“The Oman ophiolite did not form at a mid-ocean ridge” – establishing the limits of the Oman–East Pacific Rise analogy

The ~500km-long mid-Cretaceous Semail nappe of the Sultanate of Oman and United Arab Emirates (the ‘Oman ophiolite’) is the largest and best-preserved ophiolite complex known. It is of particular importance because it is generally believed to have an internal structure and composition closely comparable to that of crust formed at the present-day East Pacific Rise, and is our only known on-land analogue for ocean lithosphere formed at a fast spreading rate. On the basis of this assumption Oman has long played a pivotal role in guiding our conceptual understanding of fast-spreading ridge processes, especially of the otherwise mostly inaccessible lower crust. There remains doubt, however, about how appropriate the analogy with modern fast-spreading ridges actually is in detail. Arguments as to the geodynamic environment within which the Oman ophiolite formed, and the mechanisms by which it was obducted, have been continuing for three decades and remain heated. In contrast to most ophiolites, the axial volcanic suite and associated sheeted dyke complex in Oman have compositions similar to typical mid-ocean ridge basalt (MORB), leading many workers therefore to assume an open-ocean mid-ocean ridge origin for the ophiolite. However, in northern Oman, where the extrusive sequence is best developed, later lava sequences and associated plutonics succeed the axial magmatic phase. These have clear hydrous signatures; some, indeed, are boninitic: rare rocks that are only otherwise found in intra-oceanic forearcs. To some authors these characteristics are consistent with the entire ophiolite being formed above a subduction zone rather than open-ocean setting, perhaps in some kind of ‘pre-arc’ marginal basin rather than open-ocean setting. Most other workers, however, have instead maintained or assumed that the Oman formed at a true open-ocean mid-ocean ridge. Alternative explanations for the source of the water in the later lavas are that it is: (i) a residue from ancient subduction; (ii) derived from descending near-axis hydrothermal fluids; or (iii) a consequence of overthrusting at the initiation of obduction, which is thereby constrained to have started immediately before the later hydrous volcanic episode. In this presentation I shall review the geodynamic models for the origin of the Oman ophiolite, showing that the entire ophiolite is likely to have formed at a submarine spreading centre above a nascent subduction zone rather than at a conventional mid-ocean ridge (MacLeod et al., 2013). Water in the Oman MORB source has modified the behaviour of melts within the crust, with significant consequences for the resulting crustal accretion mechanisms. I will conclude by critically evaluating the extent to which the ophiolite may be utilised as an analogue for modern ocean lithosphere formed at fast-spreading mid-ocean ridges such as the East Pacific Rise. MacLeod, C.J., Lissenberg, C.J. & Bibby, L.E., 2013. “Moist MORB” axial magmatism in the Oman ophiolite: the evidence against a mid-ocean ridge origin. Geology, 41, 459-462, doi:10.1130/G33904.1. http://geology.gsapubs.org/content/41/4/459.full (currently open-access) The ~500km-long mid-Cretaceous Semail nappe of the Sultanate of Oman and United Arab Emirates (the ‘Oman ophiolite’) is the largest and best-preserved ophiolite complex known. It is of particular importance because it is generally believed to have an internal structure and composition closely comparable to that of crust formed at the present-day East Pacific Rise, and is our only known on-land analogue for ocean lithosphere formed at a fast spreading rate. On the basis of this assumption Oman has long played a pivotal role in guiding our conceptual understanding of fast-spreading ridge processes, especially of the otherwise mostly inaccessible lower crust. There remains doubt, however, about how appropriate the analogy with modern fast-spreading ridges actually is in detail. Arguments as to the geodynamic environment within which the Oman ophiolite formed, and the mechanisms by which it was obducted, have been continuing for three decades and remain heated. In contrast to most ophiolites, the axial volcanic suite and associated sheeted dyke complex in Oman have compositions similar to typical mid-ocean ridge basalt (MORB), leading many workers therefore to assume an open-ocean mid-ocean ridge origin for the ophiolite. However, in northern Oman, where the extrusive sequence is best developed, later lava sequences and associated plutonics succeed the axial magmatic phase. These have clear hydrous signatures; some, indeed, are boninitic: rare rocks that are only otherwise found in intra-oceanic forearcs. To some authors these characteristics are consistent with the entire ophiolite being formed above a subduction zone rather than open-ocean setting, perhaps in some kind of ‘pre-arc’ marginal basin rather than open-ocean setting. Most other workers, however, have instead maintained or assumed that the Oman formed at a true open-ocean mid-ocean ridge. Alternative explanations for the source of the water in the later lavas are that it is: (i) a residue from ancient subduction; (ii) derived from descending near-axis hydrothermal fluids; or (iii) a consequence of overthrusting at the initiation of obduction, which is thereby constrained to have started immediately before the later hydrous volcanic episode. In this presentation I shall review the geodynamic models for the origin of the Oman ophiolite, showing that the entire ophiolite is likely to have formed at a submarine spreading centre above a nascent subduction zone rather than at a conventional mid-ocean ridge (MacLeod et al., 2013). Water in the Oman MORB source has modified the behaviour of melts within the crust, with significant consequences for the resulting crustal accretion mechanisms. I will conclude by critically evaluating the extent to which the ophiolite may be utilised as an analogue for modern ocean lithosphere formed at fast-spreading mid-ocean ridges such as the East Pacific Rise. MacLeod, C.J., Lissenberg, C.J. & Bibby, L.E., 2013. “Moist MORB” axial magmatism in the Oman ophiolite: the evidence against a mid-ocean ridge origin. Geology, 41, 459-462, doi:10.1130/G33904.1. http://geology.gsapubs.org/content/41/4/459.full (currently open-access)