Neoproterozoic–Early Paleozoic gravitational tectonic collapse in the southern part of the Arabian-Nubian Shield: The Bulbul Belt of southern Ethiopia

Abstract

We present new structural and metamorphic data from the Bulbul Belt in southern Ethiopia indicating that the latest phase of the Neoproterozoic evolution of the southern part of the Arabian-Nubian Shield is characterized by the development of low-angle oblique normal-slip shear zones manifesting regional gravitational tectonic collapse. The ∼100 km long, ∼1–5 km wide, N-trending Chulul Shear Zone which represents the southern part of the Bulbul Shear Zone is characterized by the presence of moderately E-dipping mylonitic fabric, SE-plunging stretching lineation and kinematic indicators showing top-to-the-southeast tectonic transport. This shear zone separates the structurally and metamorphically contrasting Melka Guba Domain (the southern extension of the Alghe terrane) in the west from the Chulul Domain (the southern extension of the Bulbul terrane) to the east. The Melka Guba Domain comprises migmatites, hornblende–biotite gneisses and quartzo-feldspathic gneisses with metamorphic assemblages that indicate upper amphibolite metamorphism ( T of ∼650 °C and P of ∼6 kbar). In addition, the Melka Guba Domain preserves E-W trending relic gneissic and migmatitic layering that is not evident elsewhere in the region. This older planar fabric is discordantly overprinted by a regional gneissic and migmatitic layering that was subsequently folded about NNE-trending upright folds. The Chulul Domain, on the other hand, is dominated by amphibolite schist, mafic and ultramafic rocks and meta-diorites interpreted as an island arc-ophiolitic-plutonic assemblage. Metamorphic mineral assemblages in the Chulul Domain suggest greenschist facies metamorphism ( T of ∼450 °C and P of ∼3 kbar). Deformation in the Chulul Domain is dominated by N-trending schistosity similar in orientation to that in the Melka Guba Domain to the west. We explain these structural and metamorphic contrasts using a model of gravitational tectonic collapse (∼550–500 Ma) involving detachment along the Chulul Shear Zone. The collapse follows the collision between the Melka Guba and the Chulul Domains at ∼820–580 Ma after the consumption of a possibly E-dipping subduction zone. Our model offers little support to the theory that the Neoproterozoic N-trending belts in southern Ethiopia were roots of northward expulsion of the Arabian-Nubian Shield from the Mozambique Belt.