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Slab-mantle decoupling and its implications for subduction zone thermal structure, fluid
supply, and geophysical processes
Ikuko Wada1,2 and Kelin Wang1,2
[email protected] and [email protected]
1 School of Earth and Ocean Sciences, University of Victoria, Canada2 Pacific Geoscience Centre, Geological Survey of Canada
Mass and Heat Transfer in Subduction Zones
(Currie and Hyndman, 2006)
• The thermal state of the subducting slab• Slab-driven mantle wedge flow
Depth of Basalt-Eclogite Transformation
AlaskaCascadia
(Rondenay et al., 2008)
Downdip end ofa low-velocity layer
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)
Depth Range of Intraslab Earthquakes
Dehydration embrittlement at deeper depths
(Inferred from earthquakes located by Engdahl et al. 1998 and local networks)
Slab thermal parameter (102 km) = Slab age × Descent rate
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
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
Intensity of Arc Volcanism
(Crisp, 1984; White et al., 2006)
Slab thermal parameter (102 km) = Slab age × Descent rate
Arc Location
OthersEngland et al. (2004)
Syracuse and Abers (2006)
Slab thermal parameter (102 km) = Slab age × Descent rate
Costa Rica
Sharp Change in Seismic Attenuation
Low attenuationCold condition
High attenuationHot condition
(Rychert et al., 2008)
• Similarly sharp transition in Nicaragua, Alaska, central Andes, Hikurangi, and NE Japan
Modelling Approach
• 2-D steady-state finite element model
• T- and stress-dependent mantle rheology
• Metamorphic reactions and water flow are not included.
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)
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
Increasingdegree of
decoupling
Decoupling to 80-km depth
Decoupling to 120-km depth
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
MDDconstraints
Max. Depth Decoupling (MDD) of 70-80 km
(serpentine)
Petrological Models: Stability of Hydrous Phases
NE Japan (cold 100-Ma slab)Cascadia (warm 8-Ma slab)
Distance (km)
Dep
th (
km)
Cascadia(warm slab):
(Bostock et al., 2002)
Low V – Serpentinization
High V – Little serpentinizationNE Japan
(cold slab):
(Miura et al., 2005)
Model Results with the Common MDD of 70-80 km
Peak crustal dehydrationMantle dehydration
Peakcrustal dehydration
Hydratedmantle
Modelled Depths of Slab Dehydration
Peakcrustal dehydration
Antigorite stability in the subducting mantle
Downdip extent of Low-Velocity Layer
(Untransformed Basaltic Crust)
Deeper peak crustal dehydration
Peakcrustal dehydration
Hydratedmantle
Modelled Depths of Slab Dehydration
Slab thermal parameter (102 km) = Slab age × Descent rate
Depth Range of Intraslab
EarthquakesDeeper slab
dehydration
Peakcrustal dehydration
Hydratedmantle
Modelled Depths of Slab Dehydration
Slab thermal parameter (102 km) = Slab age × Descent rate
Volcanic Output Rate
Peakcrustal dehydration
Hydratedmantle
Modelled Depths of Slab Dehydration
More fluid beneath the arc
(Crisp, 1984; White et al., 2006)
Slab thermal parameter (102 km) = Slab age × Descent rate
Arc Location
OthersEngland et al. (2004)
Syracuse and Abers (2006)
Slab thermal parameter (102 km) = Slab age × Descent rate
Hot Mantle Beneath the Arc
Model-predicted max. subarc mantle temperature in the seventeen subduction zones
(serpentine)
Common Depth of Decoupling (MDD) of 70-80 km
NE Japan (cold 100-Ma slab)Cascadia (warm 8-Ma slab)
The Effects of Subduction Rate and Slab Dip on the Thermally Expected Location of the Arc
Reference Faster subduction rate Steeper slab dip
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?
Decrease in Strength Contrast with Depth
Strength contrast between antigorite and olivine decreases with increasing pressure.
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?
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.