distribution of resistive and conductive structures in nankai accretionary wedge reveals contrasting...
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Tectonophysics 611 (2014) 181–191
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Tectonophysics
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Distribution of resistive and conductive structures in Nankai accretionarywedge reveals contrasting stress paths
Marianne Conin a,⁎,1, Sylvain Bourlange b,2, Pierre Henry c, Aurelien Boiselet b,3, Philippe Gaillot d
a Universite d'Aix Marseille, CEREGE, UMR 7330, Europole de l'Arbois, 13545 Aix en Provence, Franceb Universite de Lorraine, CRPG, UPR 2300, 15 rue Notre Dame des Pauvres, 54500 Vandoeuvre-lès-Nancy, Francec CNRS, CEREGE, UMR 7330, Universite d'Aix Marseille, Saint Charles, 13331 Marseille, Franced ExxonMobil Upstream Research Company, 3120 Buffalo Speedway, Houston, TX, USA
⁎ Corresponding author.E-mail addresses: [email protected] (M. Conin), bour
(S. Bourlange), [email protected] (P. Henry), [email protected]@exxonmobil.com (P. Gaillot).
1 Now at: Universite des Antilles et de la Guyane, LARG97190 Pointe-a-Pitre, Guadeloupe, Lesser Antilles. Tel.: +590483094.
2 Now at: Universite de Lorraine, GeoRessources, UMR54506 Vandoeuvre-les-Nancy, France.
3 Now at: ENS, INRS, UMR 8538, 24 rue Lhomond, 7500
0040-1951/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.tecto.2013.11.025
a b s t r a c t
a r t i c l e i n f oArticle history:Received 29 June 2012Received in revised form 2 July 2013Accepted 19 November 2013Available online 28 November 2013
Keywords:Nankai accretionary wedgeDeformationStress pathErosionComparison logging data–core data
In this article, we study the characteristics and spatial distribution of the deformation structures along theKumano transect of theNankai accretionarywedge, and use this information to interpret the stress path followedby the sediments. Deformation structures are identified from logging while drilling (LWD) resistivity images ofthe materials surrounding the drill hole and from 3-dimensional X-ray CT-images of cores acquired during theIODP NanTroSEIZE project. The relative resistivity of the structures identified on logs and the strike, dip, and den-sity of structures identified on CT scan images are measured. The analysis of dip and strike of structures indicatesthatmost of the resistive structures identifiedon logging data correspond to compactive shear bands. Results alsoindicate that conductive structures predominate at the toe of the prism and above the main out of sequencethrust, in locations where past and recent erosion occurred. We propose several mechanisms that could explainthe relation between erosion and the absence of compactive shear bands. We conclude that sediments followeddifferent stress paths depending on their location within the wedge, and that those differences explain the dis-tribution of deformation structures within the wedge. We also show the coexistence of dilatant and compactantstructures in fault zones including the frontal thrust and mega splay fault, and we interpret the coexistence ofthese structures as a possible consequence of a transient fluid pressure.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Distributed deformation occurs at the toe of accretionary wedgesduring frontal accretion (Gonzalez-Mieres and Suppe, 2006; Henryet al., 2003; Morgan and Karig, 1995), and results in pervasive strain lo-calization structures that are observed at core and borehole scale (Karigand Lundberg, 1990; Lundberg and Moore, 1986; Maltman et al., 1993;Vannucchi and Tobin, 2000). This study examines the relationship be-tween structures identified on cores from X-ray CT scan data set and vi-sual observations, and images of the resistivity of the boreholewall fromNanTroSEIZE expeditions on the Nankai accretionary wedge. We ob-serve variations in the physical properties of these structures betweendrill sites and as a function of depth, and propose that differences instress path determine these variations.
[email protected] (A. Boiselet),
E EA 4098, Campus de Fouillole,33 684933005; fax: +590 (0)
7359, campus des Aiguillettes,
5 Paris, France.
ghts reserved.
Deformation structures on cores have been extensively studied atthe toe of the Nankai accretionary wedge. They are classified as follows:(i) shear zones (Byrne et al., 1993; Karig and Lundberg, 1990);(ii) slickenlined faults (Maltman et al., 1993), (iii) compactive shearbands (Ujiie et al., 2004), (iv) kink bands (Lundberg and Moore,1986), and (v) dewatering veins (Brothers et al., 1996). Compactiveshear bands are 1–10 mm thick roughly planar zones of semi-brittle de-formation, oblique to bedding plane and with associated porosity loss(Byrne et al., 1993; Karig and Lundberg, 1990; Ujiie et al., 2004). Theyare also distinguished by their dark color (e.g. Lewis et al., 2013). Faultsare not associated with porosity reduction, or only within a very thinlayer along the shear plane. Volume change during strain localizationin porous rocks has been taken into account in bifurcation theory(Issen and Rudnicki, 2000; Rudnicki and Rice, 1975) to explain fieldobservations and laboratory experiments on loose sandstones. Accord-ingly, structures tend to be compactive, dilatant, or neutral dependingon the location of the stress state on the yield envelope and on constitu-tive parameters (Besuelle, 2001); (Rudnicki, 2004). One result is thatcompactive shear bands can form at a greater angle than 45° from σ1,which cannot be understood with Mohr–Coulomb theory alone. Forclay-rich lithologies, relationships between the location of the stressstate on the yield surface and the sign of volumetric plastic strain havelong been considered in critical state soil mechanics (e.g., Roscoe and
182 M. Conin et al. / Tectonophysics 611 (2014) 181–191
Burland, 1968), but the characteristics (porosity variation and angle) ofthe strain localization structures have not been as thoroughly studied inthe laboratory as for sandstone. However, the characteristics of the de-formation bands observed in Nankai lead to interpret them ascompactive shear bands (Ujiie et al., 2004), and we hypothesize thatstress path and pore pressure conditions control the type of structures(compactant vs. dilatant) formed during strain localization, and theirpermeability (Bourlange et al., 2003). This assertion is also supportedby observations of structures around gallery excavations in Boom clay(Dehandschutter et al., 2004).
Conversely, planar fracture-like structures on resistivity images fromlogging while drilling were interpreted as either conductive or resistivedepending on the apparent resistivity contrast with the formation(Ienaga et al., 2006; McNeill et al., 2004). In this context it is likelythat resistive structures generally correspond to compactive shearbands and that conductive structures correspond to tensional fracturesor dilatant small faults, but this was not demonstrated, at least not atthe scale of individual structures. In the Kumano transect of the Nankaiwedge a series of boreholes drilled riserless during IODP expeditions314, 315 and 316 collected high quality fracturation measurements oncores and in situ using logging-while-drilling (LWD) technique(Kinoshita et al., 2009) (Fig. 1). Those measurements allow qualitativeassessments of deformation at four sites located at (i) the front of theprism (Site C0006), (ii) in the hanging wall of the main out-of-sequence thrust called splay fault (Park et al., 2002) (Site C0004),(iii) in imbricated thrust sheets (Site C0001) and (iv) in the KumanoForearc Basin (Site C0002).
Using borehole resistivity images and X-ray CT-scans of core, thisstudy examines deformation structures in the slope sediments and inthe shallow part of the wedge. First the correspondences between bore-hole resistive structures and core compactive shear bands, and betweenconductive structures and faults are assessed. The spatial distribution
A)
B)
Site C0004
Site C0001 500mVE = 1.05
4.0
Dep
th (
m)
2.5
3.0
3.5
SENW
IL 2675
Splay fault
Unit I: slope sediment
Unit IIaccretionary wedge
Unit IIIunderthr
sedime
Fig. 1. A. Seismic data line of the Kumano transect (Moore et al., 2009). B. Interpretation of a clo(Moore et al., 2009)). C. Interpretation of a close up on the seismic line at the deformation fron
of conductive and resistive structures within the wedge is theninterpreted as an indication of the state of consolidation of the materialduring deformation. A possible relationshipwith the distribution of ero-sion and deposition on the wedge slope — and with in-sequence andout-of-sequence thrusting — is proposed.
2. Data and data analysis
Resistive and conductive fractures are identified on LWD resistivityimages (Fig. 2A). LWD measurements, made shortly after the hole isopened with the drill bit (~few minutes for resistivity measurements)are little affected by fluid invasion and borehole instability and providereliable in situ data. At the core scale, sub-mm structures can be identi-fied fromCT scan images. After calibration of CT values into densitywithmoisture and density measurements on samples, the latter features canbe related to local density and porosity variations (Fig. 2B).
The logging while drilling induction resistivity tool (SchlumbergerGeoVISION® tool) provides measurements with 3 depths (relative tothe borehole wall) of investigation: shallow-, medium- and deep-focused buttons (corresponding to ~3 cm, ~8 cm and ~13 cm depthsof investigation respectively for a 22 cmborehole diameter). The buttonelectrodes are 2.5 cm in diameter and provide 56 measurementsaround the hole at each rotation of the tool, allowing the productionof a resistivity image of the hole. Drilling rate of penetration enabled avertical image resolution of ~5 cm. We use the resistivity images toidentify the dip, azimuth, relative resistivity (conductive versus resis-tive) and frequency of resistive and conductive structures encounteredin the logged wells. The possibility that pyrite filling could be the causeof high fracture conductivity should be discarded because pyrite ishighly visible in CT-scan data — owing to its high density of5000 kg · m−3 and high X-ray adsorption — and was almost neverfound as a fracture filling. Fluid in fractures and porosity variations in
C)Unit I : trench to slope transition
Protothrust zone
Frontal thrust
Dep
th (
km)
4.5
5.0
5.5
NW
500 mNo VE
Site C0006
4.0Site C0007
(proj)
SE
:ustnt
Unit IV: trench channel complex
Unit II: accreted trench wedge
Unit III: upper
Shikoku basin facies
se up of the seismic line above the two branches of the splay fault system (modified fromt (modified from (Moore et al., 2009)). VE = vertical exaggeration.
0 Ohm.m 3
depthmbsf
320
330
0 Ohm.m 3 Apparent dip(degree)
Site C0001B
NE S WN NE S WN
resistive structuresconductives structures
10 80 1000
1200
1400
1600
0 1 2 3 4 5 6 7
CT
co
un
t
distance
Po
rosi
ty
1.71.751.81.851.91.95
den
sity
(g
.cm
-3)
0.45
0.49
0.53
0.57
C0001F-14H-2209 mbsf
A) B)
Fig. 2.A. Example of fracture picking from logging resistivity images at Site C0001B. Tails of the tadpole on the right column (apparent dip) indicate the dip direction relative to the North.B. Example of a compactive shear band observed on a CT scan image, and the analysis of the density and porosity contrasts between the band and the surrounding rock at Site C0001F.
183M. Conin et al. / Tectonophysics 611 (2014) 181–191
compactive shear bands are themost likely causes of resistivity contrast.Structures with no resolvable resistivity contrast can only be imaged ifthey offset bedding or other structures. Regardless of resistivity con-trasts, structureswith low dip angle (b20°) are generally difficult to dif-ferentiate from beddings and thus are probably under-sampled.
A) compactive shear bands on cores at Site C0001H
depth intervals: 200 - 230 m and 270-290m
equal area
B) r
NN
Fig. 3. Lower hemispheres equal area projections of poles A. Compactive shear bands observed oand B. resistive structures observed on logging resistivity images (in blue) at Site C0001B at th
The cores are X-rayCT scannedwith amedical typeX-ray CT Scanner(GE) shortly after being recovered onboard. Volumes generated consistof a stack of 0.625 mm thick slices orthogonal to the core axis (Z) ofX-ray attenuation value (CT), with a pixel size of 0.188 × 0.188 mm inthe (X,Y) plane. Structures can be identified when they present a
equal area
esistive structures observed on logging imagesat Site C0001B
depth interval: 200 - 300m
N
n CT scan images (in black) at the 200–230 and 270–290 mdepth intervals at Site C0001He 200–300 m depth intervals.
184 M. Conin et al. / Tectonophysics 611 (2014) 181–191
density contrast with the surrounding material and/or displace layers,usually bedding, or burrows. Typically, structures with porosity lossand/or higher density have higher X-ray absorption and appear bright
Site C0004 Lo
Porosity0.4 0.6 0.8
0 - 2
305 -
N
Qua
t.P
lioce
neQ
uat.
Uni
t IIIA
Uni
t IIB
IIIU
nit I
V
100
200
300
0
Depth(m)
400
slope apron
upper accretionary prism
mass transport complexfault bounded packageunderthrust apron
0 80 160 240 320 20 40 60 80Strike (degree) Dip (degree)
Site C0006
0.3 0.5 0.7
Porosity
N
N
0 - 250
700 - 72
Log
Décollement
Plio
.M
ioce
neP
leis
toce
ne
Un
it II
IU
nit
IIU
nit
IVN
o co
res
U.I
trench slope transition Upper Shikoku Basin facies
accreted trench wedge trench wedge
400
500
600
700
800
900
100
200
300
0
Depth(m) 0 80 160 240 320 0 20 40 60 80
Strike (degree) Dip (degree)
Site C0001 Depth
(m)
Un
it I
Un
it II
Un
it II
I
100
200
300
400
500
600
0
700
800
900
No
core
s
1000
Qua
tern
ary
Plio
cène
Mio
c.
slope apron
lower accretionary prism
upper accretionary prism
0 80 160 240 320 20 40 60 80
Strike (degree) Dip (degree)
conductive fracturesresistive fractures
0.3 0.5 0.7
Porosity
interstitial porosity on sample
estimated interstitial porosity from resistivity
Fig. 4. Lithology (Kinoshita et al., 2009), porosity (Conin et al., 2011), strike and dip of structureological units or selected intervals of cores and distribution of the fractures determined from respond to major deformation zones identified from core data (Kinoshita et al., 2009).
on the images (Fig. 2B). Most bright and planar structures wider than1 mmwere identified as shear zones or compactive shear bands in thevisual core descriptions performed on board (Kinoshita et al., 2009).
gging data Cores data
0 2 4 6 8 100
4080
120160200240280320360400
Number of fracturesobserved on LWD resistivity images (10 meters intervals)
50 m
400 m
250 - 305 m
N
N
N
170 - 290 m
N
355 - 370 m
268 - 313 m 393 - 405 m
N
485 - 505 m 524 - 569 m
N
N m
470 - 900 m
N
250 - 470 m
0 m
ging data Cores data Number of fracturesobserved on LWD resistivity images (20 meters intervals)
0 5 10 15 200
4080
120160200240280320360400440480520560600640680720760800840880
N
N
0 - 200m
N
200 - 550m
550 - 1000m
N
200 -230 m270 - 300 m
faults and fracturescompactive shear bands
Logging data Cores data
0 2 4 6 8 100
4080
120160200240280320360400440480520560600640680720760800840880920960
1000
Number of fracturesobserved on LWD resistivity images (20 meters intervals)
conductive fractures
resistive fracturesunclassified fractures
s determined on resistivity images, stereographic diagram of structural orientation by lith-sistivity images for Sites C0006, C0004, C0001 and C0002. At Site C0006, blue zones corre-
Site C0002
0.3 0.5 0.7
Porosity
BSR
0 - 830 m
830 - 936 m
936- 1400 m
N
N
N
Logging data
0 2 4 6 8
0
80
160
240
320
400
480
560
640
720
800
880
960
1040
1120
1200
1280
1360
Number of fracturesobserved on LWD resistivity images (20 meters intervals)
basal (starved) basin
lower forearc basin upper accretionary prism
Un
it II
IU
nit
IV
Qua
tern
ary
Plio
c.M
ioc.
Qua
tern
ary
gap
in c
orin
gno
cor
es
Un
it I
Un
it II
100
200
300
400
500
600
0
700
800
900
1000
1100
1200
1300
1400
Depth(m)
upper forearc basin
0 80 160 240 3200 20 40 60 80
Dip (degree) Strike (degree)
conductive fracturesresistive fractures
conductive fractures
resistive fracturesunclassified fractures
interstitial porosity on sample
estimated interstitial porosity from resistivity
Fig. 4 (continued).
C0006F- 15R -1524 mbsf
10 cm
C0006E- 37X -7294 mbsf
5 cm
C0006F-22R-2591 mbsf
5 cm
A) B) C)
Fig. 5. CT scan images of Site C0006 cores. A. Example of a highly damaged core. B. Coreshowing compactive shear bands (white bands), some of which are cut by faults (bottomof the picture). C. Core showing conjugate compactive shear bands.
185M. Conin et al. / Tectonophysics 611 (2014) 181–191
Some of the observed bands change in thickness from 1 to 5 mm alongtheir length and tend to bifurcate into parallel strands (Kinoshita et al.,2009). Another type of structure appears as arrays of parallel,subvertical, planar or slightly curved sheets of 1–5 mm thickness,often sigmoidal with thin, sometime branched, endings. These struc-tures are sediment-filled veins and interpreted as fluid escape struc-tures (Kinoshita et al., 2009; Ritger, 1985). They may form from thepassage of earthquake waves through poorly consolidated sediments(Brothers et al., 1996). Some of the bright high-angle shear zones withnormal displacement may form by the coalescence of these dewateringveins, as observed at Site C0001 (Famin et al., 2008). Furthermore, someof the larger veins observed at Site C0002 also have apparent normaldisplacement (Kinoshita et al., 2009). High angle structures with smallapparent shear displacement may thus be ambiguous, but a distinctioncan generally be made between sediment-filled veins and compactiveshear bands as veins tend to form periodic patterns while compactiveshear bands are single structures that typically show a conjugate geom-etry. Open fractures appear as dark lines in the CT-images. Gasexpansion cracks are commonly observed in cores sampledwith the hy-draulic piston coring system and appear as pervasive, planar or slightlycurved, black surfaces. Natural fractures and faults also often displaylower CT count than the surrounding material, which may indicateopening (Fig. 2B). However, the fracture opening observed in both theCT scan and the cores is in great part the consequence of unloadingand does not reflect the in situ state of the fracture.
Dip and apparent azimuth in the core reference frame are deter-mined on the CT images. Azimuths can be converted to a geographic ref-erence frame when the natural remanent magnetization of the cores isknown. In practice, we used core orientation data produced on-boardfrom alternating field demagnetization data (Kinoshita et al., 2009),and only present corrected azimuths. We also verified the consistency
C0001F- 14H -2209 mbsf
5 cm
A) B)
Fig. 7. A. CT scan images of core 14H2 at hole C0001F (~209 mbsf). We observed a brightband, interpreted as compactive shear bands, cut by numerous faults. Thus those faultspostdate the formation of the band. B. The same compactive shear bands observed oncore photography. The band is dark and millimeter scale in width and is cut by a seriesof faults.
5 cm
C0004D- 49R -3 367 mbsf
C0004D-35R-1301mbsf
10 cm
A) B)
Fig. 6. CT scan images of Site C0004 cores. A. Damaged cores B. Core showing a bright banddisplaced by a shear band showing a reverse sense of shear.
186 M. Conin et al. / Tectonophysics 611 (2014) 181–191
of each structure (nature, orientations and kinematics — when it'spossible) — determined from the CT images with visual observationsmade on photography of the core and shipboard determinations alsobased on visual observations on cores (Kinoshita et al., 2009) (see com-plete database in Supplementary material, and Fig. 7). Density (ρb) iscalculated from CT counts. There is no general relationship betweenCT-scan and density because X-ray absorption at a given wavelengthdepends on chemical composition. However, it is possible to calculatedensity from CT counts for materials of identical chemical compositionor if the bulk chemical composition is correlated with the density varia-tions. This is, for instance, the case for sediments undergoing compac-tion and linear relationships may be used as an approximation tocalculate density and porosity (Ujiie et al., 2004).We calibrate a polyno-mial equation by fitting density data obtained on core samples withstandard IODP method (Blum, 1997) on CT-counts averaged over thesame interval of intact core (Gaillot et al., 2008):
ρb ¼ a� CT2 þ b� CTþ c: ð1Þ
The intact part of the core is defined as having CT-value above athreshold, which is adjusted as a function of depth. Parameters a, b andc are determined for several different lithological categories, based on vi-sual core description. We here use parameters determined for silty clayintervals (a = −7.18 × 10−5, b = 0.73194, c = 1060 kg · m−3) asthis represents the dominant lithology, and the one in which thecompactive shear bands are generally observed. The variation of densitybetween compactive shear bands and nondeformed surrounding
sediments ranges between 100 and 200 kg · m−3 (Fig. 2B) and corre-sponds to a porosity variation of 6–13% (Fig. 2B).
The sense of displacement on compactive shear bands is generallymore difficult to determine than on small faults, which may displayslickenlined surfaces. In a few cases an apparent normal or inverse mo-tion could be inferred from offset layers (Fig. 6). Compactive shearbands generally appear as early structures and may be cross cut by sev-eral generations of small faults (Fig. 7). In some cases, a fault or fracturehas formed within or along the edge of a shear band (Fig. 5B). Ascompactive shear bands are thought to represent early strain localiza-tion, occurring on the compactive yield envelope, overprinting by afault can occur on the same loading stress path when the critical stateis reached (see Fig. 9 and the discussion section). Alternatively, this su-perposition of structuresmay occur during later reactivation as a conse-quence of acquired mechanical heterogeneity.
Observations of cross cutting relationships between reorientedfaults in cores from Site C0001 and Site C0002 indicate an early com-pressive phase with thrusts striking NE–SW, consistent with accretiontectonics, followed by strike-slip and extensional deformation (Byrneet al., 2009; Lewis et al., 2013). The occurrence of extension, and its ori-entation, are roughly consistent with the present day state of stress asindicated by anisotropic strain recovery and borehole breakout analysis,that appears trench parallel at Site C0001, and trench perpendicular atC0002 (Byrne et al., 2009; Chang et al., 2010). The orientation of normalfaults at each site displays some variability. Normal faults striking E–Wappearmore common at Site C0002. At Site C0001, a majority of normalfaults strike NW–SE, but several NNE–SSW trending normal or strike-
Compactive shear bands on cores at Site C0001H depth intervals: 200 - 230 m and 270-300m
Eigen vectors
Best fit great circle: strike : 167°dip: 89.0°
equal areaN
Fig. 8. Equal area projections of poles of compactive shear bands observed on CT scan im-ages at the 200–230 and 270–290 m depth intervals at Site C0001H (black dots). The redsquares correspond to the Eigen vectors calculated from the poles of the compactive shearbands.
187M. Conin et al. / Tectonophysics 611 (2014) 181–191
slip faults are also found (Lewis et al., 2013). At Site C0004 and C0006,the same cross cutting relationships are observed (compactive shearbands, thrust faults then normal faults) but only few faults could bereoriented (Kinoshita et al., 2009).
Most compactive shear bands that could be reliably reoriented strikeE–W to NE–SW (e.g. Fig. 3 and 4). The sense of motion on individualstructures cannot be reliably inferred from orientation, but, consideringthe history of stress orientation in Nankai accretionary wedge, earlystructureswith such orientationsprobably have reverse sense. However,some of the early compactive shear bands observed in the accretionarywedge probably formed in Shikoku Basin well before accretion.Compactive shear zones as well as healed faults were observed in theinput sediments at IODP Sites C0011 and C0012 (Henry et al., 2012;Underwood et al., 2010). These healed faults are low angle and thin(1 mm maximum). The shear zones are high angle, several millimetersto cm thick, bright on the CT-images, and display relatively large dis-placements (several cm to tens of cm). These thick shear zones are pre-sumably associated with slumping events and display a wide range oforientations.
3. General relationships between borehole resistivity imaging andcore structures
Because the resolution of the CT-scan (bmm) is much better thanthe resolution of the LWD resistivity image (about 2.5 cm) it is unclearwhether the planar structures recognized on the borehole resistivityimages are the same as those observed on the CT scan images of thecores. Furthermore, structures cannot be correlated individually be-cause the resistivity images and cores were retrieved from differentdrill holes. However, the two holes are generally only a few tens of me-ters apart and it remains possible to correlate zoneswith increased den-sity of structures and compare the orientation distribution of theobserved structures. In the resistivity images the planar structures aregenerally identified as apparent discontinuities. Their resistive or
conductive character relies on a visual interpretation of small variationsof the apparent resistivity (of the order of a fewpercents) that cannot beprecisely quantified. For a first order of magnitude calculation we as-sume a linear response, such that the effect of having a conductive frac-ture in the volume explored by the electrode will be to increaseapparent conductivity by the conductivity of the fluid multiplied byfracture opening divided by the diameter of the electrode footprint(1 in.). Therefore, a fracture opening of 0.5 mm (1/50 in.) with a con-ductivity ratio between the fluid and the medium of 5 (e.g., seawater,of conductivity about 5 S · m−1 at 20 °C and a formation conductivityof about 1 S · m−1, which may be considered typical of the cases stud-ied here)would result in a 10% apparent conductivity increase. It resultsthat openings much smaller than the resolution could result in the clas-sification of a planar structure as conductive. Most natural open frac-tures identified on core CT-scans are less than 2 mm thick (this studyand Kinoshita et al., 2009), and their opening in the core may bewider than the in situ opening because of the effect of unloading andgas expansion. However it can still be seen on the resistivity images be-cause of the high resistivity contrast between fluid filled fracture andthe sediment. For compactive shear bands, we here propose an estima-tion of the resistivity contrast based on the CT-scan data. Densities of theband and the surrounding formation can be estimated by volume aver-aging of the CT-scan derived density values [Eq. (1)]. We then calculateporosity (φ) and estimate the ratio of bulk resistivity (Rb) to fluid resis-tivity (Rf) from Archie's law (resistivity index) which for our purposehere is a satisfactory approximation (Kinoshita et al., 2009):
φ ¼ ρb−ρg� �
= ρw−ρg� �
ð2Þ
and
Rf ¼ Rb � φm: ð3Þ
Core sample analysis shows that grain density, ρg, equals2650 kg · m−3; pore water density, ρw, equals 1024 kg · m−3, mequals 2.4 (Kinoshita et al., 2009). Typically, the contrast of density be-tween a compactive shear band and surrounding sediments wouldimply a resistivity variation of 30–40%. Applying the same reasoningas for open fractures, an apparent resistivity variation of 3–4% at the res-olution of the tool is expected for a 3 mm compactive shear band. How-ever, compactive shear bands associated with a lower density contrastthan 100 kg · m−3 or thinner than 3 mm are probably not seen onthe log resistivity images.
Equivalence between resistive structures and compactive shearbands, and between conductive structures and faults can also be arguedbased on the distribution of orientations over specific intervals. Struc-tures cannot be individually correlated because LWD data and corescannot be recovered from the same hole but the distance betweenholes at the same site is generally a few tens of meters. An example ofcomparison between compactive shear bands and resistive structuresis given for the 200–230 m interval and the 270–300 m interval atSite C0001. Comparison between orientation distribution resultsshows that the resistive structures and compactive shear bands bothtrend in E–W to NW–SE directions and have broadly similar geometrieswith NW dipping and SE dipping planes (Fig. 3). The differences be-tween the two data sets can be explained by the fact that the LWDdata set does not include most of the low dipping angle structures,which cannot be distinguished easily from bedding in the LWD images.
In summary, the thickest (thicker than 3 mm) compactive shearbands may be seen on LWD images and probably represent most ofthe structures interpreted as resistive fractures. On the other hand, frac-tureswith anopening of the order of only a fewhundredmicrons can bedetected as conductive fractures. Such an opening does not require fluidoverpressuring and may just result from a brittle dilatant behavior.However, many small faults do not display a distinctive density contrast
q
p’
diff.
str
ess
mean stress
K0
Critical state lin
e - Coulomb failure
yield surface
Strain localization and appearenceof compactive shear bands
yieldsurface
Consolidation increaseϕ= 60% ϕ= 40%
yield
surfa
ce
no shear bands
Critical state lin
e
mean stress
diff.
str
ess
q
p’
For normally consolidated sediments :
For overconsolidated sediments :
Fig. 9.Mean effective stress (p′) and differential stress (q) diagram showing the possible stress path creating localized ductile deformations for normally consolidate and overconsolidatesediments. The gray line is the envelope for brittle failure, the gray curve is the limit for ductile deformation onset.
188 M. Conin et al. / Tectonophysics 611 (2014) 181–191
at the resolution of the CT-scan and these are unlikely to appear on LWDimages.
4. Distribution of structures
LWD resistivity data show that conductive structures dominate inthe frontal part of theprism (Table 1, Fig. 4). Thenumber of resistive fea-tures and their percentage relative to conductive structures increase inthe upper part of the accretionary wedge at C0001 and C0002 compar-atively to the front of the prism (Site C0006) and just above the termi-nation of the mega splay fault (Site C0004) (Table 1). A similardistribution of compactive deformation band is observed on coresfrom CT scan image analysis.
Table 1Summary of the repartition of deformation along the transect.
Sites C0002 C0001 C0004 C0006
Total Accretedsediments
Number of fractures in LWD 80 48 272 153 252Number on fractures per meter 0.06 0.09 0.3 0.4 0.3Conductive 46.2% 31.2% 69.5% 88.9% 92.5%Resistive 53.8% 68.8% 28.3% 11.1% 7.5%Unclassified NA NA 2.2% 0% 0%
4.1. Site C0006 — toe of the prism
At Site C0006 at the toe of the prism, the LWD borehole crossed thedécollement and data were thus acquired in the hanging wall andfootwall. The footwall appears dominated by sands from the trench fill(Lithologic Unit IV) andwas not cored. Cores recovered slope sediments(Lithological Unit I) and accreted sediments comprising trenchturbidites (Lithologic Unit II) and Shikoku Basin hemipelagic deposits(Lithologic Unit III).
At Site C0006, 92.5% of the structures are conductive (Table 1). Thefew resistive structures are found (i) in the décollement, (ii) in majordeformation zones (Fig. 4) along with conductive ones, and (iii) in theUpper Shikoku Basin facies between 500 mand 560 m.Half of the resis-tive structures (~60%) and most (~80%) of the conductive ones below250 m trend NE–SW with an average value of N066°E, and a standarddeviation of 48°. Another population, composed exclusively of conduc-tive fractures,more common in the upper part of the borehole, between240 and 460 m, strike betweenN320°E andN360°E. Thefirst populationof fractures may correspond to compressional structures in a contextwhere the maximal horizontal stress trend NW–SE (N150°E ± 16°)(Chang et al., 2010), whereas the second population is more suggestiveof normal faulting or strike slip faulting generated by the extensionalstate of stress occurring in the shallow portion of the sediments(Byrne et al., 2009). At the base of the décollement zone (~700–720 m), the orientation of resistive structures varies significantly(Fig. 4). This could indicate changes of stress, fluid pressure or strainthrough time in that zone, possibly in relation with the seismic cycle.
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In the cores, both fractures and compactive shear bands are present.However, structures observed on CT scan images are difficult to reorientasmost of thematerial is highly fractured or tectonically brecciated, andthe coring technique (Rotary Core Barrel technique) induced a fragmen-tation of the core (Fig. 5A). Combined visual core descriptions and CTimage analyses indicate that compactive shear bands are found be-tween 240 and 560 m which correspond to the bottom of the coredhole (this study and Kinoshita et al., 2009). Many of the observedcompactive shear bands contain faults that localize within or at theedge of the band (Fig. 5B), and likely postdate the formation of thecompactive shear band. In many cases, these bands are conjugate,with angles between 45 and 60° between the bands (Fig. 5C). Most ofthem trend NE–SW, although a secondary population is also observedand trends NNE–SSW. They are thin (~1 mm) and cut by faults whichcould explain why some of them are not recognized as resistive struc-tures on LWD images.
4.2. Site C0004— splay fault
Site C0004 is located on the splay fault hanging wall and drilledthrough the splay fault zone between 258 and 308 m. Starting fromthe seafloor, 78 m of slope sediments (Lithologic Unit 1) lies on masstransport deposits and accreted sediments (Unit II) forming the thrusthanging wall (Kinoshita et al., 2009). Lithologic Unit III is defined as afault bounded package of hemipelagic sediments bearing ash layersthat correspond to the splay fault zone interval. These units are thrustover 1.6 Ma sediments (Unit IV) that were deposited in a slope basin.
At Site C0004 the core recovery was very low and the cores werehighly damaged (Fig. 6A). 89% of the structures are conductive andthe few resistive structures are found in the fault-bounded package de-fining the splay fault zone and in the upper part of the hole (above130 m). Most of the conductive structures (~85%) and half of the resis-tive structures (~55%) strike NE–SW (N20°E to N60°E), close to perpen-dicular to the maximum horizontal stress (N136° ± 15°) (Chang et al.,2010). Therefore they probably mostly correspond to thrusts, but forsome higher angles E–W structures that could be dominated by astrike-slip component. A second population of resistive structuresstrikes NW–SE (N290°E to N350°E) (Fig. 4). As at Site C0006 the trendsof the resistive structures in the splay fault zone vary significantly. A fewcompactive shear bands are observed on CT scan images and are oftencut by faults. The high fracturation of the core impeded a correct orien-tation of the structures observed on CT scan images, and only few struc-tures (15) have been reoriented from two specific zones (between170 m and 290 m, in the hanging wall and the bounded package ofthe splay fault; and between 355 and 370 m, in the footwall of thefault). The compactive structures mainly trend NE–SW in the footwall,roughly orthogonal to the convergence direction. Some compactiveshear bands are observed in the fault bounded package, but this zoneis too damaged to make any orientation analysis.
4.3. Site C0001— imbricated thrust sheet
Site C0001 was drilled landward of Site C0004 on the splay thrusthangingwall at a locationwhere slope sediments form a thicker deposits(210 m) than at C0004 (Kinoshita et al., 2009). A 1.5 Ma hiatus is foundbetween the accreted sediments and the slope apron but the porosity–effective stress curves do not indicate erosion at this site (Conin et al.,2011). The logging while drilling borehole went much deeper that thecored hole, which stopped at 457.8 mwithin accreted sediment of dom-inantly hemipelagic lithology.We observe numerous resistive structuresfrom ~200 m to 550 m and numerous compactive shear bands from thetop of the accreted sediments at ~200 m down to the base of the coredinterval (~450 m) (Fig. 4). The number of compactive and resistivestructures is significantly higher than in the twoprevious holes. Between200 and 450 m depths, most of the resistive structures and compactiveshear bands trend NE–SW (N60–80°E), almost perpendicular to the
maximum horizontal stress (N156°E ± 16°) (Chang et al., 2010). Resis-tive structures are also observed in the slope sediments (from 60 m to200 m) with variable trend and dip. These structures are unlikely to becompactive shear bands because very few were observed on cores ofslope sediments. Many of these resistive structures dip gently (b20°)and could be thin beds such as thin ash layers. High-angle ones couldbe dewatering veins. Conductive structures in theupper part of the bore-hole (down to 550 m depth) are also dominantly NE–SW. Consideringthat the state of stress recorded by sediments at this site has been eithercompressive with σ1 horizontal oriented NE–SW (nearly normal to thetrench), or extensive with σ3 parallel to the trench (Byrne et al., 2009;Lewis et al., 2013), those conductive structures likely correspond tothrusts. Their dips, relatively steep for thrust, maybe the consequenceof progressive tilting of features during accretion processes. Drilling in-duced tensile fractures are occasionally observed, notably in the 80–110 m interval. Below 550 m most of the structures are conductiveand, below ~630 m depth an abrupt change of structures strike anddip is observed with most structures sub-vertical and parallel to themaximum horizontal stress. These structures appear as bands with athickness of several pixels on the LWD images, implying either wideopenings or damaged zones on the borehole walls. This part of theLWDboreholewasdrilledwith high annulus pressure,maintained by re-peated barite mud pumping into the open borehole, and this resulted inthe suppression of the borehole breakouts. The occurrence of verticalconductive structures is presumably a result of the formation of tensilefractures and of the opening of favorably oriented preexisting fractures.Naturally elevated pore-fluid pressure in the formation is also suspectedbelow 450 m depth (Conin et al., 2011). Remarkably, high-angle con-ductive or neutral structures oriented N100–120°E are observed, mostlyfrom 550 to 620 m depths. These may correspond to right-lateral strike-slip faults.
In the cores, we observe high dip angles coalescence structuresinterpreted as veins that could correspond to some steep resistive struc-tures observed on the log (Fig. 4). The deformation structures werereoriented in two intervals (between 200 and 230 mbsf, and between270 and 300 mbsf). They strike dominantly in a close to E–W direction(Fig. 8). The poles are distributed along a great circle with two mainclusters along this great circle, that suggests the existence of two conju-gate populations of structures, which is also confirmed by the direct ob-servations of conjugate structures on CT scan images. The calculation ofthe Eigen vectors of thepoles provides 3major directions correspondingto the three principal axes of the deformation tensor. Those directionstrend N342°E, plunging 76.0°; N167°E plunging 14.0°; and N076°Eplunging 1.0°. Because the data show two conjugate populations ofstructure, the Eigen vectors can be interpreted in terms of stress tensoraxis. The intermediate principal stress (σ2) responsible for this defor-mation must be on the structure planes and thus correspond to the di-rection N076°E plunging 1.0°.
Using the same reasoning than for conductive features, the maxi-mum principal stress (σ1) must be close to horizontal and thus corre-spond to the vector trending N167°E and plunging 14.0°. The minimalprincipal stress (σ3) is thus trending N342°E and plunging 76.0°. Thisestimation of the stress tensor responsible for the formation of thosecompactive shear bands corresponds to a compressive stress regimewhich is also consistent with thrust offsets observed on some deforma-tion bands, on the visual descriptions of the cores (Kinoshita et al.,2009). The borehole breakout analysis indicates a present state of stressrather in favor of a strike slip regime at that depth (Chang et al., 2010).This suggests that a permutation of the σ2 and σ3 axes occurred be-tween the formation of those structures and the present state.
4.4. Site C0002 — forearc basin
Site C0002 was drilled in the Kumano forearc basin and comprisesthree Lithological Units (Kinoshita et al., 2009). Units I and II correspondto the Quaternary fill of the forearc basin (1.6 Ma–present), Unit III to
190 M. Conin et al. / Tectonophysics 611 (2014) 181–191
slope sediments deposited on the accretionary wedge, and Unit IV toaccreted sediments deposited in a trench setting. At Site C0002, resistivestructures dominate in the upper part of the borehole and in theaccreted sediments (Table 1; Fig. 4). Resistive structures aremost abun-dant in the starved basin sediments of Unit III, where the highest occur-rence of dewatering veins is found. As the resistive structures have highdip angles, theymay be related to those ~N060 dewatering veins. In thefore-arc basin sediment the distribution of structure orientation is rath-er scattered. In the accretionary wedge, resistive structures strikeN040°E to N080°E with variable dips (20°–80°). These could be relatedto early compressive structures in the accretionary wedge or to normalfaults associated to the recent phase of trench normal extension ob-served on seismic profiles at this site (Gulick et al., 2010), in the corestructural data (Kinoshita et al., 2009), and consistent with the orienta-tion of borehole breakouts of N41°E ± 14° (Chang et al., 2010). A groupof conductive N130°E structures at about 1200 m below seafloor maycorrespond to an earlier episode of trench perpendicular extension orto strike-slip faults.
5. Summary of results and discussion
From the toe to the inner part of the wedge, we observed an ultradominant set of conjugate structures trending NE–SW. This set com-prises conductive and resistive structures of the same orientation butin varying proportions. The proportion of resistive versus conductivefeatures substantially differs from the front to the back of the wedge.
Orientations ofmost of those structures are consistentwith an inter-pretation as reverse faults or shear bands within a compressional stateof stress induced by the shortening due to accretion, except for threezones. In the deepest part of site C0001(below 550 mbsf), a set of con-ductive structures is oriented N–S and dip very steeply. Those featuresare likely drilling induced features (as previously proposed inKinoshita et al., 2009), related to a drastic increased in drilling fluidpressure. In the slope sediments of this site, the range of structure orien-tation is wider than elsewhere, with no specific value. This could be re-lated to the fact that we are in the slope sediments, with an extensionalstate of stress (Byrne et al., 2009), and influenced by slope instability(Strasser et al., 2011).
At Site C0006, a set of faults trend NW–SE and dip toward the SouthWest. This direction is consistent with directions of normal faults ob-served on cores (Byrne et al., 2009) and could be related to the presenceof a slump scar observed at the SouthWest of the drilling Site interpretedas the signature of a seamount subduction (Moore et al., 2009).
Observations regarding the spatial and orientation distributions ofresistive structures in terms of abundance and orientation confirmthat they are generally representative of compactive shear bands, al-though compactive shear bands observed in cores are not always seenat the resolution of the resistivity image. Another type of dense struc-ture, akin to dewatering veins, may be tracked form the cores to theLWD images. Dewatering veins, sometime display some shear compo-nent, and normal shear displacement. They presumably formed at ashallow level from earthquake shaking but could have been buried sub-sequently and retained their low porosity high resistivity character atdepths.
Setting aside the dewatering veins, there are relatively lesscompactive shear bands in the basins and slope sediments than in ac-creted material. It is unclear whether these observations result fromlithological difference, or from mechanical anisotropy acquired duringcompaction. By mechanical anisotropy, we here mean that conditionsfor localization may be different in a material deforming by nearly uni-axial compaction with vertical maximum principal compressive stress(close to K0 stress state), and in a material that experienced a permuta-tion of principal stress axis, withmaximumprincipal compressive stressparallel to the compaction fabric.
The results show that resistive structures and compactive shearbands are preferentially found in the inner wedge and in the landward
edge of the outer wedge (Sites C0001 and C0002). At Sites C0004 andC0006 (splay fault/toe of prism), the lowoccurrence, or absence of resis-tive structures is striking. At Site C0004, only very few compactive shearbands have been found in cores. At Site C006, compactive shear bandscan be observed, but they are less numerous by meter interval than atSite C0001, some of them contain faults, and are in general less densethan the structures at Site C0001. The overprinting of compactiveshear bands by brittle dilatant faults may hide them from the resistivityimaging LWD tool. One particularity of those two sites is that erosion of~60 m, and ~300 m occurred at the top of the sediments within the last0.5 Ma (Conin et al., 2011). There is also evidence of past erosion, occur-ring between the accretion and the deposition of the slope sediments, atSite C0004 (Conin et al., 2011). Several processes may account for a re-lationship between erosion and the absence of compactive shear bands.Compactive shear bands are expected to form on the cap of the yieldsurface, implying a loading path with increasing effective mean stress.They are not expected to form if failure occurs on the critical state lineor on a brittle failure envelope. The recent erosion left the sedimentsoverconsolidated (Saffer et al., 2011) and overconsolidated sedimentsshow brittle behavior, and shear fractures in brittle materials can con-tinue to dilate during post peak shearing (Nygård et al., 2006). Stress re-lease caused by erosion would favor dilatancy during deformation.Compactive shear bands remaining from earlier deformation could behidden on the low resolution LWD resistivity images by the presenceof dilated fractures either parallel to them or cross cutting them. Con-trasting stress paths may thus explain why conductive structures dom-inate at Site C0004 (splay fault), and resistive ones are common at SiteC0001 (imbricated thrust sheet) within the same lithology. There are,however, different possible ways to obtain this result. (1) Erosion atSite C0004 may have imposed an unloading path for most of the defor-mation history and thus prevented formation of compactive shearbands, except at a very early stage. (2) Compactive shear bands mayhave formed only in moderately compacted sediments, at porosities ofmore than 50% (or interstitial porosity of more than 40%, taking into ac-count smectite interlayer water (Conin et al., 2011)), and be subse-quently removed by erosion at Site C0004. However, the porosity inthe lower part of the cored interval at Site C0001, where compactivestructures are still observed, is comparable to the porosity of the accre-tionary wedge material at Site C0004, although at a shallower depth.This leads to favor the first explanation. At Site C0006 (toe of prism),comparison with observations at Site 1174 on Muroto transect —
where deformation bands were mostly found in the hemipelagites ofthe distal turbidite sequence at porosities of 50% or more (Ujiie et al.,2004) — leads to consider either interpretation as valid.
At Site C0002 (forearc basin) the resistive and compactive structuresin the accretionary wedgemay have formed as early compressive struc-tures, or as late extensional structures during or after the deposition ofthe forearc basin. Burial beneath the forearc basin presumably broughtthe accretionary wedge sediment back to a normally consolidatedstate, and loading path. However, there are few resistive structures inthe forearc basin sediments and their orientation preclude that theyare recent normal faults. It appears more likely that the resistive struc-tures in the accretionary wedge are early structures, unless lithologicaldifference between wedge and forearc sediment dictates a differencein deformation style. Then thepresence of compactive shear bands in ac-creted sediments of 30–40% porosity may result from porosity loss fromburial below the forearc basin, after the formation of the shear bands.
In major fault zones (including splay fault and décollement) bothcompactive and dilatant structures are found. This was already noticedon the Muroto transect (Bourlange et al., 2003; Ujiie et al., 2003) andmay be explained by (1) preservation of early structures formed duringthe initiation of the fault zone, (2) fluid pressure cycling resulting in de-formation, or (3) the coexistence of both mechanisms when deforma-tion occurs near the critical state.
Compactive shear band are most abundant in the upper part of theaccreted sediment. This may be understood if a minimum “critical”
191M. Conin et al. / Tectonophysics 611 (2014) 181–191
porosity is required for the formation of the compactive shear bands, asobserved in sandstones (Besuelle, 2001). Alternatively, this may be ex-plained if deformation becomes undrained with increasing depth, andmean stress increase during tectonic compression and/or if burial iscompensated by a fluid pressure rise (Shi and Wang, 1985) (Fig. 9).
6. Conclusion
Differences in the compactive/dilatant style of dominant structuresdepending on their location in the accretionary wedge suggest that ac-creted sediments followed different stress paths depending on their lo-cation. Erosion of the hangingwall of the frontal thrust and of the out ofsequence thrust that evolved into the splay fault may explain the ob-served distribution. This also suggests patterns of erosion and deposi-tion may be persistent during the growth of an accretionary marginand, in the long term, result in the exhumation of overconsolidated sed-iment above active thrusts.
Acknowledgments
The authors are grateful to the ship's crew, drilling crew, and staffand technicians aboard the D/V Chikyu. Reviews from K. Ujiie andT. Byrne greatly improved the finalmanuscript. This research used sam-ples and data provided by the IntegratedOcean Drilling Program (IODP)and was funded by CNRS/INSU.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.tecto.2013.11.025.
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