Modeling slab temperature evolution through the lifetime of a subduction zone

Abstract:

The thermal structure of subduction zones enacts a first order control on metamorphic reactions, rheological strength, and the locus and magnitude of devolatilization. Given the importance of these processes, a goal of subduction research is the development of accurate predictions of subduction zone thermal structure. Modeling studies with fixed subduction geometries and imposed plate velocities have been used to map out how various subduction parameters affect the pressure-temperature (P-T) conditions of slabs. While this type of modeling approach is attractive as it can be tailored to specific subduction zones, such models are unsuited to examining how temporal variations in subduction parameters influence thermal structure. Taking a dynamic approach, we use time-dependent models to i), investigate the extent to which the P-T conditions of the slab top and slab Moho, and the associated metamorphic reactions, evolve throughout the lifetime of a subduction zone, and ii), reappraise the effects of various subduction parameters (e.g. crustal and slab strength) on P-T conditions. Our models use the ASPECT finite element code and contain highly resolved crustal channels in a whole mantle domain. We track the P-T conditions of the slab top and Moho for > 50 Myr of subduction, during which the slab transitions through three distinct phases: initiation, free-sinking, and mature evolution. The slab top exhibits rapid cooling during initiation, ~ 50 °C/Myr at a depth of 60 km, and slower cooling subsequently. This cooling is related to variability in the “decoupling depth”, which, in our models, increases with time as the overriding plate ages and thickens. The resultant variability in slab top P-T conditions, particularly during the first 10 Myr, may contribute to the wide variability of P-T conditions recorded by exhumed UHP rocks. Aside from a warming pulse during free-sinking, the slab Moho also cools with time. We couple these P-T paths to thermodynamic models of typical subduction lithologies and calculate the resultant locations of metamorphic dehydration along the slab top and Moho. This analysis reveals a strong dependence of dehydration depth and magnitude on time-evolving thermal structure. Together, our work demonstrates that dynamic variations in subduction evolution may manifest strongly in the rock record.