THERMAL MODELING OF SUBDUCTION-ZONE METAMORPHISM: DEHYDRATION REACTIONS, EARTHQUAKES, AND ARC MAGMATISM
At P=3 GPa (100 km depth), temperatures along the slab interface are predicted to be 750 °C beneath Nicaragua, 800 °C beneath NE Japan, and 960 °C beneath Cascadia. Compared to an isoviscous mantle-wedge rheology, interface temperatures calculated using the olivine rheology are 150-200 °C warmer. Predicted temperatures at the base of the subducting oceanic crust at 3 GPa range from 340 °C (NE Japan) to 400 °C (Nicaragua) to 720 °C (Cascadia). The high thermal gradients perpendicular to the slab interface permit partial melting of subducting sediments while the underlying oceanic crust dehydrates, as indicated by recent geochemical studies of arc basalts. In the Cascadia subduction zone, the blueschist to eclogite transition within the subducting oceanic crust is predicted to occur at a depth of 40-50 km, consistent with recent S-wave seismic tomography. Hydrous eclogite is predicted to persist to ~120 km depth beneath NE Japan and Nicaragua which is slightly less than the ~150 km depth indicated by observations of a dipping low-seismic-velocity wave guide. Slab dehydration reactions in the subducting lower crust and uppermost mantle correlate well with the spatial distribution of Wadati-Benioff zone earthquakes.