Tectonic evolution of the Central Steep Zone, Axum area, northern Ethiopia: inferences from magnetic and geochemical data

Abstract

AbstractNorthern Ethiopia is marked by a fanning system of thrust planes with NW-dipping structures in the east and southeast-dipping in the west. The central zone of this large-scale (200 km long) structure is formed by a c. 10 km wide zone of localized strain and amphibolites facies metamorphic conditions (680 °C and 3.4 kbar) referred to as the Central Steep Zone (CSZ). The CSZ comprises a mafic rock assemblage of amphibolite, serpentinite showing ocean-floor characteristics and calc-silicate schist. A monzonite intrusion in the central part of the CSZ post-dates the deformation and is related to partial melting of the mafic rocks. Magnetic fabric measurements reveal NE-trending (043°) steep foliations in the CSZ with vertical orientation of lineation, parallel to the axes of micro-folds. This high-strain zone is interpreted as central zone of a positive flower structure on the basis of simultaneous flattening and shear movement, typical for transpressive kinematics. The CSZ has a northern continuation into the Nafka terrane of Eritrea where it can be traced over a distance of 200 km. This high-strain belt forms a major structure in the context of Arabian–Nubian Shield (ANS) collision tectonics during the closure of the Mozambique Ocean and assembly of Gondwana.