Reappraisal of the kinematic history of Nugrus shear zone using PALSAR and microstructural data: implications for the tectonic evolution of the Eastern Desert tectonic terrane, northern Nubian Shield

Abstract

Nugrus shear zone (NSZ) is a NW-trending semiductile-semibrittle high-strain zone exhibiting obvious sinistral sense of shear. It is regarded by some workers to represent the boundary between the Central Eastern Desert (CED) and South Eastern Desert (SED) of Egypt. A variety of Neoproterozoic lithologies are remarkably affected by the NSZ, encompassing high-grade gneisses, metavolcanics and metavolcaniclastics, gneissose granite, and ophiolitic melange. Visual interpretation and automated lineament extraction have been applied on the local sigma-filtered level 1.5 PALSAR data. Results of directional filtering and line algorithm techniques support the dominance of NW and NE structural trends in the study area. Microstructural and finite strain analyses have been done for the oriented samples of gneisses, metavolcanics, metavolcaniclastics, and gneissose granite using CSS, Fabric 8, and EllipseFit software. The Rf/ϕ and Fry methods demonstrate that the strain ratios of the collected samples are closely compatible and consistent. The core of Hafafit culmination gave 1.5 strain ratio with top-to-the-NW sense of shear, whereas the two rims show 1.7 strain ratio with top-to-the-NE sense of shear. Samples collected along the flanks of Wadi Nugrus show the two types of shear but the NW is dominant with 1.8 strain ratio and sinistral sense of shear, whereas those collected from Wadi Abu Rashid and Wadi Sikiat display 1.4 strain ratio with dextral sense of movement. Finite strain results along with field relations and observations prove that the NSZ and environs experienced a prolonged tectonic history involving at least three deformations (Dl, D2, and D3). D1 phase involves NE-SW shortening and is documented by large-scale NW-trending, NE-dipping, imbricate thrust sheets with NE plunging stretching lineation. D2 was a transpressional phase (660–645 Ma), coeval with the Najd Shear System (NSS), and expressed by the NW- to NNW-trending, steeply dipping, strike-slip shear zones with frequent subvertical foliation and subhorizontal lineation. Exhumation of gneissic domes in Hafafit area was related to this phase. The D3 phase (645–540 Ma) was responsible for the formation of NE-oriented shear with dextral sense of movement. Whether the NW and NE shears are conjugate pairs or not will be a matter of much discussion.