Terrane rotation during the East African Orogeny: Evidence from the Bulbul Shear Zone, south Ethiopia Mohamed G. Abdelsalam a, ⁎ , Lulu Tsige b , Tadesse Yihunie b , Bedru Hussien b a Department of Geological Sciences and Engineering, Missouri University of Science and Technology, 129 McNutt Hall, 1400 North Bishop Avenue, Rolla, MO, 65409, USA b Department of Regional Mapping, Geological Survey of Ethiopia, Addis Ababa, Ethiopia Received 20 August 2007; received in revised form 16 April 2008; accepted 26 May 2008 Available online 4 June 2008 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. © 2008 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. Keywords: Precambrian; Southern Ethiopia; Bulbul Shear Zone; Terrane rotation; East African Orogen 1. Introduction Southern Ethiopia is underlain by Neoproterozoic–Early Paleozoic rocks, which were formed and/or deformed during the East African Orogeny associated with collision between East and West Gondwana (Stern, 1994; Fig. 1) after the closure of the Mozambique Ocean (Santosh et al., 2006; Vaughan and Pankhurst, 2008). Meert (2003) and Meert and Lieberman (in press) have proposed two phases during the assembly of Gondwana. The first phase includes the East African Orogeny (eastern Africa and Arabia) which spanned the period between 700 and 570 Ma whereas the second phase was the Kuunga Orogeny which occurred between 570 and 530 Ma and this is now preserved mainly in southeastern Africa, western and northern Antarctica, and western India. Recent 40 Ar/ 39 Ar age data from the Ross Orogen in Antarctica suggest that the final suturing between East and West Gondwana was accompanied by short-lived, but regional constriction deformation between ~515–510 and ~495 Ma (Paulsen et al., 2007). The Bulbul Shear Zone is a Neoproterozoic–Early Paleozoic structure in southern Ethiopia (Fig. 2) separates the low-grade meta-volcanic and meta-sedimentary rocks of the Bulbul Terrane in the east from the medium- to high-grade gneisses, migmatites and granulites of the Alghe Terrane in the west (Fig. 3). It is one of three (Megado, Kenticha, and Bulbul) ~15 km wide and ~100 km long, N-trending belts in southern Ethiopia (Fig. 2). These belts occur as low-grade volcano-sedimentary-ophiolite zones within Available online at www.sciencedirect.com Gondwana Research 14 (2008) 497 – 508 www.elsevier.com/locate/gr ⁎ Corresponding author. Tel.: +573 341 4100; fax: +573 341 6935. E-mail address: abdelsam@mst.edu (M.G. Abdelsalam). 1342-937X/$ - see front matter © 2008 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2008.05.001