The Bi’r Umq-Nakasib Suture Zone in the Arabian-Nubian Shield: AKey to Understanding Crustal Growth in the East African Orogen

The Bi’r Umq-Nakasib suture zone, 5-65 km wide and over 600 km long, consists of highly deformed ophiolite nappes and metavolcanic, metasedimentary, and intrusive rocks contained in one of the longest ophiolite-decorated shear zones in the Arabian-Nubian Shield. The rocks originated in a variety of juvenile oceanic environments and include assemblages formed in mid-ocean-ridge, subduction-zone, passive-margin, and continental-slope settings. Dating of the ophiolites, volcanic rocks, and preand syntectonic plutons indicates that oceanic magmatism in the region was active -870-830 Ma whereas suturing occurred -780-760 Ma. This chronology suggests that suturing involved the closure of a relatively long-lived oceanic basin and makes the Bi’r Umq-Nakasib shear zone the oldest accretionary structure known among the juvenile Neoproterozoic rocks of the northern East African Orogen. Creation of the shear zone dates the onset of arc-arc convergence in what eventually became the Arabian-Nubian shield, and marks the beginning of the complex, heterogeneous process of terrane amalgamation and continental accretion that led to the eventual accretion of East and West Gondwana.