Genesis of the gneissic core complexes in the Arabian-Nubian Shield and its tectonic implications: A regional overview

Abstract

Metamorphic core complexes (MCCs) are a domed structures cored by high grade gneiss overlain by low grade supracrustal rocks. They are characterized by some common features such as extensional fabrics, ultra high-grade metamorphic facies, low angle normal fault (detachment fault), strike-slip shear zones surrounded the core complexes and ductile mylontitic shear zone (thrust zone) that separate the overlying low-grade rocks from the lower high-grade rocks. There is a debate about the presence or absence of metamorphic core complexes in the Arabian-Nubian Shield (ANS) especially in its northern part. The gneissic complexes in the ANS are considered as strike-slip core complexes like the Qazaz and Krish Domes whereas, those in the Egyptian Nubian Shield (ENS) are interpreted as antiformal stacks (e.g. Meatiq and Hafafit) formed during thrusting, or core complexes formed during orogen-parallel crustal extension. Some metamorphic core complexes are domes or contain gneiss domes within them, but not all gneiss domes possess the essential elements of a true metamorphic core complex. The most important points that negate the existence of MCCs in the ANS are absence of ultra-high grade metamorphic facies, absence of real low-angle normal faults, not all the gneisses have a domal structures, adjacent syn-extensional basins have fill that is older than the gneissic complexes, and models of ANS core complex exhumation include strike-slip faults with slip senses recently found to be inconsistent with the models. The gneissic complexes in the ANS differ from the Cordilleran-type or Aegean-type metamorphic core complexes. The origin of gneiss domes in the ANS is controversial, and many of them are presumably produced by mechanisms other than horizontal extension. During the oblique convergence of East and West Gondwana, the gneissic complexes in the ENS evolved from pure shear to simple shear-dominated transpression due to oblique convergence between East and West Gondwana along the Mozambique belt.