1. 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

2. Wada and Wang, 2009, G3

3. 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)

4. 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 and local networks)

5. Intensity of Arc Volcanism
More magma production
Slab thermal parameter (102 km) = Slab age × Descent rate
(Crisp, 1984; White et al., 2006)

6. Survival depth of basaltic crust (blue diamond) and
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

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

8. Low seismic attenuation Low Vp/Vs Serpentinization Stagnant
Cold Forearc
Hot Arc, Back Arc
70 ~ 80 km
~ 100 km
Low seismic attenuation
Low Vp/Vs
Serpentinization
Stagnant
High attenuation
High Vp/Vs
Melting
Vigorous wedge flow

9. Northern
Cascadia
(Currie et al.
2004, EPSL)

10. inflow outflow Oceanic geotherm (plate cooling model) Landward
Temperature- and stress-dependent mantle wedge rheology
Depth
Mantle adiabat
outflow
Temperature

11. Heat Flow Measurements

12. Heat flow transect across the Cascadia subduction zone
probe
BSR
Offshore well
Land borehole
ODP hole

13. Comparison with thermal model results

15. Two primary constraints:
Blue:
Basaltic crust
Purple:
Serpentine stability
in slab or mantle wedge
Preferred
Cascadia model
Decoupling to
~ km depth
Two primary constraints:
Surface heat flow
(cold foreac)
Mantle temperature beneath arc > 1200C (hot arc)

16. Fluid content in the subducting slab
Crust (wet basalt)
Mantle
warm
cold
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)

17. End-member warm-slab and cold-slab subduction zones
N Cascadia
NE Japan
Blue:
Basaltic crust
Purple:
Serpentine stability
Basalt to eclogite ~ km depth
Feeble arc volcanism
Serpentinized mantle wedge corner
Intraslab earthquakes to ~90 km depth
Basalt to eclogite ~ 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

18. Assuming decoulping to 75 km
Wada and Wang, 2009, G3

19. Survival depth of basaltic oceanic crust (blue) and
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

20. 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

21. 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

22. ? 1 1

23. Northeast Japan 1 3

24. 2 or 3 1 Hellenic Arc Southeast Mexico
Quaternary faults (Angelier et al., 1982) and earthquake focal mechanisms (Benetatos et al., 2004) 1
2 or 3
Southeast
Mexico

25. Northern Cascadia 1
2 or 3

26. Summary of Stress Indicators

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

28. Contours of maximum shear stress Mantle wedge rheology:
Far-field
force
Contours of maximum shear stress
Mantle wedge rheology:
Dislocation creep
Effective viscosity:

29. ?   n Summary of Stress Indicators Force Balance Model
Assuming V = H, Lamb (2006) obtained   0.03 for most subduction zones
  0.05 ?

30. 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.

31. 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):
 =  0.05
orogeny model

32. 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.

33. small earthquakes in upper plate
Summary of Stresses
in Cascadia forearc
small earthquakes in upper plate
Wang, 2000, Tectonophysics

34. A 100-km line becomes shorter by 2 cm each year
Geodetic Strain Rates

35. small earthquakes in upper plate
Geodetic Strain Rates
Forearc Stresses
Wang, 2000, Tectonophysics

36. Nankai Forearc
Stresses and geodetic strain rates are similar to Cascadia
Wang, 2000, Tectonophysics

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

38. 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

39. A Stretched Elastic Band
Time 1: Tension
Contraction
Time 2: Less tension

40. 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

41. (Probability from inversion)
>20% peak slip
Entire fault
Static stress drop
(Probability from inversion)
Areas with >10%
peak slip
If deformation is elastic, it only reflects stress changes.
Cascadia geodetic shortening reflects stress increase due to interseismic locking of the plate interface.
Tohoku earthquake
Mw = 9
March 11, 2011
Great earthquake cycles cause small perturbations to forearc stress.
Simons et al., 2011

42. Margin-normal stress perturbation
Margin-parallel compression

43. Margin-normal stress perturbation
Margin-parallel compression

44. 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.

45. Cascadia: All sites move landward
Wells and Simpson (2001)
Wang, 2007, SEIZE volume

46. Alaska and Chile: Opposing motion of coastal and inland sites
Freymueller et al. (2009)
Wang et al. (2007, G3)

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

48. Japan and Sumatra: All sites move seaward
3.5 months after
M=9 quake
A few years after
M=9.2 quake
Grijalva et al (2009)

49. Based on Wang, 2007, SEIZE volume
Coast line
Inter-seismic 2
(Cascadia)
Inter-seismic 1
(Alaska, Chile)
Post-seismic
(Japan, Sumatra)
Co-seismic
Coast line
Based on Wang, 2007, SEIZE volume

50. Locking 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

51. A couple of years About four decades Three centuries

52.  M K TK = 10K/= 3 yr Central part of Sumatra mesh
Hu, 2011, PhD thesis M K 
TM = 10M/ = 60 yr
TK = 10K/= 3 yr
Central part
of Sumatra mesh

53. A couple of years About four decades Three centuries
Wang et al., in prep.

54. 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

55. 1995 Antofagasta earthquake, N. Chile (Mw = 8.0)
Displacements
(dominated by co-seismic)
Velocities
(2 years after earthquake)
Data from Klotz et al. (1999) and Khazaradze and Klotz (2003)

56. 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.

57. 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 … …

58. … …

59. In model: Coupling stress represented by ’ and h
Moho
Layer viscosity ’
Thickness h
In Earth: Interface and wedge strengths controlled by petrology and fluid
In model: Coupling stress represented by ’ and h

60. Wang and He, 1999, JGR