SHEAR HEATING IN SUBDUCTION ZONES: IMPLICATIONS FOR THERMAL MODELS AND METAMORPHIC P-T PATHS

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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The effect of temperature-dependent thermal parameters in thermal models of subduction zones

10.5194/egusphere-egu21-120 ◽

2021 ◽

Author(s):

Iris van Zelst ◽

Timothy J. Craig ◽

Cedric Thieulot

Keyword(s):

Subduction Zones ◽

Thermal Parameters ◽

Dislocation Creep ◽

Seismogenic Zone ◽

Effect Of Temperature ◽

Temperature Dependent ◽

Thermal Models ◽

Intermediate Depth ◽

Dehydration Reactions ◽

Model Setup

The thermal structure of subduction zones plays an important role in the seismicity that occurs there with e.g., the downdip limit of the seismogenic zone associated with particular isotherms (350 &#176;C - 450 &#176;C) and intermediate-depth seismicity linked to dehydration reactions that occur at specific temperatures and pressures. Therefore, accurate thermal models of subduction zones that include the complexities found in laboratory studies are necessary. One of the often-ignored effects in models is the temperature-dependence of the thermal parameters such as the thermal conductivity, heat capacity, and density.&#160;Here, we build upon the model setup presented by Van Keken et al., 2008 by including temperature-dependent thermal parameters to an otherwise clearly constrained, simple model setup of a subducting plate. We consider a fixed kinematic slab dipping at 45&#176; and a stationary overriding plate with a dynamic mantle wedge. Such a simple setup allows us to isolate the effect of temperature-dependent thermal parameters. We add a more complex plate cooling model for the oceanic plate for consistency with the thermal parameters.&#160;We test the effect of temperature-dependent thermal parameters on models with different rheologies, such as an isoviscous wedge, diffusion and dislocation creep. We find that slab temperatures can change by up to 65 &#176;C which affects the location of isotherm depths. The downdip limit of the seismogenic zone defined by e.g., the 350 &#176;C isotherm shifts by approximately 4 km, thereby increasing the maximum possible rupture area of the seismogenic zone. Similarly, the 600 &#176;C isotherm is shifted approximately 30 km deeper, affecting the depth at which dehydration reactions and hence intermediate-depth seismicity occurs. Our results therefore show that temperature-dependent thermal parameters in thermal models of subduction zones cannot be ignored when studying subduction-related seismicity.&#160;

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Correction for Kohn et al., Shear heating reconciles thermal models with the metamorphic rock record of subduction

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1820654116 ◽

2019 ◽

Vol 116 (33) ◽

pp. 16651-16653

Keyword(s):

Metamorphic Rock ◽

Thermal Models ◽

Rock Record ◽

Shear Heating

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The effect of temperature-dependent material properties on simple thermal models of subduction zones

10.31223/x5b31x ◽

2021 ◽

Author(s):

Iris van Zelst ◽

Cedric Thieulot ◽

Timothy Craig

Keyword(s):

Material Properties ◽

Subduction Zones ◽

Effect Of Temperature ◽

Temperature Dependent ◽

Thermal Models

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Deep decoupling in subduction zones: Observations and temperature limits

Geosphere ◽

10.1130/ges02278.1 ◽

2020 ◽

Vol 16 (6) ◽

pp. 1408-1424 ◽

Cited By ~ 1

Author(s):

Geoffrey A. Abers ◽

Peter E. van Keken ◽

Cian R. Wilson

Keyword(s):

Subduction Zone ◽

Shear Zone ◽

Subduction Zones ◽

Thermal Structure ◽

Shear Zones ◽

Seismic Attenuation ◽

Plate Interface ◽

Brittle Ductile Transition ◽

Wide Range ◽

Shear Heating

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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