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Forced subduction initiation recorded in the sole and crust of the Semail Ophiolite of Oman

Abstract

Subduction zones are unique to Earth and fundamental in its evolution, yet we still know little about the causes and mechanisms of their initiation. Numerical models show that far-field forcing may cause subduction initiation at weak pre-existing structures, while inferences from modern subduction zones suggest initiation through spontaneous lithospheric gravitational collapse. For both endmembers, the timing of subduction inception corresponds with initial lower plate burial, whereas coeval or delayed extension in the upper plate are diagnostic of spontaneous or forced subduction initiation, respectively. In modern systems, the earliest extension-related upper plate rocks are found in forearcs, but lower plate rocks that recorded initial burial have been subducted and are inaccessible. Here, we investigate a fossil system, the archetypal Semail Ophiolite of Oman, which exposes both lower and upper plate relics of incipient subduction stages. We show with Lu–Hf and U–Pb geochronology of the lower and upper plate material that initial burial of the lower plate occurred before 104 million years ago, predating upper plate extension and the formation of Semail oceanic crust by at least 8 Myr. Such a time lag reveals far-field forced subduction initiation and provides unequivocal, direct evidence for a subduction initiation mechanism in the geological record.

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Fig. 1: Conceptual lithospheric sections representing SSI versus ISI.
Fig. 2: Geological maps, sample locations and field relationships.
Fig. 3: Petrography of the investigated samples.
Fig. 4: Trace element content of representative garnet from samples SU-03A, WT-150 and WT-151.
Fig. 5: Geochronological results.
Fig. 6: Pressure–temperature–time evolution of the Semail metamorphic sole.

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Acknowledgements

This research was financially supported by the Natural Sciences and Engineering Research Council of Canada (Discovery Grant RGPIN-2014-05681 to C.G. and RGPIN-2015-04080 to M.A.S.), the Canadian Foundation for Innovation (Projects 34991 to C.G. and 229814 to M.A.S.) and European Research Council (Starting Grant 306810 (SINK) and NWO Vidi grant 864.11.004 to D.J.J.v.H). We thank M. Al Battashi (Sultanate of the Oman Ministry of Commerce and Industry, Directorate General of Minerals) for permission to undertake field sampling in Oman.

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Contributions

C.G. generated the project, led the field work, completed the petrological study and wrote the manuscript. M.S. conducted the Lu–Hf analyses and contributed to writing the manuscript. D.J.J.v.H. participated in the field work, and contributed to the rationale and writing of the manuscript. D.G. and F.C. completed the U–Pb geochronological analyses. B.C. planned and participated in the field work, and prepared and analysed the samples. M.M. organized and participated in the field work. O.R. participated in defining the rationale and writing the manuscript. D.S. conducted the laser ablation ICP analyses.

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Correspondence to Carl Guilmette.

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Supplementary information

Supplementary Table 1

Electron microprobe spot analyses of garnet.

Supplementary Table 2

Laser ablation ICP-MS spot analyses of garnet.

Supplementary Tables

Supplementary Tables 3 and 4.

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Guilmette, C., Smit, M.A., van Hinsbergen, D.J.J. et al. Forced subduction initiation recorded in the sole and crust of the Semail Ophiolite of Oman. Nature Geosci 11, 688–695 (2018). https://doi.org/10.1038/s41561-018-0209-2

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