Kelin Wang1,2

1Pacific Geoscience Centre, Geological Survey of Canada2School of Earth and Ocean Sciences, University of Victoria

Dealing with paradoxes

in subduction zone geodynamics

Acknowledgements: Yan Hu – deformation modeling (PhD work) Ikuko Wada – thermal modeling (PhD work)John He – computer programming

Wada and Wang, 2009, G3

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

Intensity of Arc Volcanism

More magma production

(Crisp, 1984; White et al., 2006)

Slab thermal parameter (102 km) = Slab age × Descent rate

Survival depth of basaltic crust (blue diamond)

anddepth range of intraslab

earthquakes (purple lines)

Eruption rate of arc volcanoes

(White et al., 2006)

Depth of slab beneath volcanic arc

Colour: different publications

Warm Cold

Wada and Wang, 2009, G3

Paradox 1

Subduction zones exhibit great (thermally controlled) diversity in petrologic, seismic, and volcanic processes, but they share a rather uniform slab-arc configuration.

~ 10

0 km

• Low seismic attenuation• Low Vp/Vs• Serpentinization• Stagnant

• High attenuation• High Vp/Vs• Melting• Vigorous wedge flow

Cold Forearc Hot Arc, Back Arc

70 ~

80 km

NorthernCascadia

(Currie et al.2004, EPSL)

inflo

wo

utflo

w

Temperature

Dep

th

LandwardGeotherm

Mantle adiabat

Oceanic geotherm(plate cooling model)

Temperature- and stress-dependent mantle wedge rheology

RT

PVE

An

n

exp2

1

Heat Flow Measurements

Heat flow transect across the Cascadia subduction zone

probe

BSROffshore well

Land boreholeODP hole

Comparison with thermal model results

Blue: Basaltic crust

Purple: Serpentine stability

in slab or mantle wedge

Preferred Cascadia model• Decoupling to ~ 70 - 80 km depth

Two primary constraints:• Surface heat flow (cold foreac)

• Mantle temperature beneath arc > 1200C (hot arc)

Fluid content in the subducting slab

Phase diagram from Hacker et al. (2004) Reactions from Schmidt and Poli (1998)Wet solidus: (1) Schmidt and Poli (1998), (2)

Grove et al. (2003)

Crust (wet basalt) Mantle

warmwarm

cold

coldwt% bound H2O

Blue: Basaltic crust

Purple:Serpentine stability

Basalt to eclogite ~ 40-50 km depthFeeble arc volcanismSerpentinized mantle wedge cornerIntraslab earthquakes to ~90 km depth

Basalt to eclogite ~ 100-140 kmActive arc volcanismHigh-velocity wedge cornerEarthquakes to hundreds of km

N Cascadia NE Japan

Kirby et al., 1996; Wada and Wang, 2009; Syracuse et al., 2010 ; van Keken et al., 2011

End-member warm-slab and cold-slab subduction zones

Assuming decoulping to 75 km

Wada and Wang, 2009, G3

Survival depth of basaltic oceanic crust (blue)

anddepth range of intraslab

earthquakes (purple)

Model-predicted peak dehydration depth (blue)

andantigorite stability in

subducting slab (purple)

Warm Cold

Wada and Wang, 2009, G3

Paradox 1: Subduction zones exhibit great (thermally controlled) diversity in petrologic, seismic, and volcanic processes, but they share a rather uniform slab-arc configuration.

Reconciliation: Common depth of decoupling between the slab and the mantle wedge

Weakening of slab-mantle wedge interface

• Weak hydrous minerals: (wet) serpentine, talc, brucite, chlorite

e.g. frictional coefficient of wet talc ~0.2

• Elevated fluid pressure: if = 0.2, Pf /Plith = 90%, = 0.02

? ?1

11

1

3

Northeast Japan

Southeast Mexico

1

2 or 3

1

Hellenic ArcQuaternary faults (Angelier et al., 1982) and earthquake focal mechanisms (Benetatos et al., 2004)

1

2 or 3

Northern Cascadia

Summary of Stress Indicators

Paradox 2

Subduction zones accommodate plate convergence, but few forearcs are under margin-normal compression.

RT

PVE

An

n

exp2

1

Mantle wedge rheology:Dislocation creep

Effective viscosity:

Far-fieldforce

Contours of maximum

shear stress

Summary of Stress IndicatorsSummary of Stress Indicators Force Balance Model

0.05

Assuming V = H, Lamb (2006) obtained 0.03 for most subduction zones

n

?

Red: Stress constrained by stress indicators I compiled.Blue: Megathrust stress determined by Lamb (2006) assuming V = H.

Thermal models have been developed for most sites with 0.03 for frictional heating along megathrust.

Modeling Results for Peru-Chile

Lamb (2007): 0.095

assuming V = H

Richardson and Coblentz (1994):H=25 MPa (

0.06)recognizing V > H

Sobolev and Babeyko (2005):

= 0.015 0.05orogeny model

Do Chilean-type subduction zones have a strong fault?

Paradox 2: Subduction zones accommodate plate convergence, but few forearcs are under margin-normal compression.

Explanation: Plate interface too weak to overcome gravitational tension in the forearc.

Summary of Stressesin Cascadia forearc

small earthquakes in upper plate

Wang, 2000, Tectonophysics

Geodetic Strain Rates

A 100-km line becomes shorter

by 2 cm each year

Geodetic Strain Rates Forearc Stresses

small earthquakes in upper plate

Wang, 2000, Tectonophysics

Nankai Forearc

Stresses and geodetic strain rates are similar to Cascadia

Wang, 2000, Tectonophysics

Paradox 3

At some forearcs, maximum compression is margin-parallel, but fastest geodetic shortening is roughly margin-normal.

If deformation is elastic, it only reflects stress changes and has nothing to do with absolute stress.

Cascadia geodetic shortening reflects stress increase due to interseismic locking of the plate interface.

Geodetic Strain Rates

A Stretched Elastic Band

Time 1: Tension

Time 2: Less tension Contraction

Great earthquake cycles cause small perturbations to forearc stress.

If deformation is elastic, it only reflects stress changes.

Cascadia geodetic shortening reflects stress increase due to interseismic locking of the plate interface.

Geodetic Strain Rates

If deformation is elastic, it only reflects stress changes.

Cascadia geodetic shortening reflects stress increase due to interseismic locking of the plate interface.

Great earthquake cycles cause small perturbations to forearc stress.Simons et al., 2011

Tohoku earthquakeMw = 9

March 11, 2011

>20% peak slip

Entire fault

Static stress drop(Probability from inversion)

Areas with >10% peak slip

Margin-parallel compression

Margin-normal stress

perturbation

Margin-parallel compression

Margin-normal stress

perturbation

Paradox 3: At some forearcs, maximum compression is margin-parallel, but fastest geodetic shortening is roughly margin-normal.

Explanation: The geodetic shortening only reflects small stress changes in earthquake cycles.

Cascadia: All sites move landward

Wells and Simpson (2001)

Wang, 2007, SEIZE volume

Alaska and Chile: Opposing motion of coastal and inland sites

M = 9.2 M = 9.2 19641964

Freymueller et al. (2009)

M = 9.5 M = 9.5 19601960

Wang et al. (2007, G3)

Paradox 4

Interseismic locking of subduction fault causes landward motion of the upper plate, but some areas show seaward motion.

Japan and Sumatra: All sites move seaward

Grijalva et al (2009)http://www.gsi.go.jp/cais/topic110314-index.html

3.5 months afterM=9 quake

A few years afterM=9.2 quake

Inter-seismic 2 (Cascadia)

Inter-seismic 1(Alaska, Chile)

Co-seismic

Coast line

Coast line

Post-seismic(Japan, Sumatra)

Based on Wang, 2007, SEIZE volume

Rupture

Stress relaxation

Stress relaxation

Afterslip

Locking

Characteristic timescales:Afterslip – months to a few yearsViscoelastic relaxation (transient) – a few yearsViscoelastic relaxation (steady-state) – a few decadesLocking – (centuries) length of the earthquake cycle

A couple of years About four decades Three centuries

Central part of Sumatra mesh

M

K

TM = 10M/ = 60 yr

TK = 10K/= 3 yr

Hu, 2011, PhD thesis

A couple of years About four decades Three centuries

Wang et al., in prep.

2 yr after EQ(like Japan, Sumatra)

40 yr after EQ(like Chile, Alaska)

Present

Deformation Following the 1700 Cascadia Earthquake

Hu, 2011, PhD thesis

1995 Antofagasta earthquake, N. Chile (Mw = 8.0)

1993-95 Displacements (dominated by co-seismic)

1996-97 Velocities (2 years after earthquake)

Data from Klotz et al. (1999) and Khazaradze and Klotz (2003)

Paradox 4: Interseismic locking of subduction fault causes landward motion of the upper plate, but some areas show seaward motion.

Explanation: The seaward motion is the result of afterslip and viscoelastic mantle relaxation. It will diminish with time.

Paradox 5: Mountain building at a subduction zoneParadox 6: Episodic tremor and slip

Paradox 7: Strong asperities of weak faultsParadox 8: … …

… …… …

Paradox 1000: … …… …

To be continued … …

… …

… …

Layer viscosity ’Thickness h

Moho

In Earth: Interface and wedge strengths controlled by petrology and fluid

In model: Coupling stress represented by ’ and h

Wang and He, 1999, JGR