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

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.