Slab Temperature Evolution Over the Lifetime of a Subduction Zone

Abstract

The thermal evolution of subducting slabs controls a range of subduction processes, yet we lack a robust understanding of how thermal structure develops over a subduction zone's lifetime. We investigate the time‐dependence of slab thermal structure using dynamically consistent, time evolving models. Pressure‐temperature (P–T) conditions along the slab Moho and slab top exhibit substantial variability throughout the various phases of subduction: initiation, free sinking, and mature subduction. This variability occurs in response to time‐dependent subduction properties (e.g., fast vs. slow convergence) and thermal structure inherited from previous phases (e.g., due to upper plate aging). At a given depth, the slab cools rapidly during initiation, after which slower cooling occurs. In the case of the Moho, additional cooling occurs during the free sinking phase. We explore the implications of time‐dependent thermal structure on exhumed rocks and slab dehydration. Modeled slab top P–T paths span much of the P–T space associated with exhumed rocks, suggesting a significant component of recorded variability may have dynamic origins. Coupling our P–T profiles with thermodynamic models of oceanic lithosphere, we show that dehydrating ultramafic rocks at the slab Moho provide the bulk of hydrous fluid at subarc depths during the earliest phases. Over subsequent phases, these rocks carry fluids into the deeper mantle, and it is mafic crust along the slab top that releases water at subarc depths. We conclude that varying subduction conditions, and non‐steady‐state thermal structure, challenge the utility of kinematically driven models with constant subduction parameters, particularly for investigating thermal structure in the geological past.

Timescales of subduction initiation and evolution of subduction thermal regimes

“…Subduction is one of the main facilitators of plate tectonics and yet the mechanisms responsible for subduction initiation are controversial (Stern and Gerya, 2017). How plate-scale mechanic s and kinematics allow subduction to progress and reach steady-state remains unclear (Agard et al, 2018;Anczkiewicz et al, 2004;Crameri et al, 2020;Gurnis et al, 2004;Holt and Condit, 2021). Direct geological records of subduction initiation are limited to metamorphic soles, which correspond to thin sheets of deformed and metamorphosed oceanic crust.…”

Structural and Thermal Evolution of an Infant Subduction Shear Zone: Insights From Sub‐Ophiolite Metamorphic Rocks Recovered From Oman Drilling Project Site BT‐1B

“…This distinct stage of faster cooling may indicate faster subduction, on the order of several cm/yr (cf. Holt & Condit, 2021 ; Peacock & Hyndman, 1999 ; van Keken et al., 2002 ). Rapid cooling from near‐peak T through ∼750°, 650° and 550°C is captured by zircon U‐Pb crystallization ages in the HT sole leucosomes (Rioux et al., 2016 ; Roberts et al., 2016 ; Styles et al., 2006 ; Warren et al., 2005 ), titanite U‐Pb cooling ages (Guilmette et al., 2018 ), titanite U‐Pb crystallization ages (this study and Garber et al., 2020 ), and hornblende 40 Ar/ 39 Ar cooling ages (Hacker, 1994 ), respectively (Figure 13 ).…”

“…This is especially true if subduction continues after ophiolite formation, since the spreading center—which serves as a heat source for high‐temperature metamorphism—migrates laterally away from the developing interface at the half spreading rate (∼10 cm/yr spreading rate for ≤1 Myr in Oman). Cooling leads to changes in metamorphic mineral parageneses and increased depths of prograde metamorphic dehydration of the shear zone (Holt & Condit, 2021 ; Poli & Schmidt, 1995 ; van Keken et al., 2011 ). Both of these phenomena strongly impact the operative microphysical deformation mechanisms along the interface.…”

“…Mechanical weakening and progressive strain localization must occur along the interface in order to facilitate slab‐mantle coupling and large‐scale convection (e.g., Coltice et al., 2017 ; Gurnis et al., 2004 ; Wada et al., 2008 ; Wada & Wang, 2009 ). However, even the most rigorous dynamically‐consistent models of subduction initiation have yet to reproduce observed thermal characteristics during the earliest stages (e.g., Holt & Condit, 2021 ), which points to a disconnect in our understanding of how subduction zone strength and temperature coevolve during subduction infancy. Furthermore, rates of interface cooling and consequences of cooling for metamorphic mineral stability and rock rheology are poorly constrained.…”

“…even the most rigorous dynamically-consistent models of subduction initiation have yet to reproduce observed thermal characteristics during the earliest stages (e.g., Holt & Condit, 2021), which points to a disconnect in our understanding of how subduction zone strength and temperature coevolve during subduction infancy. Furthermore, rates of interface cooling and consequences of cooling for metamorphic mineral stability and rock rheology are poorly constrained.…”