Kinematic and paleomagnetic restoration of the Semail Ophiolite (Oman) reveals subduction initiation along an ancient Neotethyan fracture zone

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.