Thermal modeling of the forearc-arc region of subduction zones and implications for subduction zone processes
Ikuko WadaSchool of Earth and Ocean Sciences, University of Victoria, Canada
Pacific Geoscience Centre, Geological Survey of Canada
At subduction zones, viscous coupling between the subducting slab and the overriding mantle drives “corner flow” beneath the forearc-backarc region. The flow brings in hot mantle material from greater depths to keep the mantle wedge warm. However, low forearc surface heat flows indicate a cold and thus stagnant forearc mantle. It is hypothesized that elevated pore fluid pressure and the presence of weak hydrous minerals along the plate interface cause slab-mantle wedge decoupling, resulting in the stagnant and cold forearc mantle. We model this system using a two-dimensional steady state finite element model. The mantle wedge is assumed to have a dislocation-creep rheology, and the effects of interface weakening is approximated by imposing a thin low-viscosity layer along the plate interface. Decoupling occurs when the interface layer is weaker than the mantle wedge, causing the wedge flow above the decoupled interface to stop. The maximum depth of decoupling controls the thermal conditions in the mantle wedge and the slab beneath it and therefore is the key to most primary thermal and petrological processes in subduction zones. We apply the model to a number of subduction zones to investigate their thermal structure and its implications to subduction zone processes. In the models, the maximum decoupling depth is constrained by surface heat flow data and the presumed subarc mantle temperature of > 1250°C. We find that the optimal maximum decoupling depth for most subduction zones is in the range of 70 to 80 km. For all subduction zones, the stagnant part of the forearc mantle wedge above the maximum decoupling depth is sufficiently cold to allow serpentine to be stable, but the actual degree of serpentinization depends on the availability of fluids. For subduction zones with a young and warm slab such as Cascadia and Nankai, dehydration of the subducting crust peaks beneath the stagnant part of the mantle wedge, providing ample fluid for serpentinization. For subduction zones with an old and cold slab such as NE Japan and Hikurangi, crustal dehydration peaks at greater depths, and significantly less metamorphic fluid is released beneath the stagnant part of the mantle wedge and lower degree of serpentinization is expected except at ocean-ocean subduction zones such as Mariana and Kermadec. Beneath the arc, cold slabs provide large fluid flux into the hot and flowing part of the mantle wedge to promote melt production and arc volcanism. In contrast, young slabs are significantly drier at subarc depths, and a lower level of arc volcanism is expected.