texture development in naturally compacted and experimentally deformed silty clay sediments from the...
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Tectonophysics 636 (2014) 125–142
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Tectonophysics
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Texture development in naturally compacted and experimentallydeformed silty clay sediments from the Nankai Trench and Forearc, Japan
Kai Schumann a,⁎, Michael Stipp a, Bernd Leiss b, Jan H. Behrmann a
a GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstr. 1-3, D-24 148 Kiel, Germanyb Geoscience Centre, University of Göttingen, Goldschmidtstr. 3, D-37077 Göttingen, Germany
⁎ Corresponding author. Tel.: +49 431 6002418.E-mail addresses: [email protected] (K. Schuma
(M. Stipp), [email protected] (B. Leiss), jbehrmann@geomar
http://dx.doi.org/10.1016/j.tecto.2014.08.0050040-1951/© 2014 Elsevier B.V. All rights reserved.
a b s t r a c t
a r t i c l e i n f oArticle history:Received 29 October 2013Received in revised form 14 August 2014Accepted 16 August 2014Available online 26 August 2014
Keywords:Crystallographic preferred orientationSynchrotron texture analysisSoft sedimentNankai Accretionary PrismSubduction
The petrophysical properties of fine-grained marine sediments to a large extent depend on the microstructureand crystallographic preferred orientations (CPOs). In this contributionwe show that Rietveld-based synchrotrontexture analysis is a new and valuable tool to quantify textures ofwater-saturatedfine-grained phyllosilicate-richsediments, and assess the effects of compaction and tectonic deformation.We studied the CPO of compositionallyalmost homogeneous silty clay drillcore samples from the Nankai Accretionary Prism slope and the incomingPhilippine Sea plate, offshore SW Japan. Basal planes of phyllosilicates show bedding-parallel alignment increas-ing with drillhole depth, thus reflecting progressive burial and compaction. In some samples calcite and albitedisplay a CPO due to crystallographically controlled non-isometric grain shapes, or nannofossil tests.Consolidated-undrained experimental deformation of a suite of thirteen samples from the prism slope showsthat the CPOs of phyllosilicate and calcite basal planes develop normal to the experimental shortening axis.There is at least a qualitative relation between CPO intensity and strain magnitude. Scanning electronmicrographs show concurrent evolution of preferred orientations of micropores and detrital illite flakes normalto axial shortening. This indicates that the microfabrics are sensitive strain gauges, and contribute to anisotropicphysical properties along with the CPO.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Sediment packages in accretionary prisms that are offscraped fromthe downgoing plate in subduction zones undergo considerabledeformation while remaining at a low degree of lithification. Thecomposition, structure and microfabrics of the sediments constrain theacoustic properties, themechanical behavior and the hydrogeological re-gime of accretionary prisms. Especially anisotropy effects are importantfor an improved understanding and interpretation of reflection seismicimages, key petrophysical parameters, and the permeability of fluidadvection pathways. Viewed in a wider context, the development ofmicrofabrics has an important bearing on the physical properties ofreservoir rocks and geological barriers especially defined by fine-grained sediments.
Published studies show that deformed sediments are highly aniso-tropic regarding acoustic velocity, permeability, and shear strength(e.g. Bennett et al., 1981; Fawad et al., 2010; Jones, 1994; Rai andHanson, 1988; Voltolini et al., 2009). Microfabrics, especially preferredalignment ofminerals, control this anisotropy,which in turn has implica-tions for seismic and hydrogeological modeling (e.g. Johansen et al.,
nn), [email protected] (J.H. Behrmann).
2004; Rai and Hanson, 1988; Voltolini et al., 2009). Fabric genesis alsochanges the frictional behavior of clay-bearing sediments (e.g. Jones,1994; Scholz, 2002) the most important factor being grain alignment(e.g. Johansen et al., 2004; Sondergeld and Rai, 2011; Voltolini et al.,2009). Crystallographic preferred orientation (termed CPO or texture inthe following text) of clay minerals is a result of burial and compactionof sediments (e.g. Bowles et al., 1969; Fawad et al., 2010; Mitchell,1956; O'Brien and Slatt, 1990) or tectonic shearing (Buatier et al., 2012;Janssen et al., 2012; Schleicher et al., 2009; Wenk et al., 2010a).Phyllosilicates become aligned perpendicular to the maximum shorten-ing direction during compaction and diagenesis (e.g. Kawamura andOgawa, 2004). Because of crystallographically controlled differences insurface charges freshly sedimented clays form cardhouse or honeycombstructures (e.g. Bennett et al., 1991;Mitchell, 1956), dominated by edge-to-face grain contacts, and explaining the initial high porosity of clays,muds and mudstones. During burial, compaction or tectonic shearing,edge-to-face contacts are replaced by repellent face-to-face contacts dur-ing the evolution of CPO and porosity reduction (e.g. Carson et al., 1982;Kawamura and Ogawa, 2004). This is the most dramatic changemicrofabrics and textures of sediments undergo. Principal effect is areduction in stiffness (Bennett et al., 1991). Furthermore, pore space col-lapse causes overpressures under natural conditions (e.g. Bennett et al.,1981; Brace, 1978; Carson et al., 1982; Kawamura and Ogawa, 2004),leading to a further decrease in friction and shear strength of sediments
126 K. Schumann et al. / Tectonophysics 636 (2014) 125–142
(e.g. Bangs et al., 2009; Brace, 1972;Moore et al., 1995; Rice, 1992; Tobinand Saffer, 2009). Understanding and quantifying processes governingfabric and texture formation are, therefore, of paramount importance.
Scanning electron microscopy (SEM) has greatly aided the charac-terization of microfabrics of fine-grained sediments (e.g. Agar et al.,1989; Fawad et al., 2010; Janssen et al., 2012; Krinsley et al., 1983;Prior and Behrmann, 1990a,1990b), but essentially remains a qualita-tive tool. X-ray texture goniometry (XTG) (e.g. Aplin et al., 2006;Baker et al., 1969; Behrmann and Kopf, 1993; Curtis et al., 1980; Hoet al., 1999; Kopf and Behrmann, 1997; Oertel, 1983; Oertel andCurtis, 1972; Schleicher et al., 2009; van der Pluijm et al., 1994) hasthe potential of overcoming these limitations, but full texture character-ization is difficult unless applied to well-lithified sedimentary rocks.Pole figures usually show maxima of poles to [001] in the direction ofmaximum shortening, be it by uniaxial compaction (e.g. Bennett et al.,1981; Bowles et al., 1969; Vasseur et al., 1995) or tectonic shearing(e.g. Janssen et al., 2012; Schleicher et al., 2009; Wenk et al., 2010a,2010b). In soft and unconsolidated, water-rich and polymineralicsediments, however, both techniques are difficult to apply because ofsmall grain size, poor crystallinity, low scattering, high stackingdisorder, broadening of diffraction peaks, and complex compositions.Additionally, porous sediments pose big problems with regard tosample preparation (e.g. Behrmann and Kopf, 1993).
During recent years, synchrotron techniques for texture analysishave become available (e.g. Lonardelli et al., 2005; Wenk et al., 2008,2010a). Because of high brilliance and high energy, low-scattering ma-terials can be investigated successfully. The high energy enables samplepenetration of several centimeters, permitting the study of entire sam-ple volumes instead of the thin surface layers investigated by XTG. Fur-ther, image plate detectors enable the determination of complex andcomprehensive texture information within a short time. Synchrotronradiation is hardly affected by water, allowing the measurement ofwet samples in “as-is conditions” without additional sample prepara-tion and drying. Hence, this method allows to minimize modificationsof the material properties by sample preparation.
Exploiting the advantages of synchrotron radiation, we have investi-gatedmicrofabrics and CPOs of a sample set of variably compacted siltyclay sediments and also of experimentally deformed samples from theNankai accretionary prism and the incoming Philippine Sea plateoffshore Japan. The main questions addressed in this study are:(1) how successfully can texture development in fine-grained deep-sea sediments be assessed quantitatively using synchrotron radiation?(2) In which way do the results reflect active margin compaction andtectonic deformation, and (3) to which degree can these naturalprocesses be reproduced by triaxial deformation experiments in thelaboratory?
2. Geological setting and sample description
At the Nankai convergent margin offshore SW Japan, the PhilippineSea Plate is subducted beneath the Eurasian Plate with a rate of~4 cm/a (Seno et al., 1993). At the plate boundary, a thick sequence ofpelagic sediments belonging to the Shikoku Basin on the incomingPhilippine Sea Plate (Ike et al., 2008a), is overlain by sand-rich trenchsediments (Moore et al., 2009; Tsuji et al., 2011). The upper part ofthe trench fill is tectonically accreted to the overriding Eurasian Plate(Fig. 1). Within this accretionary complex, several out-of-sequencethrust faults accommodate horizontal compressive deformation (Songet al., 2011). NanTroSEIZE drilling focuses on the Kumano transect,south of Kii Peninsula (Honshu island), located in the 1944 Tonankaiearthquake rupture area (Baba and Cummins, 2005; Park et al., 2000).Perpendicular to the trench, a geological transect of 13 drillsites withmultiple holes was drilled during several IODP expeditions. This studyfocuses on sites drilled during IODP Expeditions 315 (Holes C0001E,C0001F and C0001H, see Expedition 315 Scientists, 2009), 316 (HolesC0004C, C0006E, C0007C and C0008A; see Expedition 316 Scientists,
2009a, 2009b, 2009c, 2009d; Tobin et al., 2009), and 333 (HolesC0011D, C0012C and C0012E; see Expedition 333 Scientists, 2012a,2012b) (Fig. 1). IODP Expeditions 315 and 316 investigated theaccretionary prism, while IODP Expedition 333 was dedicated to theincoming plate sediments. In the following, we briefly summarize theinformation relevant to our study given by Expedition 315 Scientists(2009), Expedition 316 Scientists, 2009a, 2009b, 2009c, 2009d;Expedition 333 Scientists, 2012a, 2012b and Tobin et al. (2009). Thesample material used in this study is listed in Table 1.
Samples from two sites in the Shikoku Basin on the incoming platewere analyzed. Site C0012, the most seaward one, was drilled at thecrest of a basement knoll, whereas Site C0011 is located closer to thetrench on the northwestern flank of a prominent bathymetric high(Ike et al., 2008b, Fig. 1). The sediments are silty clays, clayey silts andclays intercalated with volcanic ash layers.
Sites C0006, C0007 and C0008 cored during IODP Expedition 316 arelocated seaward of a major fault system within the accretionarycomplex on the overriding plate, the so-called megasplay fault (Mooreet al., 2009, Fig. 1B), while Sites C0006 and C0007 are located in azone of active deformation at the accretionary prism toe. Samplesfrom both sites are trench and trench-wedge sediments, mainlyfine-grained sand, silty sand and silty clay. Site C0008 is located in thefootwall of the megasplay fault. The dominant lithology there is siltyclay and minor volcanic ash. Site C0004, cored during IODP Expedition316, and Site C0001, cored during IODP Expedition 315, were drilledinto the hanging wall of the megasplay fault, consisting mainly of siltyclay from the slope apron facies. At Site C0001 these sediments areunderlain by accretionary prism sediments, also mainly bioturbatedsilty clay to clayey silt.
Overall, samples were cored from drillhole depths between 28.9 and522.9 m below sea floor (mbsf). Natural (experimentally undeformed)core samples as well as experimentally deformed samples were usedfor texture investigation in this study. Experimental samples were de-formed under consolidated-undrained conditions in a triaxial apparatus(for further details on the experimental deformation see Stipp et al.,2013).
3. Methods
3.1. Smear slide point counting
For a first composition analysis and especially the determination ofthe ash and biogenic material contents, smear slide point counting on18 samples was conducted. After drying of the dispersed samples onmicroscopic slides using a heating plate (~60 °C), Cargille Meltmountwas added, and the sample was then covered by a cover slip. For thepoint counting, an automatic precision stepping stage device (ConwyValley Systems Limited) mounted onto a ZEISS Axioskop microscopewas used to provide a statistical distribution of ~400 counting pointsacross each smear slide. Five component groups are distinguished.Agglomerates of minerals are named “lithics” and volcanic glass shardsare named “ash”. Minerals, mainly quartz, feldspar and mica aresummarized as “minerals”, foraminifera, diatoms, sponge needles and ra-diolarians are summarized as “biogenic material”. Material, which couldnot be resolved due to its small grain size, is termed “submicroscopics”.To provide a rough estimate, submicroscopics are taken to be more orless equal to the clay content of the samples.
3.2. Shape preferred orientation (SPO) analysis
Preferred long axes orientations of minerals and pores were deter-mined on backscattered and secondary electron (BSE and SE) imagesusing a JEOL JXA 8200 electron microprobe. As we used the microprobefor our study preferentially in high resolution imaging mode in analogyto a scanning electron microscope (SEM), we term these investigationsSEM analysis in the following. Samples were dry cut, freeze-dried, and
A
B
~4.1–6.5 cm/y
135°E 136° 137° 138°
34°
33°
Kii Peninsula
km
0 50
Nankai
Trough PhilippineSea Plate
Pacific Plate
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Site C0001 Sites C0004, C0008
Sites C0006, C0007
Site C0011
Site C0012
Philippine Sea plate(Shikoku Basin)
0 10km
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C0001 C0004C0006, C0007
C0011C0012
Kumano Basin
Old accretionary sediments
Megasplay fault
Subducting Philippine Sea PlateDécollement
Megasplay fault zone SENW Zone of frontalaccretion
C0008
Slope sediment
Dep
th (k
m) Shikoku Basin sediments
−3000
−2000
−1000
0
1000
2000
3000
m
Fig. 1. (A) Geographical position and tectonic setting of the Nankai Trough (inset) and detailed bathymetric map (Becker et al., 2009) with the IODP drill sites in the Kumano Basin SE ofHonshu Island. The arrow indicates the plate convergence direction. (B) Seismic cross section along the transect of the drill sites with tectonic interpretation (after Kinoshita et al., 2012).IODP Sites C0001, C0004 and C0008 are located on both sides of theMegasplay Fault, Sites C0006 and C0007 are at the accretionary prism toe, and IODP Sites C0011 and C0012 are locatedon the incoming plate.
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the water-free pore space was stabilized by Araldite 2020 Epoxy Resinunder vacuum using a water-jet vacuum pump. This preparationmethodminimizesmicrostructural modifications by swelling or drying.To further prevent swelling of the freeze-dried clay minerals, ethanolwas used as lubricant and for cooling during grinding and polishing(see Prior and Behrmann, 1990a).
For SEM analysis, an acceleration voltage of 20 kV was applied toachieve a high resolution down to a grain size of ~1 μm. Detailed BSEimages (2000 times magnification, ~200 μm × ~180 μm in size) wereanalyzed. The long and short axes of the investigated mineral grains(mainly detrital illite) and poreswere determined using the image anal-ysis software “Scion Image” (Scion Corporation 2000–2001). Illite grains(up to 40 μm in length) were identified bymanual selection, while porespace was separated from other phases and the fine-grained matrixbased on gray-scale value thresholding (see Fig. 2). The semi-automatic thresholding procedure was controlled by energy dispersiveX-ray (EDX)microanalysis. EDX also enabled to identify mineral phasesduring imaging and shape analysis. Single mineral grains down to agrain size of 2 μm could be analyzed allowing for the differentiation ofsmectite and illite from feldspar, quartz and calcite. Fine grained“matrix” minerals (less than 1 μm) could not be analyzed by EDX.
For the BSE imageswe used a constant threshold value of 220withina 256 gray-value range to separate the pore space (Fig. 2). Further image
processing largely followed the steps described byWorden et al. (2005).Preferred orientation can qualitatively be recognized in binary images(Fig. 2). Long axes orientations of pores andmineral grainswere plottedin rose diagrams (Fig. 3) using the “StereoNett” software package.
3.3. Synchrotron texture measurements
Hard synchrotron X-ray diffraction was applied at beam line W2 ofthe HASYLAB X-ray wiggler hall operated by Helmholtz-ZentrumGeesthacht (DORIS W2) at the German Electron Synchrotron (DESY)source in Hamburg. Relatively low absorption coefficients of hardsynchrotron radiation in condensed matter allow to penetrate sampleswith a thickness of several centimeters. In contrast to neutron radiation,porewater contents ranging between 30% (Sample 316-C0006E-30X-1)and 65% (Sample 315-C0001E-11H-1) did not affect themeasurements.Additionally, synchrotron diffraction results in a relatively highresolution of 2Θ angles. This is a significant advantage for the analysisof mineralogically complex mud and mudstone samples.
To keep mechanical manipulations of the sample material small,we developed a cutter to punch cylindrical samples of 17 mmdiameter and ~18 mm length from the drill core and to transferthem into acrylic glass sample holders. The sample holders werethen sealed to avoid desiccation. The cylindrical sample shape
Table 1Sample list.a
Sample name (IODP) Subsamples Depth [mbsf] Description
Site C0001 (accretionary prism, hanging wall of the megasplay fault, landward of the megasplay fault)315-C0001E-11H-1 K003, K005 90.6 Slope apron facies, silty clay to clayey silt with volcanic ash, silt, and fine sand315-C0001F-14H-3 210.9 Upper accretionary prism facies, silty clay to clayey silt with rare volcanic ash, silt, and silty sand315-C0001H-19R-2 398.2 Upper accretionary prism facies, silty clay to clayey silt with rare volcanic ash, silt, and silty sand
Site C0004 (accretionary prism, hanging wall of the megasplay fault, landward of the megasplay fault)316-C0004C-8H-2 K004, K010 65.3 Slope apron facies, greenish gray silty clay, substantial component of calcareous nannofossils
Site C0008 (accretionary prism, footwall of the megasplay fault, seaward of the megasplay fault)316-C0008C-7H-8 K015, K018 61.0 Slope sediments, silty clay with substantial component of calcareous nannofossils, biogenic debris and volcanic ash316-C0008A-9H-3 K006, K014 76.1 Slope sediments, silty clay with substantial component of calcareous nannofossils, biogenic debris and volcanic ash
Site C0006 (accretionary prism toe, seaward of the megasplay fault)316-C0006E-8H-1 K007, K009 48.2 Sand-dominated trench wedge facies, fine-grained sand with secondary quartz and feldspar316-C0006E-20X-2 K019 128.0 Sand-mud trench wedge facies, fine-grained sand, silty sand, and silty clay316-C0006E-30X-1 G0001 221.3 Mud-dominated trench wedge facies, silty clay and minor normally graded silty clay, sand, and rare ash
Site C0007 (accretionary prism toe, seaward of the megasplay fault)316-C0007C-7X-1 K012, K016 62.9 Slope apron facies, fine to very coarse grained sand, dominantly of metamorphic and volcanic lithic fragments
Site C0011 (incoming plate, seaward of the accretionary prism, flank of a bathymetric high)333-C0011D-26X-1 206.2 Shikaku Basin sediments, silty clay, clayey silt, and clay with abundant thin (b50 cm) intercalations of volcanic ash333-C0011D-41X-4 316.0 Shikoku Basin sediments, bioturbated mudstone with minor occurrence of altered volcanic ash layers
Site C0012 (incoming plate, seaward of the accretionary prism, crest of a basement knoll)333-C0012C-4H-5 28.9 Shikoku Basin sediments, clay to silty clay333-C0012C-9H-6 75.9 Shikoku Basin sediments, bioturbated silty clay with minor contributions of volcanic ash333-C0012C-15H-5 121.6 Shikoku Basin sediments, bioturbated silty clay with minor contributions of volcanic ash333-C0012E-3X-4 522.9 Transition between Shikoku Basin sediments and igneous crust, bioturbated silty clay, relative increase of volcanic glass
a The samples investigated in this study are listed in the table, sorted by drill sites and depth. Lithology data are taken from the Expedition 315 Scientists (2009), Expedition 316 Scientists(2009a, 2009b, 2009c, 2009d) and from Expedition 333 Scientists (2012a, 2012b).
128 K. Schumann et al. / Tectonophysics 636 (2014) 125–142
ensures a constantly irradiated volumewhile rotating the samples in thebeam, and no correction for sample geometry was necessary. The DORISelectron acceleration ring provides a monochromatic high-energy X-raybeamwith a wavelength of ~0.12 Å and 100 keV energy. The beamwitha size of 1 × 1 mm was directed perpendicular to the axis of the
20 µm
core
axi
s
D Equartz
quartzillite
illiteillite
feldspar
feldspar
core
axi
s
A B
20 µm
quartz
quartz
illite
microfossil
feldspar
Fig. 2. Microstructures of the naturally (experimentally undeformed) Sample 315-C0001316-C0004C8H-2 (D–F). Core axis is vertical for all six images. A and D are mosaics of 16 detaiof A and D, respectively, and show the threshold result for pores in black and rockmaterial in wimages like these were used for the shape preferred orientation analysis of pores and detrital i
drill cores, i.e. perpendicular to the sample cylinder axis. Transmissiondiffraction patterns were recorded using a 30 × 30 cm mar345 ImagePlate Detector with a resolution of 2000 × 2000 pixels, mounted~1.3 m behind the samples. Samples were rotated in 5° steps from−90° to +90° around the core axis, with 4 s counting time per image.
20 µm20 µm
core
axi
s
core
axi
s
F
core
axi
s
cor e
axi
s
C
20 µm 20 µm
E-11H-1 (A–C) and of the experimentally deformed subsample K010 from drill coreled BSE images. Mineral grains identified by EDX are indicated. B and E are binary imageshite. C and F are binary images of A and D, showing manually outlined illite grains. Binaryllite grains.
315-
C00
01E
-11H
-1or
igin
al, i
rreg
ular
bed
ding
pores31
5-C
0001
E-1
1H-1
K00
5, 4
0%, i
rreg
ular
bed
ding
illite
A B
G H
0 0
n=1324 n=1826
0 0
n=1097 n=2755
C D0 0
n=1758 n=2122
316-
C00
08C
-7H
-8K
018,
27%
, 20°
bed
ding
poresillite
I J
K L
0 0
n=1802 n=3567
316-
C00
08A
-9H
-4K
014,
44%
, 5°
bedd
ing
O P0 031
6-C
0007
C-7
H-1
K01
1, 4
8%, 1
6° b
eddi
ng
n=1327 n=6327
E F0 0
n=853 n=2792
316-
C00
07C
-7H
-1K
016,
38%
, 16°
bed
ding
0
316-
C00
06E
-20X
-3K
013,
43%
, 31
° be
ddin
g 0
n=1493 n=6886
natu
ral s
ampl
eex
peri
men
tally
def
orm
ed s
ampl
es
expe
rim
enta
lly d
efor
med
sam
ples
0
316-
C00
04C
-8H
-2K
010,
44%
, 8°
bedd
ing 0
n=6181n=2217
M N
Fig. 3. Rose diagrams of the preferred orientations of detrital illite platelets and pores from one naturally compacted (A, B) and seven experimentally deformed IODP core samples. Coreaxes are vertical. Bedding orientations (black lines with arrowheads) in the respective drill cores are taken from shipboard observations (Expedition 315 Scientists, 2009; Expedition 316Scientists, 2009a, 2009b, 2009c, 2009d; Expedition 333 Scientists, 2012a, 2012b). Orientations of illite platelets and pore long axes are indicated by gray lines. Orientations are grouped in10°-bins. Experimental subsample names, axial shortening strains in percent, and bedding dips are indicated.
129K. Schumann et al. / Tectonophysics 636 (2014) 125–142
The resulting 37 images allow for 100% pole figure coverage (Lonardelliet al., 2005).
To assessmineralogical compositions, the 37measurements on eachsample were added to provide averaged diffraction spectra, which alsoenable easy and fast preliminary refinement of the crystal structureparameters. Such starting parameters reduce the computation time forfinal phase and texture refinement.
The plate detector images display Debye–Scherrer cones as rings, inwhich each one represents a (hkl) plane. The darker the ring, the higheris the corresponding diffraction index. Discontinuous rings indicateCPOs, so that these can be directly derived from the ring patterns onthe plate images, and in unrolled 2D-plots of the data (Fig. 4). Thedata processing strategy is as follows:
(1) Image format conversion from the detector format into a 16bittiff-format needed for further interpretation steps (programMarcvt by Marresearch GmbH).
(2) Wavelength determination of the synchrotron radiation bymeans of an Al2O3 (corundum) standard sample, beam centerdetermination and spectra summation of the data (programFIT2D, Hammersley, 1998).
(3) Final crystal parameter refinement and texture calculations bymeans of MAUD (Material Analysis Using Diffraction, Lutterottiet al., 1997), a Rietveld-based code.
Diffraction peak intensities are not only functions of the crystal struc-ture (atomic parameters), but also of the CPO (Matthies et al., 1997). TheMAUD program combines the crystal structure refinement by theRietveld method (Rietveld, 1969) with quantitative texture analysis.
Full 360° plane detector images were integrated over 5° steps into 72slices, i.e. 2Θ profiles. This procedure results in 37 × 72 = 2664 spectrawith an overall number of 3 to 3.5 million data points per sample usedfor phase and texture refinement. We used a 2Θ range between 0.45and 3.5° for our analysis, which corresponds to a range in d-spacingsbetween 1.9 Å and 20 Å. Due to the lead beam stop mounted in thecenter of the detector, 2Θ angles smaller than 0.45° are not displayed inthe plate detector images (Fig. 4). Increasing the 2Θ range did not leadto improved data because of peak superpositions. The refinement stepsgenerally followed Lonardelli et al. (2005) andWenk et al. (2008, 2010b):
(1) Background and scale parameter setting and analysis (“wizardtool” of MAUD).
(2) Refinement of the beam center, sample-detector distance andthewavelength (starting values are previously graphically deter-mined using the “beam center” function of FIT2D). The sampledetector distance (directly measured in the lab) was refined incase of an inclined sample axis, which resulted in elliptic samplerotations causing variable sample detector distances and variablyradiated sample volumes.
333-C0012E-3X-4 315-C0001E-11H-1, K005 316-C0008A-9H-8, K006A B C
020
4060
spec
trum
num
ber
(exp
erim
enta
l dat
a)
0.5 1.5 2.5 3.52-Theta (degrees)
40 80 120 160Intensity {^1/2}[Count{^1/2}]
quar
tzca
lcite
chlo
rite
/ illi
tequ
artz
smec
tite/
illit
eka
olin
ite
illite
calc
ite
calc
ite
albi
te
albi
te
quar
tz
D
quartz
beamstop
calcite
Fig. 4. Plate detector images of textures in three different samples. Variable Debye–Scherrer ring intensity shows crystallographic preferred orientation (CPO; short arrows). Smooth ringsin (A) are indicative for fine-grained samplematerial, while spotty plate detector images point to coarse-grained samples and single crystal reflections (increased number of strong pointreflections in B and C). The position of the beam stop (amassive lead block) is indicated by red circles. The straight lighter line in the lower part of the plate detector images is causedby theacrylic glass beam stop holder. (D) An unrolled plate detector image of the Sample 333-C0012E-3X-4 (also shown in A) shows variable intensities pointing to CPO of the samplematerial.
130 K. Schumann et al. / Tectonophysics 636 (2014) 125–142
(3) Refinement of the crystal parameters with the Rietveld methoduntil a good visualfit of the theoretical and the observed spectrumwas achieved (Fig. 5). Each of the refinement stepswas done in atleast five iterations.
Mineral phases for the Rietveld refinement were chosen accordingto the composition analyses found in Expedition 315 Scientists (2009),Expedition 316 Scientists, 2009a, 2009b, 2009c, 2009d; Guo andUnderwood (2012) and our own EDX analyses. Crystal structure datarequired for Rietveld refinement were taken from the American Miner-alogist Crystal StructureDate Base (Downs andHall-Wallace, 2003).Weused published structures for smectite (Gournis et al., 2008), illite(Gualtieri et al., 2008), kaolinite (Bish and Von Dreele, 1989), chlorite(Zanazzi et al., 2009), albite (Harlow and Brown, 1980), calcite(Maslen et al., 1995), and quartz (Dušek et al., 2001).
For texture refinement, an E-WIMV algorithm (see Matthies andVinel, 1982) was applied. This improves the integration of the Rietveldmethod and the texture calculations (Wenk et al., 2003). Textures
were calculated for all mineral phases of the different samples and arepresented as recalculated pole figures (equal area projection, lowerhemisphere).
Possible error sources were assessed by adding crystallographic in-formation files. This is essential for the Rietveld refinement. Especiallychemical compositions of complex low-symmetry minerals, such asclays, can only be approximated in the crystallographic informationfiles. Furthermore, crystal disorder is a common phenomenon in clayminerals (e.g. Ufer et al., 2004 and references therein). Stacking disordercauses asymmetric peaks, and enlarges the width of basal peaks(e.g. Wenk et al., 2008). We applied a turbostratic disorder model(Ufer et al., 2004) to take into account disorder of smectite, whichdoes not enhance the experimental fit. Since this study focuses on CPOwe did not concentrate on further structural refinement. The overallweighted residual index Rw (for more information see Wenk et al.,2008) is automatically calculated by the MAUD program after eachrefinement step, and can be used as an index for the quality of the re-finement. LowRw values are indicative for a good datafit. The calculated
Inte
nsit
y 1/
2 [
Cou
nt 1/
2]
100
50
1.0 2.0 3.0
chloriteillite
calcitealbite
kaolinitemontmorillonite
quartz
2-Theta [degrees]
0.6 1.0 1.4 1.8 2.2 2.6 3.0 3.4
2-Theta (degrees)
10080604020Intensity{^1/2}[Count{^1/2}]
040
8012
0
Spec
trum
# (
data
| fi
t)
A
B
Fig. 5. (A) Example forof a refined spectrum showing the experimental data (dotted line) and the fitted curve (solid line). Peak positions of the identified mineral phases are indicatedbelow the image (A). Note that the fitted solid line indicates the best fit for the whole dataset (37 ω-rotations), while the experimental data represent only a single measurement atone ω-position. Differences can be caused by preferred orientation of the mineral phases and are not necessary indications of a bad fit result. (B) 2D-Multiplot of the measured data(lower part) and the fitted data (upper part).
131K. Schumann et al. / Tectonophysics 636 (2014) 125–142
Rw values lie in the range between 7.0 and 15.2% apart from two excep-tional values of 19.1 and 19.5%. For comparison, Wenk et al. (2008)achieved Rw values between 6.2 and 9.8% for the analyzed shales. An
example for a refined spectrum is given in Fig. 5A. However, the Rw
value indicates only the bulk error of the entire refinement, the qualityof the individual fits cannot be derived and needs to be checked by
132 K. Schumann et al. / Tectonophysics 636 (2014) 125–142
optical comparison (Toby, 2006). Optical comparison of the modeledspectra and the observed spectra indicated a good refinement fit (seeexample in Fig. 5B).
4. Results
4.1. Sample compositions
Point counting data of smear slides (cf. Stipp et al., 2013) showvariable sample compositions (Table 2). The content of lithics is highlyvariable across the sample set and ranges between 1.8 and 46.8%.Biogenic material is similarly variable and ranges between 0.3 and46%. The amount of amorphous microfossils has a maximum of 1 to5% in Samples K003 and K005 and is less than 1% in the other samples,indicating a negligibly small underestimation of the silica content in ourdata. Samples from the accretionary prism toe have thehighest contentsof submicroscopic material (between 43.4 and 78%), while samplesfrom the hanging wall of the megasplay fault and from the footwall ofthe megasplay fault have lower contents ranging between 39.7 and50.9% and between 32.6 and 56%, respectively. As clay minerals aregenerally below the resolution of the optical microscope used forpoint counting, the abundance of submicroscopics more or less resem-bles the total clay contents as derived from the synchrotron data (seebelow). The hanging wall samples of the megasplay fault have thelowest contents of “minerals” (between 6.5 and 10.4%). Samples fromthe footwall of the megasplay fault and the accretionary prism toehave slightly higher contents of “minerals” ranging between 12.8 and16% and between 11.8 and 17.3%, respectively. Ash contents are lowthroughout and only slightly variable between the different settingsranging between 1% and 8.6%. No smear slides analyses of the incomingplate sediments have been carried out, but the shipboard data ofExpedition 333 Scientists (2012a, 2012b) indicate that clay mineralsand biogenic material are the most abundant constituents.
Rietveld refinement of the synchrotron data indicates 21–43%quartz, 13–33% feldspar, 24–50% total clay, and up to 24% calcite(Table 3). These compositions are characteristic for recent marinesediments (Biscaye, 1965; Petschick et al., 1996) and the results arein good agreement with the compositions reported by Expedition 315Scientists (2009), Expedition 316 Scientists (2009a, 2009b, 2009c,2009d) and Expedition 333 Scientists (2012a, 2012b). To verify ourresults, X-ray powder diffraction (XRD) analysis of three samples
Table 2Point counting results.a
Sample Location Lithics Minerals Ash Biogenic Submicroscopic
K003 HW 4 7 2 46 41K004 HW 9.7 9.8 1 42.8 37K005 HW 4.7 6.5 3.2 34.7 50.9K010 HW 10.8 10.4 5.5 33.7 39.7K002 FW 28.8 15.7 8.1 14.8 32.6K006 FW 25.4 16 6.2 12.9 38.9K014 FW 11.2 12.8 5.6 24.1 46.4K015 FW 11 15.2 6.4 11.6 56K018 FW 11 14.4 7.5 11.4 55.1K007 T 17.1 13 3.7 5.2 61.1K009 T 17.8 11.8 5.5 21.6 43.4K011 T 22.6 17.3 7.7 4.9 47.1K012 T 16.9 13.8 3.1 2 64.2K013 T 11.9 16.5 4.9 10.1 56.6K016 T 26.2 16.6 8.6 4.5 44K019 T 1.8 12.8 1.8 5.4 78G001 T 46.8 15.4 4.3 0.3 33.5
a Contents of the different components in vol.%; lithics = mineral agglomerates;minerals = mainlyquartz, feldspar,mica andcalcite; ash = volcanic glass shards; biogen-ic material = foraminifera, diatoms, sponge needles and radiolarians; submicroscopicmaterial = matrix below the resolution of the optical microscope, most probably clayminerals; HW = hanging wall of the megasplay fault; FW = footwall of the megasplayfault; T = accretionary prism toe.
(316-C0008A-9H-3, 316-C0007C-7X-1 and 316-C0006E-20X-1) wasconducted (Table 4). These results are roughly similar to the standardshipboard XRD data acquired by Expedition 315 Scientists (2009),Expedition 316 Scientists (2009a, 2009b, 2009c, 2009d), see Table 3.
Clay contents determined by synchrotron-based Rietveld refine-ment of the incoming plate hemipelagics are somewhat higher com-pared to the samples from the accretionary prism. This is confirmedby the data of Expedition 315 Scientists (2009), Expedition 316Scientists (2009a, 2009b, 2009c, 2009d) and Expedition 333 Scientists(2012a, 2012b). A significant compositional variation occurs alsowithinthe accretionary prism samples (Table 3). Slope apron facies sedimentsfrom the hanging wall of the megasplay fault (315-C0001E-11H-1 and316-C0004C-8H-2) have quite similar clay contents, while Samples315-C0001F-14H-3 and 315-C0001H-19R-2 from the upper accretion-ary prism facies (sampled at greater depth) show somewhat higherclay contents (Expedition 315 Scientists, 2009).
The slope sediments from the footwall of the megasplay fault (316-C0008C-7H-8 and 316-C0008A-9H-3; Expedition 316 Scientists, 2009d)display slightly lower clay contents in comparison to the slope apronsediments from the hanging wall of the megasplay fault. At theaccretionary prism toe, Sample 316-C0007C-7X-1 shows similar claycontents compared to the slope sediments (footwall samples) andthus is interpreted to represent deformed and overthrusted slope sedi-ments. The samples drilled at IODP Site C0006 have been characterizedas trench wedge sediments. According to Expedition 316 Scientists(2009b) Sample 316-C0006E-8H-1 consists of sand-dominated trenchwedge sediments, while Sample 316-C0006E-20X-2 representssand-mud trench wedge sediments and Sample 316-C0006E-30X-1represents mud-dominated trench wedge sediments.
4.2. Microstructures and textures of naturally deformed samples
4.2.1. Shape preferred orientation (SPO) of minerals and poresMicrostructural analyses were conducted on a naturally deformed
sample from the hanging wall to the megasplay fault (core segment315-C0001E-11H-1; Fig. 2A–C). The bedding in this core segment isdescribed as being irregular (Expedition 315 Scientists, 2009). Themaximum of illite long axis orientations is tilted 46° to the horizontal,suggesting that an SPO-forming process other than vertical compactionwas active. On the other hand long axes of the pores are nearlyrandomly oriented, discounting significant pore space anisotropy (seeFig. 3A and B).
4.2.2. TexturesBedding plane attitudes of the naturally deformed or compacted
samples vary from almost horizontal to very steep, with dip angles upto 60° (Expedition 315 Scientists, 2009; Expedition 316 Scientists,2009a, 2009b, 2009c, 2009d; Expedition 333 Scientists, 2012a, 2012b,Fig. 6). In the following description, we focus on the major mineralphases relevant for texture genesis. A detailed documentation of allmineral phases and pole figure plots of their textures can be found inSchumann (2014).
Synchrotron texture analyses can reveal weak intensity maximawell below 2 multiples of a random distribution (mrd), and topologiesthat are axially symmetric around the pole to the bedding planes (seeFig. 6). Only in the deepest sample (333-C0012E-3X-4) the intensitymaximum for poles to illite [001] is as high as 4.07 mrd (Fig. 6). Polesof [100] quartz always show random distributions (see Schumann,2014).
Five samples from the incoming platewere investigated (Sites C0011and C0012, Table 1). In the samples from Site C0012 (Table 1), the polesto [001] of illite are normal to bedding and the strength of textureincreases with depth from 1.13 mrd (sample 333-C0012C-4H-5) to4.07 mrd (sample 333-C0012E-3X-4). Poles to [001] of kaoliniteshow similar orientations, except for sample 333-C0012C-15H-5,which shows a point maximum normal to bedding, combined with
Table 3Composition of the samples.a
IODP normalized (wt%) Guo and Underwood, 2012
Sample name (IODP) Subsample Depth Clay Quartz Plagioclase Albite Calcite Total clay Smectite Illite Kaolinite Chlorite
Site C0001315-C0001E-11H-1 90.6 45.7 19.4 18.1 16.9 46 15 18 2 11
Original 30 26.3 26.5 16.9 2.1 7.9 20 0K003 34 27.4 20.6 17.7 3.9 14 12.9 3.2K005 49.4 20.6 12.9 16.8 1.5 34 13.9 0
315-C0001F-14H-3 210.9 58.4 23.2 18.2 0.1 21 21 3 12Original 30.8 30.5 33.8 4.6 2.3 12.2 16.3 0
315-C0001H-19R-2 398.2 62.4 18.6 16.8 2.2 28 22 4 9Original 48.5 22.4 24.2 4.8 8.9 28.3 11.3 0
Site C0004316-C0004C-8H-2 65.3 46.1 20.7 15.9 17.3 46 17 19 3 8
K004 39.2 26.3 20.1 14.1 2.2 18.2 17.3 1.5K010 25.9 28.9 25 20 2.2 8.1 15.2 0.4
Site C0008316-C0008C-7H-8 61 45.2 24.8 29.6 53 16 22 5 10
K015 34.4 34.7 25.6 5.1 3.2 11.7 19 0.5K018 26.4 39.4 29.3 4.7 3.1 7.9 15.4 0
316-C0008A-9H-3 76.1 43.7 30.2 21.7 4.4 44 11 16 7 9Original 26.5 35 32 6 1.5 15 10 0K006 24 42.3 29.2 4.2 3.2 4.4 16.1 0.3K014 36.3 31.2 25.6 6.6 4.4 10.5 21.3 0.1
Site C0006316-C0006E-8H-1 48.2 44.6 25.8 29.4 0.2 53 13 22 6 12
K007 25.9 35.6 32.7 6.2 1.7 13.8 10.4 0K009 39.9 27.6 25.8 6.4 3.5 24.5 11.9 0
316-C0006E-20X-2 128 50.2 26.5 23.3 0 49 9 24 0 16K019 44 23.4 23.1 9.2 6.2 7.5 30.3 0
316-C0006E-30X-1 221.3 48.2 23.4 28.4 0 48 15 17 1 16G001 42.4 32.1 20.4 4.7 2.5 19.6 20.3 0
Site C0007316-C0007C-7X-1 62.9 37 27 36 0 37 9 16 1 11
K012 31.7 35.8 32.2 0 15.4 2.8 13 0.5K016 29.7 38.1 31.8 0.1 1.4 18.5 9.8 0
Site C0011333-C0011D-26X-1 206.2 69 19 11 0
Original 34.6 32.2 27.7 5.1 3.9 17.1 13.3 0.3333-C0011D-41X-4 316 63 16 10 11
Original 33.9 30.6 15 20.5 3.6 7.3 23 0
Site C0012333-C0012C-4H-5 28.9 63 20 17 0
Original 35.1 28.3 29.8 6.6 4.7 13.8 16.6 0333-C0012C-9H-6 75.9 68 20 12 0
Original 45.9 28.1 19.5 6.2 4.7 34 7.2 0333-C0012C-15H-5 121.6 70 17 11 3
Original 41.6 30.4 23.4 4.3 5.9 11.7 24 0333-C0012E-3X-4 522.9 69 11 5 15
Original 31.2 23.9 21 23.7 6.5 12.4 12.3 0
a The composition of the samples investigated in this study is given in the table. Data printed in italic are taken from the Expedition 315 Scientists, 2009; Expedition 316 Scientists, 2009a,2009b, 2009c, 2009d, Expedition 333 Scientists, 2012a, 2012b and fromGuo andUnderwood, 2012. Values printed in bold represent data of this study. The division into subsamples relates tothe experimental deformation of the samples (see Stipp et al., 2013). “Original”means that experimentally undeformed, naturall subsampleswere analyzed. Note that the samples came fromthe same core segment and from a similar depth, but are not completely identic. Internal inhomogeneity may occur although the overall composition is rather similar.
133K. Schumann et al. / Tectonophysics 636 (2014) 125–142
a weak girdle distribution in the bedding plane (Fig. 6). The maxi-mum strength of the [001] kaolinite poles only slightly increasesfrom 1.11 mrd (sample 333-C0012C-4H-5) to 1.40 mrd (sample333-C0012E-3X-4). In sample 333-C0011D-26X-1, which comesfrom the flank of a basement high, the poles to [001] of illite and[001] of kaolinite are slightly tilted away from the bedding plane nor-mal. The [010] albite show random distribution, except for the samples333-C0012-15H-5, 333-C0011D-26X-1 and 333-C0012-3X-4, wherevery strong textures of 22 mrd and 27 mrd parallel to the core axiswere observed (see Schumann, 2014).
Samples from the accretionary prism (Table 1, Fig. 6) show texturemaxima up to 1.86mrd. Topologies of the phyllosilicate CPO are mostlysimilar to those of the samples from the incoming plate, but differ fromthem in one important aspect. Where bedding is horizontal, point
maxima or small circle maxima of poles to [001] are axially symmetricaround the bedding plane normal, and thus around the core axis(Fig. 6). Where bedding is tilted (Samples 315-C0001F-14H-3 and316-C0006E-30X-1), the CPO remains axially symmetric around normalto the bedding plane. Illite [001] CPO in sample 315-C0001F-14H-3 is al-most random, an observation that contrasts with the correspondingCPO of kaolinite (Fig. 6). This, however, may be a sampling effect dueto the low concentration of illite (12.2%). The calcite [006]- pole maxi-mum forms a small circle distribution around the core axis, just like inSample 333-C0012E-3X-4, but CPO intensity is much weaker (Fig. 6).The deepest sample from IODP Site C0001 located on the megasplayhanging wall (Sample 315-C0001H-19R-2; 398.2 mbsf) shows themost pronounced [001]-maxima of illite (1.86 mrd) and kaolinite(1.54 mrd) (see Fig. 6).
Table 4Comparison of the sample compositions determined by different methods in different studies.a
Sample 316-C0008A-9H-3 316-C0007C-7X-1 316-C0006E-20X-2
Investigator This study BGR Expedition316 Scientists
Guo This study BGR Expedition316 Scientists
Guo This study BGR Expedition316 Scientists
Guo
Smectite 1.5–4.4 7.0 11.0 1.4–15.4 15.9 9.0 6.2 23.1 9.0Muscovite 13.5 10.3 12.4Chlorite 0–0.3 2.9 16.0 0–0.5 4.2 11.0 0.0 5.5 16.0Kaolinite 10–21.3 6.7 1.0 9.8–13 1.2 1.0 30.3 1.4 0.0Illite 4.4–15 16.0 2.8–18.5 16.0 7.5 24.0Total clay 24–36.3 30.1 47.3 48.0 25.9–39.9 31.6 37.0 37.0 44.0 42.4 50.2 49.0Plagioclase (16 An) 15.2 16.0 13.7Plagioclase (50 An) 7.3 13.4 9.3Orthoclase 4.6 4.5 4.2Feldspar 25.6–32 21.7 31.8–32.2 36.0 23.1 23.3Hornblende 1.5 1.5 1.5Augite 0.8Quartz 31.2–42 36.2 35.8–38.1 30.1 27.0 23.4 26.0 26.5Calcite 4.2–6 3.4 4.4 0–0.1 1.1 0.0 9.2 1.8 0.0
a The sample composition of this study is derived from Rietveld refinement of synchrotron data of the complete samples. Data from the BGR (Bundesanstalt für Geowissenschaften undRohstoffe) are determined by x-ray powder diffraction (XRD) of the complete samples. Data from Expedition 316 Scientists, 2009a, 2009b, 2009c, 2009d are determined using XRD anal-ysis to measure total clay, quartz, plagioclase and calcite aboard D/V CHIKYU. Guo and Underwood, 2012 only used the clay sized fraction (b2 μm) for their XRD analysis and only inves-tigated the clay minerals.
134 K. Schumann et al. / Tectonophysics 636 (2014) 125–142
4.3. Experimentally deformed samples
4.3.1. Shape preferred orientation (SPO) of minerals and poresThe SPO of an experimentally deformed aliquot of core segment 315-
C0001E-11H-1 (subsample K005; 40% axial shortening) was investigat-ed; related starting material and microfabrics are shown in Figs. 2, 3Aand B). After that experimental deformation bedding has rotated about10° clockwise, and illite SPO has strengthened (compare Fig. 3A and G).The pore space anisotropy, however, has not changed significantly(compare Fig. 3B and H).
In almost all other experimentally deformed samples, however,there is a stronger SPO of illite, withmaxima of illite basal traces orient-ed at high angles to the experimental shortening axis (Fig. 3). In SampleK010 (316-C0004C-8H-2, Fig. 2D–F), there is a strong illite and poreSPO, with the long axes of the detrital illites and pores approximatelyparallel to the bedding plane, which is tilted 8° (Expedition 316Scientists, 2009a), and thus, more or less perpendicular to the axis ofmaximum shortening in the deformation experiment.
The illite long axes of experimental subsample K013 (316-C0006E-20X-2) are parallel to the inclined bedding plane, with a secondmaximum approximately perpendicular to the bedding (Fig. 3). In con-trast, the long axes of the pores have a nearly horizontal orientationmaximum, perpendicular to the experimental shortening axis. In theexperimental Samples K011 and K016 (316-C0007C-7H-1), illite longaxes have a 20–25° mismatch with bedding orientation (Expedition316 Scientists, 2009c). Pore SPO are almost normal to the experimentalshortening axis. In Sample K014 (316-C0008A-9H-3) both, the illitelong axes and the pore long axes are oriented parallel to bedding,which is almost horizontal, and, thus, normal to the experimental short-ening axis. The illite long axis maxima of Sample K018 (316-C0008C-7H-8) is oriented steeper than the bedding, and more than 30° to theexperimental flattening plane. In contrast, the pore long axes areapproximately horizontally aligned.
4.3.2. TexturesSince experimental subsamples were obtained from IODP whole
round samples, we use the numbers of the experiments for the texturedescriptions instead of the IODP core sample numbers. The connectionbetween experiment numbers and core/sample numbers is given inTable 1. Only samples from the accretionary prismwere experimentallydeformed and are described below. They are sorted by the attainedexperimental strains.
In summary, most of the phyllosilicate CPOs in the experimentallydeformed samples are somewhat stronger than those in the original
core samples. Texture maxima exceed values of 2.0 mrd only in a fewcases, though (Fig. 7). CPO intensification is a consequence of the exper-imental strains being superimposed on the natural compaction history.Strengthening of the CPO is expected and observed in case of an originalshallowdip of bedding and, thus, plane of compaction. This is true for allsamples except Samples K007, K009 and K019 (see Fig. 7).
In Samples K003, K005, K006 and K014 the bedding plane normal isparallel to the shortening axis. Poles to [001] illite of the experimentallydeformed samples show either point maxima or small circle girdle dis-tributions with a small (b20°–30°) opening angle around the shorten-ing axis, except for Samples K007, K009 and K004, where the polemaxima to [001] of illite are normal to the bedding plane or oblique tothe bedding plane normal, respectively. In Samples K016, K007 andK004 illite CPO is almost random (Fig. 7). Pole maxima to [001] of kao-linite are approximately parallel to the core axis in the Samples K003,K004, K005 and K006, and bedding is approximately normal to theshortening direction. In the other samples, the pole maxima to [001]of kaolinite are slightly oblique to the direction of maximum loadingand shortening (Fig. 7) defining either point maxima or small circle gir-dleswith a narrow (20°–30°) opening angle. In Sample K014 the kaolin-ite CPO is almost random (Fig. 7). Poles to [006] of calcite has a pointmaximum around the shortening axis in Sample K019 (Fig. 7). ThisCPO is similar to those observed in the naturally compacted samplespresented in Fig. 6. Samples K003, K004 and K005 show similar calciteCPOs, suggesting a texture-forming mechanism akin to that in thephyllosilicates. This will be discussed in more detail below. Strong[010]-maxima of albite parallel to the shortening axis are present inSamples K018 and K006 (19.57 mrd and 12.70 mrd, see Schumann,2014), akin to the albite CPO observed in some of the samples whichonly experienced natural compaction (see above).
5. Discussion and interpretation
In this section we discuss the mechanisms that create textures indeepmarine sediments involved in compaction and accompanying tec-tonic deformation in a typical convergentmargin setting. The sedimentsare all naturally compacted, and a reasonable range of burial depths isavailable. In the case of the experimentally deformed samples, the straingeometry strongly resembles that of natural compaction with one dif-ference: as the experiments were run consolidated-undrained, samplessuffered no volume loss, but horizontal extension in response to verticalaxial shortening. While diagenesis and cementation are not consideredas major players at least in the shallow parts of accretionary wedges,maybe with the exception of settings where young and hot oceanic
Illite[001]
Kaolinite[001]
Calcite [001]
incoming plateincreasing depth
333C0011D-26X-1206.2 mbsf, 19°
1.01.051.11.15
17.1%1.28 mrd
1.01.051.11.15
13.1%1.21 mrd
333C0012E-3X-4
522.9 mbsf, 10°
1.01.52.02.53.03.54.0
12.4%4.07 mrd
1.01.051.11.151.21.251.31.35
12.3%1.40 mrd
1.01.11.21.31.4
23.7%2.43 mrd
333C0012C-4H-528.9 mbsf, 40°
1.01.051.1
16.6%1.11 mrd
1.01.051.1
13.8%1.13 mrd
1.01.11.21.3
7.2%1.30 mrd
333C0012C-9H-675.6 mbsf, 60°
1.01.051.1
34%1.18 mrd
333C0012C-15H-5121.6 mbsf, 15°
1.01.051.1
24%1.17 mrd
1.01.11.21.31.4
11.7%1.44 mrd
315C0001F-14H-3210 mbsf, 28°
1.0
12.2%1.04 mrd
1.01.051.11.151.2
16.3%1.23 mrd
315C0001E-11H-190.6 mbsf, 0°
1.01.051.11.151.2
20%1.20 mrd
1.01.051.11.151.2
7.9%1.21 mrd
1.01.051.11.151.2
16.9%1.20 mrd
315C0001H-19R-2
398.2 mbsf
1.01.11.21.31.41.51.61.71.8
28.3%1.86 mrd
1.01.11.21.31.41.5
11.3%1.54 mrd
316C0008A-9H-3
76.1 mbsf, 5-12°
1.01.11.21.31.41.51.6
15%1.62 mrd
1.01.11.21.3
9.7%1.31 mrd
G001, 316C0006E-30X-1
221 mbsf
1.01.051.11.151.21.251.31.351.4
20.3%1.46 mrd
1.01.051.11.15
19.6%1.22 mrd
accretionary prismincreasing depth
max:content:
max:content:
max:content:
Illite[001]
Kaolinite[001]
Calcite [001]
max:content:
max:content:
max:content:
Fig. 6. Pole figure plots of themainmineral phases of the naturally compacted samples studied. Core and sample cylinder axes are vertical. Bedding orientation is indicated by solid blacklines, where available. Sampling depths in meters below sea floor (mbsf) and bedding dip angles are indicated for each sample. Pole figure densities are given in multiples of a randomdistribution (mrd), and contour lines below 1 are not shown. Contour levels of the pole figure plots are variable depending on maxima strength and complexity. Pole figure maximumand volume percentage for each phase are indicated below each pole figure.
135K. Schumann et al. / Tectonophysics 636 (2014) 125–142
Illit
e[0
01]
Kao
linite
[001
]K007, 316
C0006E-8H-148 mbsf55°, 22%
1.01.051.1
13.8%1.12 mrd
1.01.051.11.151.2
10.4%1.24 mrd
K019, 316C0006E-20X-2
127 mbsf31°, 23%
1.01.11.2
30.3%1.27 mrd
1.01.52.02.5
7.5%2.77 mrd
1.01.52.02.5
9.2%2.81 mrd
K018, 316C0008C-7H-8
60 mbsf15-20°, 27%
1.01.11.21.31.41.51.61.7
15.4%1.72 mrd
1.01.11.21.31.41.51.61.71.8
7.9%1.94 mrd
Cal
cite
[00
1]K006, 316
C0008A-9H-376.1 mbsf
5-12°, 31%
1.01.21.41.6
16.1%1.71 mrd
1.01.52.02.53.0
4.4%3.71 mrd
1.01.051.1
K012, 316C0007C-7X-1
62.8 mbsf
15.4%1.19 mrd
1.01.11.21.31.4
13%1.48 mrd
16°, 35%1.01.11.21.31.4
2.8%1.44 mrd
Smectite[001]
K003, 315C0001E-11H-1
90.6 mbsf5-10°, 36%
1.01.11.21.31.41.51.61.7
14%1.74 mrd
1.01.11.21.31.41.51.6
12.9%1.67 mrd
1.01.11.21.31.41.51.6
17.7%1.95 mrd
K004, 316C0004C-8H-2
65 mbsf8°, 37%
1.01.051.1
18.2%1.13 mrd
1.01.11.21.31.41.5
17.3%1.58 mrd
1.01.11.21.31.4
14.1%1.54 mrd
K016, 316C0007C-7X-1
62.8 mbsf16°, 38%
1.01.05
18.5%1.07 mrd
1.01.11.21.31.41.51.6
9.8%1.64 mrd
K015, 316C0008C-7H-8
60 mbsf15-20°, 38%
1.01.11.21.31.4
11.7%1.94 mrd
1.01.11.21.31.41.5
19%1.59 mrd
K005, 315C0001E-11H-1
90.6mbsf5-10°, 41%
1.01.11.21.31.41.5
34%1.59 mrd
1.01.11.21.31.41.51.61.71.8
13.9%1.87 mrd
K014, 316C0008A-9H-3
76.1 mbsf5-12°, 45%
1.01.21.41.61.8
10.5%2.11 mrd
1.0
21.3%1.01 mrd
K009, 316C0006E-8H-1
48 mbsf55°, 60%
1.01.52.0
24.5%2.45 mrd
1.01.11.21.31.41.51.61.7
11.9%1.79 mrd
max:content:
increasing strain
increasing strain
max:content:
max:content:
Illit
e[0
01]
Kao
linite
[001
]C
alci
te [
001]
max:
max:content:
max:content:
content:
Fig. 7. Pole figure plots of the main mineral phases of the experimentally deformed samples. Axes of cores, sample cylinders and experimental shortening are vertical, bedding dips andsampling depths are as in Fig. 6. Axial shortening strains are given in percent. Contour levels, maxima and volume percentages of phases are as in Fig. 6.
136 K. Schumann et al. / Tectonophysics 636 (2014) 125–142
137K. Schumann et al. / Tectonophysics 636 (2014) 125–142
crust is subducted (e.g. Behrmann et al., 1994), loss of porosity is ofoverriding importance for the acquisition of SPO and CPO in marinesediments.Wefinish thediscussion by reconsidering someof the gener-al advantages synchrotron texture analysis has over conventionalmethods.
5.1. Natural compaction in Nankai sediments: drivers, mechanisms, andfabric response
Perhaps the most obvious parameter to study in relation of CPOintensity is the depth of burial of the sediments. As there is a goodrange of burial depths for the naturally compacted sediments, we takethe approach to plot CPO intensity in mrd against the sampling depthbelow seafloor for the incomingplate samples and those from the accre-tionary prism (Fig. 8A). Obviously there is a wide range of CPO intensityeven within individual samples, if we take kaolinite and illite [001] andcalcite [006] pole figures into account. In Fig. 8A only pole figures areused that are derived from mineral phases with abundances of morethan 10%, because it cannot be excluded that CPO relates to incompletesampling of phases with low abundances. Samples with shallow burial
illitekaolinitecalcite
depth [mbsf]0 100 200 300 400 500 600
2
4
1
mul
tiple
s of
ran
dom
dis
trib
utio
n [m
rd]
C00
01H
-19R
-2
C00
12E
-3X
-4
C00
06E
-30X
-1
C00
12C
-15H
-5
C00
08A
-9H
-3C
0001
E-1
1H-1
C00
12C
-4H
-5
C00
12C
-9H
-6
C00
01F-
14H
-3
C00
11D
-26X
-1
axial shortening [%]0 10 20 30 40 50 60
2
3
1
mul
tiple
s of
ran
dom
dis
trib
utio
n [m
rd]
illite
smectitekaolinite
calcite
K007K019
K018 K006
K012
K003K015
K016K004
K005
K014
K009
A
B
Fig. 8. (A) CPO intensities of illite, kaolinite and calcite plotted versus depth of the incom-ing plate sediments and the accretionary prism samples. (B) CPO intensities of illite,kaolinite, smectite and calcite plotted versus the experimental shortening strain.
depths have less intense CPO than themore deeply buried ones. This re-lationship is only roughly defined, however, and it is probably becauseonly the very fine-grained clay mineral phases were sedimented in aflocculated form to produce isotropic basal plane distributions in freshlysedimented marine muds. One indication to support this is that illiteCPO is somewhat more strongly developed than kaolinite CPO. Thiscan be explained by the effect the detrital illites (see Fig. 3) have onthe overall CPO intensity. Detrital illites were probably transportedand sedimented in non-flocculated form as individual large grains,and a primary SPO will then result from sedimentary transport anddeposition processes. The results we present generally confirm theobservation of Vasin et al. (2013), who found a less pronounced CPOof kaolinite and smectite in comparison with illite in Kimmeridgeclays. Our data indicate that this effect also occurs in young sedimentsthat lack any significant diagenetic overprint.
One interesting aspect of the CPO is that maxima of poles to claybasal planes, but also of calcite [006] directions are almost always sub-perpendicular to the bedding orientations in the core (Fig. 6). Wherebedding is strongly tilted relative to the core axis (the vertical), thesemaxima are tilted in the sameway. Bedding tilt in the Nankai accretion-ary prism is primarily a result of tectonic deformation, faulting, and fold-ing. This observation proves that the CPOs are indeed compactionalfabrics, formed by progressive dewatering and porosity reduction assediments were overlain by younger sedimentary layers. This tilting ofphyllosilicate CPOmaximawas also observed in some samples analyzedby Behrmann and Kopf (1993) from ODP Site 808, drilled in the toe ofthe Nankai accretionary prism, but was then interpreted as resultingfrom tectonic deformation due to lateral contraction in the accretionaryprism toe. Our data show that tilting of compactional fabricsmay be themore likely explanation, making CPO development an early part of thefabric evolution. The tilting effect is also observed in samples fromIODP Sites C0011 and C0012 (Fig. 6), which are located far seaward ofthe Nankai deformation front (Fig. 1). Tilting in these samples is causedby hemipelagic deposition on a basement knoll (Expedition 333Scientists, 2012b) and slumping (especially Subunit IB, Samples 333-C0012C-9H-6 and 333-C0012C-15H-5; Expedition 333 Scientists,2012b).
Another parameter that varies systematically with depth is porosity.We have taken the approach to use the porosity-depth functions of thedrillholes cored and sampled in IODP Expeditions 315, 316 and 333 toassociate a value for porosity loss (from ratios of initial porosity andmeasured porosity) to each specimen. A measure of initial porosity isprovided by the average of shipboard measurements on samples fromthe uppermost 10 m of the specific drillhole. Initial porosities rangebetween 64% and 75%, and the function describing downhole porosityloss was converted to vertical uniaxial shortening (e.g., in Fig. 9) akinto the procedure described by Behrmann and Meissl (2012). PlottingCPO intensity of illite and kaolinite [001] and calcite [006] against sam-ple porosity, the CPO intensity increases with decreasing porosity forthe incoming plate sediments (Fig. 9A), while such a relationship isless obvious for the samples from the accretionary wedge (Fig. 9B).CPO intensity only slightly increases as porosity decreases. It becomesobvious, that vertical loading of the incoming plate sediments resultsin progressive flattening of the sediments accompanied by the reduc-tion of the pore space and expulsion of the pore fluid (Behrmann andKopf, 1993).
Strain can be calculated using the sample porosities (Eq. (1), Kopfand Behrmann, 1997) and using the CPO intensities of thephyllosilicates (Eq. (2)) on the basis of the March theory (Behrmannand Kopf, 1993; Kopf and Behrmann, 1997; Lipshie, 1984; Oertel,1985). Using porosities, the compactional strain (epore) is defined by
epore ¼ P0−Pdð Þ= Pd−100ð Þ ð1Þ
where P0 is the initial porosity in % at, or close to the mudline, and Pd isthe porosity at depth. Using the March model (March, 1932), the
4.0
3.5
3.0
2.5
2.0
1.5
1
75 70 65 60 55 50 45
CPO
inte
nsity
[m
rd]
porosity [%]
illitekaolinitecalcite
illitekaolinitecalcite
4.0
3.5
3.0
2.5
2.0
1.5
1
CPO
inte
nsity
[m
rd]
70 65 60 55 50 45porosity [%]
40
A B
0.0 -0.20 -0.40 -0.60
50
150
350
250
450
buri
al d
epth
[m
bsf]
ev
100
200
400
300
500buri
al d
epth
[m
bsf]
600
C D0.0 -0.20 -0.40 -0.60
ev
ev illite
ev kaolinite
ev illite
ev kaolinite
incoming plateaccretionary prism
accretionary prism
incoming plate
epore
epore
Fig. 9. (A) CPO intensities of illite, kaolinite and calcite plotted versus theporosity of the incomingplate sediments and (B) the accretionary prism sediments. (C) Strains calculated from theCPO of illite grains (diamonds), kaolinite (squares) and pores (triangles) plotted versus the burial depth of the incoming plate samples and (D) the accretionary prism samples.
138 K. Schumann et al. / Tectonophysics 636 (2014) 125–142
compactional strain (ev) from CPO intensity of phyllosilicate basalplanes and calcite c-axes is given by
ev ¼ Iv−1=2−1 ð2Þ
where Iv is the multiple of random distribution of the basal plane. Theresults obtained from the two methods of strain calculation are givenin Table 5. A diagram of the strain plotted against the sampling depthof the incoming plate samples (Fig. 9C) indicates that there is somecorrelation of strain data determined from porosity loss and from the il-lite CPOwith depth, while theMarch strains from the kaolinite CPO givesomewhat lower values. In general, porosity variationswith depth seemto reflect the compactional strain and thus support vertical loading andpore space reduction as the texture forming process for the incoming
Table 5Results of strain analyses of the incoming plate sediments.a
Sample Depth[mbsf]
CPO illite[mrd]
CPO kaolinite[mrd]
333-C0012C-4H-5 28.9 1.13 1.11333-C0012C-9H-6 75.6 1.18 1.30333-C0012C-15H-5 121.6 1.44 1.17333-C0012E-3X-4 522.9 4.07 1.40333-C0011D-26X-1 206.2 1.28 1.21316-C0008A-9H-3 76.1 1.62 1.31315-C0001E-11H-1 90.6 1.21 1.20315-C0001F-14H-3 210.0 1.04 1.23315-C0001H-19R-2 398.2 1.86 1.54316-C0006E-30X-1 221.0 1.22 1.46
a The results of the strain analyses using the CPO of illite (ev illite) and kaolinite (ev kaoliniteinitial porosity (P0) are from the Expedition 315 Scientists (2009), Expedition 316 Scientists (2further explanation.
plate samples. For the accretionary prism samples, strains computedfrom porosity loss and CPO strength do not correlate well at all(Fig. 9D), indicating that vertical compaction is not the only textureforming process. Other processes such as tectonic overprint, akin tothe inferences made by Behrmann and Kopf (1993) may influencetexture formation in these samples.
While phyllosilicate CPO as response to compaction can be easily un-derstood, the interpretation of calcite CPO, as shown by the [006] polefigures (Fig. 6) is less straightforward. Carlson and Christensen (1979)pointed out that the observed strong acoustic anisotropy of calcite-rich deep sea sediments may be related to the preferential alignmentof bio-detrital microfossil platelets parallel to bedding. Indeed,there are high-abundance taxa of calcareous micro- and nannofossils,like coccolithophoridae and discoasters, that show a highly ordered
ev illite ev kaolinite Pd[%]
P0[%]
epore
−0.06 −0.05 72 72 0−0.08 −0.12 70.9 72 −0.03−0.17 −0.08 59 72 −0.31−0.50 −0.15 48 68 −0.38−0.12 −0.09 70.2 71 −0.02−0.21 −0.13 61.3 75.19 −0.35−0.09 −0.09 64 64 0−0.02 −0.10 58.8 64 −0.12−0.27 −0.19 54 64 −0.21−0.09 −0.17 42 71.9 −0.51
) and the strain analyses using the porosity data (epore). The porosity at depth (Pd) and the009a, 2009b, 2009c, 2009d) and the Expedition 333 Scientists (2012a, 2012b). See text for
139K. Schumann et al. / Tectonophysics 636 (2014) 125–142
arrangement of calcite c-axes normal to the plate-like shapes (e.g. Black,1963, 1972). These likely constitute amajor part of the biogenic detritusdocumented in the sediment compositions (Figs. 5 and 10A), and their
A
B
C
Fig. 10. Backscattered electron micrographs of (A) microfossil shell fragments embeddedin fine-grained matrix in Sample 315-C0001E-11H-1, (B) pumice fragments (experimen-tal subsample K018) and (C) a relatively large, idiomorphic feldspar lath (experimentalsubsample K018).
preferential alignment will be just like that of phyllosilicates, andexplain the CPO of calcite observed.
Extremely strong albite [010]-maxima of up to 27.69 mrd areevident in some of the samples. These are exclusively from IODP SitesC0008, C0011 and C0012, and maxima are always oriented subnormalto bedding and parallel to the drill core axis (see Schumann, 2014). Allthe samples are from sediment sequences free of tectonic overprintfrom the incoming plate (IODP Sites C0011 and C0012) or the slopecover on the footwall of the megasplay fault (IODP Site C0008). Sincethe core axis parallel orientation is observed in some experimentalsamples as well as in the natural samples, and is absent in others thepreferred orientation of albite cannot be simply attributed to compac-tion. Several reasons for the existence or non-existence of strong [010]albite pole figuremaxima are conceivable, such as coarsemineral grainsderived from ash (Fig. 10B), localized reworking or drilling disturbance.No drilling disturbance and reworking are evident from the expeditionreports (Expedition 315 Scientists, 2009; Expedition 316 Scientists,2009a,2009b,2009c,2009d and Expedition 333 Scientists, 2012a,2012b). Furthermore, the [001] pole figure plots of the clay mineralsin the samples from the incoming plate do not indicate drilling distur-bance or re-deposition. Thus, we rule out these processes. Coarse albitegrains, probably part of the volcanoclastic input, are evident inbackscattered and secondary electron images (Figs. 2 and 10C). Glassshards were identified during point counting in all samples (Table 3),and volcanic ash is also reported by Expedition 315 Scientists (2009),Expedition 316 Scientists (2009a, 2009b, 2009c, 2009d) and Expedition333 Scientists (2012a, 2012b), from which idiomorphic albite grainscan originate (Fig. 10C). Moreover, large mineral grains can producestrong single crystal maxima, also providing a possible explanation forthis type of CPO.
5.2. Texture genesis in constant-volume shearing
Experimental deformation was conducted under consolidated andundrained conditions, which means that shearing proceeded atconstant volume, and shape and texture anisotropy, therefore, reflectan axially symmetric (biaxial) state of strain. As platy particles areprogressively reoriented during strain, increasing axial strain duringthe experiments should result in strengthening of CPO (March, 1932).In a plot of experimental strain versus CPO intensity (Fig. 8B), it can beseen that the results generally confirm this prediction of the Marchmodel.
The pore space anisotropy in response to the experimental deforma-tion is expressed by the ratio of themaximum frequency per petal divid-ed by the uniform frequency per petal (fmax / funiform) in the rosediagrams shown in Fig. 3. Generally the pore long axes are preferentiallyoriented perpendicular to the axis of shortening (Fig. 3) and thus,mimicthe maximum finite stretching in the flattening plane. Thus, pores aresensitive strain gauges, and the resulting anisotropy in the pore fabricwill induce a permeability anisotropy in the deformed fine-grainedsediments. Primary CPO and SPO of the natural samples were formedduring compaction (Fawad et al., 2010; Mitchell, 1956), and later atleast in part tilted by tectonic deformation (see Fig. 6), and thenoverprinted by the experimental deformation (Bowles et al., 1969).Such effects are also evident in tilted bedding orientations and polefigure maxima oblique to the shortening axes in Fig. 7.
The absence of quartz CPO in all samples, even after the experimen-tal deformation (see Schumann, 2014, for compilation of data) showsthat there is no orienting mechanism for rounded quartz clasts. More-over, clast shape of quartz has usually no relation to the crystal orienta-tion. Thus, even an SPO, if present, will not result in a CPO. A differenteffect is observed in the case of albite CPO, as discussed above. Threeof the experimentally deformed sediment samples show strong CPO,while all the others do not. Apart from the possibility of single crystal ef-fects, as discussed above, volcanic plagioclase derived from ash (evidentin the smear slides) usually forms albite-twinned crystal laths. Feldspar
140 K. Schumann et al. / Tectonophysics 636 (2014) 125–142
laths should therefore respond to experimental strain in a similar wayas the phyllosilicates, attenuating the CPO. We cannot distinguish inthe dataset, however, if the albite CPO in the experimentally deformedsamples is inherited from the original fabric and hence induced byburial and compaction.
5.3. Effects of sample composition
Differences in the sample composition (especially for Sample316-C0008A-9H-3) are probably due to sample heterogeneities, be-cause the data refer to different subsamples. In addition, there are alsosome methodical differences. IODP expedition shipboard scientists(Expedition 316 Scientists, 2009e) used X-ray diffraction analysis to in-vestigate the sample composition. Due to the number of poorly crystal-line mineral phases and superposed peaks (especially for the clayminerals) methodical errors can be considerable. Guo and Underwood(2012) used the grain size fraction b2 μm to determine the claymineralassemblages. In the BSE studies, we found claymineral grains exceeding2 μmanddetrital illiteflakes of up to 40 μmin size. In addition, thefilter-peel method (Moore and Reynolds, 1989) used by Guo and Underwood(2012) produces textured samples (claymineral basal planes parallel tothe filter paper; cf. Moore and Reynolds, 1989). Preferred orientationsincrease peak intensity and peak area, thus causing a misinterpretationof the clay content.
5.4. Aspects of synchrotron texture analysis
Sample preparation for the use of conventional texture analysis, i.e.X-ray diffraction (XTG) techniques is complex and has the potential toinduce many artifacts (see Behrmann and Kopf, 1993). In contrast theuse of large-volume cylindrical samplesminimizes errors linked to sam-ple preparation. Disturbance of the sample material only occurs veryclose to the sidewalls of the sample cylinder as it is punched out of thedrillcore sample. Thus, measurement artifacts are negligible, consider-ing that synchrotron texture analysis is on the whole penetrated cylin-drical sample volume consisting of sections of about 1 mm in height,instead of analyzing a very thin surface layer (standard XTG;e.g., Behrmann and Kopf, 1993). Additional advantages arise fromusing the Rietveld method of texture evaluation, because the back-ground radiation is much better approximated by a Rietveld fit in syn-chrotron texture analysis, than in simple scans across the diffractionpeaks in X-ray texture goniometry (Wenk et al., 2010b). Also, synchro-tron texture analysis offers clear advantages in distinguishing clayminerals with superposed and overlapping peak spectra by using thewhole diffraction spectrum (Wenk et al., 2008, 2010b). Because of thehigh brilliance and high energy of synchrotron radiation, low-scatteringmaterials can be investigated much better than with conventionalmethods. Image plate detectors enable the acquisition of complex textureinformation within a short time. Water in the samples slightly increasesbackground scattering, but synchrotron radiation is not significantlyaffected by water, allowing the analysis of water-saturated sedimentunder “as-is conditions” without additional sample preparation anddrying. Fast experimental procedures using high-energymonochromaticX-rays allow thematerial to remain in shape and avoid dehydration. Thishelps to minimize changes in the material properties, which arerestricted to the immediate contact zone of the sample material withthe container wall.
Synchrotron texture analysis is a new method, which means thatsynchrotron radiation for the application of this method is not yet easilyaccessible to a wide community. However, there is potential for a largeincrease in research efficiency in using this method, creating the possi-bility to generate large, comprehensive data sets. For example, the datapresented in this study required only two days of beam time at DESY,Hamburg, for complete measurement.
6. Conclusions
- Using the example of silty clay sediments from the Nankai Trenchdrilled during IODP Expeditions 315, 316 and 333, we show thatRietveld-based synchrotron texture goniometry is a new and valuableanalytical tool to quantify textures of fine-grained phyllosilicate-richsediments that have undergone very limited lithification. Themethodallows study of large volumes of water-saturated sediment,and profits from good diffraction peak definition and background-peak ratios.
- In the naturally compacted samples from the Nankai accretionarywedge and from the incoming Philippine Sea Plate [001]-planes of il-lite, kaolinite and smectite show bedding-parallel CPO, increasingwith drillhole depth, thus reflecting progressive burial and compac-tion. The strongest CPO exists in the samples with the deepest burial.In a few samples, calcite and albite display a CPO related to crystallo-graphically defined non-isometric grain shapes, or nannofossil tests.When bed tilting is observed in response to tectonic deformation,bedding-normal maxima are preserved.
- Experimental deformation performed in triaxial compression causesreorientation and/or flattening of micropores normal to the appliedaxial stress, showing that these are sensitive strain gauges.Phyllosilicate and calcite CPO develop normal to the experimentalshortening axes, and there is at least a qualitative relation betweenCPO intensity and strain magnitude. The concurrent evolution ofmicropore fabrics and CPO is seen as themain cause for anisotropiesof physical properties inmarine sediments that have not undergonesignificant diagenesis.
Acknowledgments
This study used samples and data provided by the Integrated OceanDrilling Program (IODP). The authors would like to thank the ScientificParties of IODP Expeditions 315, 316 and 333 aboard D/V CHIKYU forsample collection and data acquisition. The comments of HuguesRaimbourg and an anonymous reviewer are appreciated. We wouldlike to thank G. Tondock for the construction of a clay sampling deviceand the acrylic sample holders. Furthermore, we are grateful toL. Raue, J. Walter and K. Ullemeyer for assistance during the measure-ments, support during data analysis, and discussion. We acknowledgeK. Ufer from BGR, Hannover, and R. Kaden from University of Halle forconducting comparative phase analysis. This work was funded byDeutsche Forschungsgemeinschaft (DFG) through Grant BE1041/28 toJHB and MS.
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