Résumé
Both exhumed (mantle) peridotites and ophiolites are geological exceptions in that they are emplaced on top of lighter, mostly continental, crust. The processes leading to this configuration remains a topic of debate. In most relevant areas, extensional stresses have thinned overriding plates to a hyper-extended margin or caused total rupture of the overriding lithosphere and formation of back-arc basins. Subsequent reversals from extensional to compressional stresses inhibit spreading and trigger obduction or thrusting of oceanic/continental lithosphere. However, why back-arc basin should be formed immediately before collision and shortening remains unknown. In 3D models, we show that irregular continental margins with an oceanic embayment could be a remaining part of the puzzle: Lateral onset of collision and ongoing subduction of the oceanic embayment cause localized stresses with the overriding plate. Depending on the amount of stretching, either a hyper-extended margin or a back-arc basin if formed. Once the trench also collides with the continent within the embayment, the previously thinned/ruptured regions change from extension to compression on geologically very short time-scales. We thus provide an mechanism to form ophiolites without any external forces required. Finally, we also compare the model results to observations from the thrusted hyper-extended margin in the Alboran, as well as the ophiolite in Oman and Newfoundland to show that these areas are indeed a result of collision along an irregular continental margin.