Subduction zones are unique to Earth and fundamental in its evolution, yet we still know little on the causes and mechanisms of their initiation. Numerical models suggest far-field forcing may cause subduction initiation at weak pre-existing structures, whereas inferences from modern subduction zones suggest initiation through spontaneous lithospheric gravitational collapse. Measuring the time lag between initial lower plate burial and incipient extension in the upper plate should prove diagnostic in characterizing the subduction initiation mode. In modern systems, rocks that directly recorded initial lower plate burial should be found at the subduction interface and are inaccessible. Here we investigate a fossil system, the archetypal Semail ophiolite of Oman, which expose both lower and upper plate relics of incipient subduction stages. We show that burial of the lower plate predated upper plate extension and formation of Semail oceanic crust by at least 8 Myr, using geochronology of lower and upper plate material. Such a time lag requires far-field forced subduction initiation and provides for the first time unequivocal, direct evidence for a subduction initiation mechanism in the geological record.

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Analyzing subduction initiation is key for understanding the coupling between plate tectonics and the underlying mantle. Here we focus on suprasubduction zone (SSZ) ophiolites and how their formation links to intraoceanic subduction initiation in an absolute plate motion frame. SSZ ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intraoceanic subduction initiation. Structural, petrological, geochemical, and plate kinematic constraints on their kinematic evolution show that SSZ crust forms at fore-arc spreading centers at the expense of a mantle wedge, thereby flattening the nascent slab. This leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during SSZ crust crystallization. Former spreading centers are preserved in forearcs when subduction initiates along transform faults or offridge oceanic detachments. We show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone. Upon inception of slab pull due to, e.g., eclogitization, the sole is separated from the slab, remains welded to the thinned overriding plate lithosphere, and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge. We propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust. Our study highlights how far-field geodynamic processes and absolute plate motions may force intraoceanic subduction initiation as key toward advancing our understanding of the entire plate tectonic cycle.