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

1. Direct measurements (sub-sea wellheads: CORKs)

2. Laboratory consolidation tests to “high” stressrequires samples8-20 MPa load

3. Field data: porosity–depth trendsrequires “reference” boreholehigh-quality porosity measurements

1.3

1.35

1.4

1.45

1.5

1.55

1.6

1.65

1 10 100 1000 10000

void

ra

tio

effective stress (kPa)

rebound curve

virgin consolidation curve

Pc’

Laboratory Measurements: Permeability

and Consolidation10-15

2 2.5 3 3.5 4 4.5

10-16

10-17

10-18void ratio

k (m

2)

Site 1040

Site 1043

Ocean Crust

Margin Wedge

Sediment

1 km

SW NE4.0

4.5

5.0

Site 1039

dept

h (k

m b

sl)

Costa Rican Margin Wedge andUnderthrust Sediments

Thinning of UnitsBased on

Density Logs

Bulk density(g/cm3)

1039

1043

1040

Deformed Wedge(Terrigenous Clay)

Décollement

Upper Hemipelagic(Diatomaceous ooze)

Lower Hemipelagic(Clay)

Upper Pelagic(Chalk)

Lower Pelagic(Chalk with ash)

Gabbroic Sill

100

300

100

200

200

100

200

300

400

dep

th (

mbs

f)

1.2 1.6 2.0

1.2 1.6 2.0

1.2 1.6 2.0

pressure (MPa)

Unit I

Unit II

Unit III

1 3 5 7 9

150

200

250

300

350

400

450

500

Site 1043

Unit III

Unit II

Unit I

4 6 8 10

350

400

450

500

550

600

650

700

Site 1040

pressure (MPa)

dep

th (

mb

sf)

hyd

rostatic

hyd

rostatic

litho

static

litho

static

Unit III

Unit II

Unit I

1 2 3 4 5

0

20

40

60

80

100

120

Site 1039

effective stress (MPa)

Unit III

Unit II

Unit I

Site 1043

effective stress (MPa)

fully d

rained

Site 1040

effective stress (MPa)

Unit I

Unit II

Unit IIIfully d

rained

1 2 3 4 5 1 2 3 4 5

he

igh

t o

f s

olid

s b

elo

w d

ec

olle

me

nt

(m)

effective stress (v’)

Mechanical Implicationsd

epth

dep

th

effective stress (v’)effective stress (v’)

Increased Subduction

1.5 kmSite 1044

5.0

5.5

6.0Ocean Crust

(Site 672)

4000 6000 8000 10000 12000

500

540

580

948

pressure (kPa)2000 4000 6000 8000 10000

420

460

500

1045

pressure (kPa)2000 4000 6000 8000 10000

400

440

480

520

1046

pressure (kPa)

1047300

340

380

CO

RK

2000 4000 6000 8000 10000

pressure (kPa)

CO

RK

lithostatic

hydrostatic

Pc’ (lab)

Comparison of Subduction Zones

0

1000

2000

3000

4000

5000

6000

0 2 4 6 8 10

· LK

exc

ess

po

re p

ress

ure

(kP

a)

Nankai

Barbados

Costa Rica

dimensionless number: ratio of “geologic forcing”/”hydraulic conductivity”

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

and escape: and escape:

PorePressure

Geometry

Strain Rate Geometry

FluidSources

Permeability

PorePressure

Existing model “New” model

.

well drainedHigh K/Q:

poorly drainedLow K/Q:

TIM

E

rapid fluid escapelow pore pressures

retarded fluid escapeelevated pore pressures

shallow stable geometrysteep stable geometry

high basal shear stresswedge steepens internally

low basal shear stresswedge grows self-similarly

Proposed Model of Accretionary Wedge Evolution

Nankai Transects

Seismic Sections HereSeismic Sections HereTaper Angles Near Toe

Muroto: Taper angle ~4°

Ashizuri: Taper angle ~8-10°

- Thinner Trench-wedge turbidites

- No L. Shikoku turbidites

HypothesisHypothesis: : Differences in stratigraphy result in systematic differences in pore pressure, causing differences in taper angle along-strike.

MethodMethod: : Use numerical model of fluid flow to evaluate Use numerical model of fluid flow to evaluate whether this is plausible. If so, what conditions whether this is plausible. If so, what conditions are necessary?are necessary?

Schematic of Model DomainSchematic of Model Domain

9000

8000

7000

6000

5000

4000

3000

distance arcward from deformation front (km)

100-10-20-30-40-50

seawardlandward

decollement

De

pth

(m

bsl

)

Compaction-Driven Fluid Sources:Compaction-Driven Fluid Sources:Porosity ReductionPorosity Reduction

+

Porous sediment Compacted sediment Fluid

2

1

0

-1

-3

-2

40 30 20 10 0 -10

dept

h (k

m)

50

Porosity Distribution

10%

20%

50%

40%

30%

60%

2

1

0

-1

-3

-2

40 30 20 10 0

dept

h (k

m)

distance arcward from deformation front (km)

50

Source Distribution

log

sour

ce

Vol

/Vol

s-1

-16

-13

-14

-15

Permeability-Porosity RelationPermeability-Porosity Relation

Compiled sandstone data

10-22 10-20 10-18 10-16 10-14

0

0.2

0.4

0.6

0.8

1

permeability (m2)

Compiled data for argillaceous rocks(Neuzil, 1994)

Gulf of Mexico

Barba

dos

Shik

oku

Basi

n (In

vers

e M

odel

s)

8000

7000

6000

5000

4000

3000D

ep

th (

mb

sl)

Turbidites

Hemipelagic Clays

12000

10000

8000

6000

4000

2000

100-10-20-30-40-50

De

pth

(m

bsl

)

Hemipelagic Clays

Turbidites

MUROTO

ASHIZURI

Model Domains

ASHIZURI TRANSECT

MUROTO TRANSECT

Example Pore Pressure Results

distance from trench (km)

dep

th (

m)

dep

th (

m)

Pore Pressure and Wedge StabilityPore Pressure and Wedge Stability

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1

wedge

b

ase

= 0.45

= 0.65

Kturb = 10 x Khemi

Kturb = 3 x Khemi

Ashizuri Stability

FieldMuroto Stability Field

Tap

er a

ng

le

log (ko)

5

10

15

MODEL RESULTS

-22 -21` -20 -19 -18

20 km

; = 1

; = 1.5

; = 4

% of penetrated incoming section dominated by clay

100 020406080

; = 5

, = 13

20 km

20 km

20 km

OBSERVATIONS

Implications: Implications:

• 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

• Morphology and strength of subduction complexes a result of dynamic balance between geologic forcing and fluid escape

- strength of brittle crust in other settings?