THERMAL MODELING OF SUBDUCTION-ZONE METAMORPHISM: DEHYDRATION REACTIONS, EARTHQUAKES, AND ARC MAGMATISM

We constructed high-resolution, finite-element thermal models to illuminate the metamorphic structure of three subduction zones: NE Japan (V=100 mm/yr; age of incoming lithosphere=130 Ma), Nicaragua (80 mm/yr; 24 Ma), and Cascadia (45 mm/yr; 10 Ma). These new models incorporate a T- and stress-dependent olivine rheology for the mantle wedge that focuses hot asthenosphere into the tip of the mantle wedge as compared to an isoviscous rheology. The models predict (i) the thermal structure of the subduction zone, (ii) metamorphic P-T paths followed by subducting oceanic sediments, crust, and mantle, and (iii) the location of slab dehydration reactions. Specific predictions can be tested against the observed seismic velocity structure, distribution of Wadati-Benioff earthquakes, and arc geochemistry.

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.