Shear zone formation linked to orogen-parallel extension facilitated by localisation of stress and fluid influx at a pre-existing lithological boundary: Evidence from microstructures developed in the Abu Markhat granitoid, Egyptian Eastern Desert, Arabian-Nubian shield

An exceptionally well-exposed shear zone within the Abu Markhat granitoid, Egyptian Eastern Desert, Arabian-Nubian Shield, is investigated to constrain the parameters controlling shear zone nucleation and evolution during extensional shearing of the middle crust. The shear zone exhibits a decametre-scale strain gradient from metagranite in the pluton core to ultramylonite along the contact with deformed amphibolites. Detailed electron backscatter diffraction (EBSD)-based microstructural analyses integrated with bulk-rock and mineral chemistry reveal the dynamic interplay between stress, fluid ingress, chemical reaction and deformation. In the metagranite, protomylonite and mylonite, quartz is predominantly deformed by dislocation creep. In the ultramylonite, deformation changes to grain boundary sliding (GBS) accommodated by dislocation glide and dissolution-precipitation in monomineralic and polymineralic areas, respectively. Feldspars deform dominantly via microfracturing in the metagranite and protomylonite while deformation is controlled by syntectonic replacement reactions in the mylonite and ultramylonite. These reactions produce compositionally distinct fine-grained plagioclase and K-feldspar aggregates deforming by dissolution-precipitation creep and GBS. Microstructures suggest deformation temperature of 400 ± 50 °C throughout the whole deformed zone and strain rates of 2x10−9-6x10−11 s−1. Indicative differential stresses increase from 55 to 120 MPa towards the lithological boundary with the structurally overlying amphibolites. We propose that stress concentration at the lithological boundary played a first-order role in the nucleation of the shear zone. Fluids are locally introduced into the shear zone margin resulting in the activation of multiple deformation mechanisms that controlled its evolution. In contrast to long-lived extensional shear zones, the Abu Markhat shear zone related to orogen-parallel extension was active over a small temperature window and high strain rates indicating a notably short-lived activity of the shear zone characterized by transient high strain rate.