The Oman Ophiolite as a Record of Subduction Initiation

The Oman ophiolite is the largest and best-known ophiolite in the world. It formed in the Cretaceous (~95 Ma) in the Neotethyan ocean, but its geodynamic setting of formation has been heavily debated for over three decades. Many workers have assumed that it formed in an open ocean setting, consequently utilising the ophiolite as a direct analogue for fast-spreading oceanic crust, whereas others argue that the complex formed in a subduction setting. Here, we make the case that the Oman ophiolite records the evolution of the upper plate of a newly initiated subduction zone.

Using a database of >1200 lava and dyke analyses (‘OmanDB’), we show that the earliest lava sequence (the Geotimes unit) is systematically different to modern mid-ocean ridge basalt, and that these differences can be explained by the presence of elevated water contents. This rules out a mid-ocean ridge origin, pointing instead to a subduction-related setting. The lavas evolved from the Geotimes ‘moist MORB’ to island-arc tholeiite and boninite (the Lasail and Alley units); hence, we conclude that the entire ophiolite formed in a subduction zone. The data suggest a progressive addition of water and concomitant depletion of the mantle source. High-precision U-Pb zircon geochronology indicates that this fundamental change in magmatic source occurred within ~2 million years.

The spreading structure of the ophiolite is characterized by a series of NW-SE trending propagating rifts that crosscut earlier N-S trending ridge segments. Together with palaeomagnetic evidence, which calls for 30° clockwise rotation between Geotimes and Lasail/Alley, and 120° between Lasail/Alley and the later Salahi lavas, it suggests that construction of the lithosphere was accompanied by significant plate rotation.

Combined, the available evidence suggests that the rapid change in magmatic signature to increasingly arc-like compositions was coeval with large-scale rotational disaggregation of young ocean lithosphere and consequent reorganisation of the spreading system. This is consistent with the formation of the ophiolite above a newly initiated subduction zone, where a rapidly sinking slab changes the mantle melting regime and induces rotational stress due to differential rollback.