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1. An introductory review of the thermal structure of subduction zones: III—Comparison between models and observations

   https://doi.org/10.1186/s40645-023-00589-5  

   van Keken, Peter E.; Wilson, Cian R. (September 2023, Progress in Earth and Planetary Science)

   Abstract The thermal structure of subduction zones is fundamental to our understanding of the physical and chemical processes that occur at active convergent plate margins. These include magma generation and related arc volcanism, shallow and deep seismicity, and metamorphic reactions that can release fluids. Computational models can predict the thermal structure to great numerical precision when models are fully described but this does not guarantee accuracy or applicability. In a trio of companion papers, the construction of thermal subduction zone models, their use in subduction zone studies, and their link to geophysical and geochemical observations are explored. In this last part, we discuss how independent finite element approaches predict the thermal structure of the global subduction system and investigate how well these predictions correspond to geophysical, geochemical, and petrological observations. 

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2. An introductory review of the thermal structure of subduction zones: II—numerical approach and validation

   https://doi.org/10.1186/s40645-023-00588-6  

   Wilson, Cian R.; van Keken, Peter E. (November 2023, Progress in Earth and Planetary Science)

   Abstract The thermal structure of subduction zones is fundamental to our understanding of the physical and chemical processes that occur at active convergent plate margins. These include magma generation and related arc volcanism, shallow and deep seismicity, and metamorphic reactions that can release fluids. Computational models can predict the thermal structure to great numerical precision when models are fully described but this does not guarantee accuracy or applicability. In a trio of companion papers, the construction of thermal subduction zone models, their use in subduction zone studies, and their link to geophysical and geochemical observations are explored. In this part II, the finite element techniques that can be used to predict thermal structure are discussed in an introductory fashion along with their verification and validation. Steady-state thermal structure for the updated subduction zone benchmark. a) Temperature predicted by TF for case 1; b) temperature difference between TF and Sepran using the penalty function (PF) method for case 1 at f_m=1 where f_mrepresents the smallest element sizes in the finite element grids near the coupling point; c) slab top temperature comparison for case 1; and d)–f) as a)–c) but now for case 2. The star indicates the position or temperature conditions at the coupling point. 

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3. Subduction-Zone Fluids

   https://doi.org/10.2138/gselements.16.6.395  

   Manning, Craig E.; Frezzotti, Maria Luce (December 2020, Elements)

   null (Ed.)

   Fluids are essential to the physical and chemical processes in subduction zones. Two types of subduction-zone fluids can be distinguished. First, shallow fluids, which are relatively dilute and water rich and that have properties that vary between subduction zones depending on the local thermal regime. Second, deep fluids, which possess higher proportions of dissolved silicate, salts and non-polar gases relative to water content, and have properties that are broadly similar in most subduction systems, regardless of the local thermal structure. We review key physical and chemical properties of fluids in two key subduction-zone contexts—along the slab top and beneath the volcanic front—to illustrate the distinct properties of shallow and deep subduction-zone fluids. 

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4. Deep decoupling in subduction zones: Observations and temperature limits

   https://doi.org/10.1130/GES02278.1  

   Abers, Geoffrey A.; van Keken, Peter E.; Wilson, Cian R. (October 2020, Geosphere)

   null (Ed.)

   Abstract The plate interface undergoes two transitions between seismogenic depths and subarc depths. A brittle-ductile transition at 20–50 km depth is followed by a transition to full viscous coupling to the overlying mantle wedge at ∼80 km depth. We review evidence for both transitions, focusing on heat-flow and seismic-attenuation constraints on the deeper transition. The intervening ductile shear zone likely weakens considerably as temperature increases, such that its rheology exerts a stronger control on subduction-zone thermal structure than does frictional shear heating. We evaluate its role through analytic approximations and two-dimensional finite-element models for both idealized subduction geometries and those resembling real subduction zones. We show that a temperature-buffering process exists in the shear zone that results in temperatures being tightly controlled by the rheological strength of that shear zone’s material for a wide range of shear-heating behaviors of the shallower brittle region. Higher temperatures result in weaker shear zones and hence less heat generation, so temperatures stop increasing and shear zones stop weakening. The net result for many rheologies are temperatures limited to ≤350–420 °C along the plate interface below the cold forearc of most subduction zones until the hot coupled mantle is approached. Very young incoming plates are the exception. This rheological buffering desensitizes subduction-zone thermal structure to many parameters and may help explain the global constancy of the 80 km coupling limit. We recalculate water fluxes to the forearc wedge and deep mantle and find that shear heating has little effect on global water circulation. 

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5. Thermal modeling of subduction zones with prescribed and evolving 2D and 3D slab geometries

   https://doi.org/10.1186/s40645-024-00611-4  

   Sime, Nathan; Wilson, Cian R.; van Keken, Peter E. (March 2024, Progress in Earth and Planetary Science)

   Abstract The determination of the temperature in and above the slab in subduction zones, using models where the top of the slab is precisely known, is important to test hypotheses regarding the causes of arc volcanism and intermediate-depth seismicity. While 2D and 3D models can predict the thermal structure with high precision for fixed slab geometries, a number of regions are characterized by relatively large geometrical changes over time. Examples include the flat slab segments in South America that evolved from more steeply dipping geometries to the present day flat slab geometry. We devise, implement, and test a numerical approach to model the thermal evolution of a subduction zone with prescribed changes in slab geometry over time. Our numerical model approximates the subduction zone geometry by employing time dependent deformation of a Bézier spline that is used as the slab interface in a finite element discretization of the Stokes and heat equations. We implement the numerical model using the FEniCS open source finite element suite and describe the means by which we compute approximations of the subduction zone velocity, temperature, and pressure fields. We compute and compare the 3D time evolving numerical model with its 2D analogy at cross-sections for slabs that evolve to the present-day structure of a flat segment of the subducting Nazca plate. 

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