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Slab-mantle decoupling and its implications for subduction zone thermal structure, fluid supply, and geophysical processes Ikuko Wada 1,2 and Kelin Wang.

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Presentation on theme: "Slab-mantle decoupling and its implications for subduction zone thermal structure, fluid supply, and geophysical processes Ikuko Wada 1,2 and Kelin Wang."— Presentation transcript:

1 Slab-mantle decoupling and its implications for subduction zone thermal structure, fluid supply, and geophysical processes Ikuko Wada 1,2 and Kelin Wang 1,2 and 1 School of Earth and Ocean Sciences, University of Victoria, Canada 2 Pacific Geoscience Centre, Geological Survey of Canada

2 Mass and Heat Transfer in Subduction Zones (Currie and Hyndman, 2006) The thermal state of the subducting slab Slab-driven mantle wedge flow

3 Temperature- and Fluid-Dependent Processes

4 Depth of Basalt-Eclogite Transformation AlaskaCascadia (Rondenay et al., 2008) Downdip end of a low-velocity layer

5 Max. Depth of a Low-Velocity Layer Deeper basalt-eclogite transformation and peak crustal dehydration Slab thermal parameter (10 2 km) = Slab age × Descent rate (Fukao et al., 1983;Cassidy and Ellis, 1993; Bostock et al., 2002; Hori et al, 1985; Hori, 1990; Ohkura, 2000; Yuan et al., 2000; Bock et al., 2000; Abers, 2006; Rondenay et al., 2008; Matsuzawa et al., 1986; Kawakatsu and Watada, 2007)

6 Depth Range of Intraslab Earthquakes Dehydration embrittlement at deeper depths (Inferred from earthquakes located by Engdahl et al and local networks) Slab thermal parameter (10 2 km) = Slab age × Descent rate

7 Episodic Tremor and Slip (ETS) Nankai (warm slab) Cascadia (warm slab) ETS-like events in Mexico, Alaska, and Costa Rica No ETS in NE Japan and Hikurangi

8 Mantle Wedge Serpentinization Cascadia (Bostock et al., 2002) Serpentinization in Nankai, Kyushu, Alaska, Chile, Costa Rica, and Mariana Minor degree of serpentinization in NE Japan and Hikurangi

9 Intensity of Arc Volcanism (Crisp, 1984; White et al., 2006) Slab thermal parameter (10 2 km) = Slab age × Descent rate

10 Arc Location OthersEngland et al. (2004) Syracuse and Abers (2006) Slab thermal parameter (10 2 km) = Slab age × Descent rate

11 Costa Rica Sharp Change in Seismic Attenuation Low attenuation Cold condition High attenuation Hot condition (Rychert et al., 2008) Similarly sharp transition in Nicaragua, Alaska, central Andes, Hikurangi, and NE Japan

12 Forearc-Arc Thermal Structure Decoupled Coupled Cold & stagnant

13 Modelling Approach 2-D steady-state finite element model T- and stress-dependent mantle rheology Metamorphic reactions and water flow are not included.

14 Free slip: Furukawa (1993) Kelemen et al. (2003) Velocity discontinuity: Kneller et al. (2005, 2007) Free slip or velocity discontinuity Rigid corner Peacock and Wang (1999) van Keken et al. (2002) Currie et al. (2004) Conder (2005) (improved version)

15 Interface Layer Approach

16 Full coupling Mantle either does not flow or flows at full speed, resulting in a bimodal flow behaviour. There is a strong thermal contrast between stagnant and flowing parts. Flow Velocity and Thermal Fields Northern Cascadia model with an 8 Ma-old slab and 4.5 cm/yr subduction rate Reduced coupling Lower temperature Stronger mantle Greater strength contrast Increasing degree of decoupling Decoupling to 80-km depth Decoupling to 120-km depth

17 Sharp Thermal Transition in the Mantle Wedge Generic model Attenuation In Costa Rica Hot Cold

18 Model Simplification: Truncation of the Interface Layer

19 Seventeen Subduction Zones Investigated in This Study

20 Maximum Depth of Decoupling (MDD): Cascadia Cascadia (warm 8-Ma slab) >1200°C Low surface heat flow in the forearc High mantle temperature (> 1200°C) beneath the arc MDD constraints Max. Depth Decoupling (MDD) of km

21 NE Japan (cold 100-Ma slab)Cascadia (warm 8-Ma slab) Max. Depth Decoupling (MDD) of km

22 (serpentine) Petrological Models: Stability of Hydrous Phases NE Japan (cold 100-Ma slab)Cascadia (warm 8-Ma slab)

23 Distance (km) Depth (km) Cascadia (warm slab): (Bostock et al., 2002) Low V – Serpentinization High V – Little serpentinization NE Japan (cold slab): (Miura et al., 2005)

24 Common Max. Depth of Decoupling (MDD) of km

25 Model Results with the Common MDD of km Peak crustal dehydration Mantle dehydration Peak crustal dehydration Hydrated mantle Modelled Depths of Slab Dehydration Peak crustal dehydration Antigorite stability in the subducting mantle

26 Downdip extent of Low-Velocity Layer (Untransformed Basaltic Crust) Deeper peak crustal dehydration Peak crustal dehydration Hydrated mantle Modelled Depths of Slab Dehydration Slab thermal parameter (10 2 km) = Slab age × Descent rate

27 Depth Range of Intraslab Earthquakes Deeper slab dehydration Peak crustal dehydration Hydrated mantle Modelled Depths of Slab Dehydration Slab thermal parameter (10 2 km) = Slab age × Descent rate

28 Stable Thermal Condition for Serpentinization Nankai Costa Rica SC ChileSumatra

29 Serpentinization at Ocean-Ocean Margins KermadecMariana Chrysotile/Lizardite

30 Episodic Tremor and Slip Nankai Cascadia

31 Volcanic Output Rate Peak crustal dehydration Hydrated mantle Modelled Depths of Slab Dehydration More fluid beneath the arc (Crisp, 1984; White et al., 2006) Slab thermal parameter (10 2 km) = Slab age × Descent rate

32 Arc Location OthersEngland et al. (2004) Syracuse and Abers (2006) Slab thermal parameter (10 2 km) = Slab age × Descent rate

33 Hot Mantle Beneath the Arc Model-predicted max. subarc mantle temperature in the seventeen subduction zones

34 (serpentine) Common Depth of Decoupling (MDD) of km NE Japan (cold 100-Ma slab)Cascadia (warm 8-Ma slab)

35 The Effects of Subduction Rate and Slab Dip on the Thermally Expected Location of the Arc ReferenceFaster subduction rateSteeper slab dip

36 Future Research: What Controls the MDD? Metamorphic phase changes of material along the interface? Strengthening of minerals, particularly antigorite, along the interface with depth? Uniform heat supply from the backarc?

37 Decrease in Strength Contrast with Depth Strength contrast between antigorite and olivine decreases with increasing pressure.

38 Future Research: What Controls the MDD? Metamorphic phase changes of material along the interface? Strengthening of minerals, particularly antigorite, along the interface with depth? Uniform heat supply from the backarc?

39 Concluding Remarks The flow in the mantle wedge is bimodal, and the change in the decoupling-coupling transition is sharp. The bimodal flow behaviour results in sharp thermal contrast in the forearc mantle wedge. Most, if not all, subduction zones share a common maximum depth of decoupling (MDD) of km. The common MDD explains the observed systematic variations in the petrologic, seismological, and volcanic processes. The common MDD also explains the uniform location of the thermal transition in the forearc mantle wedge and the uniform configuration of subduction zones.


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