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Development of permeability anisotropy of antigorite serpentinite gouge during shear deformations

　　　Keishi  Okazaki1※,  Ikuo  Katayama1,  Hiroyuki  Noda2,  Miki  Takahashi3  ※okazakikeishi@hiroshima-­‐u.ac.jp　          1Earth  and  planetary  system  science,  Hiroshima  University,  Japan,        2InsGtute  for  Research  on  Earth  EvoluGon  (IFREE),  Japan  Agency  for  Marine-­‐Earth  Science  and  Technology  (JAMSTEC),  Japan          3Geological  Survey  of  Japan,  Advanced  Industrial  Science  and  Technology,  Japan

Serpen&nite  in  subduc&on  zone    and    earthquake,  slow  earthquake  

Experimental  apparatus  

:Antigorite serpentinite from Nomo metamorphic rocks, Japan

Pressure vessel Control panel

“Gas  pressure  medium  high  temperature  high  pressure  triaxial  deformaGon  apparatus”  (Wibberlery  and  Shimamoto,  2003)  

Max.  Pc:  220MPa,  Max.  Pp:  200MPa,  Temp.800℃,  Advantages：  1.  Accurate  measurements  of  axial  load  and  fluid  flow  2.  Pore  pressure  control  →DeformaGon  experiments  under  hydrothermal  condiGon  and  with  conGnuous  permeability  measurement  during  deformaGon  are  possible.      

p Mineral  composiGon:  AnGgorite  (~98%),  Spinel,  MagneGte,  Diopside  and  no  olivine  relict.    

p Crushed  and  sieved  to  extract  grains  less  than  100  μm  in  diameter.  Mean  grain  diameter:  1.51  μm  (d50),  aspect  raGo:  0.74  (measured  using  Morphologi  G3,  Malvern  Instruments  Ltd).  

Summary:    1.  Permeability   in  three  orthogonal  direcGons  of  anGgorite  serpenGnite  gouge  was  measured  during  pre-­‐cut  fricGonal  

experiments.   2.  PermeabiliGes  in  all  direcGons  decreases  by  one  order  of  magnitude  at  iniGal  compacGon  by  increasing  mean  stress  

without  showing  significant  anisotropy.   3.  At   the  steady  state   in   terms  of  shear  stress,  permeability  anisotropies  kx/kz  and  ky/kz   stayed  at   their   steady  state  

value  as  high  as  nearly  one  order  magnitude.   4.  Microstructures  of   recovered  samples   suggest   that   the  permeability  anisotropy   is   caused  by  developments  of  R,  Y  

and  P-­‐shear  structures  that  may  prevent  fluid  flow  normal  to  the  fault  in  serpenGnite  gouge.   5.  Permeability  anisotropies  may  enhance  fluid  flow  along  subducGon  plate  interface  and  acGve  fault  zones.  

Permeability  anisotropy  and  fluid  flow  in  fault  zones  

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40

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80

100

120

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ss [M

Pa] GR606

GR609GR610GR611GR614GR633GR623 Ar � ~ 0.49

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Increasing Pp

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“Effects of fluids on rock deformation“  =one  of  the  largest  uncertainGes  in  the  subducGon  zone!!   ・Absorp7on  of  water  on  mineral  surface  ・SerpenGnizaGon  (hydraGon)  of  ultramafic  rock  ・Decreasing  in  rock  fricGon  and  flow  stress  (Morrow  et  al.,  2000;  Giger  et  al.,2008  )  

・AlteraGon  of  brille-­‐ducGle  transiGon  zone  ・EffecGve  pressure  low  (e.g.  Terzaghi,  1923)  ・Thermal  pressurizaGon  (Sibson,  1973)  ・Fault-­‐valve  behavior  (Sibson  ,1992)  

Slow  earthquakes  (Obara,  2002,  etc…）:  occur  in  high  Vp/Vs  raGo  (~high  fluid  pressure  =  low  Pe)  zone  of  subducGon  zone    　　↘  Serpen7nized  mantle  wedge?  

Lower  plane  of  the  double  seismic  zone  :  p dehydraGon  embrillement  of  serpenGnite?  

(Kirby  et  al.,  1996;  Peacock,  2001)    

p reacGvates  outer-­‐rise  fault?(Nakajima  et  al.,  2011)  

DEPSSDEPSSDepartment of Earth and Planetary Systems Science Hiroshima University, JAPAN

_Hiroshima

→How  is  fluid  kept  along  fault  zone?  →Permeability  anisotropy  must  act  an  important  role  keeping  fluid  pressure  along  fault  zones!!  

Alumina precut spacer withPp hole

Antigorite gouge sample

Polyolefinjacket

Porousalumina

WCspacer

Aluminaspacer

Hole forporepressure

k// k�k-

20mm

Figure 1. Okazaki et al., 2012

k// k-­‐ k⊥

○Riedel  shears  (R1,  Y  and  P)  are  developed  normal  to  the  plane  including  the  fault  normal  and  slip  direcGons.  But  they  are  not  straight  as  recognized  in  a  secGon  normal  to  the  slip  direcGon.  ○Len7cular  structure  is  developed  in  the  direcGon    normal  to  the  slip  direcGon  in  the  fault.    →They  prevent  fluid  flow  normal  to  the  fault  in  serpen7nite  gouge.  

PermeabiliGes  in  all  direcGons  decreases  by  one  order  of  magnitude  unGl  shear  stress  reaches  steady-­‐state  (apparent  slip  ~  1  mm)  without  showing  significant  anisotropy.  Ater  the  shear  stress  reaches  steady-­‐state,  anisotropy  of  permeability  becomes  remarkable.  

Structure    development Steady  state?

Permeability  anisotropies  kx/kz  and  ky/kz  stayed  at  their  steady  state  value  as  high  as  8  at  γ  =3.    →Fluids  are  likely  to  move  parallel  to  the  fault  surface  and  might  be  kept  around  fault  zone  with  minimal  loss.   →This  value  seems  to  be  not  enough  to  maintain  excess  pore  pressure  from  previous  models  (Rice,  1992,  Katayama  et  al.,  2012).    

Fault  healing(e.g.  Tenthorey  et  al,  2003)  and  cap  rocks  (e.g.  Peacock  et  al.,  2011;  Katayama  et  al.,  2012)  potenGally  act  important  roles  to  increase  permeability  gap  and  to  maintain  excess  pore  pressure.    

Moho Oceanic crust

Mantle wedge(Peridotite)

Serpentinizedmantle wedge

Megathrust earthquake

Slow earthquakes(SSE, LFE, NVT)

Oceanic lithosphere(Philippine Sea plate)

Oceanic ridge

? ?

Outer-rise earthquake

?

?

Inland fault

Intra-slabearthquake

?

Shear Strain (γ)

Serpen&nite  in  subduc&on  zone  and  its  poten&al  significance  in  regular  and  slow  earthquakes:

10 1 100 101 1020

0.5

1

1.5

2

2.5

3

3.5

4

Effective Circle Diameter, +m

Num

ber D

ensi

ty N

orm

aliz

ed b

y lo

gbo

xcar

Starting material

Student Version of MATLAB

Effective Grain Diameter, μm

−1 −0.5 0 0.5 1 1.5 20

0.2

0.4

0.6

0.8

1

log10(Effective Grain Diameter), µm

Asp

ect R

atio

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0

2

4

6

8

10

12

Student Version of MATLAB

0

0.2

0.4

0.6

0.8

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0 1 2 3 4 5 6Axial shortening [mm]

0

0.2

0.4

0.6

0.8

GR642 (k//)GR655 (k//)GR645 (k-)GR657 (k-)GR654 (k�)GR664 (k�)GR663 (k//)

1x10-19

1x10-18

1x10-17

1x10-16

Perm

eabi

lity

[m ]

1x10-19

1x10-18

1x10-17

1x10-16GR642 (k//)GR655 (k//)GR645 (k-)GR657 (k-)GR654 (k�)GR664 (k�)GR663 (k//)

0 1 2 3 4 5 6Axial shortening [mm]

0.7

0.9

1.1

1.3

1.5

Gou

ge th

ickn

ess

[mm

]

0.7

0.9

1.1

1.3

1.5

Perm

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[m2 ]

Gou

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ess

[mm

]Fr

ictio

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effic

ient

a

b

cHit pointGR663 (k//)GR642 (k//)GR645 (k-)

GR657 (k-)GR654 (k

�)

GR655 (k//)GR664 (k

�)

Lz = 1.184 - 0.325 da0.210

displacement [mm]

GR663 (k//)GR642 (k//)

GR645 (k-)GR657 (k-)GR654 (k

�)GR655 (k//)

GR664 (k�)

Figure 2. Okazaki et al., 2012

GR663 (k//)GR642 (k//)

GR645 (k-)GR657 (k-)GR654 (k

�)

GR655 (k//) GR664 (k�)

Experimental  condi7on:  Pc  =  150MPa,  Pp  =  100MPa  Slip  rate  =  0.575  μm/s,  Pore  fluid:  water,  Temp.  =  RT  

0 1 2 3 4 5 6"Axial displacement [mm]"

0

0.2

0.4

0.6

0.8

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0

0.2

0.4

0.6

0.8

GR642 (k//)GR655 (k//)GR645 (k-)GR657 (k-)GR654 (k�)GR664 (k�)GR663 (k//)

0 1 2 3 4 5 6Axial shortening [mm]

0.7

0.9

1.1

1.3

1.5

Gou

ge th

ickn

ess

[mm

]

0.7

0.9

1.1

1.3

1.5

Gou

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[mm

]Fr

ictio

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Hit pointGR663 (k//)GR642 (k//)GR645 (k-)

GR657 (k-)GR654 (k

�)

GR655 (k//)GR664 (k

�)

Lz = 1.184 - 0.325 da0.210

displacement [mm]

GR663 (k//)GR642 (k//)

GR645 (k-)GR657 (k-)GR654 (k

�)

GR655 (k//) GR664 (k�)

Permeability  measurement  on  An&gorite  serpen&nite                  gouge  during  shear  deforma&on  

Microstructures  of  recovered  samples  

L2

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ki=

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2

kx+Ly2

ky+Lz2

kz* k: permeability, L: length of each component

0 0.2 0.4 0.6 0.8 10

0.5

1

1.5

2

2.5

3

Aspect ratioNum

ber D

ensi

ty N

orm

aliz

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nifo

rm D

istri

butio

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Starting material

Student Version of MATLAB

Aspect Ratio

Shear Strain (γ)