism was synchronous with D3 shearing. Fluid incursion was concentrated within sheared domain boundaries and caused isotopic re-equilibration. The WES records a long-lived but episodic tectonothermal event that can be explained in terms of arc–continent collision. Melting of a subducting slab and associated intrusion caused metamorphism and the formation of an oceanic island-arc complex. Continued plate convergence caused severe E–W shortening and basin closure. Further attenuation gave rise to transcurrent shearing, fluid influx and a second thermal event during accretion and orogenic closure.