Rethinking the Anatomy and Architecture of the Neoproterozoic Gneiss Domes in the Nubian Shield: Are they Extensional Metamorphic Core Complexes?

Abdullah T. Mohammad, Yahia El Kazzaz

The Nubian Shield features amphibolite-grade, gneiss-cored domes surrounded by greenschist-grade island arc and ophiolitic assemblages. These structures exhibit anomalous intense ductile deformation and higher metamorphism than the surrounding rock units; yet, their nature, geometry, and origin remain controversial. The present study integrates field data, remote sensing, and detailed structural and microstructural analyses to reevaluate the nature, architecture, and formation of these metamorphic domes, focusing on the Meatiq dome in Egypt's Central Eastern Desert. Our findings reveal: (1) Meatiq Dome comprises a domed thrust shear zone originating at mid-crustal depths, initially oriented sub-horizontally. Microstructural analysis shows the dominance of crystal plastic deformation with non-coaxial strain and varying metamorphic conditions. (2) The granite plutons in the dome (Um Baanib and Abu Ziran) were emplaced syn-kinematically during NW-ward tectonic transport, strongly supported by the sub-magmatic microstructures that reflect concurrent magmatic and solid-state deformation. Both plutons exhibit sheet-like geometries and were emplaced along distinct rheological contacts. (3) While the Meatiq Dome shares some attributes with metamorphic core complexes (MCCs), such as low-angle shear surfaces and mylonitic rocks, it lacks typical MCC features like low-angle and rotational normal faults. Structural analysis indicates that the domiform geometry of these structures is better explained by thrust-related folding rather than by MCC models. The deformation history of the Meatiq Dome comprises five key phases related to different tectonic stages with ages ranging from Precambrian to Cenozoic. The results provide new insights into the 3D geometry and structural history of the gneiss domes in the Nubian Shield, with implications for deformation processes in mid-crustal shear zones.