Thermal and dynamical evolution of the upper mantle in subduction zones

We present results from two-dimensional (2-D) numerical experiments on the thermal and dynamical evolution of the subducting slab and of the overlying mantle wedge for a range in subduction parameters. These include subduction rate and the age and rheology of both subducting and overriding plates. Experiments also consider the influence of slab forcing conditions (from purely kinematic to purely dynamic) on the evolution of both the slab and mantle wedge. One goal is to determine how different parameters control thermal evolution of the slab-wedge interface, from just after subduction initiation up through roughly 500-600 lan of subduction, where temperatures are approaching steady state. An additional goal is to define optimal conditions for the melting of slab sediments and crust. Results show slab surface temperatures (SSTs) depend strongly on subduction velocity, plate thermal structure, and upper mantle (or wedge) viscosity structure. Fast subduction beneath a thick (>70 km) overriding plate results in the coolest SSTs. Maximum SSTs are recorded as an early transient event for cases of Slow subduction (<3 cm/yr) beneath young, thin lithosphere (<45 km). The latter result supports a model for melting of slab sediments, and possibly crust, early on in cases where young plates subduct beneath thin lithosphere, such as in the Cascades. Maximum wedge temperatures are recorded at higher subduction rates and are found to be strongly dependent on factors influencing return flow into the wedge, such as age of the overriding plate and the ratio of retrograde to longitudinal slab motion. Assuming a model for are magma genesis driven by fluids migrating into the wedge, these results predict higher-temperature, Mg-rich melts coming up beneath subduction zones with fast, steep slabs and young overriding plates, such as in Japan. The influence of variable viscosity is most pronounced in the slab-wedge corner, which tends to stagnate, or freeze out, with time. Moreover, a region of highly viscous mantle develops above the slab at intermediate depths (>100 km) which deflects the zone of maximum shear away from slab-wedge interface.