Time dependent slab temperatures, metamorphism, and mechanical properties: Insights from dynamic subduction models

The thermal structure of subduction zones enacts a first-order control on many geological processes and properties, including the locus and degree of slab devolatilization, and the associated densities and strengths of subducting material. Modeling studies with fixed subduction geometries and plate velocities have been used to map out how various subduction parameters affect the pressure-temperature conditions of slabs and, in turn, the depths of major dehydration reactions. However, there is abundant geological evidence that slab properties, and the associated temperatures, evolve over few-Myr timescales. In this study, we use numerical subduction models to target this time dependence. Specifically, we focus on the styles and drivers of thermal transience and the imprint of this on subducting slab dehydration and slab strength.Specifically, we have developed 2-D and 3-D subduction models that enable slab properties to evolve through time in a dynamically consistent fashion using the ASPECT finite element code1-3. We use these models to investigate: i) the extent to which slab thermal conditions – and the associated metamorphic reactions and slab strength – evolve throughout the lifetime of a subduction zone, ii) the effects of first-order subduction zone properties on this evolution, and iii) the degree to which three-dimensionality (i.e., the presence of a slab edge) impacts this evolution. Regardless of imposed basic subduction parameters (e.g., plate ages, crustal strengths), our model subduction zones exhibit highly time-dependent thermal evolutions. The slab top, for example, exhibits rapid cooling during initiation and slower cooling subsequently, with along-strike temperature variations of up to ~40°C in the 3-D models. This thermal transience has fundamental implications for the geophysical and geochemical evolution of subduction zones; it manifests in a strong time dependence of dehydration depths and magnitudes and, in turn, substantial variability in slab strength.  1: Bangerth, W., Dannberg, J., Gassmoeller, R., & Heister, T. (2020). ASPECT v2.1.0, Zenodo. https://doi.org/10.5281/ZENODO.39246042: Heister, T., Dannberg, J., Gassmöller, R., & Bangerth, W. (2017). High accuracy mantle convection simulation through modern numerical methods - II: Realistic models and problems. Geophys. J. Int., 210(2), https://doi.org/10.1093/gji/ggx1953: Holt, A. F., & Condit, C. B. (2021). Slab temperature evolution over the lifetime of a subduction zone. Geochem., Geophys., Geosys., 22, doi:10.1029/2020GC009476.