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1. Metamorphic Dehydration, Fluid Pressure, and the Frictional‐Viscous Transition Along Subduction Megathrusts: Case Study in Cascadia and Implications for Slow Earthquakes

   https://doi.org/10.1029/2025AV002000  

   Ozawa, So; Dunham, Eric M; Condit, Cailey B (June 2026, AGU Advances)

   Abstract Relative plate motion in subduction zones transitions from frictional slip to viscous flow with increasing depth and temperature. The frictional‐viscous transition can control the depth extent of megathrust earthquakes and episodic tremor and slip (ETS). Pore fluid pressure is a critical control on the transition, but models for its depth dependence are lacking. Here, we present a steady‐state modeling framework to calculate the fluid pressure and shear stress profile along the subduction interface. We consider fluid production from dehydration reactions in the subducting oceanic lithosphere, calculated from thermodynamic equilibrium models. These fluids are channeled updip, though in some models we allow for fluid loss into the overriding plate. The fluid pressure is calculated from Darcy's law, with permeability depending on effective stress, temperature, and slip rate. We allow for both rate‐state frictional sliding and thermally activated linear viscous flow, and solve for the partitioning of deformation between them. We apply the modeling framework to the Cascadia subduction zone. Our results show nearly uniform effective stress in the seismogenic zone, below which it decreases with depth and fluid pressure approaches lithostatic pressure. The frictional‐viscous transition spans a wide range of depths, and mixed frictional‐viscous deformation is predicted at the ETS source depth. A model with fluid leak‐off into the overlying plate produces a more heterogeneous effective stress, with a local minimum near the major dehydration depth. Our results provide important insight into earthquake hazards and the mechanism of ETS in Cascadia, and the modeling framework is applicable to global subduction zones. 

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2. 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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3. Sumatra Subduction Zone

   https://doi.org/10.14379/iodp.proc.362.2017  

   McNeill, L.C.; Dugan, B.; Petronotis, K.E. (October 2017, Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   Drilling the input materials of the north Sumatran subduction zone, part of the 5000 km long Sunda subduction zone system and the origin of the Mw ~9.2 earthquake and tsunami that devastated coastal communities around the Indian Ocean in 2004, was designed to groundtruth the material properties causing unexpectedly shallow seismogenic slip and a distinctive forearc prism structure. The intriguing seismogenic behavior and forearc structure are not well explained by existing models or by relationships observed at margins where seismogenic slip typically occurs farther landward. The input materials of the north Sumatran subduction zone are a distinctively thick (as thick as 4–5 km) succession of primarily Bengal-Nicobar Fan–related sediments. The correspondence between the 2004 rupture location and the overlying prism plateau, as well as evidence for a strengthened input section, suggest the input materials are key to driving the distinctive slip behavior and long-term forearc structure. During Expedition 362, two sites on the Indian oceanic plate ~250 km southwest of the subduction zone, Sites U1480 and U1481, were drilled, cored, and logged to a maximum depth of 1500 meters below seafloor. The succession of sediment/rocks that will develop into the plate boundary detachment and will drive growth of the forearc were sampled, and their progressive mechanical, frictional, and hydrogeological property evolution will be analyzed through postcruise experimental and modeling studies. The large penetration depths with good core recovery and successful wireline logging in the challenging submarine fan materials will enable evaluation of the role of thick sedimentary subduction zone input sections in driving shallow slip and amplifying earthquake and tsunami magnitudes at the Sunda subduction zone and globally at other subduction zones where submarine fan–influenced sections are being subducted. 

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4. Expedition 362 Preliminary Report: Sumatra Seismogenic Zone

   https://doi.org/10.14379/iodp.pr.362.2017  

   Dugan, B.; McNeill, L.; Petronotis, K. (June 2017, Preliminary report)

   null (Ed.)

   Drilling the input materials of the north Sumatran subduction zone, part of the 5000 km long Sunda subduction zone system and the origin of the Mw ~9.2 earthquake and tsunami that devastated coastal communities around the Indian Ocean in 2004, was designed to groundtruth the material properties causing unexpectedly shallow seismogenic slip and a distinctive forearc prism structure. The intriguing seismogenic behavior and forearc structure are not well explained by existing models or by relationships observed at margins where seismogenic slip typically occurs farther landward. The input materials of the north Sumatran subduction zone are a distinctively thick (as thick as 4–5 km) succession of primarily Bengal-Nicobar Fan–related sediments. The correspondence between the 2004 rupture location and the overlying prism plateau, as well as evidence for a strengthened input section, suggest the input materials are key to driving the distinctive slip behavior and long-term forearc structure. During Expedition 362, two sites on the Indian oceanic plate ~250 km southwest of the subduction zone, Sites U1480 and U1481, were drilled, cored, and logged to a maximum depth of 1500 meters below seafloor. The succession of sediment/rocks that will develop into the plate boundary detachment and will drive growth of the forearc were sampled, and their progressive mechanical, frictional, and hydrogeological property evolution will be analyzed through postcruise experimental and modeling studies. Large penetration depths with good core recovery and successful wireline logging in the challenging submarine fan materials will enable evaluation of the role of thick sedimentary subduction zone input sections in driving shallow slip and amplifying earthquake and tsunami magnitudes, at the Sunda subduction zone and globally at other subduction zones where submarine fan–influenced sections are being subducted. 

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5. Shear heating reconciles thermal models with the metamorphic rock record of subduction

   https://doi.org/10.1073/pnas.1809962115  

   Kohn, Matthew J.; Castro, Adrian E.; Kerswell, Buchanan C.; Ranero, César R.; Spear, Frank S. (November 2018, Proceedings of the National Academy of Sciences)

   Some commonly referenced thermal-mechanical models of current subduction zones imply temperatures that are 100–500 °C colder at 30–80-km depth than pressure–temperature conditions determined thermobarometrically from exhumed metamorphic rocks. Accurately inferring subduction zone thermal structure, whether from models or rocks, is crucial for predicting metamorphic reactions and associated fluid release, subarc melting conditions, rheologies, and fault-slip phenomena. Here, we compile surface heat flow data from subduction zones worldwide and show that values are higher than can be explained for a frictionless subduction interface often assumed for modeling. An additional heat source––likely shear heating––is required to explain these forearc heat flow values. A friction coefficient of at least 0.03 and possibly as high as 0.1 in some cases explains these data, and we recommend a provisional average value of 0.05 ± 0.015 for modeling. Even small coefficients of friction can contribute several hundred degrees of heating at depths of 30–80 km. Adding such shear stresses to thermal models quantitatively reproduces the pressure–temperature conditions recorded by exhumed metamorphic rocks. Comparatively higher temperatures generally drive rock dehydration and densification, so, at a given depth, hotter rocks are denser than colder rocks, and harder to exhume through buoyancy mechanisms. Consequently––conversely to previous proposals––exhumed metamorphic rocks might overrepresent old-cold subduction where rocks at the slab interface are wetter and more buoyant than in young-hot subduction zones. 

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