Marine Geology 295-298 (2012) 1–13

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Permeability contrasts between sheared and normally consolidated sediments in theNankai accretionary prism

Matt J. Ikari ⁎, Demian M. SafferDepartment of Geosciences, The Pennsylvania State University, University Park, PA, USA

⁎ Corresponding author at: MARUM, Center for Marinversity of Bremen, 28359 Bremen, Germany. Tel.: +49

E-mail address: mikari@marum.de (M.J. Ikari).

0025-3227/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.margeo.2011.11.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 May 2011Received in revised form 15 November 2011Accepted 16 November 2011Available online 8 December 2011

Communicated by D.J.W. Piper

Keywords:permeabilityfluid flowpore pressurefaultssubduction zoneNankai Trough

At subduction zones, the permeability of major fault zones influences pore pressure generation, controls fluidflow pathways and rates, and affects fault slip behavior and mechanical strength by mediating effective normalstress. Therefore, there is a need for detailed and systematic permeability measurements of natural materialsfrom fault systems, particularly measurements that allow direct comparison between the permeability ofsheared and unsheared samples from the same host rock or sediment. We conducted laboratory experimentsto compare the permeability of sheared and uniaxially consolidated (unsheared) marine sediments sampledduring IODP Expedition 316 andODP Leg 190 to the Nankai Trough offshore Japan. These sampleswere retrievedfrom: (1) The décollement zone and incoming trench fill offshore Shikoku Island (the Muroto transect);(2) Slope sediments sampled offshore SW Honshu (the Kumano transect) ~25 km landward of the trench, in-cluding material overriden by a major out-of-sequence thrust fault, termed the “megasplay”; and (3) A regionof diffuse thrust faulting near the toe of the accretionary prism along the Kumano transect. Our results showthat shearing reduces fault-normal permeability by up to 1 order of magnitude, and this reduction is largestfor shallow (b500 mbsf) samples. Shearing-induced permeability reduction is smaller in samples from greaterdepth, where pre-existing fabric from compaction and lithificationmay be better developed. Our results indicatethat localized shearing in fault zones should result in heterogeneous permeability in the uppermost few kilome-ters in accretionary prisms, which favors both the trapping of fluids beneath and within major faults, and thechanneling of flow parallel to fault structure. These low permeabilities promote the development of elevatedpore fluid pressures during accretion and underthrusting, and will also facilitate dynamic hydrologic processeswithin shear zones including dilatancy hardening and thermal pressurization.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The permeability of compacting and sheared accreted sediments inaccretionary complexes mediates pore fluid pressure in the wedge inte-rior andbeneath fault zones, and thus exerts a first order control on fluidflow and drainage patterns (Screaton et al., 1990; Henry and Wang,1991; Moore and Vrolijk, 1992; Bolton and Maltman, 1998; Saffer,2010; Saffer and Tobin, 2011). Fluid pressures act against the overbur-den stress, and thereby reduce the stress necessary for shear failure(e.g., Hubbert and Rubey, 1959). As a result, pore pressure plays a keyrole in controlling wedge taper angle (e.g., Davis et al., 1983; Dahlen,1990; Le Pichon et al., 1993). Pore pressure is also hypothesized to me-diate fault slip behavior. For example, sustained elevated pore pressuremay allow faults to slip at low shear stresses, limiting the size of earth-quake stress drops and favoring aseismic slip (e.g., Sholz, 1998; Beeler,2007). High pore pressures have also been invoked to explain slowslip, low frequency earthquakes (LFE), and tremor (Kodaira et al.,

e Environmental Sciences, Uni-421 218 65517.

rights reserved.

2004; Shelly et al., 2006; Liu and Rice, 2007; Saffer and Tobin, 2011).Over short timescales, low permeability results in undrained behavior,which may lead to rupture stabilization due to dilatancy hardening(Segall and Rice, 1995; Samuelson et al., 2009; Liu and Rubin, 2010;Segall et al., 2010) or slip weakening caused by thermal pressurization(e.g. Wibberley and Shimamoto, 2005; Rice, 2006).

Within accreted andunderthrust sediments at subduction zones, per-meability varies spatially due to the combination of heterogeneity in ac-creted sediment lithofacies (e.g. Moore et al., 1991; Underwood, 2007),consolidation in response to increasing effective stress (Saffer andMcKiernan, 2005), and shearing in localized zones (e.g., Maltman et al.,1992; Faulkner et al., 2010). For example, previous experimental studieshave demonstrated that sediment permeability decreases dramaticallywith progressive consolidation, by up to four orders of magnitude as po-rosity is decreased from ~50% tob~15% (e.g., Bolton et al., 1999; Safferand McKiernan, 2005; Long et al., 2008; Gamage et al., 2011). Majorfault zones also exert a primary control on fluid flow and drainage pat-terns in these systems (e.g. Moore, 1989). In addition to localized perme-ability reduction in shear zones (e.g., Maltman et al., 1992; Brown et al.,1994), most brittle fault zones contain pervasively fractured damagezones a few meters to tens of meters wide, which exhibit substantially

2 M.J. Ikari, D.M. Saffer / Marine Geology 295-298 (2012) 1–13

higher permeability than the surrounding country rock (Vrolijk, 1987;Caine et al., 1996; Evans et al., 1997; Gudmundsson et al., 2001;Faulkner et al., 2010). In cases where subduction fault zone permeabilityhas been directly measured parallel to the fault by single or cross-welltests, it is >2–3 orders of magnitude higher than that of the surroundingsediment (e.g., Fisher et al., 1996; Screaton et al., 2000). This is consistentwith structural observations from drill core, which document brecciatedand fractured damage zones tens of meters thick (Maltman et al., 1997;Moore et al., 2001). It is also consistent with inverse modeling studiesof several subduction–accretion complexes, which show that long-termaveraged along-fault permeabilities at the regional scale must be~1×10−17 to 1×10−14 m2 in order to maintain realistic pore pressures(Saffer and Bekins, 1998; Spinelli et al., 2006; Saffer and Tobin, 2011).Similarly high fault zone permeability is inferred from drainage patternswithin adjacent sediments (Skarbek and Saffer, 2009) and from geo-chemical and thermal anomalies that require rapid flow consistentwith fault zone permeabilities of ~10−16 to 10−13 m2 (e.g., Bekinset al., 1995; Henry, 2000; Spinelli et al., 2006).

At a smaller scale, in experiments using synthetic fault material theaccumulation of shear strain along slip zones substantially reducesfault-perpendicular permeability (Zhang et al., 1999; Takahashi et al.,2007; Crawford et al., 2008; Ikari et al., 2009a). This may cause the de-velopment of strongly anisotropic permeability within faults that trapsfluids and potentially guides flow or allows pore fluid pressure transla-tion (e.g. Arch and Maltman, 1990; Faulkner and Rutter, 1998). Perme-ability anisotropy is also observed in naturally sheared fault rocks (Kopf,2001). As a result of low fault-normal permeability, fluids may betrapped or channelized beneath faults within subjacent permeabledamage zones or the footwall (Byerlee, 1990; Moore et al., 1990;Maltman et al., 1992; Tobin et al., 2001; Zoback et al., 2011). The extentto which this process occurs depends on the permeabilities of bothshear zones and the surrounding wall rock. Yet the magnitude of per-meability reduction in sheared naturalmaterial fromwithin and aroundtectonic fault zones relative to unsheared, compacting sediment in thewall rock represents a gap in our knowledge of these systems.

Here, we report on the results of permeability measurements con-ducted on natural sediment and fault zonematerial sampled by drillingat several sites in the Nankai Trough offshore SW Japan. The accretedmaterial and the faults that develop within it are predominantly com-posed of clay-rich pelagic and hemipelagic sediments (ShipboardScientific Party, 2001a; Underwood, 2007; Screaton et al., 2009a)(Figs. 1–3). We focus on measuring the permeability of unsheared andsheared sediment using samples of fault and wall rock from three

Kyushu

Shikoku Island

JAPAN

0

1946

32°

34°N

132°E 134°

33°

Muroto

Transect

Site 11744

Nankai Trough DeformationFront

Fig. 1.Map of the Nankai trough showing locations of the Muroto and Kumano transects, and(dashed boxes) and epicenters (stars) of the 1944 Tonankai and 1946 Nankaido earthquak

distinct faulting environments: the décollement, the megasplay, andthe frontal thrust (Figs. 2 and 3). We measure the vertical permeabilityof intact, unsheared wall rock (sediment) under uniaxial loading condi-tions up to 87MPa effective stress, and compare these datawith perme-abilities measured across experimentally sheared specimens fromwithin the adjacent fault zones, deformed in a direct shear configura-tion at effective normal stresses of up to ~60 MPa. We then investigatethe roles of accumulated shear strain and structural positionwithin andacross the fault zones in controlling permeability. Finally, we extrapo-late our results to estimate the in situ permeability of fault zonematerialand adjacent sediments in the active accretionary complex, and discussthe implications for regional-scale fluid flow, excess pore pressure de-velopment, and the mechanics of fault slip.

2. Geologic setting and sample descriptions

The Nankai accretionary complex is formed by subduction of thePhilippine Sea plate beneath the Eurasian plate off the east coast ofSW Japan (Fig. 1). The subduction margin has been extensively stud-ied by drilling and geophysical studies (e.g., Moore et al., 1990, 2001,2009; Taira et al., 1992; Ashi et al., 2009; Screaton et al., 2009a; Tobinet al., 2009). A wide range of fault slip behaviors have been documen-ted, including historical great earthquakes (Mw>8.0) and associatedtsunamis (Ando, 1975; Satake, 1993; Kikuchi et al., 2003), slow slipevents (Davis et al., 2006), non-volcanic tremor (Obana andKodaira, 2009), and very low frequency earthquakes (VLFE) (Ito andObara, 2006). Our study focuses on samples obtained from two dril-ling transects: the Muroto transect drilled during ODP Leg 190; andthe Kumano transect drilled during IODP Expeditions 315 and 316as part of the Nankai Trough Seismogenic Zone Experiment (NanTro-SEIZE) (Fig. 1). Samples were obtained from three distinct structuralregions: (1) The décollement and incoming trench sediments alongthe Muroto transect (Sites 1173 and 1174, ODP Leg 190); (2) Hangingwall, footwall, and fault material from a major out-of-sequence thrust(OOST) fault termed the “megasplay” that cuts the accretionary prismalong the Kumano transect (Sites C0001, C0004 and C0008, IODP Ex-peditions 315 and 316); and (3) The frontal thrust zone along theKumano transect (Site C0007, IODP Expedition 316) (Figs. 2 and 3).

2.1. Décollement and incoming trench sediment, Muroto Transect

Drilling during ODP Leg 190 penetrated the décollement zone alongthe Muroto Transect at Site 1174, and the incoming sediment package

PPEP

PSP

NAP

Kum

ano

Transect

Site C0004

Kii Peninsula

Tokai

km

200100

1944

138°136°

Site 1173

Site C0008

Site C0007

ODP and IODP drillsites where samples for this study were collected. The rupture areases are shown for reference (modified from Screaton et al., 2009a).

Fronta lOOST

Décollemen t

Site1174

Fronta lSlopebasin

Distance from deformation front (km)

60 50 40 30 20 10 0

Dep

th (

km)

0

2

4

6

8

10

Décollemen t

thrust

-10

Site1173

Unit I: Slope apron

Unit IIA: Axial trench wedge

Unit IIB: Outer trench wedge

Unit IIC: Trench to wedge

Unit III: Upper Shikoku Basin

Decollement Zone

`

Unit V: Volcaniclastic

Unit IV: Lower Shikoku Basin

100

200

300

400

500

600

700

800

900

1000

1100

hemipelagicmud/mudstonesandymud/mudstone

silt turbidites

sand turbidites

volcanic ash

slumpfolds

Site 1174

Unit IA: Outer trench wedge

Unit IB: Trench to basin

Unit II: Upper Shikoku Basin

Unit III: Lower Shikoku Basin

Unit IV: Volcaniclastic

Unit V: Basalt basement

100

200

300

400

500

600

700

Site 1173

Muroto Transect

gravel, pebblymud/mudstone

Accretionary prism

hemipelagicmud/mudstone

sandy mud/mudstone

sand turbidites

volcanic ash

chaotic stratification,slump folds

silt turbidites

gravel, pebblymud/mudstone

Fig. 2. Profile of the accretionary prism along the Muroto transect, showing the location and lithostratigraphy of drill sites 1173 and 1174 (Modified from Shipboard Scientific Party,2001a,b,c). Diamonds indicate locations of samples used in this study (blue for sheared samples, red for unsheared samples).

3M.J. Ikari, D.M. Saffer / Marine Geology 295-298 (2012) 1–13

seaward of the trench at Site 1173 (Fig. 2). At Site 1174, the drilled sec-tion includes hemipelagic mudstones of the Pliocene to QuaternaryUpper Shikoku Basin (USB) facies (483–661 mbsf) andMiocene to Plio-cene Lower Shikoku Basin (LSB) facies (661–1102 mbsf) (ShipboardScientific Party, 2001a,c). The USB is composed ofmonotonous hemipe-lagic mudwith interbeds of volcanic ash, andmaintains nearly constantporosity downsection. The LSB is a relatively homogeneous silty clays-tone/clayey siltstone in which porosity decreases with depth followinga normal consolidation trend. The boundary between the USB and LSBcoincides with a diagenetic boundarymarked by the alteration of volca-nic ash to siliceous claystones containing smectite and zeolite. Thedécollement is defined as a ~32 m-thick zone of fractured and brecciat-ed mudstone extending from 808 to 840 mbsf formed entirely withinthe LSB. Near-surface heat flow on the incoming plate is ~180 mW/m2

in this area (Shipboard Scientific Party, 2001a), and is anomalouslyhigh for the ~15 Ma ocean crust (Yamano et al., 1992, 2003). Bottom-

hole temperature at Site 1174 is estimated at 130–140 °C (ShipboardScientific Party, 2001a; Yamano et al., 2003).

Samples for our shearing experiments were taken from depthsof 660–998 mbsf at Site 1174. The samples include 3 specimensfrom within the décollement zone, 1 sample from the Upper ShikokuBasin facies mudstone (USB), and 6 from the LSB (Fig. 2, Table 1).Samples for permeability measurements on unsheared materialwere taken from the LSB at Site 1173 at depths of 427 and 465 mbsf(the depth of the USB–LSB boundary is 344 mbsf at Site 1173)(Shipboard Scientific Party, 2001a,b) (Fig. 2; Table 2). Bulk clay contentdetermined by X–ray diffraction (XRD) ranges from 37 to 67 wt.%, andis ~45–55 wt.% for most of our samples (Shipboard Scientific Party,2001b,c). Analysis of the clay-sized fraction shows that the total sedi-ment is composed of 5–23 wt.% smectite, 18–40 wt.% illite, and7–23 wt.% chlorite (+ minor kaolinite) (Steurer and Underwood,2005).

Site C0004

Dep

th (

km)

Distance from deformation front (km)

2

4

6

8

10

12

60 50 40 30 20 10 0

Accretionary prism

Forearc basin

Subducting oceanic crust

Decollement

Megasplay fault

Line 5Plate interface

Site C0001

Site C0008

Qua

t.P

lioce

neQ

uate

rnar

y

Unit I: Slope apron

Unit IIA: Mass-transport complex

Unit IIB: Upper accretionary prism

Unit III: fault-bounded package

Unit IV: Underthrust slope apron

Site C0004

100

200

300

400

mbsf

sandy silt

silt turbidite

hemipelagic mud

volcanic ash sand turbidite

breccia

100

200

300

400

Unit I: Slope apron

Unit II:Upper accretionary prism

mbsf

Qua

tern

ary

Plio

cene

Mio

c.

Site C0001

unconformity

sandy silt

silt turbidite

hemipelagic mud

volcanic ash

Unit II: Accreted trench wedge

100

200

300

Plio

c.

Unit IB: Mass-transport deposits

Unit IA: Slope apron

Ple

isto

cene

mbsf

Site C0008

sandy silt

silt turbidite

hemipelagic mud

volcanic ash sand turbidite

conglomerate

Hole A

Kumano Transect

Ple

isto

cene

Unit I: Trench-to-slope transition

Unit II: Accreted trench wedgeHole D

mbsf

100

Site C0007

Unit IV: Accreted trench wedge

Unit III:Upper Shikoku Basin facies

Plio

cene

200

300

400

Ple

ist.?

Ple

isto

cene

sandy silt

silt turbidite

hemipelagic mud

volcanic ash sand turbidite

gravel

HolesA/B/C

Site C0007

Fig. 3. Profile of the accretionary prism along the Kumano transect, showing the location and lithostratigraphy of drill sites C0001, C0004, C0007 and C0008 (Kumano transect profilemodified fromKimura et al., 2008, lithostratigraphy fromM.B. Underwood, unpublished data). Diamonds indicate locations of samples used in this study (left side and in blue for shearedsamples, right side and in red for unsheared samples).

4 M.J. Ikari, D.M. Saffer / Marine Geology 295-298 (2012) 1–13

2.2. Slope sediments and megasplay fault zone, Kumano Transect

Drilling at Site C0004 penetrated a major out-of-sequence thrustfault known as the megasplay (Park et al., 2002; Moore et al., 2007,2009). The megasplay branches from the main décollement ~50–55 km landward of the deformation front and terminates near the sea-floor ~25 km landward of it (Fig. 3). It forms amajor structural boundaryin the accretionary wedge, and has been implicated in coseismic slipduring great earthquakes (Cummins and Kaneda, 2000; Tanioka andSatake, 2001; Kikuchi et al., 2003), earthquake afterslip (Sagiya andThatcher, 1999), and in recently discovered very low frequency earth-quakes (VLFE) (Ito andObara, 2006). At shallowdepths (b~2 km), accre-tionary prismmaterial is thrusted over younger slope sediments, whichcomprise the footwall and are incorporated into the fault zone. Shallowheat flow at Sites C0001, C0004, and C0008 ranges from 50 to 60 mW/m2, resulting in estimated in situ temperatures of ~19–23 °C at thebase of these boreholes (Harris et al., 2011). For samples from themegasplay fault zone area at Site C0004, we measured permeability of:(1) 12 experimentally sheared samples, including 1 sample from amass-transport complex (78–118 mbsf), 2 from the hangingwall thrustsheet (100–258 mbsf), 5 from within the megasplay fault zone(258–308 mbsf), and 4 from the overridden slope apron sedimentsin the footwall; and (2) 2 intact samples from the footwall (Tables 1and 2). In addition, we tested several samples of slope sediment fromSites C0001, C0008, and the uppermost ~100 m at Site C0004 whichserve as reference material for the footwall of the megasplay (Fig. 3).This included tests on 4 samples under uniaxial consolidation conditions(unsheared), and13 sheared samples. This suite of samples is dominant-ly composed of silty clay and clayey silt; the clay content of our samplesranges from 25 to 65 wt.%, with most ranging from ~40 to 60 wt.% clay

(Expedition 315 Scientists, 2009; Expedition 316 Scientists, 2009a,c)(Tables 1 and 2). The smectite content of these samples is 7–25 wt.%,similar to those from the Muroto transect. Illite and chlorite (+ kaolin-ite) contents are 13–26 wt.% and 9–16 wt.% respectively, lower than inthe Muroto transect samples (Guo and Underwood, in press).

2.3. Frontal thrust, Kumano Transect

Drilling at IODP Site C0007 sampled the frontal thrust zone near thetrench along the Kumano transect (Fig. 3) (Screaton et al., 2009a). Thisregion is characterized by diffuse thrust faulting; three major faultswere penetrated at depth intervals of 238–259 mbsf, 342–362 mbsf,and 399–446 mbsf (Expedition 316 Scientists, 2009b) (Fig. 3). Shallowheatflowat Site C0007 is 70 mW/m2, and estimated in situ temperatureat the bottom of Hole C0007D is ~22 °C (Harris et al., 2011). We mea-sured the permeability of 7 experimentally sheared samples, including3 from the trenchwedge facies (stratigraphic Unit II) and 4 from accret-ed sediments composed of Pliocene silty clay with volcanic ash layers(stratigraphic Unit III) that extend from 362 to 439 mbsf (Fig. 3).Unit III is considered the stratigraphic equivalent of the USB facies sam-pled at Sites 1173 and 1174 (Expedition 316 Scientists, 2009b; Screatonet al., 2009a) (Table 1). One of our samples was located in the shallow-est fault zone and 1 was located in the deepest fault zone. We com-pare our measurements on sheared samples with permeabilities ofintact core from 401 mbsf reported by Guo et al. (2011). With the ex-ception of one sand-rich sample (C0007C-11X-4), total clay mineralcontent of these samples ranges from 50 to 65 wt.% (Expedition 316Scientists, 2009b). Smectite content in the bulk sediment is18–28 wt.%, illite content is 17–24 wt.%, and chlorite (+ kaolinite)content is 13–15 wt.%. These clay contents are similar to those of the

Table 1Samples tested and experiment parameters for permeability measurements on sheared samples.

Sample # Experiments Stratigraphic location Depth(mbsf)

σn′ γ State % Clay

1174B-55R1 p2720 Upper Shikoku 660.35 26 6.8 Intact 451174B-57R-CC p2721 Lower Shikoku 688.07 26 15.6 Intact 491174B-67R3 p2642 Lower Shikoku 777.24 26 18.0 Intact 531174B-72R1 p2705 Décollement Zone 822.20 26 11.4 Remolded 371174B-72R2 p2704 Décollement Zone 824.53 26 14.9 Remolded 431174B-73R2 p2706 Décollement Zone 834.60 26 12.5 Remolded 531174B-73R-CC p2707 Lower Shikoku 841.47 26 17.7 Intact 531174B-74R2 p2690, p2753, p2763, p2764 Lower Shikoku 843.30 11, 22, 43, 61 0–18.8 Remolded 471174B-80R3 p2738 Lower Shikoku 902.00 26 15.0 Intact 671174B-87R1 p2752 Lower Shikoku 967.80 26 15.3 Intact 55C0004C-2H-5 p2950 Slope sediments 10.98 11, 21 2.5–4.5 Remolded 49C0004C-4H-1 p2951 Slope sediments 25.64 11, 21 3.8–6.4 Remolded 45C0004C-7H-7 p2957 Slope sediments 60.89 8, 21 2.8–5.1 Remolded 59C0004C-12X-8 p2955 Mass Transport Complex 96.97 11, 21 1.7–3.6 Remolded 53C0004C-15X-2 p2118, p2956 Accretionary prism 119.28 8, 25 2.0–6.1 Remolded 54C0004D-22R-1 p2627 Accretionary prism 243.10 26 20.5 Remolded 60C0004D-27R-1 p2102 Megasplay fault zone 266.16 23 16.0 Remolded 65C0004D-28R-1 p2919 Megasplay fault zone 270.66 12 3.1 Remolded 59C0004D-29R-2 p2069 Megasplay fault zone 275.73 25 14.4 Remolded 61C0004D-30R-1 p2067 Megasplay fault zone 278.75 25 0–19.9 Remolded 58C0004D-34R-1 p2068 Megasplay fault zone 297.55 25 15.7 Remolded 62C0004D-41R-CC p2917 Underthrust slope seds 330.63 11 3.4 Remolded 49C0004D-42R-3 p2074, p2121 Underthrust slope seds 335.42 25, 26 10.5–10.8 Remolded 25C0004D-47R-2 p2112 Underthrust slope seds 357.11 25 3.7 Intact 53C0004D-51R-2 p2918 Underthrust slope seds 375.08 11 3.5 Intact 53C0001E-2H-3 p2952 Slope sediments 7.91 11, 21 2.2–3.9 Remolded 31C0001E-4H-2 p2333, p2337 Slope Sediments 25.13 8, 11, 16, 21 5.0–11.2 Remolded 39C0001E-7H-7 p2334, p2336 Slope sediments 58.25 8, 11, 17, 22 5.6–15.9 Remolded 39C0001E-10H-5 p2100, p2205 Slope sediments 85.22 8, 11, 16, 21 5.7–18.8 Intact, remolded 48C0001E-13H-6 p2367, p2368 Slope Sediments 114.88 8, 11, 16, 21 7.9–23.2 intact, remolded 43C0008A-3H-1 p2922, p2923 Slope sediments 16.05 8, 11, 15, 20 1.8–4.8 Remolded 46C0008A-6H-7 p2953 Slope sediments 51.56 11, 21 3.1–5.5 Remolded 54C0008A-7H-8 p2931, p2932 Slope Sediments 62.86 8, 11, 16, 21 3.0–4.6 Remolded 31C0008A-10H-8 p2560, p2626 Slope sediments 90.17 8, 11, 16, 21 5.1–14.6 Remolded 46C0008A-15H-3 p2954 Slope sediments 121.29 11, 21 3.2–5.4 Remolded 52C0008A-21H-3 p2933 Slope sediments 162.45 8, 11 2.7–4.5 Remolded 58C0007C-11X4 p2654 Outer trench wedge 103.61 25 7.0 Remolded 31C0007D-9R-2 p2777 Fault zone 249.37 22 14.6 Remolded 50C0007D-16R2 p2640 Marginal trench wedge 315.42 26 10.0 Intact 53C0007D 23R2 p2639 Upper Shikoku equiv. 381.14 26 14.0 Intact 65C0007D-24R-1 p2655, p2656 Upper Shikoku equiv. 389.28 26 21.0, 22.3 Intact, remolded 64C0007D-27R1 p2644 Fault zone 418.42 26 12.4 Remolded 61C0007D-29R-1 p2641 Fault zone 437.19 26 29.3 Remolded 64

5M.J. Ikari, D.M. Saffer / Marine Geology 295-298 (2012) 1–13

Kumano slope sediments, but lower than those of the Muroto transectsamples (Guo and Underwood, in press).

3. Experimental methods

3.1. Permeability of intact, unsheared core samples

To determine the permeability of unsheared core samples, weconducted constant rate of strain (CRS) consolidation tests parallel

Table 2Samples tested and best-fit curve parameters for permeability of unsheared sediments. Inte

Sample # Stratigraphic location Depth(mbsf)

σ(

1173-45X-CC Lower Shikoku 427.10 31173-49X-CC Lower Shikoku 464.71 3C0007D-25R-2 Frontal thrust 400.60 8C0004D-47R-2 Megasplay FW 357.11 4C0004D-51R-2 Megasplay FW 375.08 7C0001E-7H-7 Slope apron 58.25 5C0001E-10H-5 Slope apron 85.22 0C0001E-13H-6 (1) Slope apron 114.88 5C0001E-13H-6 (2) Slope apron 114.88 0C0008C-9H-5 Slope apron 71.64 0

to the core axis (vertical direction) in a uniaxial consolidation appara-tus (Fig. 4A) (e.g., Olson, 1986; Saffer and McKiernan, 2005). Thesetests provide continuous measurement of permeability and volumet-ric compressibility as a function of effective stress (and thus as a func-tion of porosity) during progressive loading (e.g., Olson, 1986; Safferand McKiernan, 2005; Dugan and Germaine, 2008). Right-cylindersamples were trimmed from whole-round cores to 25, 36, or 50 mmdiameter and 20 cm height. Samples were placed in a fixed-ring con-solidation system, backpressured to 500 kPa for 24 h, and deformed

rcept (A) and slope (B) are the parameters A and B in Eq. (2).

v′

MPa)% Clay A B R2

.5–87.2 52 −16.6 −1.9 0.97

.0–20.9 56 −17.2 −2.0 0.99

.0–23.9 63 −11.9 −5.0 0.87

.7–44.0 53 −15.3 −2.0 0.99

.0–43.5 53 −14.9 −2.4 0.99

.0–38.9 39 −16.1 −1.1 0.98

.5–45.1 48 −15.6 −1.5 0.95

.0–24.7 43 −15.4 −1.6 0.94

.5–42.2 43 −16.3 −1.2 0.95

.5–18.5 54 −16.3 −1.2 0.99

Pore FluidOutlets Confining Fluid

Pore FluidInlet

Vertical Load

NormalLoad

Vertical Load

Pore FluidOutlet

UndrainedBoundary Condition

SampleSample

A B

Fig. 4. Schematic illustrations of A. Uniaxial consolidation apparatus, and B. Biaxial deformation system.

6 M.J. Ikari, D.M. Saffer / Marine Geology 295-298 (2012) 1–13

at a constant rate of axial strain to peak stresses of ~20 MPa (50 mmdi-ameter specimens), ~45 MPa (36 mm diameter), or ~90 MPa (25 mmdiameter).

During deformation, the sample is undrained at its base, and is opento a constant and controlled backpressure at its top boundary (e.g.Olson, 1986; ASTM, 1996). We continuously monitored sample height(H, in m), axial stress (σv, in Pa), and basal excess pore pressure (μb, inPa). We calculate the coefficient of consolidation (cv, in m2/s) by:

cv ¼ −H2 log σv2

σv1

h i

2Δt log 1− μbσv

h i ð1Þ

where Δt is time (s), and σv1, σv2, and σv are the initial, final, and aver-age effective stresses at the sample top for each time increment (ASTM,1996). Permeability is given by:

kv ¼ cvmvν ð2Þ

where mv is the sample volumetric compressibility (Pa−1), calculatedfor each time increment frommeasured changes in height and effectivestress, and ν is the viscosity of water (1×10−3 Pa s at 20 °C).

3.2. Permeability of sheared specimens

To measure the permeability of sheared sediments, we conductedexperiments in a double-direct shear configuration using a biaxial de-formation apparatus under true-triaxial stress conditions (Ikari et al.,2009a; Samuelson et al., 2009) (Fig. 4B). In all cases, the samples weresheared experimentally, both for wall rock and samples from withinfault zones. When possible, samples were tested as intact “wafers”trimmed from whole-round cores and sheared in a direction perpen-dicular to the core axis (i.e., parallel to bedding). Other samples wereobtained as soft sediment, or as brecciated mudstone fragments. Wedried these samples at low temperature, and powdered and remoldedthem for our tests (Ikari et al., 2009a) (Table 1).

In each experiment, we sheared the samples at a constant displace-ment rate boundary condition (11 μm/s). Effective normal stress rangedfrom 7 to 61 MPa, and represents the combined effects of confiningpressure (Pc), externally applied normal load, and two independentlycontrolled pore pressures, one at each sample boundary (inlet pressurePpa, outlet pressure Ppb). In order to approximately match the observedin situ pore fluid chemistry for the samples, we used 3.5 wt.% NaCl brine(e.g., Ashi et al., 2009; Screaton et al., 2009a). In these tests, both the

applied normal stress and the fluid flow direction were perpendicularto the experimental shear zone. These stress and hydrologic boundaryconditions are comparable to those for the CRS consolidation tests,allowing a straightforward comparison of the two data sets.

In each experiment, we measured permeability after shearing at agiven effective normal stress by flow-through, typically at shear strainsof ~3–30. For eachmeasurement, we imposed a constant head gradientnormal to the layer, and calculated permeability (k, in m2) from theresulting steady state flow rate according to Darcy's law:

k ¼ QνdxA Ppa−Ppbð Þ ð3Þ

where Q is the volumetric flow rate (m3 s−1), dx is the layer thickness, Ais the cross-sectional area forflow(0.005 m2), and the quantity (Ppa–Ppb)is the imposedfluid pressure difference across the sample.WedefineQ asthe average of measured flow rates at the sample inlet and outlet. Atsteady state, the two flow rates differ by less than 5%.

4. Results

4.1. Permeability of unsheared sediment

The permeability of our unsheared samples decreases systematicallywith increasing effective axial stress (Fig. 5). The two samples from Site1173 exhibit lower permeability than samples from the Kumano tran-sect, decreasing from ~9×10−19 m2 to ~6×10−18 m2 at 3 MPa effec-tive stress, to ~2×10−20 m2 at 21 MPa (1173-49X), and~1×10−20 m2 at 87 MPa (Sample 1173-45X) (Fig. 5A). These valuesare consistentwith permeabilities of ~2×10−19 to ~2×10−18 m2mea-sured by Gamage and Screaton (2006) for LSB samples at low effectivestress (b1 MPa).

The permeability of slope apron sediments from Sites C0001 andC0008 decreases from ~2×10−16 m2 atb1 MPa effective stress, to~4×10−19 m2 at 45 MPa. Permeability values are highly consistent be-tween tests,with inter-sample variability of one half to one order ofmag-nitude (±~2.5×10−18 m2 at 15 MPa effective stress) (Fig. 5B). For theoverridden slope sediments that comprise the footwall of the megasplayfault at Site C0004, permeability ranges from ~4×10−17 m2 to ~2–4×10−19 m2 as effective stress is increased from 5 to 44 MPa (Fig. 5C).These values are comparable to those for the slope apron sedimentfrom Sites C0001 and C0008 (Fig. 5B). We also measured the permeabil-ity of one sample in the biaxial apparatus prior to shear (sample C0004D-30R), and obtained a value of 6.5×10−19 m2 at 25 MPa effective normal

Kumano Slope ApronSites C001/C0004/C0008

C0001E-13HC0001E-7HC0001E-10HC0008C-9HC0001 sheared (intact)C0001/4/8 sheared (remolded)A

DC

B

10-15

10-16

10-17

10-18

10-19

10-20

10-21

10-15

10-16

10-17

10-18

10-19

10-20

10-21

0 20 40 60 80 100

Lower Shikoku BasinSites 1173/1174

1173-45X1173-49X1173/1174 [Gamage & Screaton, 2006]1174 sheared (intact)1174 sheared (remolded)

Per

mea

bilit

y (m

2 )P

erm

eabi

lity

(m2 )

Effective Stress (MPa) Effective Stress (MPa)

Effective Stress (MPa) Effective Stress (MPa)

0 10 20 30 40 50

0 10 20 30 40 50

Megasplay Footwall, Site C0004

C0004D-47RC0004D-51RSheared (intact)Sheared (remolded)Unsheared (remolded)

0 5 10 15 20 25 30

Kumano Frontal ThrustSite C0007

C0007D-25R [Guo et al., 2011]Sheared (intact)Sheared (remolded)

Fig. 5. Comparison of sheared (large orange triangles and large red circles) and unsheared (small black and gray circles) sediment permeability for: A. The Lower Shikoku Basinfacies, Muroto transect, B. Slope apron sediments, Kumano transect, C. The megasplay fault zone and footwall, Kumano transect, and D. The frontal thrust, Kumano transect. Thelarge blue diamond in panel C shows permeability of intact (unsheared) wafer measured in the double-direct shear configuration, as described in text.

7M.J. Ikari, D.M. Saffer / Marine Geology 295-298 (2012) 1–13

stress, consistent with those obtained from CRS tests on intact samplesfrom the same section (Fig. 5C). Permeabilities from the USB equivalentunit at Site C0007 near the frontal thrust are similar to those of the

-21

-20

-19

-18

-17

-16

-15

0 20 40 60 80

Kum ano S lope Sedim ents(Unsheared)

C0 004D -47RC0 004D -51RC0 001E-7HC0 001E-10HC0 001E-13H (1)C0 001E-13H (2)C0 008C -9H

Lo

g 10

(k)

Effective Normal Stress (MPa)

A

Fig. 6. Best-fit curves to permeability data as a function of effective stress for unsheared sedilement zone.

slope sediments at Sites C0001, C0004, and C0008, and decrease from~8×10−15 to ~4×10−19 m2 over a range of effective stress from 8 to23 MPa (data from Guo et al., 2011; Fig. 5D).

0 20 40 60 80

B100

M uroto Decollem ent(Unshea red)

1173-45X1173-49X

Effective Normal Stress (MPa)

`

ment from: A. Kumano transect slope sediments, and B. Muroto transect LSB and décol-

8 M.J. Ikari, D.M. Saffer / Marine Geology 295-298 (2012) 1–13

The systematic reduction in permeability with increased effectivestress observed in our CRS tests on unsheared specimens is primarilya result of porosity loss, and defines a systematic log-linear relation-ship between permeability and porosity (e.g., Neuzil, 1994; Safferand McKiernan, 2005; Gamage and Screaton, 2006; Gamage et al.,2011). For a progressive loading path (i.e. monotonically increasingeffective stress), the sediment permeability can also be expressed afunction of effective stress (Fig. 6):

logk ¼ Aþ B logσv0 ð4Þ

This is consistent with two common observations during consolida-tion: (1) Permeability varies log-linearly with porosity (e.g., Neuzil,1994; Dewhurst et al., 1999; Chapuis and Aubertin, 2003; Gamageet al., 2011; Rowe et al., 2011); and (2) Porosity varies log-linearlywith effective stress (e.g., Karig and Hou, 1992; Hart et al., 1995). Inter-cepts (A), coefficients (B), and correlation coefficients (R2) for fits to ourdata using Eq. (4) are reported in Table 2.

4.2. Permeability of sheared samples

LSB samples from Site 1174 sheared at effective normal stresses of11–61 MPa exhibit permeabilities of ~1−6×10−19. These values fallwithin the range of permeabilities we measured for unsheared spec-imens at comparable effective stresses (Fig. 5A). The exceptions areall from sample 1174B-74R, which was tested as a remolded powderat multiple effective stresses and shear strains. In these instances, thepermeability of sheared material is slightly higher than that of the in-tact unsheared samples. The highest permeability values were allmeasured at low shear strains (γ=~0–3); however, we also observethat permeability decreases as a function of shear strain (Fig. 7). Weanticipate that with further shearing, the permeability of these spec-imens would decrease substantially (e.g., Ikari et al., 2009a).

The permeability of sheared samples of slope sediment from theKumano transect is consistently >~2×10−19 m2 (Fig. 5B). These valuesare generally higher than those for sheared hemipelagic mudstonesfrom the Muroto transect (maximum k≤~6×10−20 m2). However, thisdifference between the two transects is more pronounced in unshearedspecimens. For samples obtained from within the décollement shearzone along the Muroto transect, permeabilities range from ~2–6×10−20 m2. For samples from within the megasplay fault zone alongthe Kumano transect, values range from~2×10−20 to 2×10−19 m2. Per-meability is lower for samples from within the megasplay shear zone(ranging down to 2.2×10−20 m2) compared with that of sheared

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25 30

1174B-74R-2 (43 MPa)Illite Shale (25 MPa)

Nor

mal

ized

Per

mea

bilit

y (k

/ko)

Shear Strain

Fig. 7. Permeability normalized to the initial (unsheared) value as a function of shear strain,for sample 74R-2 from Site 1174 at 43MPa effective normal stress. Illite shale permeabilityas a function of shear strain at 25MPa (Ikari et al., 2009a) is shown for comparison.

samples from outside the fault zone at Site C0004 (k≥9.2×10−20 m2)(Ikari et al., 2009b). Permeabilities of samples from the frontal thrustarea exhibit considerable scatter, and are independent of stratigraphicposition.

For samples from the Kumano transect sites, the permeability ofsheared samples is generally ~10–30 times lower than that of the in-tact samples (Fig. 5). At a given effective stress, permeability of thesheared samples varies by up to an order of magnitude because it isalso a function of shear strain (Fig. 7, Table 1). Both of these observa-tions are consistent with previous experimental work on syntheticfault gouges (Brown and Moore, 1993; Zhang et al., 1999; Takahashiet al., 2007; Crawford et al., 2008; Ikari et al., 2009a). For the Kumanotransect sites, we do not observe any systematic difference in perme-ability, or permeability reduction from shearing, between remoldedsamples and those trimmed as wafers from whole-round cores.

5. Discussion

5.1. Factors controlling permeability

We find that increasing effective stress and concomitant porosityloss strongly reduce permeability, consistent with several previousstudies (Neuzil, 1994; Bolton et al., 1999; Dewhurst et al., 1999;Chapuis and Aubertin, 2003; Saffer and McKiernan, 2005; Gamage andScreaton, 2006; Gamage et al., 2011; Rowe et al., 2011). Permeabilityfollows a log-linear decrease with decreasing porosity (Fig. 6), whichcan be defined empirically using laboratory datasets, and is commonlyextrapolated to depths>3–5 km in regional-scale fluid flow models(e.g., Bekins et al., 1995; Saffer and Bekins, 1998). We also find thatshearing reduces fault-perpendicular permeability by up to ~1–1.5 or-ders of magnitude for the slope sediments along the Kumano transect.This permeability reduction is strain dependent and occursmost rapidlyduring the early stages of shearing (e.g., Takahashi et al., 2007;Crawford et al., 2008; Ikari et al., 2009a) (Fig. 7). Previous studies of ex-perimentally sheared sediment suggest that permeability evolutionduring shearing is related to the development of shear-parallel fabricand alignment of platy clay grains (e.g. Arch and Maltman, 1990;Dewhurst et al., 1996; Haines et al., 2009). For samples from theMurototransect, the permeability of our experimentally sheared specimensfalls within the range of values for the unsheared sediment at compara-ble effective normal stresses (Fig. 5A), despite its similar clay content tothe Kumano slope apron samples (52–56 wt.% clay at Site 1173,39–63 wt.% at Kumano).

The difference in permeability behavior between the two tran-sects may be related to differences in thermal state. Along theKumano transect, we estimate that in situ temperatures at our sampledepths are b~25 °C based on bottom water temperature, heat flow,and thermal conductivity measurements (Harris et al., 2011). This isconsiderably colder than for our samples from the Muroto transect(~95–125 °C) (Shipboard Scientific Party, 2001c. Along the Murototransect, the high heat flow outboard of the trench (Kinoshita et al.,2003; Yamano et al., 2003; Shipboard Scientific Party, 2001c) drivessmectite-to-illite transformation in the incoming sediment (Steurerand Underwood, 2005; Saffer et al., 2008). In the lowermost sedi-ments at Site 1174, interlayered smectite–illite contains as much as70–80% illite, indicating that the reaction has almost reached comple-tion (Steurer and Underwood, 2005). Illitization leads to strong tex-tural anisotropy (e.g., Charpentier et al., 2003). Heated uniaxialconsolidation experiments (Hüpers and Kopf, 2009) and permeabilitytests (Kato et al., 2004) have also shown that elevated temperatureresults in enhanced consolidation and decreased porosity at a giveneffective stress, which has been linked to both enhanced fabric devel-opment and decreased permeability. We suggest that along the Mur-oto transect, fabric developed during advanced consolidation anddiagenesis (driven by higher temperatures there) results in loweroverall sediment permeability than along the Kumano transect. For

9M.J. Ikari, D.M. Saffer / Marine Geology 295-298 (2012) 1–13

these sediments, shearing has a smaller effect because a strong initialfabric is already developed during burial and consolidation at temper-atures of ~95–125 °C and this signal dominates the permeability evo-lution of the sediment even when it is sheared. A similar argumentmay explain the low frictional strength of intact specimens fromdepths>500 mbsf along the Muroto transect (Ikari and Saffer, 2011).

5.2. Spatial distribution of permeability

Using our experimental results, we estimate the in situ permeabil-ity of the mudstone matrix and sheared sediment in fault zones as afunction of burial depth (Fig. 8). For the slope sediments, we limitour extrapolation to ~2.5 km depth, which marks the maximumdepth extent to which overridden slope sediments are clearly dis-cernible in seismic reflection data (Moore et al., 2009) (Fig. 3). In ap-plying our experimental data to the active accretionary complex, wecompute vertical effective stress gradients along each transect usingshipboard bulk density measurements (Shipboard Scientific Party,2001a; Expedition 315 Scientists, 2009; Expedition 316 Scientists,2009a,b,c). For the Kumano transect, we assume hydrostatic porepressure (pore pressure ratio of λ=~0.50), based on drilling observa-tions (Screaton et al., 2009b) and the results of consolidation experi-ments on slope sediments (Dugan and Daigle, 2011; Song et al.,2011), both of which indicate well-drained conditions. The resultingeffective stress gradient is ~10 MPa/km. For the Muroto transect, weuse an effective stress gradient of 5.5 MPa/km, corresponding to avalue of λ=~0.75, based on pore pressure estimates from numerical

Fig. 8. Estimated in situ permeability profiles across fault zones. Curves representing waunsheared samples (Eq. (4)). For the megasplay fault zone, measurements at 12 MPa and 25a vertical effective stress gradient of 10 MPa/km (hydrostatic conditions). A similar projectiomeasurements at 25 MPa are projected to a fault depth of 4.7 MPa, assuming a vertical effe

models (Skarbek and Saffer, 2009) and inversion of P-wave intervalvelocities (Tobin and Saffer, 2009).

Permeability reductiondue to consolidation ismost rapid at loweffec-tive stress, and therefore should be most pronounced in the uppermost~3 km and outermost ~50 km of the forearc. Shearing significantly re-duces fault-normal permeability in the megasplay fault zone, but hasonly a small effect on permeability for sediment in the vicinity of the fron-tal thrust (Figs. 5 and 8). Shearing has little or no effect on the permeabil-ity of LSB mudstones from the Muroto transect. On the basis of theseobservations, we expect a marked permeability contrast between shearzones and the surrounding mudstones at shallow depths (b500 mbsf)and in the outer ~50 km of the forearc. For sediments at greater depthsand distances from the trench (> ~50 km), shearing is likely to be less ef-fective in reducing permeability for two reasons: (1) shear-driven per-meability reduction is highest early in the strain history; and(2) permeability reduction as a result of preferred grain alignment andrecrystallization during consolidation and diagenesis reduces the impactof shearing at greater depth.

Reduced fault-normal permeability and high permeabililty contrastbetween fault zone and wall rock sediment should facilitate trappingof fluids within the footwall, and could potentially lead to substantialfluid overpressures (e.g., Gamage and Screaton, 2006; Tobin and Saffer,2009). Low fault-normal permeability also facilitates focused fluid flowparallel to and beneath major fault zones, because localized zones ofhighly sheared material will act as effective barriers that channel flowwithin permeable damage zones (Screaton et al., 1990; Bekins et al.,1995; Caine et al., 1996; Evans et al., 1997; Saffer and Bekins, 1998;

ll rock permeability are derived from effective stress-permeability relationships forMPa are projected to fault depths of 1.2 km (A) and 2.5 km (B), respectively, assumingn is used for the frontal thrust zone (C). For the Muroto transect décollement zone (D),ctive stress gradient of 5.5 MPa/km (λ=0.75).

10 M.J. Ikari, D.M. Saffer / Marine Geology 295-298 (2012) 1–13

Wibberley and Shimamoto, 2003; Spinelli et al., 2006; Faulkner et al.,2010). Therefore, channelized flow may be expected along the megas-play fault at the Kumano transect, where the permeability difference be-tween sheared and unsheared material is highest (Fig. 8). However,drilling in this region documented no evidence of focused fluid flowalong the frontal thrust or megasplay (Screaton et al., 2009b). This ap-parent discrepancy can be reconciled if the permeability of the sur-rounding sediment is sufficiently high and the overburden issufficiently thin to allow fluid escape from within and beneath faultzones, resulting in diffuse seepage to the seafloor (e.g., Saffer andBekins, 1999). For example, vertical permeability of the sedimentsalong the Kumano transect may be up to ~5×10−16 m2 (Rowe et al.,2011), which would allow fluid flow to be dispersed over large areaseven if permeability contrasts between sheared and unsheared sedi-ment are relatively large.

5.3. Implications for pore pressure and mechanical processes

Excess pore pressures in the accretionary wedge and along its baseare two key parameters, along with the frictional strengths of thewedge and décollement, that control accretionary prism geometry(e.g., Davis et al., 1983). Therefore, differences in permeability mayultimately lead to along-strike variations in accretionary wedgetaper angle (Davis et al., 1983; Dahlen, 1990; Le Pichon et al., 1993;Saffer and Bekins, 2002, 2006). For example, if the décollement zoneexhibits low fault-normal permeability, it will act as a barrier to ver-tical flow and may result in elevated pore pressures within the under-thrust section and along the décollement itself (e.g., Gamage andScreaton, 2006; Tobin and Saffer, 2009). This would in turn reducethe shear strength of the décollement and contribute to a lowertaper angle (Kopf and Brown, 2003). The taper angle along the Mur-oto transect is lower than that of the Kumano transect (Moore et al.,2001; Kimura et al., 2007), which is consistent with both: (1) our ob-servations of consistently lower permeability at Site 1174 comparedto that at Site C0007; and (2) elevated pore pressures beneath thedécollement along the Muroto transect compared with well-drainedconditions along the Kumano transect (e.g., Screaton et al., 2002;Screaton et al., 2009b; Tobin and Saffer, 2009). However, permeabilityis one of several parameters that vary along strike, and it is likely thatthe differences in taper angle between the Muroto and Kumano tran-sects are controlled by a combination of factors that affect both drain-age state and fault zone friction coefficients.

The low permeability of material from the décollement zone andthe megasplay fault zone also hold implications for fault slip behavior.On short timescales (i.e. seconds to hours), the low permeability ofshear zones will result in undrained conditions, and thus facilitate dy-namically driven pore pressure fluctuations associated with rapidfault slip. At the onset of failure, fault slip is usually accompaniedby dilation (e.g. Teufel, 1981; Marone et al., 1990; Beeler, 2007). Thisdrives a transient decrease in pore pressure, resulting in an increase inshear strength known as dilatancy hardening (Rudnicki and Chen,1988; Segall and Rice, 1995; Samuelson et al., 2009). Because dilatancyhardening increases fault strength in response to increasing slip veloci-ty, it suppresses both the nucleation and propagation of seismic slip. It isalso hypothesized as a mechanism for slow slip events and earthquakeafterslip (e.g., Rubin, 2008; Segall et al., 2010). Dilatancy hardening iscounteracted by thermal pressurization, which occurs when frictionallygenerated heat drives thermal expansion of pore fluids within anundrained or poorly drained slip zone (Wibberley and Shimamoto,2005; Rice, 2006; Ujiie and Tsutsumi, 2010). Thermal pressurization re-sults in dynamic slip weakening and may facilitate coseismic rupturepropagation (Mase and Smith, 1987; Rice, 2006). The extent to whichthermal pressurization or dilatancy hardening dominates during slipdepends on the slip rate-dependent frictional properties and width ofthe slip zone, gouge permeability, and the extent of dilation (e.g.,Segall and Rice, 1995; Sleep, 1995; Rice, 2006; Segall and Rice, 2006).

Our data, together with careful measurements of frictional propertiesand dilation, provide key quantitative data to constrain models ofthese processes (e.g. Sleep, 1995; Rice, 2006; Samuelson et al., 2009).

6. Conclusions

We compare the permeability of unsheared consolidating sedi-ment and sheared samples of the same material from the Nankai ac-cretionary complex. We find that shearing reduces permeability byup to ~1–1.5 orders of magnitude relative to unsheared sedimentfor shallow samples (b500 mbsf) from the megasplay fault zone,frontal thrust, and slope apron along the Kumano transect. For deepersamples from the décollement zone and incoming trench sediment(660–968 mbsf) along the Muroto transect, however, the permeabil-ity of sheared samples is in the same range as that of unsheared sed-iment. We also find that permeabilities of samples from the Murototransect are lower overall than those from the Kumano transect.

Shearing should effectively reduce fault-perpendicular permeabilityalong themegasplay and frontal thrust along the Kumano transect. Thismay allow channelizing of flow in subjacent damage zones or along thefault itself and facilitate the development of excess fluid pressure in thefootwall, though we expect diffuse flow to the seafloor in areas wherethe absolute permeability of both wall rock and sheared sediment ishigh (e.g., Screaton et al., 2009b). The lack of a permeability contrast be-tween sheared and unsheared samples from the Muroto transect sug-gests that the effect of shearing on permeability is limited at greaterdepth where temperatures are higher, possibly due to fabric develop-ment and diagenesis. The low overall permeability of wall rock sedi-ments along the Muroto transect could facilitate overpressuring offluids. Permeabilities of sheared natural sediment and fault zone mate-rial of less than ~10–19 m2 along both transects should also lead toundrained behavior during slip events. The resulting pore pressuretransients may be linked to slow slip events, earthquake afterslip, orpropagation of coseismic slip.

Acknowledgements

This work was supported by NSF awards EAR-0746192, EAR-0752114, and OCE-0648331 to DS. We thank Matt Knuth for help withsamples and running experiments, and Junhua Guo for providing perme-ability data for Site C0007. We also thank Editor David Piper and twoanonymous reviewers for providing helpful suggestions that improvedthe manuscript.

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