Terrane rotation during the East African Orogeny: Evidence from the Bulbul Shear Zone, south Ethiopia

Abstract

The 100 km long, N-trending, Neoproterozoic–Early Paleozoic Bulbul Shear Zone in southern Ethiopia is marked by sheared ophiolites at the interface between the Arabian–Nubian Shield in the north and the Mozambique Belt in the south. This shear zone separates the low-grade meta-volcanic and meta-sedimentary rocks of the Bulbul Terrane in the east from the medium- to high-grade gneissic, migmatites and granulites of the Alghe Terrane in the west. Stretching lineations along the Bulbul Shear Zone vary from NE-plunging in the northern part, shallowly N- and S-plunging in the central part, to SE-plunging in the south. These lineations are developed along N-trending mylonitic foliation that is moderately to steeply E-dipping. The northern part of the Bulbul Shear Zone is dominated by SW-verging fold and thrust belt indicating top-to-the southwest tectonic transport. The central part is characterized by dextral strike-slip displacement. The southern part is dominated by E-dipping oblique normal-slip planes associated with top-to-the southeast tectonic transport. Down dip stretching lineations along E-dipping slip planes are well-developed in the eastern part of the Alghe terrane and the western part of the Bulbul terrane. We interpret the along-strike variation of stretching lineations and kinematic indicators as due to NE–SW directed oblique collision between the Bulbul Terrane and the Alghe Terrane accompanied by anti-clockwise rotation of the Bulbul Terrane. Such collision and rotation are manifested by SW-verging fold and thrust belt in the north, N-trending dextral strike-slip shear zone in the center, and SE-directed normal-slip displacement in the south. This tectonic event might have occurred between 820 and 580 Ma. This was followed by E-ward slipping of the Bulbul Terrane relative to the Alghe Terrane, probably between 580 and 500 Ma.