Dynamical mechanisms controlling formation and avalanche of a stagnant slab

Abstract

We performed a numerical study to understand the dynamical mechanism controlling the formation and avalanche of a stagnant slab using two-dimensional dynamical models of the integrated plate-mantle system with freely movable subducting and overriding plates. We examined slab rheology as a mechanism for producing various styles of stagnating or penetrating slabs that interact with the 410-km and 660-km phase transitions. The simulated results with the systematically changed rheological parameters are interpreted using a simple stability analysis that includes the forces acting on the stagnant slab. Slab plasticity that memorizes the shape produced by past deformation generates slab stagnation at various depths around the 660-km phase transition. The slab stagnates even beneath the 660-km phase boundary, with a gentle Clapeyron slope. Feedbacks between trench backward migration and slab deformation promote each other during the slab stagnation stage. Slab viscosity also determines the final state of the subducted slab, that is, it continues stagnation or initiates penetration. A low-viscosity slab can finally penetrate into the lower mantle because the growth time of the Rayleigh-Taylor instability is shorter. After the avalanche, the direction of the trench migration changes depending on the lower mantle slab viscosity.

Continental flood basalts derived from the hydrous mantle transition zone

“…Assembly of supercontinents therefore potentially leads to accumulation of a large volume of hydrated oceanic slabs within the MTZ 11,12 and may possibly change the mode of mantle convection 11 . The time delay of 140-150 Myr between the final assembly and initial breakup of Pangea 53 and Rodinia 54 is in good consistent with the lifespan of stagnant slabs within the MTZ constrained by slab viscosity 42 and phase transformation 41 . Such similarity may hint at a possible causal relationship between slab avalanche (and/or mantle overturn) and breakup of supercontinents.…”

“…The slow kinetics of the pyroxenegarnet transformation 41 and slab viscosity 42 within the MTZ enables the Pacific slab to reside for a long time in this zone (over 10 8 year) and hence lose water by dehydration reactions within the MTZ 11 . Because feedbacks between trench backward migration and slab deformation are integrated during the slab stagnation 42 , the declining rate of Pacific-Eurasia convergence from a late Cretaceous convergence rate of 120-140 mm year to a minimum in the Eocene of 30-40 mm year 43 may also have contributed to the Pacific slab stagnation. After completion of the pyroxene-garnet transformation 41 , the stagnant Pacific slab is likely to eventually fall into the lower mantle (slab avalanche) and release most of its water by dehydration melting when it penetrates the 660-km discontinuity 19 .…”

“…After completion of the pyroxene-garnet transformation 41 , the stagnant Pacific slab is likely to eventually fall into the lower mantle (slab avalanche) and release most of its water by dehydration melting when it penetrates the 660-km discontinuity 19 . The viscosity reduction due to slow grain growth may also induce slab penetration 42 . Combining geophysical and experimental studies has therefore demonstrated the presence of extensive dehydration within the MTZ beneath this region 25 .…”

Dynamics of back‐arc extension controlled by subducting slab retreat: Insights from 2D thermo‐mechanical modelling

“…This dynamic process may be caused by the fact that the slab starts to penetrate into the lower mantle before the weakening and opening of the continental lithosphere, due to the fast slab going down and strong continental lithosphere structure obstruct being stretched open. Once the slab starts to penetrate into the lower mantle, the transition zone buoyancy compensates the effects of the driving force, and then the subduction ceases (Nakakuki et al, ; Zhong & Gurnis, ). This leads to no back‐arc extension occurs eventually.…”

“…Slab retreat plays an important role in the stagnant slab occurrence (Billen, ; Tagawa, Nakakuki, & Tajima, ; Torii & Yoshioka, ). Slab retreat extends the distance between the trench and the subducting slab tip and thus reduces the slab dipping angle, which favours stagnant slab (Nakakuki, Tagawa, & Iwase, ). The aforementioned studies imply that the slab retreat is a critical important driving mechanism for back‐arc extension development.…”