Evolution of the Al Amar-Idsas suture, eastern Arabian Shield: a strain analysis perspective

Finite strain analysis is used to decipher the tectonic evolution of the Al Amar–Idsas suture in the eastern segment of the Neoproterozoic Arabian Shield. Asymmetrical porphyroblasts and sigmoidal structural markers from three areas along the strike of the suture, namely the Nakhyilan, Al Amar, and Jabal Al Tays, are envisaged to reveal their strain factors. The Rf/φ and Fry techniques are employed to quartz and feldspar porphyroclasts, hornblende, and biotite flakes in 24 samples from the Abt schist, metavolcano-sedimentary rocks, and sheared granitoids. The results revealed that these rocks have experienced shape flattening and accumulation of the finite strain and an overall volume loss. The strain magnitude increases apparently across the nappe contacts with the neighboring lithologies. Development of subhorizontal foliation was presumably synchronistic with thrusting and nappe stacking, nearly parallel to the tectonic contacts. Ductile strain accumulation/crustal shortening during thrusting along the Al Amar-Idsas suture most likely encompassed a significant pure shear component. Progressive thrusting by superimposed transpression and oblique convergence is manifested by stacked nappes and diagonal stretching lineations along the thrust planes. The widespread subhorizontal lineation may represent the latest activity in the evolution of the suture. The results of the present study argue against the widely accepted model which relates the orogen-scale structures in the Arabian Shield to oblique convergence with dominant simple shear deformation. A substantial pure shear component/crustal thickening increment should have played an important role in the evolution of the suture and thus in the overall deformation history of the Shield.