Neoproterozoic structural evolution of the NE-trending Ad-Damm Shear Zone, Arabian Shield, Saudi Arabia

• Ad-Damm Shear Zone (AdSZ) marks the boundary between Jeddah and Asir terranes.
• Shear sense indicators reveal a consistent regional dextral shearing along AdSZ.
• AdSZ represents one of the conjugate shears of the sinistral Najd Shear System.
• AdSZ evolved during three Neoproterozoic deformation phases.

The Ad-Damm Shear Zone (AdSZ) is a major NE- (to NNE-) trending fault zone separating Jiddah and Asir tectonic terranes in the Neoproterozoic Juvenile Arabian Shield (AS). AdSZ is characterized by the development of dextral transcurrent shear-sense indicators and moderately to steeply NW plunging stretching lineations. It is mainly developed under high amphibolite-to greenschist-facies conditions and extends ∼380 km, with an average width ∼2–4 km, from the conspicuous Ruwah Fault Zone in the eastern shield to the Red Sea Coastal plain. It was believed to be one of the conjugate shears of the NW- to NNW-trending sinistral Najd Shear System. This assumption is, based on the noteworthy dextral shear criteria recorded within the 620 Ma mylonitic granite of No’man Complex. A total shear-zone strike length exceeding 117 km is carefully investigated during this study to reconstruct its structural evolution. Shear-sense indicators and other field observations including overprinting relations clearly demonstrate a complicated Neoproterozoic history of AdSZ, involving at least three phases of deformations (D1–D3). Both D1 and D2 phases were of contractional regime. During D1 phase a NW–SE compression led to the formation of NE-oriented low-angle thrusts and tight-overturned folds. D2 is represented by a NE–SW stress oriented that led to the development of an open folding. D3 is expressed by the NE–SW intensive dextral transcurrent brittle–ductile shearing. It is overprinting the early formed fabrics and played a significant role in the creation of AdSZ and the mega-scale related folds. Such deformation history reflects the same Neoproterozoic deformation regime recognized in the NE-trending shear zones in the Arabian–Nubian Shield (ANS).