Permeability: An Overlooked Control on the Strength of Subduction Megathrusts?
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Transcript of Permeability: An Overlooked Control on the Strength of Subduction Megathrusts?
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?