contemporaneous multi-scale structures in the Montagne des Mémises ( Préalpes du chablais , Haute-savoie , France )

the Montagne des Mémises, which lies to the south of Lake Geneva between thonon-les-bains (France) and the rhone Valley (switzerland), belongs to the frontal part of the Préalpes du chablais klippe. It is composed of an upright syncline with a curved axis that is bordered by two left-lateral strike-slip faults, the bonnevaux-col du Pertuis fault to the west, and the rhone Valley fault to the east. the south-western part of the syncline is characterized by tilted blocks separated by listric and sinistral strike-slip faults that trend N 20° E and dip West. these faults are well developed and clearly visible only along the north-western part of the syncline, that is, along the outer part of the fold. Moreover, asymmetrical and conical metre-sized folds are present within early cretaceous limestones close to the N to NNEtrending faults. bedding plane relationships and fault population analyses confirm that the compressive-strike slip structures and the normal faults are contemporaneous. the faults were formed under a strike-slip regime with σ1 between N 150° E and N 160° E. the strike-slip movement was mostly along sinistral faults oriented N to NNE. A population analysis of the normal faults indicates directions of extension parallel to the fold structures (σ3 between N 40° E and N 60° E). It shows that the extension is the result of the permutation of the σ1 and σ2 stresses of the strike-slip phase (σ3 remaining constant). During the multiphase tectonic evolution of the Prealps, the deformations described in the present article contributed, within a continuum, to the formation of the klippe and created an exemplary set of structures in the Massif des Mémises. From a more general point of view, it appears likely that such contemporaneous multi-scale structures exist along the entire length of the western Alpine arc. résuMé Le présent travail est une contribution à la connaissance structurale des Préalpes du chablais (France). Il concerne la partie frontale de la klippe et plus précisément le massif des Mémises (Préalpes Médianes Plastiques). ce massif est essentiellement formé par un synclinal perché, à axe courbe à concavité tournée vers l’est, plongeant vers le sud-ouest à l’ouest et vers le nord-est à l’est, et limité par deux accidents décrochants senestres, la faille col de bonnevaux – col du Pertuis à l’ouest et l’accident du rhône à l’est. Les nouvelles études multidisciplinaires d’ordre tectonique, hydrologique et hydrogéologique que nous avons menées révèlent trois grands types de structures, de nature et d’échelle différentes, dont l’analyse démontre la contemporanéité ou du moins la compatibilité mécanique. La structure synclinale des Mémises apparaît, à plus petite échelle, découpée en blocs basculés par des failles listriques. celles-ci sont quasi perpendiculaires à l’axe du pli et particulièrement bien observables sur son flanc nord. Des mouvements décrochants sénestres sont par ailleurs identifiés sur ces fractures et sur d’autres, toujours de direction N à NNE. ces accidents s’accompagnent enfin de petits plis métriques asymétriques et coniques, bien individualisés dans les calcaires en plaquettes du Néocomien. L’analyse structurale, et notamment l’analyse des populations de failles, accrédite la compatibilité mécanique et la probable contemporanéité des structures compressives-décrochantes et extensives. L’extension locale résulterait alors d’une adaptation de la déformation plicative. Les failles ont fonctionné en régime décrochant avec σ1 compris entre N 150° E et N 160° E et un mouvement décrochant principalement sur des failles sénestres orientées N à NNE. L’analyse des failles normales révèle une direction d’extension parallèle aux structures plicatives (σ3 compris entre N 40° E et N 60° E). Elle montre que l’extension résulte de la permutation des contraintes σ1 et σ2 de la phase décrochante (σ3 restant constant). Dans l’évolution tectogénétique polyphasée des Préalpes, les déformations décrites participent, au cours d’un continuum, à la structuration de la klippe et confèrent au massif des Mémises un cadre exemplaire d’observations. D’un point de vue plus général, il est vraisemblable que de telles structures multi-échelles, contemporaines, existent tout au long de la courbure de l’arc alpin occidental. 1.  Introduction A large number of structural studies have been carried out in the Préalpes Médianes romandes during the last three decades (Plancherel & Weidmann 1972; Plancherel 1976, 1979; Mosar 1988, (a & b), 1989, 1991, 1994, 1997, 1999, 2002, Mettraux & Mosar 1989; Mosar & bore l992, 1995; Mosar et al. 1996). However, most of the studies carried out in the Préalpes Médianes du chablais during this period were large scale (Plancherel 1990; borel & Mosar 2000) or specifically contemporaneous multi-scale structures in the Montagne des Mémises (Préalpes du chablais, Haute-savoie, France) Anne GuyomArd 1, 2, *, JeAn-yves Josnin 1, JeAn-ClAude Hippolyte 3 & serGe FudrAl 1

AbstrAct the Montagne des Mémises, which lies to the south of Lake Geneva between thonon-les-bains (France) and the rhone Valley (switzerland), belongs to the frontal part of the Préalpes du chablais klippe. It is composed of an upright syncline with a curved axis that is bordered by two left-lateral strike-slip faults, the bonnevaux-col du Pertuis fault to the west, and the rhone Valley fault to the east. the south-western part of the syncline is characterized by tilted blocks separated by listric and sinistral strike-slip faults that trend N 20° E and dip West. these faults are well developed and clearly visible only along the north-western part of the syncline, that is, along the outer part of the fold. Moreover, asymmetrical and conical metre-sized folds are present within early cretaceous limestones close to the N to NNEtrending faults. bedding plane relationships and fault population analyses confirm that the compressive-strike slip structures and the normal faults are contemporaneous.
the faults were formed under a strike-slip regime with σ 1 between N 150° E and N 160° E. the strike-slip movement was mostly along sinistral faults oriented N to NNE. A population analysis of the normal faults indicates directions of extension parallel to the fold structures (σ 3 between N 40° E and N 60° E). It shows that the extension is the result of the permutation of the σ 1 and σ 2 stresses of the strike-slip phase (σ 3 remaining constant).
During the multiphase tectonic evolution of the Prealps, the deformations described in the present article contributed, within a continuum, to the formation of the klippe and created an exemplary set of structures in the Massif des Mémises. From a more general point of view, it appears likely that such contemporaneous multi-scale structures exist along the entire length of the western Alpine arc. résuMé Le présent travail est une contribution à la connaissance structurale des Préalpes du chablais (France). Il concerne la partie frontale de la klippe et plus précisément le massif des Mémises (Préalpes Médianes Plastiques). ce massif est essentiellement formé par un synclinal perché, à axe courbe à concavité tournée vers l'est, plongeant vers le sud-ouest à l'ouest et vers le nord-est à l'est, et limité par deux accidents décrochants senestres, la faille col de bonnevaux -col du Pertuis à l'ouest et l'accident du rhône à l'est.
the aim of the present study was to establish the relationships and age of the multi-scale structures observed in the syncline that forms the Montagne des Mémises, and to relate these structures to large-scale tectonic structures. Our structural mapping was complemented by fault striation measurements (Guyomard 2006). Paleostress reconstructions were performed by a combination of two methods, originally proposed by Angelier (1990Angelier ( , 1991 and by Etchecopar et al. (1981).
We begin by describing the geographical and tectonic setting of the Montagne des Mémises, and then we present our macrostructural data and their relationships. Paleostress analysis was used to constrain the geodynamics and to reveal the chronological relationships between the macro-structures. Finally, we discuss the results within the regional tectonic setting.

The geological setting of the Montagne des Mémises.
the Montagne des Mémises, which lies on the southern shore of Lake Geneva, east of thonon-les-bains (France), forms part of the Préalpes Médianes. On the basis of sedimentary facies and structural criteria, the Préalpes Médianes have been di-vided into two different units (Lugeon & Gagnebin 1941): the outer Préalpes Médianes Plastiques and the inner Préalpes Médianes rigides (Fig. 1). the Préalpes Médianes Plastiques is a marl-rich formation consisting of triassic gypsum horizons and dolomitic limestones, Liassic spathic and siliceous limestones, Dogger marly horizons, Malm and Neocomian limestones and Late cretaceous to tertiary nodular and well-bedded horizons with marls and fine-grained sandstones. the whole succession is up to 1,700 meters thick (sartiges 1978). the Montagne des Mémises, which is composed of Liassic and Malm units, belongs to the frontal part of the Préalpes Medianes Plastiques. the variation in lithology and rheology of the sediments in the sequence influenced the deformation style of the area.

Large- to meso-scale structures in the Montagne des Mémises.
superficially, the Montagne des Mémises appears to consist of a series of simple folds (from south to north: the Lanches anticline, the Memises syncline and the thollon anticline, Fig. 2) that are covered to a greater or lesser extent by Quaternary deposits. However, detailed investigation shows that these structures are much more complex.   the Lanches anticline (Fig. 2) is a tight, northward dipping conical anticline with an axis that trends N 60° E in the area to the west of the col des Lanches, but that dips sW and trends N 90° E to the east of the col. On the western flank of the Haut des Lanches area, south-dipping thrusts have injected cornieules into rhaetian calcareous coquina beds. thrust faults and small asymmetric folds can be observed. the axes of these folds are oriented N 55° E and plunge towards NE. the Lanches anticline is very likely a fault-propagation fold similar to structures described within the Préalpes Médianes romandes (Mosar 1991(Mosar & 1994.

The Mémises syncline.
three areas are interesting from a structural point of view: the southern flank of the Montagne des Mémises at Grand Mottay, the faces of the north ridge of the Montagne des Mémises, and the northern flank of the Mémises syncline.

1) the southern flank of the Montagne des Mémises at Grand
Mottay is marked by an eastward-dipping to sub-vertical thrust together with meter-scale flexural-slip duplexes (Fig. 3). the thrust trends N 140° E and dips 25° south. Examination of calcite steps indicates a top-to -N sense of movement. A small number of fibrous calcite veins are present. tectonic stretching of this flank caused thinning of the Malm sequence. In the area between the Vallon des Lanches and the Mémises syncline, the upper part of the cherty Malm sequence lies directly upon Argovian ammonite-bearing nodular limestones and the whole Malm-Argovian sequence is only 35-cm thick, which is in marked contrast to the one hundred metres or more recorded in other parts of the study area. 2) On the northern flank of the Montagne des Mémises, we observed asymmetrical metre-scale folds. these folds were also observed by badoux and Mercanton (1962) and by sartige (1978). similar structures have been described in the south-western part of the Préalpes romandes and interpreted as second-order disharmonic folds (Mosar 1991(Mosar , 1994. We observed N 55° E-trending disharmonic folds in raethian layers within anticlines, but the asymmetric folds that occur in the Late cretaceous formations probably have another origin, as these folds are conical, trend N 40° E to N 80° E and occur close to strike-slip faults (Fig. 4). the folding is contemporaneous with the development of the schistosity and we therefore infer that the folds are the result of strike-slip faulting. 3) the northern flank of the Mémises syncline is affected by normal faults (Fig. 4) that sole out westward within the Dogger calcareous marls. they run perpendicular to the axis of the western part of the Mémises syncline and divide the syncline into numerous blocks that give an eastward tilt to the stratigraphic sequence. the fact that they are perpendicular to the syncline suggests that they result from lateral accommodation of the folding (i.e. reflect orogen-parallel extension during convergence). Moreover, to either side of the syncline, normal faults disappear and compressional deformation prevails, which supports the hypothesis of local strain accommodation.

Method
the study area included seven localities suitable for fracture analysis, and two more were found 15 km and 25 km further west. these nine localities are labeled A to I (Fig. 5). At each locality, between 8 and 25 striated fault planes with clear striation and direction indicators were measured, thus giving large enough data sets to allow accurate paleostress calculations to be made. In order to achieve a complete analysis, we used two calculation methods to determine the standard deviations of the stress axis orientations: the Direct Method (Etchecopar 1981) and the Direct Inversion Method (INVD, Angelier 1990Angelier , 1991. both methods are based on the angle between the calculated shear stress and the measured striations; but the Direct Inversion Method also takes into account the relative friction on the fault planes. these methods were used to determine the four components of the stress tensors, that is to say, the orientations and dips of the three principal stress axes and the ratios between the stress values. the principal stress axes and the striated fault planes were plotted on the lower hemispheres of stereograms (Fig. 5). We only present results for which there was agreement between the two calculation methods. the diagrams in Fig. 5 are labeled using two letters to identify the locality name and the type of faulting (e.g., n for normal faulting). A third letter was added to differentiate between different diagrams for a single locality (e.g., Fna and Fnb).
Our data provides the first paleostress results for the Montagne des Mémises. It should be noted that we did not find any evidence of polyphase deformation, suggesting that the reconstructed paleotress axes have not been tilted by successive events. Although we observed striations with slightly different orientations on different parts of a single fault plane, there was never any clear superposition of these striations.

Results
Our paleostress reconstruction reveals two types of stress regime: a strike-slip to compressional regime with σ 1 trending NNW-ssE, and an extensional regime with σ 3 almost parallel to the fold axes of the Mémises structure.
the other is vertical. this is compatible with the near surface orientation of stress axes that is either parallel or perpendicular to earth's surface. the orientations of the stress axes are similar from one locality to another, despite the wide variety of bedding plane orientations, showing that stress orientation and faulting were not governed by structure. As the striations

Stereogram Map
on the strike-slip faults are almost horizontal, and because the paleostress axes have not been rotated by folding, we can infer that this stress regime postdates the folding. A similar stress regime was found at localities A, c, D and F, but here the principal stress axes are not horizontal and vertical (Fig. 5). In diagrams A, c, D and F, σ 1 and σ 3 are along the bedding plane, or between the bedding plane and the horizontal. this consistent relationship indicates that faulting occurred before or during folding. For instance, when faulting occurs along horizontal bedding planes under a strike-slip stress regime, the resulting striations and paleostresses are similar to those described as phase 1 in Fig. 6, that is to say, the striations and the two stress axes are horizontal. If folding occurs, previous fractures are tilted but striation due to the constant stresses still occurs horizontally. Hence, typical syn-folding striations are obtained and the pitch of the slickensides varies from horizontal to parallel to the bedding plane (Fig. 6). the average syn-folding stress tensor reconstructed from the Mémises faults shows characteristic tilted stress axes. In summary, when plotted in stereographic projection, we have: The bedding is horizontal, stresses 1 and 3 are horizontal and the striations on the fracture planes have pitches close to zero.

Stereogram B:
The compression under the strike-slip regime rotated the striation plane (here 10°). The phase 1 striations were rotated at the same time. The pitches of these striations are close to the bedding plane, that is to say 10°. New fracture planes then formed. The pitches of the phase 2 striations are close to zero.

Stereogram C:
The compression under the strike-slip regime continues. Here, the rotation of the bedding plane is 20°. The pitches of the phase 1 striations are close to those of the bedding plane, that is to say, 20°. The pitches of the phase 2 striations are close to 10° (rotation between B and C). New fracture planes then formed. The pitches of the phase 3 striations are close to zero..

1)
In the case of the syn-folding strike-slip faulting, the slickensides and the σ 1 axes lie between the bedding plane (great circle) and the horizontal. 2) In the case of the post-folding strike-slip faulting, the slickensides and the σ 1 axes are horizontal.
Diagrams A and D (Fig. 5) indicate syn-folding striation and stress regimes. this is probably also the case for diagrams c and F, although it is more difficult to draw a firm conclusion because of the low dip. It could be argued that an alternation of strike-slip faulting and compressive folding would produce a similar result. However, such a multi-phase solution is probably too complex to be realistic and we therefore favour the syn-folding hypothesis.
these diagrams suggest that the faulting in the Montagne des Mémises was contemporaneous with the large-scale folding, which is compatible with other paleostress studies in the Préalpes romandes (Mosar & borel 1992), where a similar strikeslip regime was characterized using the right dihedra method. However, previous studies did not include bedding plane orientations, thereby limiting the comparisons that can be made with our results. two types of stress regime have been recognised in Late Miocene rocks in the Jura Mountains (Homberg 1997, Homberg et al. 2002: a strike-slip regime (σ 1 = NW-sE and σ 3 = NE-sW) and a reverse regime (σ 1 = NW-sE and σ 2 = NE-sW). Due to the low dip of the bedding planes, it was impossible to carry out such an analysis for the Montagne des Mémises localities.

Extensional stress regime
six of the diagrams in Fig. 5 (bn, cna, cnb, Fna, Fnb, Gn) show an extensional stress regime. In diagrams bn and Fnb the stress axes are vertical and horizontal; in the other four diagrams the stress axes are tilted. In all six cases the extension is from NNE-ssW to NE-sW. this extension direction is similar to the σ 3 trend of the strike-slip regime (e.g., cna and cnb, Fig. 5), suggesting that these two stress regimes are related through a simple stress permutation, with a switch of the σ 1 and σ 2 axes, and σ 3 remaining constant.
It should also be noted that some of the diagrams show vertical and horizontal stress axes (e.g., bn and Fnb), whereas others show tilted stress axes (e.g., cna, Gn). the former correspond to post-tilt faulting, while the later correspond to pre-tilt faulting. As for the strike-slip stress regime, they indicate that faulting occurred during folding.
As this extensional deformation was observed only on the northern edge of the folds (Mémises syncline for bn, cna and cnb; De Lyaud anticline for Fna and Fnb; Vacheresse anticline for G), we believe that it corresponds to local stress permutations (of σ 1 and σ 2 ) and that extension occurred during folding. the only cretaceous normal fault in the area (Hable 1997) runs parallel to the axis of the syncline and is thus different from the normal faults described here. this indicates that the normal faults in the Mémises syncline are of Alpine or post-Alpine age (Mosar 1999(Mosar , champagnac 2004. Moreover, the faults, the stress tensors and bedding plane relationships clearly show that the normal faulting described here occurred before the folding (e.g., Fna), during the folding (e.g., bn) and after the folding (e.g., Fnb).
It has been shown that a change from one stress regime to another may be brought about by simple permutation of the σ 1 and σ 2 stress axes, as occurred here between the strike-slip and the extensional stress regimes. (Angelier & bergerat 1983). this type of stress permutation can occur during a single tectonic event (Hippolyte et al. 1992). similarly, we propose that both the strike-slip and normal faulting in the Montagne des Mémises occurred during the same large-scale folding event. similar extension, parallel to the fold axes (like σ 3 ), has been observed in the Préalpes romandes (Mosar 1991, Mettraux & Mosar 1989) and in the Jura (Homberg 2002), but in these cases it was not possible to analyze the bedding plane-faulting rela- tionships. In the Montagne des Mémises, the relation between the bedding planes and the faulting suggests that the faulting was contemporaneous with the folding. In the Jura, normal faulting in horizontal bedding planes has been interpreted as being related to the Oligocene extension (Homberg 2002). Even though the Montagne des Mémises lies just on the other side of the Molasse basin to the Jura, such an interpretation is not possible here, as the Alpine thrust units were decoupled from their foreland and from the Jura when the Molasse basin was active. Our analysis of striated faults indicates a different interpretation for the normal faulting observed in the Montagne des Mémises. In addition, extensional structures associated with large strike-slip faults, similar to the tilted blocks of the Montagne des Mémises, have been identified by microstructural analyses within the sub-Alpine Aravis Massif (Danet 2003).

The age of the observed structures
We do not have any direct data on the ages of the observed structures. We can only say that the Eocene extension demonstrated by Hable (1997) occurred prior to this deformation and that erosion and exhumation took place after the deformation.
Further east, in the Préalpes romandes, Mosar (1988aMosar ( -b, 1989Mosar ( , 1991 has shown that the deformation of the Prealps was a multi-phase process. In our study area, there is no evidence of an earlier phase of deformation to the one studied here, but we do not know whether this is because there was no such earlier phase or because the effects of the earlier deformation have been masked by more recent events. this raises the question of whether the Montagne des Mémises is structurally distinct or whether similar studies in the frontal part of the Préalpes romandes would give equivalent results.

Conclusion
Analyses of fault-slip data and paleostress reconstructions show that most of the faulting in the Mémises area is related to a strike-slip compressional regime with σ 1 trending from N 150° E to N 160° E. this regime also produced the area's large-scale folds and small-scale conical folds. some of the fault sites have provided sufficient data to show that the strike-slip faulting and tilting were contemporaneous. reverse faulting was not found, probably because the shortening was mostly absorbed by the folding.
A small proportion of the fault planes measured indicate the occurrence of local normal faulting, at different scales, in particular along the northern edge of the folds. this extension is also contemporaneous with the folding deformation and is the result of stress permutation between the σ 1 and σ 2 axes, with σ 3 remaining constant for both the strike-slip and extensional regimes.
the folding of the Préalpes du chablais took place under a strike-slip, and locally extensional, stress regime with a com-mon orientation of the maximum principal stress axis (σ 1 or σ 2 , N 150° E to N 160° E). the small-scale conical folds were created by strike-slip faulting with σ 1 also trending N 150° E to N 160° E.