Shear heating during rapid subduction initiation beneath the Samail Ophiolite

Abstract

Metamorphic soles beneath ophiolites are thought to record subduction initiation. However, there is ambiguity about the tectonic and thermal mechanisms operative during subduction initiation, arising partly from uncertainty in the duration of sole metamorphism. Here we use chemical mapping and diffusion speedometry of garnet crystals from the metamorphic sole of the Samail Ophiolite (Oman and United Arab Emirates) to show that high-temperature (≥750 °C) metamorphism was rapid, lasting ≤1 Myr (potentially ≤100 kyr) at peak temperature conditions. The short durations are supported by zircon U–Pb ages and new garnet–whole-rock–zircon Lu–Hf data from the same rocks, contrasting with previous inferences for ≥8 Myr metamorphic durations. These observations are nominally consistent with the spontaneous sinking of a dense lower plate. However, the rapid metamorphic timescales cannot be accounted for solely by conductive thermal equilibration with juxtaposed oceanic mantle. One potential explanation is dissipative heating driven by relative motion across the nascent plate interface. This interpretation accounts for the timescales, the spatial pattern of metamorphism and the global similarities in sole pressure–temperature conditions independent of other geodynamic variables.