Three-dimensionality of Slab Thermal Structure in Dynamic Subduction Models

Understanding the evolution of subduction zone thermal structure is crucial to describing processes such as arc magmatism, seismicity and dehydration reactions in the down-going slab. Time-evolving 2D models show that slab temperatures can vary over few-Myr timescale (Holt and Condit, 2021). Since magmatism, seismicity and devolatilization vary not only through time but also along strike (e.g., Kincaid and Griffiths, 2003), we explore 3D dynamic models to understand the degree to which this three-dimensionality is also time-dependent. To explore this, we developed 3D dynamic subduction models using the ASPECT finite element code (Kronbichler et al., 2012). We focus on the thermal conditions at the plate edge vs. the center, and the impact of plate width (1000 to 3000 km) and trench mobility (free vs. fixed). All free-trench models show significant along-strike temperature variability (e.g., slab top temperature difference from plate center to edge, T, of ~ 40C at depth = 145 km for the 2000 km width case) and a decrease in T over time both along the slab top and in the mantle wedge. All models show an increase in slab top temperature towards the slab edge followed by an abrupt decrease at the very edge. Increasing the plate width causes T to increase significantly (to ~120C in the 3000 km plate width case). Fixing the trench strongly affects slab dynamics and causes T to remain approximately constant through time. The high along-strike variability we see in our free-trench models is in contrast to relatively insignificant variation in along-arc temperature predicted by some purely kinematic models (e.g. Morishige & van Keken's [2014] non-oblique models) but in general agreement with the time-dependent laboratory models of Kincaid & Griffiths (2003). Our models suggest that a dynamic setting is crucial for understanding 3-D thermal structure, which results in high temperature variability both along strike and with time and may be reflected in variations in arc magmatism and/or dehydration depths. Holt, A. F., & Condit, C. B. (2021). Geochem., Geophys., Geosys., 22(6) Kincaid, C., & Griffiths, R. W. (2003). Nature, 425(6953). Kronbichler, M., Heister, T., & Bangerth, W. (2012). Geophys. J. Int., 191(1). Morishige, M., & van Keken, P. E. (2014). Geochem., Geophys., Geosys., 15(6).