Deep decoupling in subduction zones: Observations and temperature limits

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.

Upper Plate Structure and Megathrust Properties in the Shumagin Gap Near the July 2020 M7.8 Simeonof Event

“…Fluids may also contribute to the observed band of reflectivity (Behr & Burgmann, 2021; Calvert et al., 2020). The Alaska subduction zone is much too cold at ∼30 km depth to expect significant dehydration reactions (Abers et al., 2020; van Keken et al., 2011). However, updip migration of fluids and/or expulsion of free water may contribute fluids to the plate boundary at this depth.…”

The Global Range of Temperatures on Convergent Plate Interfaces

“…Abers et al. (2020, Equation 7) appear to have noticed the error of van Keken et al. (2019) but, in attempting to correct it, gave the wrong sign in the denominator of the right‐hand side of Equation 6 of Molnar and England (1995).…”

On the Stability of Talc in Subduction Zones: A Possible Control on the Maximum Depth of Decoupling Between the Subducting Plate and Mantle Wedge

“…Downdip of the MDD, the subduction shear zone is no weaker than the overlying mantle wedge, and the consequent full coupling between the subducting slab and overlying mantle induces corner flow to advect heat from greater depths and the back-arc, resulting in a hot convecting mantle wedge capable of generating arc magmas. Understanding what controls the MDD remains an outstanding issue in subduction-zone dynamics (Abers et al, 2020;Wada & Wang, 2009). The MDD must involve a nonlinear feedback between interface strength, thermally controlled mantle wedge rheology, and mantle wedge flow (Wada & Wang, 2009).…”

“…mineralogy of the shear zone may occur at 70-80 km depth. Most metamorphic dehydration reactions in the subducting slab and downdragged mantle-wedge depend primarily on temperature (T) not pressure (P), and therefore do not provide a satisfactory explanation for the common depth of the MDD (e.g., Abers et al, 2020). In this study, we investigate the possibility that a metamorphic P-dependent reaction, which occurs in H 2 O-undersaturated ultramafic rocks, controls the abundance of talc along the plate interface and possibly the MDD.…”

“…The thermal structure of a subduction zone varies with time, and the locus of slab dehydration reactions will vary accordingly, with the mantle wedge recording the time-integrated history of slab dehydration over millions of years of subduction. Hydration is likely concentrated in a layer, several kilometers thick, at the base of the mantle wedge (Abers et al, 2020;Deschamps et al, 2013). This study focuses on the partially to fully hydrated base of the forearc mantle wedge, directly above the subduction interface.…”