Thermal structure, fluid activity and earthquake mechanisms of oceanic subduction zones

As the most seismically active regions in the world, oceanic subduction zones show contrasting seismicity in different regions. To investigate the relationships among the thermal structure, metamorphism, deformation, and fluid activity of oceanic subduction zones, we summarized progress in numerical modeling of oceanic subduction zones, pressure and temperature paths of high-and ultrahigh-pressure metamorphic rocks and ophiolites, deformation of the subduction plate interface, seismic observations in subduction zones, and stabilities of hydrous minerals. The thermal structure of subduction zones not only controls depths of dehydration embrittlement of hydrous minerals, but also affects the mechanical coupling state of the subduction plate interface, eclogitization of the subducted oceanic crust, and phase transition of metastable olivine in the subducted lithospheric mantle. Dehydration embrittlement of hydrous minerals is the primary mechanism of intermediate-depth earthquakes in subduction zones. Earthquakes in warm subduction zones predominantly occur at shallow to intermediate depths,where most hydrous minerals dehydrate at depths of 80–160 km beneath the arc and the amounts of earthquakes decrease sharply below 160 km. By contrast, earthquakes in cold subduction zones distribute continuously to ～300 km and hydrous minerals release water at greater depths. Nominally anhydrous minerals and dense hydrous magnesium silicates could carry water down to depths >300 km, resulting in localized water enrichment in the mantle transition zone. More experimental and seismic evidence is needed to decipher how fluid activity triggers slow earthquakes and deep-focus earthquakes. Knowledge about the origins of ophiolites and fossil earthquakes in ancient subduction zones will provide new insights into the tectonic evolution, the deep water cycle, and earthquake mechanisms of oceanic subduction zones.