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Permeability: An Overlooked Control on the Strength of Subduction Megathrusts?. Insights from shallow drilling, lab experiments, and numerical models. I. In Situ Pore Pressure Estimates: Methods Direct measurements (sub-sea wellheads: CORKs) Laboratory consolidation tests to “high” stress

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slide1
Permeability: An Overlooked Control on

the Strength of Subduction Megathrusts?

Insights from shallow drilling, lab experiments,

and numerical models

slide2
I. In Situ Pore Pressure Estimates: Methods
  • Direct measurements (sub-sea wellheads: CORKs)
  • Laboratory consolidation tests to “high” stress
      • requires samples
      • 8-20 MPa load
  • Field data: porosity–depth trends
    • requires “reference” borehole
    • high-quality porosity measurements
slide3


10-15

1.65

10-16

k (m2)

1.6

10-17

void ratio

10-18

1.55

2

2.5

3

3.5

4

4.5

1.5

1.45

1.4

1.35

1.3

Laboratory Measurements:

Permeability

and Consolidation

Pc’

virgin consolidation curve

void ratio

rebound curve

1

10

100

1000

10000

effective stress (kPa)

slide4
Sediment

Margin Wedge

Ocean Crust

Costa Rican Margin Wedge and

Underthrust Sediments

SW

NE

Site 1040

4.0

Site 1043

Site 1039

4.5

depth (km bsl)

5.0

1 km

slide5
1040

Thinning of Units

Based on

Density Logs

B

u

l

k

d

e

n

s

i

t

y

100

3

(

g

/

c

m

)

Deformed Wedge

(Terrigenous Clay)

200

1043

300

100

1039

Décollement

400

Upper Hemipelagic

200

(Diatomaceous ooze)

Lower Hemipelagic

100

(Clay)

Upper Pelagic

(Chalk)

depth (mbsf)

200

Lower Pelagic

(Chalk with ash)

300

Gabbroic Sill

1.2

1.2

1.2

1.6

1.6

1.6

2.0

2.0

2.0

slide6
150

350

200

400

250

450

300

500

350

550

400

600

450

650

500

700

1

3

5

7

9

4

6

8

10

Site 1043

Site 1040

Unit I

Unit I

Unit II

Unit II

Unit III

Unit III

depth (mbsf)

lithostatic

lithostatic

hydrostatic

hydrostatic

pressure (MPa)

pressure (MPa)

slide7
Site 1039

Site 1043

Site 1040

Unit I

Unit I

Unit I

0

20

Unit II

Unit II

Unit II

40

Unit III

Unit III

Unit III

60

fully drained

fully drained

80

100

120

1

1

1

2

2

2

3

3

3

4

4

4

5

5

5

effective stress (MPa)

effective stress (MPa)

effective stress (MPa)

height of solids below decollement (m)

slide8
s

1

s

s

1

1

s

1

s

s

1

1

Mechanical Implications

Increased Subduction

depth

depth

effective stress (sv’)

effective stress (sv’)

effective stress (sv’)

slide9
948

1045

1046

1047

500

400

300

420

CORK

CORK

440

340

lithostatic

540

460

hydrostatic

480

380

580

500

Pc’ (lab)

520

2000

4000

6000

8000

10000

4000

6000

8000

10000

12000

2000

4000

6000

8000

10000

2000

4000

6000

8000

10000

pressure (kPa)

pressure (kPa)

pressure (kPa)

pressure (kPa)

Site 1044

1.5 km

(Site 672)

5.0

5.5

Ocean Crust

6.0

slide10
Comparison of Subduction Zones

6000

5000

4000

excess pore pressure (kPa)

Nankai

3000

2000

Barbados

1000

Costa Rica

0

0

2

4

6

8

10

G · L/K

dimensionless number:

ratio of “geologic forcing”/”hydraulic conductivity”

ii a hydro mechanical balancing act geometry as a response to fluid sources and escape

Ii. “A Hydro-mechanical balancing act”: geometry as a response to fluid sources and escape:

Pore

Pressure

Geometry

Strain Rate

Fluid

Sources

Pore

Pressure

Geometry

Permeability

Existing model

“New” model

slide12
Proposed Model of Accretionary Wedge Evolution

.

well drained

High K/Q:

poorly drained

Low K/Q:

retarded fluid escape

elevated pore pressures

rapid fluid escape

low pore pressures

high basal shear stress

wedge steepens internally

low basal shear stress

wedge grows self-similarly

TIME

steep stable geometry

shallow stable geometry

seismic sections here
Taper Angles Near ToeSeismic Sections Here

Muroto: Taper angle ~4°

Ashizuri:

Taper angle ~8-10°

slide15
Thinner Trench-wedge
  • turbidites
  • - No L. Shikoku turbidites
slide17
Hypothesis:

Differences in stratigraphy result in systematic

differences in pore pressure, causing differences

in taper angle along-strike.

Method:

Use numerical model of fluid flow to evaluate

whether this is plausible. If so, what conditions

are necessary?

schematic of model domain
Schematic of Model Domain

3000

4000

decollement

5000

Depth (mbsl)

6000

7000

8000

9000

-50

-40

-30

-20

-10

0

10

distance arcward from deformation front (km)

landward

seaward

slide19
Compaction-Driven Fluid Sources:

Porosity Reduction

+

Porous sediment

Compacted sediment

Fluid

slide20
60%

-13

50%

40%

-14

30%

20%

-15

10%

-16

2

Porosity Distribution

1

0

depth (km)

-1

-2

-3

50

40

30

20

10

0

-10

2

Source Distribution

1

0

depth (km)

log source

Vol/Vols-1

-1

-2

-3

50

40

30

20

10

0

distance arcward from deformation front (km)

permeability porosity relation
Permeability-Porosity Relation

1

Compiled data for

argillaceous rocks

(Neuzil, 1994)

0.8

0.6

Gulf of Mexico

Shikoku Basin (Inverse Models)

0.4

Compiled

sandstone data

Barbados

0.2

0

10-22

10-20

10-18

10-16

10-14

permeability (m2)

slide22
Model Domains

MUROTO

3000

Turbidites

4000

Depth (mbsl)

5000

Hemipelagic Clays

6000

7000

8000

ASHIZURI

2000

Turbidites

4000

Depth (mbsl)

6000

Hemipelagic Clays

8000

10000

12000

-50

-40

-30

-20

-10

0

10

slide23
Example Pore Pressure Results

MUROTO TRANSECT

depth (m)

l

ASHIZURI TRANSECT

depth (m)

distance from trench (km)

slide24
1

0.95

0.9

0.85

0.8

0.75

0.7

0.65

0.6

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

Pore Pressure and Wedge Stability

Muroto Stability Field

m = 0.65

Ashizuri Stability Field

l base

m = 0.45

Kturb = 3 x Khemi

Kturb = 10 x Khemi

l wedge

slide25
MODEL RESULTS

OBSERVATIONS

15

a

,

= 13

20 km

a

;

= 4

10

20 km

a

;

= 5

5

a

;

= 1.5

20 km

a

;

= 1

20 km

-22

-21`

-20

-19

-18

100

80

60

40

20

0

log (ko)

% of penetrated incoming section dominated by clay

Taper angle

implications

Implications:

  • Morphology and strength of subduction complexes
  • a result of dynamic balance between geologic forcing
  • and fluid escape
  • - strength of brittle crust in other settings?
  • Permeability and plate convergence are important
  • - affect pore pressure
  • - influence stable taper angle, fault strength
  • Other factors also important
          • - incoming sediment thickness
          • - fault zone permeability, hydraulic fracture
          • - systematic variation in stratigraphic section
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