A review and synthesis of strain studies in the Egyptian Nubian Shield: materials, methods, problems and prospects

Abstract

More than seventy strain studies in the Egyptian Nubian Shield (ENS) are reviewed in this contribution. For the Hammamat basin strain studies, Rf/ϕ and Fry methods have been applied to pebble sections, mudcracks and raindrop prints. Consideration of pebble fabrics in undeformed outcrops suggests that RXZ estimates of less than 1.7 based on pebble ellipse data are doubtful. Extracted pebbles are also popular strained objects for analysis. Average Wadell sphericity and percentage of blades assist recognition of pebble strains in the Kareim, Queih, Igla and Seleimat basins. Feiran gneissic complex strains, measured using boudins and folds are much higher (X/Z > 15) than the reported strains for the Hafafit gneissic complex (X/Z usually < 2.5) that used metamorphic grain shapes and centres. The low Hafafit strains may result from progressive resetting of grain shapes and relocation of grains by diffusional effects during HT metamorphism. The Barramiya-Mubarak thrust belt provides a good example of focused study on ENS shear zones strain, with variable correlations between Rf/ϕ and Fry results, but consistently low k values. South Eastern Desert (SED) shear zone prolate strain ellipsoids record the effects of multiple deformation. The extensional shear zone overlying the Meatiq complex shows extreme strain and strongly prolate strain ellipsoids. ENS vorticity studies, applying PAR and RGN methods, have yielded higher Wm (> 0.5) for transpressional shears (Barramiya-Mubarak belt and Atalla shear zone) than for transtensional shears (Wm < 0.5). Volume strains have rarely been considered in the ENS, though evidence exists for negative (due to pressure solution) and positive (due to microveining) volume change during deformation. The ENS strain database can be improved by better documentation of sampling methods; better knowledge of primary fabrics; and by testing strain results using a range of materials. AMS-strain studies may broaden the range of lithologies suitable for strain measurement.