Slab-mantle decoupling and its implications for subduction zone thermal structure, fluid supply, and geophysical process

1. Slab-mantle decoupling and its implications for subduction zone thermal structure, fluid supply, and geophysical processes Ikuko Wada1,2 and Kelin Wang1,2 ikukow@uvic.ca and kwang@nrcan.gc.ca 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. Downdip end of a low-velocity layer Depth of Basalt-Eclogite Transformation Cascadia Alaska (Rondenay et al., 2008)

5. Deeper basalt-eclogite transformation and peak crustal dehydration Max. Depth of a Low-Velocity Layer 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. Dehydration embrittlement at deeper depths Depth Range of Intraslab Earthquakes Slab thermal parameter (102 km) = Slab age × Descent rate (Inferred from earthquakes located by Engdahl et al. 1998 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 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. Cold & stagnant Decoupled Coupled Forearc-Arc Thermal Structure

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

15. Interface Layer Approach

16. Reduced coupling Lower temperature Stronger mantle Greater strength contrast Flow Velocity and Thermal Fields Full coupling Northern Cascadia model with an 8 Ma-old slab and 4.5 cm/yr subduction rate • 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

19. Seventeen Subduction Zones Investigated in This Study

20. >1200°C • Low surface heat flow in the forearc • High mantle temperature (> 1200°C) beneath the arc MDD constraints Maximum Depth of Decoupling (MDD): Cascadia Max. Depth Decoupling (MDD) of 70-80 km Cascadia (warm 8-Ma slab)

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

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

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

24. Common Max. Depth of Decoupling (MDD) of 70-80 km

25. Peak crustal dehydration Mantle dehydration Modelled Depths of Slab Dehydration Peak crustal dehydration Peak crustal dehydration Hydrated mantle Antigorite stability in the subducting mantle Model Results with the Common MDD of 70-80 km

26. Deeper peak crustal dehydration Modelled Depths of Slab Dehydration Peak crustal dehydration Hydrated mantle Downdip extent of Low-Velocity Layer (Untransformed Basaltic Crust) Slab thermal parameter (102 km) = Slab age × Descent rate

27. Deeper slab dehydration Depth Range of Intraslab Earthquakes 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 SC Chile Sumatra

29. Serpentinization at Ocean-Ocean Margins Mariana Kermadec Chrysotile/Lizardite

30. Episodic Tremor and Slip Cascadia Nankai

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

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 70-80 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.