3D POST-ACCRETIONARY TECTONIC STRAIN IN THE ARABIAN-NUBIAN SHIELD: EXAMPLE FROM THE SUPERIMPOSITION OF THE OKO SHEAR ZONE ON THE NAKASIB SUTURE, RED SEA HILLS, SUDAN

We have calculated the three-dimensional (3D) tectonic strain associated with Neoproterozoic post-accretionary deformational belts as exemplified by the Oko Shear Zone. For this we have used deformed conglomeratic clasts exposed along the Neoproterozoic Nakasib Suture and the Oko Shear Zone. The Nakasib Suture is a NE-trending fold and thrust belt. It is offset by the crosscutting NNW- to NW-trending sinistral strike-slip faults of the Oko Shear Zone by c. 10 km. The structures of the Nakasib Suture were formed as a result of the collision between the Haya terrane and the Gebeit terrane c. 750 Ma ago and the Oko Shear Zone formed as a result of the shortening of the Arabian-Nubian shield caused by the collision between East and West Gondwana c. 670-610 Ma ago. Our analysis indicates that: (1) the Nakasib Suture is dominated by a flattening strain with the flattening plane of the associated strain ellipsoid striking at 21° and dipping at 77° to the SE. This flattening deformation is interpreted to be associated with nappe emplacement from North to South; (2) some areas in the Nakasib Suture are characterized by constriction strain that might be due to refolding of the early nappes; (3) the Oko Shear Zone is characterized by constriction strain, with the flattening plane of the strain ellipsoid striking at 171° and dipping at 68° to the E. The strain ellipsoid associated with the Oko Shear Zone manifests the superimposition of E-W shortening on the structures of the Nakasib Suture; (4) the tectonic strain of the Oko Shear Zone, which was superimposed over the structures of the Nakasib Suture, is characterized by a strain ellipsoid whose flattening plane has a strike of 21° that dips 49° to the W and which has a major axis with a quadratic elongation of 3.632 and a trend of 357° that plunges 25°, an intermediate axis with a quadratic elongation of 1.191 and a trend of 231° that plunges 30°, and a minor axis with a quadratic elongation of 0.245 and a trend of 115° that plunges 18°. This work suggests that a post-accretion deformation in the Arabian-Nubian Shield was imposed as a NW-SE directed shortening that created early N-S shortening zones and late NW-trending sinistral strike-slip faults.