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Mohamed Abdelsalam, Missouri University of Science and Technology (United States)
Lulu Tsige , Geological Survey of Ethiopia (Ethiopia)
Tadesse Yihunie, Geological Survey of Ethiopia (Ethiopia)
Bedru Hussien, Geological Survey of Ethiopia (Ethiopia)
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The 100 km long, N-trending, Neoproterozoic - Early Paleozoic Bulbul Shear Zone in southern Ethiopia is partially marked by sheared ophiolites and crops out at the interface between the Arabian-Nubian Shield in the north and the Mozambique Belt to the south. This shear zone separates the low-grade green-schist facies 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 to the west. Stretching lineations along the Bulbul Shear Zone vary from NE-plunging in the north, to shallowly N- and S-plunging in its central part, to SE-plunging in the south. These lineations are enclosed within N-trending mylonitic foliation that is moderately to steeply E-dipping.
Kinematic indicators show that: (1) The northern part of the Bulbul Shear Zone is dominated by SW-verging fold and thrust belt indicating top-to-the southwest tectonic transport. (2) The central part of the shear zone is characterized by dextral strike-slip displacement. (3) The southern part of the shear zone is dominated by E-dipping oblique normal-slip planes associated with top-to-the southeast tectonic transport.
In addition, various parts of the shear zone as well as the eastern and western margins of the Alghe and Bulbul Terranes, respectively show down-dip, E-plunging stretching lineation enclosed within E-dipping slip planes. We interpret the along-strike variation of stretching lineation 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 have translated into 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.
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