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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