Interaction of subducted slabs with the mantle transition‐zone: A regime diagram from 2‐D thermo‐mechanical models with a mobile trench and an overriding plate

F Garel,S C Kramer,C R Wilson,D R Davies,S Goes,J H Davies

doi:10.1002/2014gc005257

Abstract

AbstractTransition zone slab deformation influences Earth's thermal, chemical, and tectonic evolution. However, the mechanisms responsible for the wide range of imaged slab morphologies remain debated. Here we use 2‐D thermo‐mechanical models with a mobile trench, an overriding plate, a temperature and stress‐dependent rheology, and a 10, 30, or 100‐fold increase in lower mantle viscosity, to investigate the effect of initial subducting and overriding‐plate ages on slab‐transition zone interaction. Four subduction styles emerge: (i) a “vertical folding” mode, with a quasi‐stationary trench, near‐vertical subduction, and buckling/folding at depth (VF); (ii) slabs that induce mild trench retreat, which are flattened/“horizontally deflected” and stagnate at the upper‐lower mantle interface (HD); (iii) inclined slabs, which result from rapid sinking and strong trench retreat (ISR); (iv) a two‐stage mode, displaying backward‐bent and subsequently inclined slabs, with late trench retreat (BIR). Transitions from regime (i) to (iii) occur with increasing subducting plate age (i.e., buoyancy and strength). Regime (iv) develops for old (strong) subducting and overriding plates. We find that the interplay between trench motion and slab deformation at depth dictates the subduction style, both being controlled by slab strength, which is consistent with predictions from previous compositional subduction models. However, due to feedbacks between deformation, sinking rate, temperature, and slab strength, the subducting plate buoyancy, overriding plate strength, and upper‐lower mantle viscosity jump are also important controls in thermo‐mechanical subduction. For intermediate upper‐lower mantle viscosity jumps (×30), our regimes reproduce the diverse range of seismically imaged slab morphologies.

Highlights

Much of the cold material at Earth’s surface slides back into the underlying mantle at subduction zones, where one tectonic plate (‘‘slab’’) subducts beneath a second (‘‘overriding’’) plate, providing the main driving force for plate tectonics [e.g., Forsyth and Uyeda, 1975]
The end-member slab morphologies imaged seismically [e.g., Bijwaard et al, 1998; Li et al, 2008; Fukao et al, 2009] range from: (i) an almost constant dip from upper to lower mantle with possibly some broadening in the transition zone (e.g., Central America, Aegean); (ii) near-vertical slabs that appear to thicken upon interaction with the base of the upper mantle (Marianas, Kermadec); (iii) slabs that have a constant small dip in the upper mantle and flatten at the base of the transition zone (e.g., Japan); to (iv) steep slabs through the upper mantle, which flatten in the transition zone (e.g., Izu-Bonin, Tonga)
As the slab starts to interact with the high-viscosity lower mantle, VSP reduces to 2 cm/ yr

Summary

Introduction

Much of the cold material at Earth’s surface slides back into the underlying mantle at subduction zones, where one tectonic plate (‘‘slab’’) subducts beneath a second (‘‘overriding’’) plate, providing the main driving force for plate tectonics [e.g., Forsyth and Uyeda, 1975]. It has been inferred that, in some cases, subducted material extends below 700 km into Earth’s lower mantle, while in other regions, slabs bend, buckle, or flatten out in the transition zone, in response to phase and viscosity changes in this depth range [e.g., Isacks and Molnar, 1971; van der Hilst et al, 1991; Fukao et al, 1992; Gudmundsson and Sambridge, 1998; Li et al, 2008; Hayes et al, 2012] This variability in slab deformation is accompanied by significant variations in earthquake potential and basin formation or mountain building [Uyeda and Kanamori, 1979]. The end-member slab morphologies imaged seismically [e.g., Bijwaard et al, 1998; Li et al, 2008; Fukao et al, 2009] range from: (i) an almost constant dip from upper to lower mantle with possibly some broadening in the transition zone (e.g., Central America, Aegean); (ii) near-vertical slabs that appear to thicken upon interaction with the base of the upper mantle (Marianas, Kermadec); (iii) slabs that have a constant small dip in the upper mantle and flatten at the base of the transition zone (e.g., Japan); to (iv) steep slabs through the upper mantle, which flatten in the transition zone (e.g., Izu-Bonin, Tonga).

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Discussion

Conclusion