Backarc Lithospheric Thickness and Serpentine Stability Control Slab‐Mantle Coupling Depths in Subduction Zones. (25th June 2021)

• Record Type: Journal Article
• Authors: Kerswell, Buchanan C.; Kohn, Matthew J.; Gerya, Taras V.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 22:Number 6(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-06-25
• Subjects: subduction -- backarc -- coupling -- heat flow -- serpentine -- numerical model; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2020GC009304
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: A key feature of subduction zone geodynamics and thermal structure is the point at which the slab and mantle mechanically couple. This point defines the depth at which traction between slab and mantle begins to drive mantle wedge circulation and also corresponds with a rapid increase in temperature along the slab‐mantle interface. Here, we consider the effects of the backarc thermal structure and slab thermal parameter on coupling depth using two‐dimensional thermomechanical models of oceanic‐continental convergent margins. Coupling depth is strongly correlated with backarc lithospheric thickness, and weakly correlated with slab thermal parameter. Slab‐mantle coupling becomes significant where weak, hydrous antigorite reacts to form strong, anhydrous olivine and pyroxene along the slab‐mantle interface. Highly efficient (predominantly advective) heat transfer in the asthenospheric mantle wedge and inefficient (predominantly conductive) heat transfer in the lithospheric mantle wedge results in competing feedbacks that stabilize the antigorite‐out reaction at depths determined primarily by the mechanical thickness of the backarc lithosphere. For subduction zone segments where backarc lithospheric thickness can be inverted from surface heat flow, our results provide a regression model that can be applied with slab thermal parameter to predict coupling depth. Consistently high backarc heat flow in circum‐Pacific subduction zones suggests uniformly thin overriding plates likely regulated by lithospheric erosion caused by hydration and melting processes under volcanic arcs. This may also explain a common depth of slab‐mantle coupling globally. Plain Language Summary: Subduction is a process where two semi‐rigid tectonic plates of Earth's outer shell converge, and one dives beneath the other into the mantle. Subduction zones produce the world's largest earthquakes and form new crust through volcanism. Understanding earthquakes and volcanism requires understanding how the two plates move with respect to each other. With increasing depth, the sinking plate transitions from sliding past the overriding plate to "gripping" the base of the overriding plate. This gripping (coupling) reduces earthquakes and causes warm rock to flow upwards, enabling volcanism. We used two‐dimensional computer simulations of subduction to test what controls the depth of coupling between the plates. We found that coupling depth is primarily controlled by the thickness of the overriding plate and provide an equation to predict coupling depth in natural subduction zones. Overriding plates in subduction zones worldwide appear to have similar thicknesses, so coupling depths are also predicted to be similar. We do not yet fully understand why the overriding plates are uniformly thin globally, but perhaps this is related to hydration and melting under volcanic arcs. Key Points: The depths of antigorite destabilization and mechanical coupling are primarily dependent on backarc lithospheric thickness Coupling depths in natural subduction zones can be predicted given the slab age, convergence velocity, and backarc lithospheric thickness Consistently high backarc heat flow in natural subduction zones may indicate a common depth of mechanical coupling globally at ca. 82 km