Paleo-deviatoric stress magnitudes from calcite twins and related structural permeability evolution in minor faults: Example from the toarcian shale of the French Causses Basin, Aveyron, France

Tectonophysics 429 (2007) 79 – 97 www.elsevier.com/locate/tecto

Paleo-deviatoric stress magnitudes from calcite twins and related structural permeability evolution in minor faults: Example from the toarcian shale of the French Causses Basin, Aveyron, France Joël Constantin a,⁎, Philippe Laurent b , Pierre Vergély a , Justo Cabrera c b

a Orsayterre, FRE 2566, Université de Paris Sud-Orsay, 91405 Orsay Cedex, France Laboratoire de Tectonophysique, UMR 5568, Université de Montpellier II, 34095 Montpellier Cedex, France c Laboratoire d'Etudes Hydrodynamique et Géotechnique, Institut de Radioprotection et de Sûreté Nucléaire, 92265 Fontenay-aux-Roses, France

Received 29 August 2005; received in revised form 21 September 2006; accepted 28 September 2006 Available online 14 November 2006

Abstract The relationship between deformation and so-called fluid paleotransfers in minor faults has been analysed in an argillaceous formation located in the Causses Basin in France. The fluid paleotransfers are related to the fault activity to a large extent. We attempt to estimate the intensity of paleo-deviatoric stress magnitudes under which the fault activity may have occurred and consequently, the change in the structural fault permeability. The paleo-deviatoric stress magnitudes were calculated with the inverse method of Etchecopar applied to calcite twinning. The measured crystals are contained within the core zone of minor faults and this study is based on a previous complete microtectonic and microstructural analysis of the faults. In this paper, analysis of calcite twinning has been applied for the first time to vein fillings associated small faults in a context of relatively weak deformation, a condition ensured by the tectonic and structural analysis. Calculation and discussion of the paleo-deviatoric stress tensors in relation to the evolution of the structural fault permeability and to the hydraulic behaviour of the faults are the aim of this paper. The analysed faults, created and active during the same tectonic event, were permeable under a (σ1–σ3) mean value of 40–50 MPa. On the other hand, the reactivation of faults during a second tectonic event implies mean (σ1–σ3) value higher than 40–50 MPa, especially for the faults that are poorly oriented with respect to the principal tectonic stress directions. The core zone of these faults remained sealed and impermeable or became permeable by development of microcracks inside the pre-existing fillings. © 2006 Elsevier B.V. All rights reserved. Keywords: Fault; Calcite; Twinning; Structural Permeability

1. Introduction

⁎ Corresponding author. Tel.: +33 1 69 156795. E-mail addresses: [email protected] (J. Constantin), [email protected] (P. Laurent), [email protected] (P. Vergély), [email protected] (J. Cabrera). 0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2006.09.014

The evolution of the structural fault porosity, and potentially the structural fault permeability, is a topic of interest in the case of long-term radioactive waste confinement in deep geological formations having low intrinsic permeability, such as the argillaceous formations. The occurrence of natural discontinuities – particularly tectonic fractures – might change the effectiveness of the

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geological barrier and might therefore constitute weakness zones in terms of containment properties (Pusch, 1998). The structural fault porosity may have varied in the past and it is interesting to attempt to constrain the conditions under which these variations occurred, particularly in terms of state of stress. This kind of information is essential for studies of repository safety. The hydraulic behaviour of faults – conduit, barrier and conduit–barrier (Antonellini and Aydin, 1994; Caine et al., 1996) – may change with time and space in relation to the deformation processes associated with faulting and fault development (Knipe, 1992; Knipe et al., 1998; Lonergan et al., 1999). Generally the faults correspond to a volume that can be divided into the damage zone (DZ) and the core zone (CZ) (Sibson, 1990; Antonellini and Aydin, 1995; Cox, 1995; Evans et al., 1997; Seront et al., 1998; Heynekamp et al., 1999; Moretti et al., 2000; Gudmundsson et al., 2001; Gray et al., 2005). In these structural compartments, structures of different scales impact the structural porosity. However, the changes in structural porosity within the fault zone are primarily controlled by the deformation mechanisms depending mainly on the deviatoric stress. Consequently, the valuation of the differential stress is required to estimate the conditions for variation in the hydraulic behaviour of the fault. Our approach for the valuation of the paleostress magnitudes relies on the analysis of the twinning of calcite grains according to the method developed by A. Etchecopar (1984) and P. Laurent (1984; Laurent et al., 1981). This technique has been validated experimentally and in various contexts (Lacombe and Laurent, 1992; Lacombe et al., 1993; Rocher et al., 1996; Laurent et al., 2000). This paper makes an original application since the analysed calcite grains are not contained in the matrix of the host rock but in the calcitic syn-tectonic fillings sealing the CZ of faults. This investigation and the (micro)tectonic and (micro)structural analyses already published (Constantin et al., 2002, 2004) allow to discuss the conditions in which the hydraulic behaviour of the fractures, and consequently the containment capacities of the argillaceous rock, might be changed.

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2. Geological setting (framework) The argillaceous formation, dated Domerian–Toarcian (175–184 Ma), is located in the Causses basin (southern France; Fig. 1). More precisely, this study concerns the western border of the basin, on the Tournemire area, where the thickness of the argillaceous formation reaches 250 m. The formation is constituted of marls and shale and its mineralogy includes a predominant clay fraction (40– 50%) made up of illite, kaolinite, chlorite and smectite, calcite (10–30%), quartz (10–20%), dolomite and siderite (3–5%) and pyrite (3%) (Cabrera Nunez et al., 2001). The study area corresponds to a block trending E/W, limited by two reverse regional faults between which the structure is slightly monoclinal with a dip of 5° to the North. Tectonic fractures were characterised and the tectonic events responsible for their formation, their dynamic development and their possible reactivation were identified (Constantin et al., 2002). In summary, fractures result from two main tectonic stress regimes. The first is extensive and occurred during the Mesozoic. Fracturing associated with this extensive phase occurred at two periods: (1) as early as the rock compaction, during Dogger, when σ3 was NW–SE, and (2) during Upper Jurassic and probably Lower Cretaceous (Bles et al., 1989; Martin and Bergerat, 1996), when σ3 was N–S. This second event has mainly produced conjugate normal faults trending E–W and two perpendicular sets of extensional joints trending E–W and N–S. The second main tectonic stress regime corresponds to the Eocene– Pyrenean compression. The directions of σ1 varied from NE/SW to NW/SE, with two major pulses striking 020– 030 and 160–170. Fractures associated with this tectonic phase are vertical joints trending 020 and 160 and reverse faults with low dip. Joints may have been reactivated during this same phase as sinistral strike-slip faults. As for the pre-existing fractures, the normal E/W faults and the joints may have been reactivated respectively as reverse or dextral strike-slip faults and as dextral or sinistral strike-slip faults. The analysis of the calcite twinning was carried out on eight small faults. These faults have pluri-metric

Fig. 1. Location of the studied area. (A) Simplified geological map of the Causses basin. (B) Geologic map of the Tournemire area indicating the location of sampling sites and geologic cross-section. The double black arrows indicate, on the map, the directions of maximum horizontal tension (σ3) or compression (σ1), as constrained by the inversion of the fault slip data. More details below. (C) Principal paleostress axes computed from fault slip data; each computed paleostress tensor is defined by its principal stress directions σ1, σ2 and σ3, respectively diamond, triangle and square on the stereographic diagram — lower hemisphere projection; the double black arrows indicate the directions of maximum horizontal tension (σ3) or compression (σ1), resulting from the inversion (σ1 direction for a strike-slip or reverse stress tensor and σ3 direction for an extensive stress tensor); the stress ellipsoid shape ratio is defined by ϕ = (σ2 − σ3)/(σ1 − σ3) (with ϕ = 1 − R, R = (σ2 − σ3)/(σ3 − σ1), given by the Carey's method used for the inversion computation of the fault data (in Constantin et al., 2002)). The histogram shows the angular deviation for each fault plane between the measured (s) and the computed (τ) slip-vector.

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dimensions and metric offset (Constantin et al., 2002); they are intra-argillaceous and contain calcite cementation indicating fluid paleotransfers (Fig. 2A). Among these faults (Table 1), we distinguish five normal faults (faults 1, 2, 3, 4, and 5) – of which two (faults 2 and 4) show kinematic indicators (striae) of reactivation as strike-slip faults – two reverse faults (faults 6 and 7) and one strike-slip fault (fault 8). The mean deviatoric stress tensors have been derived from inversion of fault slip data, collected respectively on sites A, C and D (Fig. 1; Table 2). This computation used the algorithm of Carey (1979) which assumes that sliding occurs on the fault plane in the direction of the resolved shear stress. 3. Structural permeability evolution of the minor faults The microstructural analysis of the fault zones is described in detail in Constantin et al. (2004). Eighteen faults were sampled (Fig. 2A) and analysed to understand the deformation mechanisms having controlled the variation of the structural permeability through time. The calcite twin analysis was carried out on eight of these faults. Note that the numbering of faults was changed with respect to our previous study (Constantin et al., 2004) for simplicity. Table 1 gives the correspondence between the two orderings. The microstructures have been defined using two planes: one is perpendicular to the fault plane and to the striation and the other is perpendicular to the fault plane and parallel to the striation. It has been shown that the “hydraulic state” of the faults was controlled by the nature and the architecture of microstructures and by the variations of the petrophysical properties of the host rock within the core zone (CZ) and the damage zone (DZ). In the DZ, the evolution of the structural fault

permeability is associated with (1) microcracking (Fig. 2B and C) and (2) deformation related to a probable ductile behaviour of the shale (Charpentier et al., 2003 and Fig. 9 in Constantin et al., 2004). The structural fault permeability in the CZ is associated with the development of voids generated by (1) dilation (Fig. 2), (2) shearing and opening in extensional stepover or (3) microcracking in pre-existing calcite fillings (Figs. 2 and 3). Microcrack, solution surface (inside few CZ of normal faults; Fig. 2D), rare opening of cavity (observed solely in the fault 3, Fig. 2D) and calcite twinning are the possible deformation features entailed by the compression phase in the CZ of the normal faults. Episodic ruptures, followed by cementation and sealing, lead to cycles of permeability enhancement and reduction. When the crystallisations sealed the total structural porosity both in CZ and DZ, the faults became impermeable. The minor faults were permeable when the development of a new structural porosity occurred in both CZ and DZ. They were “semi-permeable” when only the CZ was sealed and impermeable. Indeed, the observed DZ and CZ are not necessarily associated with the same tectonic phase. In the example of fault 1 (Fig. 2B), we distinguish (1) the CZ composed of sheared calcite bands formed during the extension phase (striae on the surface of the calcite bands) and (2) the DZ consisting in sealed microcracks. The microcracks are sub-horizontal and are related to the compression phase with a growth direction parallel to σ1 horizontal direction and perpendicular to σ3 vertical direction (Tapponnier and Brace, 1976). The deformation occurred inside the DZ whereas the calcite grains of the CZ were deformed by twinning without occurrence of shearing reactivation. This observation involves that the structural permeability in the DZ was able to develop under stress intensity weaker than that necessary to the development of the structural permeability in the CZ. A

Fig. 2. Fault zone architecture and observed deformation features. (A) Photograph of sampled normal fault. The visible white cementation corresponds to the core zone (CZ). (B) Fault zone in which the damage zone (DZ) and the CZ are not associated with the same tectonic phase (fault 1). Photographs correspond to two thin sections perpendicular to the fault plane and respectively parallel and perpendicular to the striation. The microphotographs (a) and (b) show a magnification of the CZ and the DZ respectively. The black arrows indicate the sliding sense. The photographs are tilted in the orientation of the fault dip. The CZ is composed by sheared calcite bands formed during the extension phase and the DZ is composed by sealed sub-horizontal microveins that appeared during the compression phase and sealed by a red orange calcite in cathodoluminescence (microphotograph c). (C) Photograph of a fault zone in which the DZ and the CZ are associated with the same tectonic phase (fault 7). The CZ is composed by sheared calcite bands formed during the compression phase and the DZ is composed by sealed sub-horizontal microveins developed during the compression phase too. (D) Normal fault zone in which the compressive phase entailed the deformation of the pre-existing fillings (fault 3). The drawings correspond to two perpendicular thin section views (respectively parallel and perpendicular to the striation). The black arrows indicate the sliding sense. The CZ consists of a network of parallel calcite bands limited by shearing surfaces formed during the extensive phase. The deformation features associated with the compression are: microcracking, stylolithic surfaces, one cavity and calcite twinning. The microphotograph shows two calcite generations. The calcite of the parallel calcite bands is non-luminescent in cathodoluminescence and the calcite sealing the microcracks and the cavity is red orange.

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strain partitioning process was able to take place in the CZ between the fault surface and the calcite infilling and between the DZ and the CZ too.

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A schematic model of the hydraulic fault behaviour in this type of rock is given, in which the observations and inferences were included (Fig. 4). The e-twin analysis

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Fig. 2 (continued ).

J. Constantin et al. / Tectonophysics 429 (2007) 79–97 Table 1 Sampling site (A, B, C and D on Fig. 1), fault orientation, fault type Site

Number fault

Direction

Dip

Pitch

Fault type

A B C

8 7 1 4

N010 N070 N085 N095

80°E 30°S 32°S 44°S

6 (3) 2 (2)

N140 N115

40°S 55°N

3 (3) 5 (5)

N080 N095

55°N 52°S

34°N 60°W 85°W 90 0° 55°S 80°E 24°W 80°E 83°E

Dextral Reverse Normal Normal Dextral Reverse Normal Dextral Normal Normal

D

(11) (D) (1) (4)

For each fault, the number in brackets corresponds to the fault number in the article of Constantin et al. (2004) for which a complete microstructural analysis has been developed.

aims at adding values (or range of values) of deviatoric stress to this model. The microstructure types observed in each CZ are gathered in the Fig. 5, which also indicates the tectonic event associated with the microstructure development as well as the deformation process. Calcite crystallisations presently seal these microstructures. The calcite grains are twinned and did not undergo important finite rotation (coaxial strain), which allows to make use of the calcite twinning analysis. For each fault, the analysed crystals exhibit the same characteristics through cathodoluminescence analyses and are associated with the same microstructure kind (dominoes and sheared calcite bands). The analysed crystallisations took place during the same tectonic event, but are not necessarily related to the same deformation increment since the deformation mechanisms – “crack–slip, then seal” and alternance of crack–seal and slip – are repetitive. 4. Determination of the paleo-deviatoric stress from calcite twin lamellae Several methods for determining stress or strain components from calcite twin lamellae have already been proposed (see complete review of Burkhard, 1993). Stress is obtained from strain via the stress– strain relationship for the material. For this study, we used the inverse Etchecopar method (Etchecopar, 1984; Lacombe and Laurent, 1996; Laurent et al., 2000) because it is well designed to the study of polyphased samples. This method allows the direct determination of the reduced stress tensor (RST) responsible for the twinning. This RST is defined by four parameters (Angelier, 1984, 1989) which are the three principal stress orientations and the stress ellipsoid shape ratio

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(SESR) ϕ = (σ2 − σ3)/(σ1 − σ3). This method is quite similar to that using striated fault sets. Basically the RST is not a deviatoric stress tensor, as its principal stress values are respectively: σ1 = 1; σ2 = ϕ and σ3 = 0. But, a simple calculation leads to the deviatoric part of the RST (Laurent, 1984; Lacombe, 2001). The determination of the actual RST is made possible by introducing a scaling parameter, namely the critical resolved shear stress for twinning (CRSS) τs. This parameter is supposed to be constant for all the calcite grains and independent on the temperature and normal stress (see discussion in Laurent et al., 2000). Hence, the actual differential stress values (σi − σj) are proportional to τs. This method makes use of the whole e planes data set (twinned and untwinned planes), without any separation prior to the calculation (Fig. 6). This method has been successfully applied to numerous geological samples in many different geological areas (Lacombe et al., 1993, 1996; Lacombe, 2001), but the key point which has been pointed out is the calibration of the critical resolved shear stress value for twinning (CRSS) τs. After the famous paper of Turner et al. (1954), and Tullis (1980), most of the authors have used a constant value for τs that is equal to 10 MPa (eg. Craddock et al., 1993; Lacombe et al., 1996; Gonzalez-Casado and Garcia-Cuevas, 1999). However, in the slightly deformed platform carbonates occurring in the foreland areas of mountain chains, it has been shown that this value was too high (Lacombe et al., 1991, 1993). In 2000, the experimental work of Laurent et al. established that (1) the CCRS value for twinning on e in calcite grains was somewhat dependent on both temperature and strain; (2) for incipient twinning associated with very low deformation, the critical resolved shear stress value for twinning τs is more likely comprised between 5 and 7 MPa (see also discussion in Lacombe,

Table 2 Parameters of the stress tensors determined from fault slip analysis Sites NF σ1

A A C C D

5 10 8 6 8

σ2

σ3

Az. (°)

Dip. (°)

Az. (°)

Dip. (°)

Az. (°)

Dip. (°)

217 159 028 177 153

72 14 80 07 82

112 318 274 025 256

05 75 04 83 02

020 068 183 268 346

17 05 09 03 08

MD ϕ (°)

7.5 14 6.2 5.8 2.8

0.695 0.252 0.416 0.449 0.132

NF: number of faults used for computation; σ1, σ2 and σ3 the principal axes of the stress tensor; MD: mean angular deviation; ϕ: the stress ellipsoid shape ratio, (σ2 − σ3)/(σ1 − σ3). For details, see Fig. 1.

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Fig. 3. Photomicrographs of twinned calcite grains observed in the core zone of the faults (photomicrographs 1 and 2: natural light; 3 and 4: polarized light). Twin lamellae (T) are along the {01ī2} crystallographic planes. (A) and (B) Examples of twinned calcite grains into a sheared calcite band. (C) and (D) Twinned calcite grains and microcracking: inhomogeneous deformation with relative sliding along the grain boundaries and intra/intergrain microcracks sealed by pyrite (crystallisation in black on the photographs). The twin lamellae are straight and thin but they appear more or less thick due to their dip with respect to the plane of the thin section.

2001). In this paper, the calculation of the actual differential stress values (σi − σj) will be done using these two values (see Table 3). For each sample, calcite twins were collected using a Universal Stage within two mutually perpendicular thin sections (perpendicular to the fault plane respectively parallel and perpendicular to the fault striation). The lack of crystallographic preferred orientation is a basic condition to apply the stress inversion method. This condition is verified for all the samples except that of fault 5, which shows a weak preferred orientation of the optical axes (Fig. 7). For the eight faults, eleven reduced stress tensors were calculated from the e-twin analysis.

5. Results and discussion 5.1. Paleostresses orientation The paleostress tensors are discussed with respect to the different tectonic regimes recognised by the classical microtectonic analysis (Fig. 8). The evaluation of the uncertainties of the stress inversion method from e-twin analysis is estimated to be +/− 10° for the orientations of σ1, σ2 and σ3 and +/− 0.1 for the ϕ ratio (Rocher et al., 1996; Rocher, 1999). The stress tensors inferred for faults 1, 3 and 5 are consistent with an extensive regime (the principal compressive stress σ1 is vertical). The direction of extension

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Fig. 4. Schematic model of the hydraulic behaviour of minor fault in argillaceous rock, as inferred from the microstructural analysis (after Constantin et al., 2004). The proposed evolution occurs between two seismic events during the stress loading (Δσ, the deviatoric stress). The curved dashed-line arrows indicate the fluid transfers from the host rock to the damage zone (DZ). The curved full-line arrows indicate fluid transfers within the DZ. The smallest full-line arrows indicate the fluid transfers from the DZ to the core zone (CZ) and within the CZ. Three main hydraulic states are distinguished: (1) the CZ and DZ are sealed and impermeable; (2) the CZ is impermeable and the permeability increases in the DZ (by microcracking and plastic deformation of the rock); (3) the CZ and the DZ are momentarily permeable (in consequence of the rupture of the sealed core zone).

Fig. 5. Deformation features in each fault core zone. C: microstructure appeared during the compressive phase. E: microstructure appeared during the extensive phase.

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Fig. 6. Projection of the normal stress and the resolved tangential stress components. Example of fault 1. Three paleo-reduced stress tensors were calculated; respectively 1A, 1B and 1C. σn is the normal stress ranging from 0 to 1. τs is the resolved shear stress ranging from − 0.5 to +0.5. Star denotes untwinned plane, void square stands for cases where the observed twinned plane is inconsistent with the solution and filled diamond stands for those cases where the observed twinned plane is consistent with the solution. Below, the continuous and dashed curves correspond to the classified twinned planes and the classified untwinned planes respectively. s is the minimum value of the resolved shear stress τs for the whole set of twinned planes accounted for by the reduced stress tensor solution ((σ1 − σ3) is scaled to 1). The percentage of compatible twinned planes is given for each solution (for details, see Laurent, 1984).

σ3 is close to N–S corresponding to the Mesozoic extension (Fig. 8). The principal directions of the stress tensor inferred from the fault 4 define an extensive regime with σ3 trending NW/SE, which shows a significant angular deviation compared to the stress state obtained with the fault data. The normal fault 2 provides a stress tensor compatible with reverse faulting: σ1 direction is oriented close to NNE/SSW. A second extensive stress tensor, with σ3 trending N/S and compatible with the observed motion on the fault plane, has been determined, but there are not enough twinned planes to convincingly establish the result. When examining the computed data for the normal faults 4 and 5, it appears that some e twinning is also compatible with a compressive regime. Neverthe-

less the twins are not numerous enough and too diverse in orientation to obtain a reliable stress tensor. Stress tensors compatible with a N/S compression were derived from faults 1, 2 and 3. Fault 1 provides three stress tensors. This is the only one that reveals entirely the main tectonic events that had undergone syn to post-formation of the analysed normal faults. The obtained σ1 directions are also close to those revealed by the microtectonic analysis. We distinguish one strike-slip regime characterised by σ1 direction close to N160 (N150 for fault 1) and one reverse regime defined by σ1 direction close to N020 (N008 for fault 1). Fault 3 provides two stress tensors. The first one defines the extension trending N–S but the second tensor is more difficult to correlate with a tectonic event and

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Table 3 Results from the calcite twin analysis (fault orientation in Table 1) Faults

n/p/q

σ1

σ2

σ3

F

ϕ

s

(σ1–σ3) (MPa)

(σ2–σ3) (MPa)

Fault 1

1A, 106/38/63 1B, 43/38/23 1C, 20/38/13 114/36/68 3A, 135/36/54 3B, 81/36/40 84/27/42 67/20/40 101/25/75 60/21/33 125/34/75

150–13 043–67 188–00 220–18 008–64 175–35 182–73 139–86 347–17 212–15 187–20

022–71 271–15 098–03 126–12 098–00 039–45 048–12 269–02 254–09 306–09 005–71

243–15 177–16 288–87 001–69 188–27 283–25 315–13 359–03 138–70 064–72 098–01

0.31 0.27 0.52 0.69 0.01 0.08 0.02 0.26 0.79 0 0.47

0.140 0.589 0.306 0.513 0.171 0.346 0.415 0.208 0.535 0.142 0.672

0.1025 0.2590 0.1335 0.1507 0.1361 0.1350 0.1509 0.1298 0.1261 0.1248 0.1367

49/68 19/27 37.5/52.5 33/46.5 37/51.5 37/52 33/46.5 38.5/54 40/55.5 40/56 36.5/51

7/9.5 11/16 11.5/16 17/24 6/9 13/18 14/19 8/11 21/30 6/8 24.5/34.5

Fault 2 Fault 3 Fault 4 Fault 5 Fault 6 Fault 7 Fault 8

• n/p/q respectively, number of measured twinned planes/number of measured untwinned planes/number of twinned planes compatible with the solution. • σ1, σ2 and σ3, principal paleostress directions (strike and dip). • F: penalization function. • ϕ: stress ellipsoid shape ratio, ϕ = (σ2 − σ3)/ (σ1 − σ3). • s: lowest τs value sustained by the twinned planes accounted for by the tensor solution computed from the reduced stress tensor: σ1 = 1; σ2 = P; σ3 = 0; 0 b s b 0.5. • (σ1 − σ3) = τs / s with τs = 5 MPa and τs = 7 MPa (for details: see text). • (σ2 − σ3) = ϕ × (σ1 − σ3).

seems to trend to a strike-slip regime related to the compression phase (σ1 direction close to N/S). The stress directions computed from calcite twinning associated with the faults 7 and 6 characterise a reverse tectonic regime consistent with the activity of the analysed faults. The σ1 directions, N167 and N032, are different for the two reverse faults and may correspond to the two tectonic events characterising the Pyrenean phase (middle to upper Eocene). The stress state obtained from the fault 8 defines a strike-slip regime with σ1 close to the N/S direction. This direction is sub-parallel to the fault plane and thus not really compatible with the observed motion. This stress state recorded by the twinned calcite indicates a local stress re-orientation of the σ1 direction. Then, the direction of the remote σ1 stress direction was N160, compatible with the second tectonic event of the compression phase. Anyhow the deformation by twinning is most certainly associated with the compression phase. The angular differences between the stress orientations obtained from the fault slip data and twinning data probably result from local (microscopic scale) stress reorientations inside the fault volume. One of the three stress directions may trend to a direction parallel to the fault. It occurs for σ1 in faults 8, 7, 1 and 3 (first tensor). On the other hand, σ1 tends to be orthogonal to the fault for faults 2 and 3 (second tensor). These stress reorientations may be related to the level of friction on the shearing surfaces and to the fault geometry (Ohlmacher and Aydin, 1997; Connoly and Cosgrove, 1999) evolving during the dynamic development. The stress tensors calculated from the faults and the twinning can be correlated in terms of paleostress orientation and the stress ellipsoid shape ratio as well. The

differential stress ratios are similar except for fault 8 and the first tensor of fault 1. The correlation between the results of the different analyses can be made, even if sometimes it is more difficult to distinguish the tectonic events of the compression phase. The analysed minor faults were active or not under the given states of stress, which has governed the evolution of the structural permeability. The agreement between the twinning analysis, the fault slip analysis and the microstructural analysis is essential to the interpretation in terms of stress dependent permeability. 5.2. Differential stress and structural permeability The actual differential stress values depend on the critical resolved shear stress for twinning (CRSS) τs. For incipient twinning associated with very low deformation, the critical resolved shear stress value for twinning τs is low and probably comprised between 5 and 7 MPa (Laurent et al., 2000). These two values will be used to calculate the actual differential stress (Table 3). (σ1 − σ3) is determined as follows: ðr1 − r3 Þ ¼ ss =s with s, the lowest τs value is sustained by the twinned planes accounted for by the tensor solution computed from the reduced stress tensor (σ1 = 1; σ2 = P; σ3 = 0; 0 b s b 0.5), and τs is between 5 and 7 MPa. (σ2 − σ3) is obtained by (σ2 − σ3) = ϕ × (σ1 − σ3) where ϕ corresponds to the stress differential ratio. 5.2.1. Significance of the differential stress values The stress states revealed by the twinning analysis were correlated to tectonic events under which the fault

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Fig. 7. Lower hemisphere projection of optical axes of calcite crystals (projection in the thin section plane parallel to the striation). The samples show no preferential crystal orientation except for fault 5 which shows a weak preferred orientation.

dilation/sliding occurred or not. Indeed the twinning of the calcite grains may have occurred even though the fault was not active. It implies that the differential stress inducing the twinning may be lower than that inducing the shearing and the fault activity, because otherwise, no twinning would be observed.

Subsequent to the failure, the formation of a new structural porosity and an increase of permeability occur within the fault zone. The deviatoric stress and the fluid pressure decrease and the fluid transfers occur followed by the sealing of the structural porosity. In consequence of the sealing, the decrease of permeability and the

J. Constantin et al. / Tectonophysics 429 (2007) 79–97 Fig. 8. Lower hemisphere projection of the principal stress axes calculated from faults and calcite twins (the sampled fault with its slickenside lineation is projected with the axes obtained from the calcite twins). Black diamond, triangle and square are respectively σ1, σ2 and σ3. ϕ corresponds to the differential stress ratio. The black arrows indicate the direction of compression or extension.

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strength recovery of fault zone occur (Tenthorey et al., 2003). The strengthening of the faults is verified, since the crack–seal process is associated with the deformation mechanisms arising within the core zone. The faults are sealed by calcite between two failure events. Then,

the deviatoric stress and the fluid pressure – if the permeability in the fault zone is lower enough to impede the fluid drainage – re-increase (1 and 2 on the Mohr diagrams of the Fig. 9A). That may induce the fracture reactivation consisting either of reopening according to

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the hybrid extensional-shear failure (fluid pressure up to and beyond the normal stress; formation of the calcite bands) or of the slip displacement according to the shear failure (fluid pressure below the normal stress and deviatoric stress up to the shear strength of the fault; shearing of the parallel calcite bands or formation of dominoes). The microstructures responsible for changes of the structural permeability within the CZ proceed from the repetition of this fault rupture mechanism in which the competition between the stress build-up and the fluid pressure build-up controls the deformation type (dilation or shearing). During the inter-seismic period, the stress build-up entails the formation of mechanical twins, increasingly numerous in terms of direction and number. Experimental works show that the twin density and the volume fraction increase accordingly with the differential stress (σ1–σ3) (Rowe and Rutter, 1990). This deformation progress by twinning is consistent with a low temperature of deformation. The temperature can be estimated owing to recent geochemical investigations performed in the study area and the south of Massif Central (Barbarand et al., 2001; Peyaud et al., 2005, 2006). The maximum burial of the toarcian sediments occurred during the basin formation at upper Jurassic or lower Cretaceous periods. In the study area, the maximum thickness of the formations surrounding the toarcian shale is estimated to be close to 2000 m (Peyaud et al., 2005). On the south of Massif Central, Barbarand et al. (2001) estimate that 2000 to 2500 m of rocks dating Jurassic–Cretaceous were eroded before the upper Cretaceous. Thus we can assume, with a geothermal ranging from 30 °C/km to 40 °C/km, that the deformation temperature did not exceed 100–110 °C. This range of temperature is consistent with the temperature obtained by the analysis of fluid inclusions contained in the calcite infillings of similar faults sampled on the study area (Peyaud et al., 2005). The average homogenisation temperatures measured in the aqueous two-

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phase inclusions contained in the calcite infillings formed during the extensive phase, ranging from 95 °C to 110 °C. The fluid trapping temperature is lower than 70 °C (the fluid inclusions are monophase) inside the calcite infillings formed during the compressive phase (Eocene). At this range of temperature, the strain process in the calcite grains was achieved first by the formation of new thin twins parallel to the same twin plane (e1) and second by the development of other twin planes (on the e2/e3) at higher differential stress values (Ferril, 1998; Laurent et al., 2000). The obtained values of differential stress are independent of the fluid pressure (Rowe and Rutter, 1990) and we can infer that the calculated value is the maximal value reached during the deformational cycles undergone by the calcite grains during the same tectonic event (the peak differential stress, Fig. 9B). The calculated differential stress corresponds to a local value which is not necessary indicative of the far field stress conditions and could eventually correspond to a high stress concentrations related to asperities along the fault. But the obtained values are very close (Table 3, Fig. 10), which seems to indicate the lack of high stress concentrations. Anyhow, the values of differential stress are indicative locally to the conditions for the failure. From the structural data and the e-twin analysis, the following assumptions concerning the permeability of the CZ of the fault can be given (Fig. 9B). (1) When twinning and dilation/sliding are established for a given tectonic event, the value of the differential stress is close to the peak value under which the dilation and sliding occurred, inducing the formation of a new structural porosity and possibly a change of the structural permeability inside the CZ of the fault. (2) When twinning only occurred for a given tectonic event (no dilation/ sliding), the differential stress needed to trigger fault activity was not reached. In this case, when no microcracks are observed, the CZ of the faults remained sealed and thus impermeable. But, when microcracks

Fig. 9. Stress cycling, microstructures and twins development in the core zone of the faults (illustration with normal faults). (A) Mohr diagrams of shear stress, τ, against effective normal stress, σn′ with the Griffith–Coulomb failure envelope for intact rock and the failure envelope for reshear. The diagrams show evolution of the stress states leading up to the formation of parallel sheared calcite bands or of dominoes. The sheared calcite bands are interpreted like the product of successive events of opening (followed by sealing) and shearing. This alternance opening/shearing may be principally controlled by the fluctuations of the level of fluid pressure within the fault zone. The strength recovery (recovery of the initial cohesive strength, C) of the fault can explain the change of fault behaviour for small fluctuations of fluid pressure. The motion along the shear surfaces controls the opening (followed by sealing) of the dominoes in transtension zones located between two shear surfaces. During the inter-seismic period, the increase of the differential stress (1 on the diagram; the diameter of the circle increases) and the sealing of the structural porosity by calcite may lead to the development of fluid overpressures (2 on the diagram; migration of the Mohr circle toward the failure envelope). The dashed arrows indicate fluid transfers on the drawings. The development of the core zones resulted from the repetition of the stress cycling and the deformation mechanisms. The development of the twins occurs during the inter-seismic periods. (B) Interpretation of the differential stress obtained from the calcite twin analysis. (1) Twinning and fault activity occurred during the same tectonic event. The stress conditions for shearing fault reactivation may vary during the deformation period. The differential stress value would correspond to the peak stress reached during the deformation period. (2) The twinning is only observed meaning that the stress level required for the shearing fault reactivation was not reached.

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Fig. 10. Differential stress obtained from the calcite twin analysis. (A) Extension phase. (B) Compression phase.

were observed inside the pre-existing fillings of the CZ, the calculated differential stress corresponds to the stress conditions for the microcracking. The observed microcracks are associated with the compressive phase and are always located within normal faults (faults 1, 2 and 3, Fig. 5 and Fig. 8). Calcite grains of these faults were already twinned when the compressive phase occurred. The twin density of these calcite grains was already important. If the twinned calcite grains were well oriented with respect to the stress directions of the new tectonic regime, once more they were twinned even if a “strain hardening” effect was possible. Twinning was able to appear within the grains as far as the limit of twin density was not reached. Once the limit reached, the deformation was able to continue by microcracking (Rowe and Rutter, 1990). Microcracking might be equally associated with pressure solution (Renard et al., 2000). We observe a few solution surfaces within the CZ containing microcracks. But this association is not always verified. That assumes that the pressure solution crack sealing was able to be active but was not a major mechanism. Moreover this mechanism cannot explain the microcracking inside the damage zone, since the host rock contains only 10–30% of calcite (Cabrera Nunez et al., 2001) and shows no trace of pressure solution surface. Concerning the structural permeability of the DZ, we assume that microcracking occurred under a level of stress lower than that required for the activity of the fault. During the inter-seismic period the increase of the differential stress entails the microcracking of the shale within the damage zone before the sliding on the shear surfaces which requires a greater stress differential. This

assumption is validated by the observation of microcracks that appeared during the compression phase in the volume of no reactivated pre-existing normal faults. 5.2.2. Stress dependent permeability As for the Mesozoic extension phase, the mean values of the peak differential stress of faults 3, 4 and 5 are close to 40–50 MPa (Fig. 10A). These values correspond to the fault activity and the subsequent formation of structural porosity within the CZ. The value obtained with fault 1 is about 20–30 MPa. The dip of this fault is lower than the other normal faults, which probably infers a larger fluid pressure in the fault zone. The distribution of the fluid pressure could be heterogeneous inside the argillaceous formation during the period of deformation and also inside the fault zones, in relation to syn to postdiagenetic chemical and physical processes (Freed and Peacor, 1989; Sibson, 1990; Knipe, 1992; Byerlee, 1993; Hickman et al., 1995; Teige et al., 1999; Tenthorey et al., 2003). The isotopic analyses of calcite from similar normal faults in the study area show that fluids related to the extensive phase were shale pore water (Peyaud et al., 2005). The ‘‘intra-shale’’ origin indicates the no-connection with another formation. The stress states revealed by our study did not allow fluid transfers from or toward another formation through the minor faults inside the argillaceous formation. During the Cenozoic compression phase, the calcite grains of the CZ of some pre-existing normal faults were affected by twinning and microcracking. From the structural observations, we deduce that the differential stress required for the shearing reactivation was not

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reached for these faults. It leads us to infer that the value for shearing reactivation is greater than 40–50 MPa for these faults (Fig. 10B). The CZ of these pre-existing fractures either remained sealed and impermeable or became permeable when microcracking occurred. Then the permeability developed when the set of microcracks was dense enough to allow for connections and when the percolation threshold was reached, since the growth of the stress-induced microcracks enhances the connectivity and the permeability (Zhu and Wong, 1999). The isotopic investigation of the analysed-fault fillings shows that the fluids related to the compression phase resulted from a mixing between residual diagenetic water and meteoric water (Peyaud et al., 2005). The stress states revealed by our study were able to entail fluid transfers along the minor faults and connection to another formation. The containment capacities of the argillaceous formation were changed through the minor faults during the compression phase. The differential stress values for reverse and strikeslip faults (Fig. 10B) are close to 40–50 MPa and thus happen to lie in the same range than those obtained from the normal faults for the extension phase. Then the faults that appeared during a given tectonic event were active under similar values of differential stress except for fault 1. The differential stress values are slightly different for the reactivation of faults during a second tectonic event. For example, faults 2 and 4 were reactivated as strikeslip faults. Contrary to fault 2, fault 4 shows no microcracking and no value was calculated. However, the differential stress value obtained for the reactivation of fault 2 is estimated to be 35–45 MPa. This level of differential stress is lower than those obtained for faults 1 and 3 which are not reactivated faults. We thus infer that the parameters for reactivation were different according to the fault and that the mechanical behaviour of the fractures – reactivation or not – may be linked to the structural heritage (dominoes, slip surfaces, cementation rate, grain size, width of calcite bands…). In addition, this difference of mechanical behaviour when the tectonic context is modified is characterised by the occurrence or not of microcracking in the fault volume. The differential stress values associated with this deformation process range from 35 to 65 MPa. Moreover, the fracture and geometry orientation with respect to the new stress field is also a decisive parameter (Lisle and Srivastava, 2004). A fracture optimally oriented, like the reverse fault 6 or the strike-slip fault 8 (an angle close to 30° between the fracture direction and the new σ1 direction) during the compression phase, is favourable to the shearing reactivation. The pre-exist-

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ing, non-optimally oriented normal faults (angle between the fault direction and σ1 ranging in the 55°–70° interval for the first compressive tectonic event for example) involve a greater level of stress for the shearing reactivation (Sibson, 1985). Concerning the DZ, we deduced from the structural analysis that it was principally developed during the inter-seismic periods (Fig. 5). This involves a longer period of deformation than for the CZ. For each fault, the applied deviatoric stress was probably sufficient to develop structural permeability in the DZ. At this time, the fluid transfers occur in the DZ and we assume that the obtained values give the stress level inducing the formation of the DZ round the fractures and the eventual “semi-permeable” character of the tectonic fractures. 6. Conclusion The paleofluid flow pathways in an argillaceous formation may be related to the stress states arising inside the fractures volume. These states can be measured from the analysis of the twinning of calcite contained within the fault core zone. In the studied case, we show that the faults created during a tectonic event became permeable (both CZ and DZ) for a peak of differential stress magnitude (σ1–σ3) estimated close around 40–50 MPa. Lower values were obtained indicating probably variations of the frictional strength. The paleofluid flows inside the DZ have occurred for (σ1–σ3) values lower than 40–50 MPa even though the CZ remained impermeable. The reactivation of fractures during a second tectonic event is associated with (σ1–σ3) values upper to 40–50 MPa. The fractures then remained sealed and constituted barriers or became more permeable through the development of microcracks in the pre-existing fillings. These higher levels of differential stress and the difference of mechanical behaviour of the pre-existing fractures are linked to the fracture orientation with respect to the stress field and the structural content (heritage) of the fracture volume. Acknowledgments This work was supported by the Tournemire project of the French Institute for Nuclear Safety and Radioprotection. We would like to thank Jose M. GonzálezCasado and the anonymous reviewer for providing helpful review comments on the manuscript and Jean Pierre Burg for his editorial help. Special thanks go to Meredith Calandra, Jean Xavier Dessa and Emmanuel Baroux.

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