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, 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

Temperature- and Fluid-Dependent Processes

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

Forearc-Arc Thermal Structure

Decoupled

Coupled

Cold &stagnant

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)

Interface Layer Approach

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

Sharp Thermal Transition in the Mantle Wedge

Generic model

AttenuationIn Costa Rica

Hot

Cold

Model Simplification: Truncation of the Interface Layer

Seventeen Subduction Zones Investigated in This Study

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

NE Japan (cold 100-Ma slab)Cascadia (warm 8-Ma slab)

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)

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

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

Stable Thermal Condition for Serpentinization

Nankai Costa Rica

SC Chile Sumatra

Serpentinization at Ocean-Ocean Margins

KermadecMariana

Chrysotile/Lizardite

Episodic Tremor and Slip

Nankai

Cascadia

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.