1. Slab-mantle decoupling and its implications for subduction zone thermal structure, fluid supply, and geophysical processes
Ikuko Wada1,2 and Kelin Wang1,2
and
1 School of Earth and Ocean Sciences, University of Victoria, Canada
2 Pacific Geoscience Centre, Geological Survey of Canada 1

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
Cascadia
Alaska
Downdip end of
a low-velocity layer
(Rondenay et al., 2008)

5. Max. Depth of a Low-Velocity Layer
Deeper basalt-eclogite transformation and
peak crustal dehydration
Slab thermal parameter (102 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
Slab thermal parameter (102 km) = Slab age × Descent rate
(Inferred from earthquakes located by Engdahl et al and local networks)

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

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

9. Intensity of Arc Volcanism
Slab thermal parameter (102 km) = Slab age × Descent rate
(Crisp, 1984; White et al., 2006)

10. Arc Location England et al. (2004) Syracuse and Abers (2006) Others
Slab thermal parameter (102 km) = Slab age × Descent rate
England et al.
(2004)
Syracuse and Abers (2006)
Others

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

12. Forearc-Arc Thermal Structure
Cold &
stagnant
Decoupled
Coupled

13. Modelling Approach 2-D steady-state finite element model
T- and stress-dependent mantle rheology
Metamorphic reactions and water flow are not included.
Add “-” between thermally and controlled.
Animate the insertion of the thin layer. Add eta e and eta’ in the layer. 13

14. velocity discontinuity
Rigid corner
Peacock and Wang (1999)
van Keken et al. (2002)
Currie et al. (2004)
Conder (2005) (improved version)
Free slip:
Furukawa (1993)
Kelemen et al. (2003)
Velocity discontinuity:
Kneller et al. (2005, 2007)
Free slip or
velocity discontinuity
Add “-” between thermally and controlled.
Animate the insertion of the thin layer. Add eta e and eta’ in the layer. 14

15. Interface Layer Approach
Add “-” between thermally and controlled.
Animate the insertion of the thin layer. Add eta e and eta’ in the layer.

16. Flow Velocity and Thermal Fields
Full
coupling
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
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.
Decoupling to 80-km depth
Increasing
degree of
decoupling
Decoupling to 120-km depth

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

18. Model Simplification: Truncation of the Interface Layer
Add (MDD) after prescribed maximum depth of decoupling

19. Seventeen Subduction Zones Investigated in This Study
Put red rectangles for N. Cascadia and NE Japan.

20. Maximum Depth of Decoupling (MDD): Cascadia
Max. Depth Decoupling (MDD) of km
Cascadia (warm 8-Ma slab)
>1200°C
Use blue solid lines for Moho, Slab surface, and slab Moho, and label all.
Low surface heat flow in the forearc
High mantle temperature (> 1200°C) beneath the arc
MDD
constraints

21. Max. Depth Decoupling (MDD) of 70-80 km
Cascadia (warm 8-Ma slab)
NE Japan (cold 100-Ma slab)

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

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

24. Common Max. Depth of Decoupling (MDD) of 70-80 km
Put red rectangles for N. Cascadia and NE Japan. 24

25. Model Results with the Common MDD of 70-80 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 (102 km) = Slab age × Descent rate

27. Depth Range of Intraslab
Earthquakes
Deeper slab dehydration
Modelled Depths of Slab Dehydration
Peak
crustal dehydration
Hydrated
mantle
Slab thermal parameter (102 km) = Slab age × Descent rate

28. Stable Thermal Condition for Serpentinization
Nankai
Costa Rica
Figure 7 a and c of Pozgay et al. [2009]
SC Chile
Sumatra

29. Serpentinization at Ocean-Ocean Margins
Mariana
Kermadec
Figure 7 a and c of Pozgay et al. [2009]
Chrysotile/Lizardite

30. Episodic Tremor and Slip
Cascadia
Figure 7 a and c of Pozgay et al. [2009]
Nankai

31. Modelled Depths of Slab Dehydration More fluid beneath the arc
Volcanic
Output Rate
(Crisp, 1984;
White et al., 2006)
Modelled Depths of Slab Dehydration
Peak
crustal dehydration
More fluid beneath the arc
Hydrated
mantle
Slab thermal parameter (102 km) = Slab age × Descent rate

32. Arc Location England et al. (2004) Syracuse and Abers (2006) Others
Slab thermal parameter (102 km) = Slab age × Descent rate
England et al.
(2004)
Syracuse and Abers (2006)
Others

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

34. Common Depth of Decoupling (MDD) of 70-80 km
Cascadia (warm 8-Ma slab)
NE Japan (cold 100-Ma slab)
(serpentine)

35. The Effects of Subduction Rate and Slab Dip on the Thermally Expected Location of the Arc
Reference
Faster subduction rate
Steeper 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.