EVOLUTION OF THE BULBUL SHEAR ZONE IN SOUTHERN ETHIOPIA: FROM TERRANE COLLISION TO TECTONIC COLLAPSE

The Neoproterozoic-early Paleozoic N-trending Bulbul Shear Zone (BSZ) in the southern part of the Arabian-Nubian Shield in southern Ethiopia preserve early oblique terrane collision structures in the form of SW-verging folds and thrusts followed by development of N-trending dextral strike slip faults and culminated with regional gravitational tectonic collapse manifested by low-angle oblique normal-slip shear zones. The >100 km long, ~5 km wide, BSZ juxtaposes the structurally and metamorphically contrasting Alghe Terrane (AT) in the west and the Bulbul Terrnae (BT) to the east. The AT comprises various gneisses with mineral assemblages in the upper amphibolite facies metamorphism (T= ~650„aC and P= ~6 kbar). The N-trending gneissic layering is folded about tight N-trending upright folds. The BT is dominated by amphibole schist, ultramafics, and metadiorites interpreted as an island arc-ophiolitic-plutonic assemblage. BT is dominated by W-verging folds and mesoscopic thrusts that deform the regional N-trending schistosity. Mineral assemblages in the BT suggest greenschist facies metamorphism (T= ~450„aC and P= ~3 kbar). Kinematic indicators preserved in the BT indicate that the overall tectonic transport direction was from NE to SW where the BT was obliquely thrust over the AT across the BSZ due to terrane collision after the consumption of an E-dipping subduction zone. The BSZ itself is dominated by mylonitic fabric that dips steeply to the E in the northern part of the shear zone but gently dip to the E in the south. The mylonitc fabric contains NE-plunging stretching lineation in the northern part of the BSZ, down-dip or sub-horizontal lineation in the central part, and SE-plunging lineation in the southern part of the BSZ. Although the BSZ preserves some kinematic indicators of the terrane collision, many other kinematic indicators from the BSZ and the eastern margin of the AT show normal-slip shearing to the E and SE indicating that the BT had detached and gravitationally collapsed to the E.. We relate the structural style to gravitational tectonic collapse at ~550-500 Ma across the BSZ following oblique collision between the AT and the BT at ~750-600 Ma. Detachment occurred at the interface between the basement (AT) and the cover (BT).