The archetypal Semail ophiolite of Oman has inspired much thought on the dynamics of initiation of intra-oceanic subduction zones. Current models invoke subduction initiation at a mid-oceanic ridge located sufficiently close to the Arabian passive margin to allow initiation of continental subduction below the ophiolite within ~10-15 Myr after the 96-95 Ma age of formation of supra-subduction zone crust. Here, we perform an extensive paleomagnetic analysis of sheeted dyke sections across the Semail ophiolite to restore the orientation of the supra-subduction zone ridge during spreading. Our results consistently indicate that the ridge was oriented NNE-SSW, and we infer that the associated trench, close to the modern obduction front, had the same orientation. Our data are consistent with a previously documented ~150° clockwise rotation of the ophiolite, and we reconstruct that the original subduction zone was WNW-ward dipping and NNE-SSW striking. Initial subduction likely occurred in the ocean adjacent and parallel to the Arabian transform margin that formed the original underpinnings of rocks in the Zagros fold-thrust belt, now underthrust below Iran. Subduction thus likely initiated along an ancient, continental margin-parallel fracture zone, as also recently inferred from near-coeval ophiolites from the eastern Mediterranean and NW Arabian regions. Subduction initiation was therefore likely induced by (WN)W-(ES)E contraction, and this constraint may help the future identification of the dynamic triggers of Neotethyan subduction initiation in the Late Cretaceous.
- plate reconstruction