SHEAR HEATING DURING RAPID SUBDUCTION INITIATION BENEATH THE SAMAIL OPHIOLITE

Metamorphic soles of ophiolites record intraoceanic thrusting that may eventually lead to self-sustained subduction. In the archetypal Samail Ophiolite, the timing and duration of sole metamorphism have been controversial, because different methods yield divergent results. For example, zircon U-Pb TIMS dates suggest that granulite-facies sole metamorphism beneath the ophiolite occurred over ≤1 Myr (Rioux et al., 2023) and was roughly synchronous with ophiolite crystallization, consistent with spontaneous or rapidly induced SI. In contrast, garnet Lu-Hf isochron dates suggest that sole metamorphism was lengthier (~8–10 Myr) and initiated well-prior to ophiolite crystallization (Guilmette et al., 2018), favoring induced SI. These differing results lead to considerable ambiguity in the tectonic and thermal mechanisms of SI in the Samail system.

To characterize the duration of metamorphism in the Samail sole independently of isotopic dates, we analyzed granulite-facies garnet using EPMA and LA-ICPMS chemical mapping, coupled to thermobarometry, diffusion speedometry, and zircon U-Pb and garnet Lu-Hf geochronology. The garnet mapping data shows preserved prograde major and trace-element zoning through 750–800°C peak temperatures, with subsolidus, concentric REE zoning in rare garnet cores transitioning to suprasolidus, oscillatory-zoned rims and new grains. There are several examples of sharp growth- and resorption-related zoning retained in major and trace elements. Using published diffusion parameters, such sharp elemental discontinuities suggest abrupt high-T metamorphism (0.1–1.0 My at >700°C). The short durations are further supported by new garnet-WR-zircon Lu-Hf data and are consistent with our previous zircon U-Pb dates from the same rocks. These observations are consistent with spontaneous sinking of a dense lower plate, with numerous similarities to the Izu-Bonin-Mariana forearc system. However, the rapid metamorphic timescales cannot be accounted for solely by conductive equilibration with juxtaposed oceanic mantle; one potential explanation is shear heating driven by relative motion across the nascent plate interface. This interpretation accounts for the timescales, spatial pattern of metamorphism, and the global similarities in sole P-T conditions independent of other geodynamic variables.