Structural evidence for the allochthonous nature of the Bulbul terrane in southern Ethiopia: A west-verging thrust nappe

Abstract

The Neoproterozoic basement of southern Ethiopia links the low-grade Pan-African province of the Arabian–Nubian Shield to the high-grade Mozambique Belt to the south. In this intervening area, a northward terminating low-grade metavolcano–sedimentary and mafic–ultramafic sequence of the Bulbul terrane gently overlies moderately to steeply dipping granitic migmatites of the Alghe gneissic terrane. The contact between the two terranes is a right lateral thrust. In the Bulbul terrane, rocks are part of an overturned sequence with a gently east-dipping composite D1/D2 foliation containing downdip and NE-plunging stretching lineations and westerly verging intrafolial folds. These structures are interpreted to have been developed during westward thrusting. The Bulbul sequence was therefore detached and tectonically transported to the west as a thrust nappe of which the lower inverted limb is still preserved. Structural evidence suggest yet another smaller nappe sequence (terrane) is present in southern Ethiopia and further suggest that Neoproterozoic obducted crust of the Arabian–Nubian Shield in eastern Ethiopia may be located beneath Phanerozoic cover.