Cataclastic deformation of triaxially deformed , cemented mudrock 1 ( Callovo Oxfordian Clay ) : an experimental study at the micro / nano scale 2 using BIB-SEM 3

9 The macroscopic description of deformation and fluid flow in mudrocks can be improved by a better 10 understanding of microphysical deformation mechanisms. Here we use a combination of Scanning 11 Electron Microscopy (SEM) and Broad Ion Beam (BIB) polishing to study the evolution of micro 12 structure in samples of Callovo-Oxfordian Clay that were previously tested in the lab. Digital Image 13 Correlation (DIC) enabled for the measurement of strain fields in the specimens, which were used as a 14 guide to select regions in the sample for BIB-SEM analysis. Microstructures show evidence for 15 dominantly cataclastic mechanisms (intergranular, transgranular, intragranular cracking, grain rotation, 16 clay particle bending) down to nmscale. At low strain, the dilatant fabric contains individually 17 recognizable open fractures, while at high strain the reworked clay gouge contains broken non-clay 18 grains, with a clear change towards smaller pores than the undeformed material and corresponding 19 resealing of initial fracture porosity. 20 This study might provide a first step towards a micro scale basis for constitutive models of 21 deformation and fluid flow in cemented mudstones 22

1.25 µm/s, i.e., a strain rate of 2.5 10 -5 s -1 (see Bésuelle & Hall 2011 for further details).Figure 2a shows the evolution of the differential stress (σ 1σ 3 ) vs. axial strain (specimen shortening divided by its initial height).The curve shows a first stress peak at 0.02 axial strain, followed by a strong stress drop.Then, a slow stress increase is observed, followed by a second stress drop at 0.42 axial strain.After, the stress is quite constant.As shown in Figures 2b and 2c, these two stress drops are associated with major failure by faulting in the specimen.The crack that appeared during the second drop is conjugate to the first crack set, which appeared at the first drop.This set of conjugate fractures, at an angle of 20° to 45° about the direction 1, will be referred to as "main synthetic fractures" in the following sections.The DIC-derived strain fields in Figures 2b and 2c also show that the development of each single conjugate fracture is accompanied by relay zones with a set of antithetic fractures.Moreover, the fracture appearing during the second stress drop (Fig. 2c) is also reactivating the first fracture and its associated antithetic fractures.At this resolution (pixel size is 10 µm), the set of conjugate fractures and the associated antithetic fractures are the major features of localized deformation: they represent zones where the sample was sheared with damaged zones having a thickness of Solid Earth Discuss., doi:10.5194/se-2016-131, 2016 Manuscript under review for journal Solid Earth Published: 4 October 2016 c Author(s) 2016.CC-BY 3.0 License.
(2) The cylindrical sample COX-10MPa (10 mm in diameter and 20 mm in height) was sheared in triaxial compression at a confining pressure of 10 MPa.The test was carried out under tomographic monitoring at the European Synchrotron Radiation Facility (ESRF) in Grenoble, (France), using an original experimental set-up developed at Laboratoire 3SR at the University of Grenoble Alpes (France).. Complete 3D images of the specimens were recorded throughout the test using x-ray microtomography (voxel size was 14 µm).The test was displacementcontrolled, with a displacement rate of 0.05 µm/s, i.e., an axial strain rate of 2.5 10 -6 s -1 .The stress-strain curve (Figure 3.a) shows only one stress peak at an axial strain of 0.04.The peak stress is followed by a major stress drop corresponding to the formation of a shear fracture (referred to as "main synthetic fracture" in the following sections) oriented at an angle of 30-40° about the direction of the principal stress σ 1 (the DIC-based maximum shear strain fields are given in Fig. 3b).The DIC-derived volumetric strain fields (not shown here, see Lenoir 2006) indicate that the shear fracture is accompanied by some slight dilatancy.

Methods: BIB-SEM imaging of deformed microstructures
Sub-samples were selected to represent areas with different strain history based on the DIC analysis.
For COX-2MPa, three BIB cross sections were prepared around the conjugate fractures in areas with different amount of diffuse strain (at the resolution of DIC), antithetic fractures (ROI-2, Figures 4.b,6.b,c,d and 7) and a fourth one in a region without measurable strain (ROI-1; Sub-samples were first stabilized with epoxy, extracted with a low speed diamond saw in dry conditions, pre-polished dry using SiC polishing papers and BIB polished by using a JEOL SM-09010 cross-section polisher (for 8 h, 1.10 -3 -1.10 -4 Pa, 6 kV, 150 µA).BIB cross-sections are all prepared parallel both to the principle stress (σ 1 ) direction and perpendicular to the shear displacement plane.

Overview of microstructures
The sub-sample without measurable strain (i.e.ROI-1_COX-2MPa, Figure 6a) shows non-clay minerals in a clay matrix with a weak shape preferred orientation parallel to bedding (perpendicular to the experimental σ 1 ).The clay matrix contains submicron pores typical of compaction and diagenesis, with a power law distribution of pore sizes.Pores commonly have very high aspect ratio, with the long axis oriented sub-parallel to the bedding.These characteristics are very similar to those in the undeformed COX sample (Figure 5, cf. Robinet et al. 2012).
In all other BIB cross-sections (Figures 6.c-f and 7), both samples show damaged microstructures.At the sample scale, three different types of fracture are identified: (i) the main synthetic fracture (section 2), (ii) antithetic fractures (Figure 6) and (iii) joints sub-parallel to the main fracture.The material between the fractures zones has very similar microstructure to the undeformed COX.

Arrays of antithetic fractures
In COX-2MPa the antithetic fractures (Figure 7) are of two different types.Type I is located only in the clay matrix (Figure 8.a), with apertures up to few micrometres, with boundaries closely matchingsuggesting that these are opening mode fractures (Mode I).Type II fractures consist of a damage zone with thickness up to 25 µm (Figure 8.e, f, g, h, i) containing angular fragments of non-clay minerals and clay aggregates, (Figure 8.h), sometimes with preferred orientation parallel to the fracture.The transition between the damage zone and the undeformed host rock is sharp (Figure 8.f,g,h,i).In relay zones the fracturing becomes so intense that the clay matrix is fragmented into clay-size fragments (Figure 8.i).Porosity in these relay zones is locally much higher and pores much larger than in undeformed COX.Fracture boundaries usually do not match (Figure 8.h). Figure 8.e shows examples where parts of broken non-clay minerals can be matched.COX-10MPa, we observed the two types of antithetic fractures mentioned above.Antithetic fractures of Type I are very similar (indicated in Figure 6.f) to those in COX-2MPa but rare, whereas antithetic fractures of Type II contain a wider damage zone in comparison to those in COX-2MPa, in which the average grain size and the pore size is significantly smaller, consistent with stronger cataclasis at high confining pressure.In parts of the damage zones interpreted to be restraining sections, pores in the reworked clay aggregates cannot be resolved in the SEM.

Synthetic fractures
The synthetic fractures are the regions that localized most of strain and have the thickest damage zone (Figures 2 and 3).Here, COX-2MPa and COX-10MPa show very similar microstructures.The grain (fragment) size of non-clay minerals is significantly smaller than in the host rock and their sizes are poorly sorted.Non-clay minerals have also angular or/and chipped edges (Figures 9,10 and 12).
Locally, grains in the damaged zone show trans-granular fractures (Figure 10.c and 12.a).In parts of the damage zone, dilatancy and a strong increase in connected porosity (ROI-4_COX-2MPa, Figure 9 and ROI-2_COX-10MPa, Figure 10) is indicated by epoxy impregnation.In other parts, (ROI-1_COX-10MPa) strongly reworked clay matrix is not impregnated and shows no pores visible at the resolution of image (83.8 nm pixel size in Figure 11.b, c).
For COX-2MPa, the DIC analysis shows that the conjugated synthetic fractures form a complex network of fracture's branches at region where they both intersect (Figure 2

Artefacts caused by drying and unloading
Claystones are sensitive to changes in hydric conditions that can lead to the shrinkage or the swelling of the clay matrix (Galle, 2001;Kang et al., 2003;Soe et al., 2007;Gasc-Barbier & Tessier, 2007;Cosenza et al., 2007;Pineda et al., 2010;Hedan et al., 2012;Renard, 2012;Desbois et al., 2014).6, 7 and 8.f-g).One exception is the antithetic fractures of Type I observed in ROI-1_COX-10MPa (Figure 6.e), which is parallel to bedding.Mode I fractures sub-parallel to the main synthetic fractures are less easy to interpret: they may be related to the rotation of blocks between the antithetic fractures (Kim et al., 2004).Cryogenic techniques to preserve wet fabrics combined with ion beam milling and cryo-SEM (Desbois et al., 2008(Desbois et al., , 2009(Desbois et al., , 2013(Desbois et al., , 2014) ) is the dedicated technique to solve this question in the future.

Deformation mechanisms
The first, and perhaps surprising conclusion based on the observations above is the dominantly cataclastic deformation in Callovo-Oxfordian Clay at confining pressures up to 10 MPa.
Microfracturing, producing fragments at a range of scales and reworking into a clay-rich cataclastic gouge during frictional flow are the main processes in both samples.This is accompanied by dilatancy by microfracturing of the original fabric, but also by progressive decrease of porosity and pore size in the gouge with the non-clay particles embedded in reworked clay.Microstructures in the main synthetic fractures, both in COX-2MPa (Figure 9) and COX-10 MPa (Figures 10 and 12), are similar.Angular non-clay minerals in the reworked clay matrix have a wide range of grain sizes, smaller than those in the host rock.These characteristics are typical for cataclasis (Passchier & Trouw, 2005).COX-2MPa the cataclastic gouge seems to be more porous than in COX-10 MPa; this is as expected for the lower mean stress, but firm conclusion require further study to exclude that this is an unloading and drying effect.For COX-10MPa, the porosity in clay matrix is clearly reduced in comparison to the one in the host rock: most pores, if present, are below the resolution of SEM (Figure 10 and 11).The mechanism of this compaction during shearing is interpreted to be a combination of cataclasis of the cemented clay matrix, and shear-induced rearranged the clay particles around the fragments of non-clay particles.

Conclusions
The integration of bulk stress-strain data, the analysis of displacement fields by 3D digital image correlation (DIC) with Broad Ion Beam cutting and Scanning Electron Microscopy (BIB-SEM) is a powerful multiscale method to study the deformation behaviour of mudstones.At low strain, the dilatant fabric contains individually recognizable open fractures, while at high strain in shear fractures the reworked clay gouge evolves towards smaller pores than the undeformed material and corresponding resealing of initial fracture porosity.This shear induced resealing is more important at the higher confining pressure.
This study provides a first step towards a microphysical basis for constitutive models of deformation and fluid flow in cemented mudstones, with an improved extrapolation of these models for long time scales.
In the future, the microstructures on experimentally deformed specimens needs to be compared with the microstructures in naturally deformed claystones (Laurich et al.;2014) in order to extrapolate the constitutive models to long time scales.incremental volumetric strain fields (VSF) and maximum shear strain fields (SSF) fields for deformation increment 1-2 and 3-4 indicated in (a) after DIC.After Bésuelle et al. (2011).

Solid
Earth Discuss., doi:10.5194/se-2016-131,2016 Manuscript under review for journal Solid Earth Published: 4 October 2016 c Author(s) 2016.CC-BY 3.0 License.8/30 In both samples, the fragments between the arrays of antithetic fractures show only minor deformation indicated by fractured grains of organic matter (Figure 8.b), calcite (Figure 8.d, c) or quartz (Figure 8.d).Visible relative rotation of parts of fractured grain is rare (Figure 8.d).
.c).The ROI-3_COX-2MPa sub-sample (Figure 4.c) covers two of these branches.Microstructural analysis of these two branches in ROI-3_COX-2MPa show similar microstructures, with only the fracture apertures being different (Figure 6.c).In both COX-2MPa and COX-10MPa, the damage zone of the synthetic fractures contains an open fracture (Figures 9, 10 and 12), with apertures of 50 -70 µm.These large open fractures are filled with epoxy, have matching boundaries and never crosscut the non-clay minerals in the damage zone.Similar fractures are found in COX-2MPa but parallel to the antithetic fractures, with jagged morphologies, matching walls never crossing the non-clay minerals (Figure 8.b, c, e).These fractures are not resolved by DIC.
SolidEarth Discuss., doi:10.5194/se-2016-131,2016   Manuscript under review for journal Solid Earth Published: 4 October 2016 c Author(s) 2016.CC-BY 3.0 License.9/30TheDIC analysis is not affected by this because the images were acquired during deformation of preserved (wet) samples.SEM analysis is done on samples which have been deformed and unloaded, followed by slow drying and further dehydration in the high vacuum of the BIB and SEM.In COX-10MPa, this is illustrated by Figures 4.d and 4.e. Figure 4.d shows the sample at the end of the deformation experiment, whereas Figure 4.e shows the same sample but about 10 years later, both imaged with X-ray.The comparison of Figures 4.e and 4.e shows that cracks developed parallel to the bedding and that the apertures of fractures developed during the deformation became larger.These are interpreted to result from unloading and shrinkage during drying and aging of specimens.We infer that similar changes occurred in COX-2MPa: the wider damage zones in conjugate synthetic fractures imaged by SEM (Figure 6.c, d) compared the width estimated from the DIC analysis corroborates this interpretation.The fractures in the fragments between the arrays of antithetic fractures (Figure 8.b,c,d) are not present in the low strain ROI-1_COX-2MPa, and they are sub parallel to σ 1 , strongly suggesting they are formed by experimental deformation.In contrast, antithetic fractures of Type II (Figures 6, 7 and 8.e-i) are interpreted to develop during deformation because: (i) the internal microstructures and fabrics are damaged and (ii) DIC recorded a clear localization of strain in these.Though the antithetic fractures of Type I are not clearly recognized at the resolution of DIC, most of these in COX-2MPa (Figure 8.a) are interpreted to develop during deformation because they are oblique to the bedding and parallel to the antithetic fractures of Type II (Figure Solid Earth Discuss., doi:10.5194/se-2016-131,2016 Manuscript under review for journal Solid Earth Published: 4 October 2016 c Author(s) 2016.CC-BY 3.0 License.10/30 Although in many cases the initial fractures propagate around the hard non-clay grains, there is also significant fracturing of the hard non-clay minerals (e.g. Figure 8.b-d).This can be due to local stress concentrations at contacts between adjacent non-clay clay minerals, or because the clay matrix is so strongly cemented that it can transmit stresses sufficient to fracture calcite and quartz grains.Broken non-clay minerals can displace or rotate with respect to each other (Figure 8.d) with local dilatancy during deformation (Figure 2.b), in agreement with the interpretation of DIC measurements in Bésuelle & Hall (2011) and Lenoir et al. (2007).In COX-2MPa, the propagation of antithetic fractures of Type I (Figure 8.a) is predominantly in the clay matrix (Figure 8.a).This is in agreement with the smaller strain in comparison to antithetic fractures of Type II.Antithetic fractures of Type II contain angular non-clay grains with size smaller than those in the host rock.We interpret these as evidence for comminution by grain fracturing.Matching broken grains (Figure 8.e) are rare and in agreement with high strain cataclastic flow.Fragments of clay aggregates in the antithetic fractures of Type II are much less coherent (Figure 8.h) and more porous than the undeformed COX (Figure 8.i), indicating strong remolding by cataclastic flow, and perhaps also plastic deformation of phyllosilicates.Here, pore morphologies are not compatible with drying -induced deformation only, and we infer that these developed during deformation.

Solid
Earth Discuss., doi:10.5194/se-2016-131,2016 Manuscript under review for journal Solid Earth Published: 4 October 2016 c Author(s) 2016.CC-BY 3.0 License.11/30 We studied samples of Callovo-Oxfordian Clay (COX) subjected to triaxial compression at 2 MPa and 10MPa confining pressure.DIC was used to locate regions deformed to different states of strain and BIB-SEM allows microstructural investigations of mineral and porosity fabrics down to nanometre scale.Microstructures show evidence for dominantly cataclastic mechanisms (intergranular, transgranular, intragranular cracking, grain rotation, clay particle bending) down to nm-scale.

Figure 1 :
Figure 1: Drawing of the experimental concept used for the investigation of experimentally deformed finegrained mudrocks from bulk-scale to nm-scale.The example is based on a triaxial deformation test (10 MPa confining pressure) performed on a cylindrical Callovo-Oxfordian Clay, where displacement fields were followed by volumetric DIC on X-ray microtomography images (after Lenoir et al., 2007;).

Figure 2 :
Figure 2: Results of deformation test done on sample COX-2MPa.(a): deviator stress vs. axial strain response.The red star indicates the state of sample when BIB-SEM microstructural analyses are done.(b) and (c):

Figure 3 :
Figure 3: Results of deformation test done on sample COX-10MPa.(a): deviator stress vs. axial strain response.The red star indicates the state of sample when BIB-SEM microstructural analyses are done.(b) and (c): incremental maximum shear strain fields for deformation increment 1-2 and 2-3 indicated in (a) interpreted after DIC.After Lenoir et al. (2007).

Figure 4 :
Figure 4: Selection of differently strained areas (ROI) highlighted from DIC analysis on samples COX-2MPa (a-b) and COX-10MPa (c-e), for BIB-SEM microstructural analyses.(b): for COX-2MPa, four ROI were analysed: three at conjugate synthetic fractures in areas with different amount of diffuse strain and antithetic fractures (ROI-2, ROI-3 and ROI-4) and one in a region without measurable strain.(e): for COX-10MPa, two ROI were analysed both around the single synthetic shear fracture.(d) shows the X-ray radiography of the sample taken directly at the end of the deformation test.(e) shows the X-ray radiography of the same sample but taken about 10 years after the end of the deformation: drying cracks developed following the bedding and the aperture of the single shear fracture became larger.Orientation of σ 1 and of the bedding are indicated in red.

Figure 5 :
Figure 5: (a): BSE SEM micrograph of the typical mineral fabric in undeformed COX.(b): SE2 SEM micrograph of a detail from (a) showing the typical pore fabric in undeformed COX.

Figure 6 :
Figure 6: BSE SEM micrographs of the BIB cross-sections' overviews of COX-2MPa (a-d) and COX-10MPa (e-f) at differently strained areas (ROI) highlighted from DIC analysis in Figure 4. High strained ROI (c-f), display damaged microstructures, where three different types of fracture are identified: (1) the main synthetic fracture, (2) antithetic fractures oriented about 60º to the main fracture and (3) joints sub-parallel to the main synthetic fracture.These fracture are respectively indicated by 1, 2, 3 numbers in the figure.Orientation of the principle stress (σ 1 ) is indicated in red.Dashed yellow lines indicate the boundaries of the BIB polished areas.

Figure 7 :
Figure 7: Larger field of BSE SEM micrograph of the BIB cross-section's overview at ROI-1 in COX-2MPa sample.It shows the network of antithetic fractures (indicated by number 1) oblique to the principle main synthetic fracture (indicated by number 2).Orientation of the principle stress (σ 1 ) is indicated in red.Dashed yellow lines indicate the boundaries of the BIB polished areas.

Figure 8 :
Figure 8: Detailed microstructures in sample COX-2MPa.(a): a fracture running parallel to the antithetic fractures and at the interfaces between non-clay mineral and clay matrix.(b) and (c): intragranular fractures (i) and transgranular fractures (ii) at impingement of non-clay minerals.(d): a broken quartz grain showing evidence for rotation of its broken fragments.(e): incipient of flow of broken non-clay mineral within the antithetic fractures.(f) and (g): parts of antithetic fractures displaying thick damaged fabrics made of broken grains and clay matrix fragments.(h): Detail from (g). (i): Detail from (f) showing the denser and deformed fabric of a part of the clay matrix squeezed between a quartz grain located in the damaged fabric and the boundary with the host rock.In (f-i), the damaged zone is related to a higher porosity in comparison to the host rock.Orientation of σ 1 is indicated in red.Dashed yellow lines indicate the boundaries between the damaged fabric and the host rock, and also some grain boundaries.Qtz: quartz; Cc.: calcite; OM: organic matter.Black squares in (f) are missing pictures.

Figure 9 :
Figure 9: Detailed microstructure close the main fracture (indicated by number 1) in sample COX-2MPa.The main fracture displays internal damaged fabric made of fragments of broken non-clay minerals and clay matrix.Close to the main synthetic fracture, the host rock displays jagged joints sub-parallel to the main synthetic fracture (indicated by number 3) starting and ending at antithetic fracture (indicated by number 2).Orientation of the principle stress (σ 1 ) is indicated in red.The dashed yellow line indicates the boundary between the damaged fabric and the host rock.

Figure 10 :
Figure 10: Microstructures of ROI-1 in sample COX-10MPa.(a-c): The damaged fabric within the main fracture (1) is made of fragments of non-clay minerals derived from the dense, tight clay matrix.(a): the large open fracture in the middle of the main fracture (black) is interpreted to develop after the experiment by unloading or/and drying (see Section 5.1 for details).(c): some grains within the damaged fabric, but close to the boundary between the damaged fabric and the host rock, show transgranular fracturing (ii).Orientation of the principle stress (σ 1 ) is indicated in red.The dashed yellow lines indicate the boundaries between the damaged fabric and the host rock.

Figure 11 :
Figure 11: Detail of Figure 10.b. Microstructures (ROI-1_COX-10MPa) showing detail of porosity in BSE SEM micrograph (a) and SE2 SEM micrograph (b).At the resolution of the SEM micrograph, the damaged fabric appears very low porous in comparison to the host rock.The dashed yellow line indicates the boundary between the damaged fabric (left) and the host rock (right).

Figure 12 :FiguresFigure 1
Figure 12: Detailed microstructures at ROI-2 in sample COX-10MPa.(a-c): The damaged fabric within the main synthetic fracture (indicated by number 1) is made of fragments of non-clay minerals and clay matrix derived from the host rock.(a): some grains within the damaged fabric, but close to the boundary between the damaged fabric and the host rock, show transgranular fracturing (ii).Detailed observations in (b) and (c) (SE2 SEM and BSE SEM micrographs of the same sub-area, respectively) show that parts of the damaged fabric display (1) porous island, where pores are between the fragments of non-clay and clay matrix; whereas other parts display (2) low porous islands made of fragment of non-clay minerals embedded in a dense, tight clay matrix.Pores within the porous island can be either filled with epoxy (in deep black pixel values) or not.