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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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Permeability: An Overlooked Control on

the Strength of Subduction Megathrusts?

Insights from shallow drilling, lab experiments,

and numerical models


  • Field data: porosity–depth trends

    • requires “reference” borehole

    • high-quality porosity measurements


  • 

    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)


    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


    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


    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)


    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)


    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’)


    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


    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


    Proposed Model of Accretionary Wedge Evolution response to fluid sources and escape:

    .

    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


    Nankai Transects response to fluid sources and escape:


    Seismic sections here

    Taper Angles Near Toe response to fluid sources and escape:

    Seismic Sections Here

    Muroto: Taper angle ~4°

    Ashizuri:

    Taper angle ~8-10°



    Hypothesis response to fluid sources and escape: :

    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 response to fluid sources and escape:

    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


    Compaction-Driven Fluid Sources: response to fluid sources and escape:

    Porosity Reduction

    +

    Porous sediment

    Compacted sediment

    Fluid


    60% response to fluid sources and escape:

    -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 response to fluid sources and escape:

    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)


    Model Domains response to fluid sources and escape:

    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


    Example Pore Pressure Results response to fluid sources and escape:

    MUROTO TRANSECT

    depth (m)

    l

    ASHIZURI TRANSECT

    depth (m)

    distance from trench (km)


    1 response to fluid sources and escape:

    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


    MODEL RESULTS response to fluid sources and escape:

    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: response to fluid sources and escape:

    • 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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