Dealing with paradoxes
in subduction zone geodynamics
Kelin Wang1,2
1Pacific Geoscience Centre, Geological Survey of Canada
2School of Earth and Ocean Sciences, University of Victoria
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
Slab thermal parameter (102 km) = Slab age × Descent rate
(Inferred from earthquakes located by Engdahl et al. 1998 and local networks)
Intensity of Arc Volcanism
More magma
production
Slab thermal parameter (102 km) = Slab age × Descent rate
(Crisp, 1984; White et al., 2006)
Warm
Cold
Survival depth of basaltic
crust (blue diamond)
and
depth range of intraslab
earthquakes (purple lines)
Eruption rate of arc
volcanoes
(White et al., 2006)
Depth of slab beneath
volcanic arc
Colour: different publications
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.
Cold Forearc
70 ~ 80 km
Low seismic attenuation
Low Vp/Vs
Serpentinization
Stagnant
• High attenuation
• High Vp/Vs
• Melting
• Vigorous wedge flow
~ 100 km
•
•
•
•
Hot Arc, Back Arc
Northern
Cascadia
(Currie et al.
2004, EPSL)
Oceanic geotherm
(plate cooling model)

n
2A
 E  PV 

 RT 
 1n exp 
Depth
Temperature- and
stress-dependent
mantle wedge rheology
outflow
inflow
Landward
Geotherm
Temperature
Heat Flow Measurements
Heat flow transect across the Cascadia subduction zone
probe
ODP hole
BSR
Offshore well
Land borehole
Comparison with thermal model results
Preferred
Cascadia model
• Decoupling to
~ 70 - 80 km depth
Two primary
constraints:
• Surface heat flow
(cold foreac)
• Mantle temperature
beneath arc > 1200C
(hot arc)
Blue:
Basaltic crust
Purple:
Serpentine stability
in slab or mantle wedge
Fluid content in the subducting slab
Crust (wet basalt)
Mantle
wt% bound H2O
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)
End-member warm-slab and cold-slab subduction zones
N Cascadia
NE Japan
Blue:
Basaltic crust
Purple:
Serpentine stability
Basalt to eclogite ~ 40-50 km depth
Feeble arc volcanism
Serpentinized mantle wedge corner
Intraslab earthquakes to ~90 km depth
Basalt to eclogite ~ 100-140 km
Active arc volcanism
High-velocity wedge corner
Earthquakes to hundreds of km
Kirby et al., 1996; Wada and Wang, 2009; Syracuse et al., 2010 ; van Keken et al., 2011
Assuming decoulping to 75 km
Wada and Wang, 2009, G3
Warm
Cold
Survival depth of basaltic
oceanic crust (blue)
and
depth range of intraslab
earthquakes (purple)
Model-predicted peak
dehydration depth (blue)
and
antigorite stability in
subducting slab (purple)
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
?
1
?
1
Northeast Japan
3
1
Hellenic Arc
Quaternary faults (Angelier et
al., 1982) and earthquake focal
mechanisms (Benetatos et al.,
2004)
2 or 3
1
Southeast
Mexico
1
Northern Cascadia
2 or 3
1
Summary of Stress Indicators
Paradox 2
Subduction zones accommodate plate convergence, but
few forearcs are under margin-normal compression.
Far-field
force
Mantle wedge rheology:
Dislocation creep
Effective viscosity:

n
2A

1 n
 E  PV 
exp 

 RT 
Contours of
maximum
shear stress
Summary
Summary
of Stress
of Stress
Indicators
IndicatorsForce Balance Model
  n
Assuming V = H, Lamb
(2006) obtained   0.03
for most subduction zones
  0.05
?
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.
Do Chilean-type subduction zones have a strong fault?
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  0.05
orogeny model
Paradox 2: Subduction zones accommodate plate
convergence, but few forearcs are under marginnormal compression.
Explanation: Plate interface too weak to
overcome gravitational tension in the forearc.
Summary of Stresses
in Cascadia forearc
small earthquakes
in upper plate
Wang, 2000, Tectonophysics
A 100-km line
becomes shorter
by 2 cm each year
Geodetic Strain Rates
small earthquakes
in upper plate
Geodetic Strain Rates
Forearc Stresses
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 marginparallel, but fastest geodetic shortening is roughly marginnormal.
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
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.
Geodetic Strain Rates
Static stress drop
(Probability from inversion)
If deformation
is elastic, it
Entire fault
only reflects
stress changes.
Areas with >10%
peak slip
Cascadia geodetic
shortening reflects
stress
>20% peak slip
increase due to interseismic
locking of the plate
interface.
Tohoku earthquake
Mw = 9
March 11, 2011
Simons et al., 2011
Great earthquake cycles
cause small perturbations
to forearc stress.
Margin-normal
stress
perturbation
Margin-parallel compression
Margin-normal
stress
perturbation
Margin-parallel compression
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
1964
Freymueller et al. (2009)
Wang et al. (2007, G3)
M = 9.5
1960
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
3.5 months after
M=9 quake
A few years after
M=9.2 quake
http://www.gsi.go.jp/cais/topic110314-index.html
Grijalva et al (2009)
Coast line
Inter-seismic 2
(Cascadia)
Inter-seismic 1
(Alaska, Chile)
Post-seismic
(Japan, Sumatra)
Co-seismic
Based on Wang, 2007, SEIZE volume
Coast line
Rupture
Afterslip
Stress
relaxation
Stress relaxation
Characteristic timescales:
Afterslip – months to a few years
Viscoelastic relaxation (transient) – a few years
Viscoelastic relaxation (steady-state) – a few decades
Locking – (centuries) length of the earthquake cycle
A couple of years
About four decades
Three centuries
Hu, 2011, PhD thesis
TK = 10K/= 3 yr
Central part
of Sumatra mesh
TM = 10M/ = 60 yr


M
K
A couple of years
About four decades
Three centuries
Wang et al., in prep.
Deformation Following the 1700 Cascadia Earthquake
2 yr after EQ
(like Japan, Sumatra)
40 yr after EQ
(like Chile, Alaska)
Present
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 zone
Paradox 6: Episodic tremor and slip
Paradox 7: Strong asperities of weak faults
Paradox 8: … …
……
……
Paradox 1000: … …
……
To be continued … …
……
……
Moho
Layer viscosity ’
Thickness h
In Earth: Interface and wedge strengths controlled by petrology and fluid
In model: Coupling stress represented by ’ and h
Wang and He, 1999, JGR