Editorial: Geology and Tectonic Setting of the Arabian-Nubian Shield

Abstract

The Arabian-Nubian Shield (ANS) lies at the northern part of the East African Orogen (EAO) or East African Antarctic Orogen (EAAO) and represents the upper crustal equivalent of the high-grade Mozambique Belt (MB). The juxtaposition of the ANS low-grade basement rocks and the MB high-grade rocks is documented in various domains, particularly in southern Ethiopia and Kenya. The ANS itself is considered by some workers as the largest tract of juvenile Neoproterozoic continental crust on Earth, and is regarded by others as a major suture zone separating east-and west-Gondwanalands. It is dominated by Neoproterozoic juvenile arcs, post-amalgamation depositional basins (≈ forty basins allover the ANS), voluminous granitoid and gabbroic intrusions, and enclaves of pre-Neoproterozoic crust. This collage was evolved during much debatable late Cryogenian-Ediacaran (650-542 Ma) depositional, tectonic, metamorphic and intrusive events. The closing 100 Ma of the Neoproterozoic crustal growth of the ANS has attracted the attention of Precambrian structural geologists and at least four models or paradigms have been proposed; infracrustal-, Turkic-type-, hot-spot-, and arc-assembly-orogenic models. Infracrustal model, whereby ophiolites and island-arc volcanics and volcaniclastics were thrusted over an old craton consisting of high-grade gneisses, migmatites and remobilized equivalents; the contact is Pan-African thrusting. The high-grade rocks are suggested to be exposed in a number of tectonic windows, forming domal-like structures or swells; e.g. Beitan, Hafafit, Meatiq, Sibai and El-Shalul in the Nubian segment of the ANS. However, recent isotopic and geochronologic studies challenge this interpretation. According to the Turkic-type orogenic model, much of the ANS were formed in broad fore-arc complexes. The Hot-spot Model envisaged the evolution of the ANS due to accretion of oceanic plateaux formed by upwelling mantle plumes. The arc assembly model is favored by most ANS §Guest Editor researchers, whereby EAO (EAAO) juvenile crust was generated around and within a Pacific-sized ocean (Mozambique Ocean). Within the frame of this model, the ANS is regarded in recent publications as an "accretion-obduction-type Orogen" in which stacking of thinskinned nappes was achieves by oblique (transpressional) convergence of boundary plates. Moreover, numerous attempts have been made to classify the northern extension of the EAO (i.e. ANS) into multiple amalgamated tectonic terranes, such as: Halfa, Bayuda, Barka, Hager-Tokar, Ghedem, Haya,Gebeit-Gabgaba, Gerf (South Eastern Desert), Asir, Jeddah, Hijaz, Madyan, Afif and Rayn. Contacts separating these terranes comprise original suture zones (ophiolites-decorated mega shears), cryptic major shear zones and post-amalgamation fault zones. Several ANS terrane accretion (assembly/suturing/amalgamation/ convergence) events have been discriminated and precisely dated. Among these events, the oldest was that creating the Bir Umq-Nakasib Suture; the oldest accretionary zone throughout the entire ANS. The youngest event was responsible for the formation of the Late Neoproterozoic brittle-ductile shears (e.g. Yiba and Hamisana Shear Zones), and the NW-to NNW-oriented sinistral transpressional Najd Shear System, as well as its conjugate NE-to NNE-oriented dextral transpressional mega shears (e.g. Ad-Damm and Wadi Fatima Shear Zones). The deformation patterns and structural fabrics observed within, and in the vicinity of, sutures and shear zones revealed that the ANS experienced final accretion to the Saharan Metacraton (existed to the west) concurrent with Gondwana assembly through progressive oblique convergence of East-and West-Gondwanalands. In this special issue, we invited Peter Johnson to write a review on the tectonic setting of the ANS and the entire EAO. He has a preference to write a brief review on current geochronologic and isotopic datasets under the title "Review of an expanding geochronologic and isotopic data base: importance for tectonic modeling in the Arabian-Nubian Shield". We were pleased to start this special issue with Johnson's excellent review. The author illustrates the critical contribution that these types of data make to robust,