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International audience; Cet ouvrage présente une histoire inhabituelle et originale de la Grande Guerre, sur le front occidental, vue sous l’angle de la géologie et des géologues, observateurs ou protagonistes « privilégiés » du conflit. Il est écrit par des historiens des sciences et des géologues d’aujourd’hui.Le temps qu’il fait, le paysage, le sol, la végétation… les soldats font abondamment référence à leur environnement immédiat dans leurs journaux et leurs correspondances de guerre. Dessins, peintures et photographies témoignent aussi de leurs difficiles conditions de vie : la corvée d’eau, la boue des tranchées, les champs de bataille… Cet environnement qui conditionne le « fait guerrier » est en grande partie contraint par la composition et la structure du sol et du sous-sol. Pour naturel qu’il soit, ce lien étroit entre la géologie et la Grande Guerre n’a cependant été abordé, jusqu’à présent, en France, que dans très peu d’ouvrages. Ce volume vise à combler cette lacune.Ce livre traite aussi de la place des géologues dans le conflit avec leur rôle tant sur le front, où il fut valorisé ou négligé suivant les armées, qu’à l’arrière et dans les régions envahies où la vie scientifique, forcément bouleversée, a dû s’adapter à la mobilisation et/ou à l’occupation. Leur engagement patriotique influença les relations scientifiques internationales encore plusieurs années après la fin du conflit. Enfin, dans cette première guerre industrielle, le rôle des géologues – trop souvent ignoré – a été déterminant pour subvenir aux besoins des armées en eau et pour découvrir de nouvelles ressources minérales : matériaux d’empierrement, fer, charbon, pétrole.Cent ans après, les effets de la guerre sur l’environnement sont encore perceptibles dans les zones de l’ancienne ligne de front : habitat reconstruit et modifié, paysage rural transformé et surtout sols et eaux contaminés. Les approches modernes de la géologie permettent d’aborder ici cette histoire environnementale. La Grande Guerre est toujours présente au cœur de la nature comme dans la mémoire des hommes !L’Association des Géologues du Bassin de Paris (AGBP), le Comité Français d’Histoire de la Géologie (COFRHIGEO) et la Société Géologique du Nord (SGN) ont réalisé un ouvrage consacré aux interactions entre la géologie et les opérations militaires.Labellisé par la Mission du Centenaire, l’ouvrage est également placé sous le patronage de l’Académie des Sciences et de la Délégation française pour l’Unesco.Bergerat F. (dir.) avec la collaboration de P. Gaviglio, J.P. Gély, N. Ginsburger, J. Labourguigne, P. Le Vigouroux,J. Lorenz et M.J. Roulet (éds) (2018) – 14-18, La Terre et le Feu. Géologie et géologues sur le front occidental.Co-édition AGBP - COFRHIGÉO - SGN, Mém. hors-série n°10 de l’AGBP, 480 p.

Jacques Angelier was born 2 March 1947 in Ales, France. He passed away on 31 January 2010. Jacques was a student at ‘Ecole Normale Superieure’ of Saint-Cloud from 1966 to 1970. He obtained his PhD in 1970 and his DSc in 1979. From 1970–71 Jacques was assistant lecturer at the University Pierre & Marie Curie in Paris (Paris VI), and from 1971–76 assistant lecturer at the University of Orleans. Following his stay in Orleans, Jacques returned to University Pierre & Marie Curie in 1976 as an assistant professor and, from 1981, as a full professor. Jacques worked in Paris until 2003, then moved to the Oceanological Observatory of Villefranche-sur-Mer, also a part of University Pierre & Marie Curie. The main research activities of Jacques Angelier were in the fields of structural geology and geodynamics, with focus on brittle deformation. He was a leader in analysing crustal stresses, that is, in developing methods to understand the various stress fields to which the Earth’s crust has been subjected through time. The basic techniques are inversion of fault-slip data from minor fault sets measured in the field and of focal mechanisms (fault-plane solutions) of earthquakes in a given area to calculate the state of stress. Inversion of fault data is primarily used to infer the so-called “paleostresses”, that is, the state of stress that existed in the past at the time of fault slips. This method relies much on measurements and interpretation of striations on fault planes in the field. Focal mechanisms of earthquakes are used to infer current stresses in active fault zones. Jacques developed computer programs to calculate the stress tensors from these various types of data. Jacques Angelier applied his methods to carry out regional and local brittle-tectonic studies in many countries and regions. These include North Africa, Greece, Turkey, …

Physical properties of in situ rock mass are usually estimated from results obtained through laboratory tests on intact rock samples because the access to in situ rock may be quite challenging. This approach however raises some questions concerning the number of samples needed for reliable result, the validity of the extrapolation of the parameters from centimetre scale to a large rock mass and finally the effect of discontinuities contained in the rock mass. An underground quarry in Bougival with easy access to metre-scale pillars and the possibility to collect large number of samples has been chosen to analyse the scale effect and the anisotropy of the Campanian chalk. Different experiments have been designed to determine the dynamic elastic properties (Young’s modulus and Poisson’s ratio) based on geophysical approaches: ultrasonic measurements on laboratory samples, and “hammer” seismic measurements in situ. The static Young’s modulus and Poisson’s ratio have been determined through uniaxial compression tests on centimetre core samples. Pillars with and without visible discontinuities, as well as with various overburden rock thicknesses, have been chosen in order to analyse the possible impact of different heterogeneities on the elastic properties. Core samples of intact chalk, with 40mm to 100mm diameters, have been studied in laboratory. The high dispersion observed on the different results suggests that if only a few tests are analysed, the conclusions may not be representative. A statistical approach is more appropriate to analyse the mechanical properties of the chalk. The dynamic Young’s Modulus and Poisson’s ratio calculated from laboratory samples (centimetres) and in situ rocks (about ten metres) do not reveal any clear impact of size on these elastic properties. The presence of discontinuities has a major impact on both the dynamic Young’s modulus and Poisson’s ratio. Decreasing values of these properties have been observed where discontinuities (fractures, flints) have been detected. Finally, the overburden rock thickness above the underground quarries (from 14m to 50m) seems to have no effect on the mechanical properties; the uncertainty of the measurements, partly due to the heterogeneity of the chalk mass, is likely to be more important than the effect of load on the pillars.

International audience; We review the evolution of concepts on and methods of estimating the state of stress from fault movements. Theories of failure in isotropic materials suggested a simple geometrical construction of optimal principal stress directions from a fault plane and its associated slip. These optimal directions align shear stress and slip directions and maximize the difference between shear stress and frictional resistance on the fault plane. Optimal stress directions for calcite twinning are obtained by a similar construction, with the difference that they maximize shear stress. Force representation of seismic sources independently introduced pressure, P, and tension, T, axes at positions that also maximize shear stress on both nodal planes. Frictional slip theory and the constraint that slip and shear stress directions be parallel allowed to address reactivation of pre-existing faults. This suggested that stress could also be inverted from reactivated fault and slip data or earthquake focal mechanisms. Early methods relied on geometrical constructions as a substitute for calculations, whereas later methods relied on software as these calculations became tractable with the help of computers. Similar methods were developed for the inversion of stress from crystal twin gliding with non-optimal geometry, with a different criterion that relies on a threshold of the component of shear stress along the gliding line. Even though these methods seek a common stress tensor compatible with fault and slip data, their main use is to separate polyphase data into homogeneous subsets and help deciphering complex tectonic histories. Fault and slip data can also be analyzed to constrain the strain rather than the stress tensor. In most cases this involves a summation and yields an average strain for the considered rock volume. Stress inversion thus appears better suited for differentiating heterogeneous data whereas strain analysis appears better suited for homogenizing them.

Accurate GPS mapping and field measurements have been carried out along the surface traces of three N–S right-lateral historical and prehistorical seismic strike-slip faults in the South Iceland Seismic Zone: the Minnivellir, Tjorvafit, and Rettarnes faults. We used these data and those previously collected on the Selsund and Leirubakki faults to characterise the faulting behaviour. This study illustrates a characteristic multi-scale segmentation including, from the longer to the smaller fault elements: (i) fault (corresponding to the total length of the seismic fault), (ii) fault segments, (iii) arrays of fractures and (iv) individual fractures. The most distinctive feature of such right-lateral fault is its left-stepping en-echelon arrangement at various scales, with push-up structures accommodating the motion in the relay zones. This study also highlights the role of the nature and thickness of the uppermost layer (lava flow, thin or thick grassy soil) with respect to the type and length of fractures at the surface. Our mapping reveals complex fault geometry which, however, is consistent with the general left-lateral transform behaviour of the South Iceland Seismic Zone. Some left-lateral conjugate fault branches are present along these major right-lateral faults and participate to the deformation. The estimation of the strike-slip offsets (1.3–2 m) based on the analysis of push-up structures allow to infer the total rupture lengths (45–70 km) for these faults and to estimate the magnitudes (7.0–7.2) of the related earthquakes, bringing confirmation that events significantly larger than those of June 2000 (Ms = 6.6) have occurred in the SISZ, which plays an important role for the seismic hazard assessment.

International audience

The Palaeozoic to recent evolution of the Tethys system gave way to the largest mountain chain of the world extending from the Atlantic to Pacific oceans - the Alpine- Himalayan Mountain chain, which is still developing as a result of collision and northwards con- vergence of continental blocks including Apulia in the west, the Afro-Arabian Plate in the middle and the Indian Plate in the east. This Special Publication addresses the main problems of the middle part of this system incorporating the Balkans, Black Sea and Greater Caucasus in the north and the Afro-Arabian Plate in the south. Since the Early Mesozoic a number of small to large scale oceanic basins opened and closed as the intervening continental fragments drifted northwards and diachro- nously collided with and accreted to the southern margin of the Eurasian Plate. Despite the remark- able consequences of this, in terms of subduction, obduction and orogenic processes, little is known about the timing and palaeogeographic evolution of the region. This includes the amounts of short- ening and interplay between synconvergent extension and compression, development of magmatic arc and arc-related basins and the timing and mechanism of their deformation. The chapters pre- sented in this Special Publication present new information that help to fill some of the gaps of the puzzle.

In chalk, strong deformation at the grain level is associated with faulting. Within a 5-10 cm thick fringe on each side of normal faults, texture modifications were documented and analysed in samples from the Campanian White Chalk. We used SEM observations, image analysis, reconstruction of the porous media and physical measurements. Dissolution and cementation features, together with a reorganisation of the pore space, can be explained by massive fluid transfers. Faulting is interpreted as ductile shearing, which involves slip and dissolution first in shear deformation bands and then along a single fault plane. (C) 2008 Elsevier Ltd. All rights reserved.

10 pages; International audience; The Oolithe Blanche Formation (Bathonian, Middle Jurassic) is one of the deep saline aquifers of the Paris Basin in France. The spatial distribution of its reservoir properties (porosity, permeability, tortuosity, etc.) is now better known with relatively homogeneous properties, except for some levels in the central part of the basin, where permeability exhibits higher values. This spatial distribution has been correlated with diagenetic events (variability of cementation) and palaeo-fluid flow circulation phases leading to variable cementation. In this paper, numerical simulations of reactive transport are performed. They provide a preliminary quantitative analysis of the Oolithe Blanche Formation, the type of fluids involved, the duration of fluid flow, and the time required to reduce the primary porosity of the Bathonian sediments by 10% due to cementation. Our results from the reactive transport simulations along a flow line, and a parameter sensitivity analysis suggest that diagenesis processes driven by meteoric water recharge do not exclusively cause the 10% decrease in porosity. Other geochemical and hydrogeologic processes must be involved.

A detailed study of normal faults in the calcarenites and chalks of the Mons Basin reveals that pressure-solution processes play a significant role in fault zones, which affects the determination of the amounts of extension. Drag folding associated with normal simple shear and rock dissolution concurred to produce layer flexuring in the hanging wall of the normal faults, whereas the footwall remained almost intact. Analysing the distribution of fossil belemnite rostra in and away from fault zones enabled us to evaluate these effects. The sense and amount of belemnite tilt constrain the minimum thickness of dissolved hanging wall fringe, revealing significant pressure-solution in addition to drag folding. We developed 2-D and 3-D analytical models of along-fault belemnite rotations induced by drag folding and dissolution, and we applied these models to our data from the Mons Basin. The results show that the pressure-solution effect is significant, implying a smaller ratio between horizontal and vertical components of motion than expected. It follows that the across-fault amount of extension is smaller than that predicted from the single fault geometry, suggesting that although the existence of Campanian-Maastrichtian-Danian extension in the Mons Basin area is beyond doubt, its amplitude should not be overestimated. (c) 2005 Elsevier Ltd. All rights reserved.

(2005). Interactions between magmatism and tectonics in Iceland: a review. Geodinamica Acta: Vol. 18, No. 1, pp. 1-9.

The tectonic evolution of the region adjacent to the SW border of the East European craton is crucial for understanding the Palaeozoic amalgamation of Europe and its later evolution. We present a structural analysis of the brittle deformation (fault-slip data inversion) and folding carried out in the Cambrian to Cenozoic rocks of the European platform in SE Poland to characterize the Late Palaeozoic, Maastrichtian–Palaeocene and Miocene tectonics of this region. The Variscan (Late Carboniferous) palaeo-stress pattern was characterized by N–S to NNE–SSW-oriented compression, as inferred from the folding of the Palaeozoic (mainly Devonian) rocks. The compression was associated with the northwesterly (present-day coordinates) motion of Gondwana towards the northerly situated Laurussia. Subsequently, the Mid-Polish Trough (MPT) developed in an extensional regime. The stress regime changed to compressional at the Maastrichtian–Palaeocene transition in response to the Africa–Europe plate kinematics. The compressional regime induced the tectonic inversion of the Mid-Polish Trough. The stress pattern was characterized by NE–SW-oriented compression in the Holy Cross Mts. (HCM) area and in the Radomsko High. Folds and faults in the Mesozoic cover revealed a sinistral reactivation of the Holy Cross Fault (HCF) and of NW–SE-trending faults. The Alpine stress patterns were related to the subduction of the European platform beneath the Carpathian orogen. The flexural bending of the subducted lithosphere induced a homogeneous Middle Miocene N–S extension in the Carpathian foreland. Subsequently, the northward propagation of the Carpathian front resulted in late Middle Miocene N–S compression, restricted to the proximal area of the frontal thrusts, implied by a strain partitioning at a low friction contact between the Carpathian nappes and the European platform. Since the Palaeozoic, the Teisseyre–Tornquist Zone (TTZ) behaved as lithospheric weak zone, which has been reactivated during the Maastrichtian–Palaeocene change in stress regime. It resulted in the tectonic inversion of the Polish Trough. Inherited faults conditioned superficial stress perturbations and focused the deformation.

Outcrop-scale structural data and seismic section interpretation are combined to unveil a very complicated Tertiary deformation history of a once Tethyan margin: the Mecsek–Villany area of Hungary. This combination of data helped to reconstruct the possible activity of individual fault zones. At least four ENE–WSW striking zones—the Northern Imbricates, the South Mecsek zone, the Gorcsony–Mariakemend ridge and the Villany Mountains—were confirmed as regional long-lived transpressive zones with very complicated internal deformation, frequently with oppositely dipping thrust faults. Tertiary structural history began with a roughly N–S-directed shortening in the South Mecsek zone. It was followed by a NE–SW-directed transpression activating practically all important wrench zones together with perpendicular transfer faults. Basins were created along some of these deformation zones, but were also affected by major tilts due to inversion. After a relatively quiescent period in the Middle Miocene, the Late Sarmatian inversion followed. Shortly after, this event was relayed by a NE–SW-directed extension–transtension. An important inversion period characterised by NW–SE compression occurred in Late Pannonian (Messinian), when all the former wrench zones were reactivated as right-lateral shear. This event is responsible for the present topography of the region.

Based on an analysis of 8000 minor fault-slip data in the Jura Mountains (France), we discuss the influence of pre-existing discontinuities on the development of fold-and-thrust belts. We present palinspastic maps showing the stress fields and active faults during the Cenozoic pre-orogenic events in the Jura belt prior to the main Late Miocene fold-and-thrust tectonics. During the Eocene, a N–S strike-slip regime produced a few NNE–SSW sub-vertical strike-slip faults in the central external Jura and a few E–W reverse faults in the eastern Jura near the future frontal thrust. During the Oligocene, an average WNW–ESE extension, with irregular stress trajectories, resulted in normal faulting along N–S to NE–SW trends in the external part of the belt, WNW–ESE trends along the future northern and northeastern frontal thrust, and NW–SE trends in the internal Jura. The Late Miocene tectonics began with a strike-slip regime with a fan-shaped compressional trajectory. It was followed by a stress field with similar stress direction, but local σ 2 / σ 3 stress permutation resulted in strike-slip regime domains contrasting with reverse regime domains. Stress deflections and permutations occurred near inherited cover and basement discontinuities. Major deformation zones, like the Jura frontal thrust onto the foreland, the thrust of the internal central Jura onto the external Jura, and the narrow deformation bands within the flat-lying plateaus formed close to the inherited faults. The structural style of the Jura belt thus partly mimics the pre-orogenic fault pattern. Stress deflections point to the pre-orogenic faults, express the indentation process of the Jura by its hinterland, and highlight successive slip events along major faults during the fold-and-thrust tectonics. This case study illustrates the relevance of minor fault-slip studies for characterizing both the pre-orogenic tectonics and the kinematics of the deformation.

The Husavik–Flatey Fault (HFF) is an oblique dextral transform fault, part of the Tjornes Fracture Zone (TFZ), that connects the North Volcanic Zone of Iceland and the Kolbeinsey Ridge. We carry out stress inversion to reconstruct the paleostress fields and present-day stress fields along the Husavik–Flatey Fault, analysing 2700 brittle tectonic data measured on the field and about 700 earthquake focal mechanisms calculated by the Icelandic Meteorological Office. This allows us to discuss the Latest Cenozoic finite deformations (from the tectonic data) as well as the present-day deformations (from the earthquake mechanisms). In both these cases, different tectonic groups are reconstructed and each of them includes several distinct stress states characterised by normal or strike-slip faulting. The stress states of a same tectonic group are related through stress permutations ( σ 1 − σ 2 and σ 2 − σ 3 permutations as well as σ 1 − σ 3 reversals). They do not reflect separate tectonic episodes. The tectonic groups derived from the geological data and the earthquake data have striking similarity and are considered to be related. The obliquity of the Husavik–Flatey Fault implies geometric accommodation in the transform zone, resulting mainly from a dextral transtension along an ENE–WSW trend. This overall mechanism is subject to slip partitioning into two stress states: a Husavik–Flatey Fault-perpendicular, NE–SW trending extension and a Husavik–Flatey Fault-parallel, NW–SE trending extension. These three regimes occur in various local tectonic successions and not as a regional definite succession of tectonic events. The largest magnitude earthquakes reveal a regional stress field tightly related to the transform motion, whereas the lowest magnitude earthquakes depend on the local stress fields. The field data also reveal an early extension trending similar to the spreading vector. The focal mechanism data do not reflect this extension, which occurred earlier in the evolution of the HFF and is interpreted as a stage of structural development dominated by the rifting process.

Two different paleomagnetic directions have been obtained in several formations in northwestern Bulgaria. One of them, found only in Upper Permian–Lower Triassic red sandstones, is very likely a primary magnetization. A secondary magnetization of Eocene age appears as either pre-, syn- or post-folding depending on the site. The identification of the timing of remagnetization with respect to folding allows us to distinguish areas among the first deformed during Tertiary tectonics. Within each superimposed structural unit of the West Balkan, the deformation spread from south to north. The boundary between the Srednogorie and Balkan zones appears to be a major tectonic structure in Bulgaria. The Balkan likely underwent a clockwise rotation relative to stable Europe since the beginning of the main Middle Eocene orogenesis.

Extension in an oceanic rift domain is accommodated by fissuring and faulting processes. The fissuring, including dike injection, involves extension and an increase in crustal volume, consistent with magma supply, as in oceanic ridge and hot spot contexts. The faulting mainly involves thinning and extension. The relationship between these two deformation modes is analysed in the northern rift of Iceland. In the active rift zone, deformation and volcanism occur within 2–3-km-wide fissure swarms separated by 10-km-wide zones where deformation is minor. Based on topographic and geological mapping, the local structure in a major fissure swarm was analysed. The fracture density increases near the faults, but also beyond their tips. The geometrical analysis of structures revealed that for the last 10,000 years, block faulting and tilting accommodated about one half of the extension, the other half resulting from fissuring. This distribution shows that even in an active volcanic zone with widespread magmatism, the stretching resulting from normal shear can play an important role. As estimated from the ages of eroded surfaces and lava flows, the deformation rates range between 1.5 and 15 cm/year. These values are compared with other rates estimated for different time intervals in the same area.

Seismic Zone from the surface traces of the Arnes and Hestfjall faults. Two major earthquakes (M s = 6.6) occurred on 17 and 21 June 2000 in the South Iceland Seismic Zone. This paper presents characteristic examples of surface traces of these seismic faults, called the Arnes and Hestfjall faults. The surface trace of the Arnes fault at Mykjunes shows a conjugate strike-slip pattern with a N30°E trending right-lateral fault and a N60°E trending left-lateral fault. Each of these faults is formed by en-echelon' arrays of fractures and push-ups. The surface trace of the Hestfjall fault near Bitra reveals a N50°-60°E trending left-lateral strike-slip fault including large open fractures, en-echelon' fractures and some push-ups. These observations show that the actual earthquake-related fault pattern is not restricted to the north-south-trending dextral strike-slip revealed by focal mechanisms and north-south alignment of aftershocks, but includes conjugate systems consistent with the structural pattern of the SISZ.

This paper presents the main characteristics of the historical seismicity in Iceland, as well as some examples of historical earthquakes in the South Iceland Seismic Zone. The geological aspects of the major earthquakes and their social impact are shown. The relationships with the volcanic phenomena and the presence of a hot spot beneath Iceland are discussed. Finally, the contribution of the knowledge of historical seismicity to the understanding of prevention and forecasting is highlighted.

Brittle tectonic analysis of Cretaceous–Paleogene sediments at a total of 17 sites located in the Isle of Wight (U.K.) enables four main tectonic events that occurred prior to and after the folding to be identified and successive palaeostress tensors to be determined using the inversion method. Three of the events can be shown to have occurred prior to the folding: (1) a syn-sedimentary extension of Upper Cretaceous age; (2) a strike-slip faulting regime with an ESE–WNW direction of compression; (3) a compressional regime, marked by strike-slip faulting, with an NNE–SSW to N–S direction of compression. The fourth and last compressional event took place after the folding and is characterised both by reverse and strike-slip faulting, with a dominant N–S direction of compression. Syn-folding faults also developed between the third and fourth events. All four events can be connected to the extensional tectonics and different steps of structural inversion, both of which were integral to the development and evolution of the Wessex basin.

The inversion of ∼ 1000 fault slip data collected in the Flateyjarskagi Peninsula allows reconstruction of four main tectonic regimes. These include normal and strike-slip faulting modes and are related to the general behavior of the Husavik-Flatey Fault (HFF), a major structure of the Tjornes Fracture Zone connecting the Kolbeinsey Ridge and the North Icelandic Rift. The two most important regimes (E-W and NE-SW extensions), consistent with the right-lateral motion along the Husavik-Flatey Fault, constitute the main tectonic group. The two others (NW-SE and N-S extensions), forming the subordinate tectonic group, are incompatible and result from drastic stress permutations. The relationships between these stress regimes imply not only σ1/σ2 and σ2/σ3, stress permutations but also σ1/σ3 reversals. A critical review of other data available, such as lava bedding, dike, and major fault attitudes, allows us to complete the structural pattern of the Flateyjarskagi peninsula and to highlight the mechanism of the transform zone. The complex pattern of dikes and faults in the northern part of Flateyjarskagi can be explained by the superposition of several processes: (1) a transform-perpendicular extension (E-W to ESE-WNW trends), (2) a simple shear (NNE-SSW to NE-SW trends), and (3) a stress perturbation due to the transform motion (NW-SE trends). An important factor controlling the transform mechanism is the variation of coupling along the HFF. The obliquity between the direction of transform motion and the trend of extension for the two main regimes may vary between 20° and 90°, reflecting repeated changes of the coefficient of friction along the HFF. Such change from very low mechanical coupling (weak fault) to moderate friction may occur very rapidly since it takes place several times in a few years, as shown by focal mechanisms of earthquake analysis.

Tectonic studies near major fault zones often reveal multiple tectonic regimes. Do these regimes indicate multiphase tectonism with distinct episodes, or do they reflect single-phase tectonism with time-space perturbations along lithospheric weakness zones? Based on tectonic analyses in Flateyjarskagi, North Iceland, we reconstruct the late Cenozoic tectonic regimes related to right-lateral transform motion along the Tjornes Fracture Zone, which connects the Kolbeinsey Ridge and the North Iceland Rift. Rifting and transform motion have induced eight normal and strike-slip regimes, four of which are inconsistent with the overall kinematics (as a probable result of stress drop, elastic rebound and dyke injection). For the consistent regimes, contrasting angles between extension and transform trends reflect repeated changes from moderate (25°) to very low mechanical coupling (85°) across the transform zone. Thus, the tectonic regimes need not be interpreted in terms of numerous tectonic episodes but rather as a consequence of variable coupling across the transform zone.

The South Iceland Seismic Zone (SISZ) trends E–W between two major volcanic rift zones of Southern Iceland. It is 20–60 km wide and 70 km long, and characterized by important seismotectonic activity. Major destructive earthquakes in the SISZ were reported throughout the centuries, from the initial settlements in Iceland until now. We carried out a structural study in this area, including analysis of aerial photographs, local observation of major faults and collection of minor fault slip data in outcrops, as well as an analysis of the focal mechanisms of earthquakes. At the regional scale, the main fault trends are approximately NNE–SSW and NE–SW. ENE–WSW, NW–SE and WNW–ESE trending faults are also detected in aerial photographs and in the field. All these faults are normal or strike-slip in character. Some of the historical major earthquake fractures are observed in the post-glacial lava flows in the SISZ: most are right-lateral and trend roughly N–S. The fault trace left by the last major single earthquake in the study area, a M7 earthquake in 1912, is mentioned as a case example. In addition to the large-scale faults, we analyzed more than 700 minor faults at 25 sites. Most sites are located in rocks of Upper Pliocene–Pleistocene age. Inversion of fault slip data sets enabled us to reconstruct local paleostress tensors, hence to define the major tectonic regimes which have prevailed in the SISZ. Examples of characteristic sites are given, and the main results are presented. Two main groups of faulting mechanisms reveal two distinct stress regimes, with perpendicular directions of extension, NW–SE (primary) and NE–SW (secondary). Both groups, however, display inhomogeneous data sets, related to extensional and to strike-slip faulting. The primary stress regime is in agreement with both the general behaviour of the SISZ as a left-lateral transform zone and the opening of the rift segments. The secondary stress regime, incompatible with the primary stress regime, is interpreted in terms of stress permutations. A population of 231 double couple focal mechanisms (M>1 and depth > 2 km) was also analyzed in terms of stress states. The results show great similarity in terms of stress directions. The present-day stress field mainly inferred from analyses of earthquake focal mechanisms is consistent with the present behaviour of the SISZ as a left-lateral transform zone. However, the proportion of strike-slip faulting within the present-day seismic activity (71%) is significantly higher than that revealed by the geological observation of Quaternary faults (50%). This contrast is interpreted in terms of development and evolution of the transform fault zone. The paleostress fields identified in the Upper Pliocene–Pleistocene formations of the SISZ reflect both the previous behaviour of the area, when it was located inside the rift zone, and its present behaviour as a transform zone, thus illustrating the local evolution from rifting to transform motion.

The present study was carried out in the Holy Cross Mountains (HCM) of south-central Poland and includes computation of palaeostresses following Angelier's method and field structural analysis. The Palaeozoic basement of the HCM comprises two tectonic units separated by the major WNW–ESE-striking Holy Cross Fault (HCF). Fold analysis indicates a N–S to NNE–SSW direction of Variscan shortening. Micro-structures and fold analysis from Upper Devonian rocks further reveal: (1) a brittle tectonic event due to a NW–SE compression preceding folding that could be related to pre-Late Carboniferous tectonics, due to block transport within the Tornquist–Teisseyre Zone (TTZ), and (2) polyphase Variscan folding comprising (a) an early stage of N–S shortening marked by north-verging ramps, (b) a main folding event and axial cleavage formation involving N–S to NNE–SSW shortening, and (c) a late stage of shortening deforming older folds and cleavage. A mainly extensional tectonic regime dominated from the Permian until the Cretaceous, during which time the HCF was reactivated as a normal fault. Large NW–SE faults bordering the Mid-Polish Trough (MPT) developed. Subsequent tectonic inversion of the MPT resulted in basin uplift (`Mid-Polish Swell', MPS). Palaeostress computations from Mesozoic strata suggest a NE–SW direction for the main Maastrichtian–Paleocene shortening phase, in addition to two minor brittle events resulting from N–S and E–W compression. Analysis of local folds in the Mesozoic cover indicates a causal relationship with the Maastrichtian–Paleocene reactivation of older faults. In particular, en-echelon folds in the Radomsko Elevation suggest a sinistral reactivation of the Palaeozoic HCF. Folds in the southwestern part of the HCM argue for reactivation in the reverse mode of a NW–SE-trending fault bordering the MPS that originated in the Mesozoic. In Palaeozoic strata, post-Variscan brittle deformation and micro-fault reactivation are attributed to the tectonic events of Maastrichtian–Paleocene age. Reactivation of N–S fractures is demonstrated in the Rzepka Quarry, where fractures in Devonian dolomites are filled by calcite, probably formed during a late Variscan event, and by younger Late Permian–Early Triassic sediments. They were subsequently reactivated as dextral strike-slip faults during the NE–SW Maastrichtian–Paleocene compression.

Two transform zones connecting the Icelandic rift segments and the Mid-Atlantic ridge are described and compared in terms of faulting and stress patterns. One is a diffuse shear zone of Riedel type. The other, formed by three major lineaments, reflects an alternance of moderate and very low mechanical couplings. Comparing these two transform zones at different stages of evolution shows that (1) the trend of the transform motion, as expressed in major structures, appears only when the zone is mature, and (2) a rather simple stress pattern exists at early stages of the process whereas a complex stress pattern prevails at later stages.

The South Iceland Seismic Zone (SISZ) is located at the junction of three rift segments in southwestern Iceland. The presence of different types of faulting and of differently orientated subgroups in Upper Pliocene to Holocene formations indicate polyphase tectonism. We measured 736 minor faults at 25 sites. Two types of relationships between stress regimes are represented. The first type, named IDS (inhomogeneous data set), is characterized by the presence of two types of fault mechanisms, normal and strike-slip, consistent with a single direction of extension. The second type, named OSR (opposite stress regimes), is characterized by the presence of perpendicular directions of extensions for a single type (normal or strike-slip) of faulting. Because of contradictory chronological criteria, we infer that the OSR alternated during the brittle tectonic activity of the SISZ. Two stress regimes, primary and secondary, are characterized by directions of extension NW-SE and NE-SW, respectively. The general fracture pattern characterized for the primary stress regime in the SISZ includes NNE-SSW trending right-lateral strike-slip faults, conjugate ENE-WSW trending left-lateral faults and NE-SW normal faults. This distribution is quite consistent with a Riedeltype model of fault pattern in a left-lateral shear zone. The stress states characterized based on analysis of both the earthquake focal mechanisms and the recent faulting show great similarity in terms of stress directions. The main difference is the larger ratio of strike-slip motions representing 71 % of the total population in the case of earthquake focal mechanisms, whereas for the whole set of faults the proportion of strike-slip faulting was 50 %. We explain that a temporal evolution of the tectonic regime in the SISZ region, accompanied by a gradual change in stress field, starts with rift-type pure extension and progressively leads to development of preferentially strike-slip structures in the kinematic context of leftlateral transform motion.

Significant matrix strains were associated with normal faulting in the Campanian chalk of the Mons basin. These transformations, mainly due to pressure solution, located against the fault planes, brought about systematic changes in the porous network: a reduction in volume against the fault planes, a change in access diameter and an increase in heterogeneity of the material. The measurements of other physical properties (elastic waves velocity, capillary rise, permeability) of the chalk provide good evidence for estimating the width of the transformed zone: it ranges between 100 and 150 mm. These transformations cannot be detected when the medium is observed at the grain scale: the most visible evidences of transformation are limited in a 50 mm thick fringe. The transformation may have occurred either in a closed or in an open system. If we follow the first assumption we can estimate the change in volume, and therefore the change in horizontal dimension of the faulted blocks: it is small compared to the horizontal extension. The second assumption is supported by a few evidences: tilted and striated rostra of belemnites along fault planes. In that case the geometrical change can be estimated too and reveals that the horizontal shortening is in the same range as the horizontal extension. It means that the extensional deformation of the Mons basin might have been partially hindered by chemically induced matrix strains.

Three large (kilometric-scale) caves were studied in the Buda hills and the main directions of cave corridors, fault planes and mineralized veins were measured. Different stages of mineralizations are recognised: calcite scaleno-hedrons, baryte, silica, gypsum. New investigations of fluid inclusions in the baryte suggest a crystallization temperature of 50 °C and a freshwater fluid source. Microtectonic analysis allows the reconstruction of the successive tectonic events: (1) a NE-SW extensional phase at the Late Eocene-Early Oligocene limit (phase I), (2) a strike-slip phase with NW-SE compression and NE-SW extension during the Late Oligocene-Early Miocene (phase II), (3) a NW-SE transtensional phase (phase III) and finally (4) a NE-SW extensional phase of Quaternary age (phase IV). The major phase is the strike-slip one, characterized by an important dextral strikeslip zone: the Ferenc-hegy zone.

The largest earthquakes in Iceland are associated with strike-slip faults in the South Iceland Seismic Zone (SISZ). Major destructive earthquake sequences occur in this zone at an average interval of 80 years, the last two sequences being in 1784 and 1896. The last major single earthquake in the zone was an earthquake of magnitude 7 ( M ) in 1912. The seismicity is mostly associated with a conjugate system of NNE-trending (mainly) right-lateral faults and ENE-trending (mainly) left-lateral faults, but in addition there are many NW-trending faults in the SISZ. Particular attention is paid to the comparison between present-day fault activity (earthquake mechanisms) and Quaternary faulting (fault-slip data). This comparison reveals a general similarity despite the temporal difference between these subsets independently collected. The fault populations are well exposed in the hyaloclastite mountain Vordufell, near the central part of the South Iceland Seismic Zone, where considerable fault slip occurred during the earthquake sequences of 1784 and 1896. The cumulative displacement on most of these faults is 1–6 m. In addition to the large-scale faults, we measured 79 minor faults (displacements of the order of centimetres) in the Vordufell Mountain. Most of the minor faults are left-lateral. Analysis of these minor faults recording palaeostresses in rocks ranging in age from 3.1 to 0.7 Ma, as well as 50 focal mechanisms from a swarm of present-day microearthquakes occurring beneath the Vordufell Mountain, gave consistent results in terms of seismotectonic stress (orientations of stress axes and ratios of principal stress differences). This shows that the tectonic regime remained unchanged through recent times. The results indicate, however, that there are two contrasting stress regimes in this part of the SISZ. The primary subset indicates NW–SE tension and NE–SW compression, whereas the secondary subset indicates NW–SE compression and NE–SW tension. We propose that the primary subset is consistent with the dominating time-averaged regional stress field in the SISZ. This dominating stress field can be largely explained as a consequence of simple plate pull parallel with the spreading vector in South Iceland. The secondary subset and the associated stress field, however, may be partly related to stress release and rebound and partly to dike injections in the nearby segments of the rift zone in South Iceland.

The Vestfirdir Peninsula of northwestern Iceland mainly consists of tholeiitic lava flows, 8–14 Ma old, gently dipping to the southeast. A detailed study of strike-slip and normal faulting allowed identification of two main paleostress regimes. Two sets of normal faults were recognized. The largest set trends ENE-WSW to NNE-SSW; the minor set trends NW-SE to WNW-ESE. Concerning the major extension, the reconstruction of paleostress trajectories shows a gradual change in trend from ESE-WNW, in the northern half of the peninsula, to NNW-SSE to the south. The minor extension also shows a gradual change from NNE-SSW to ENE-WSW trends, from north to south. The nearly constant perpendicularity between the major and minor trends of extension is accounted for by permutation of stress axes within the general pattern of extension related to oceanic rifting. The progressive azimuthal change of the major extension trend, from northeast to southwest across the peninsula, is interpreted as the expression of a c...

The Jurassic sedimentary rocks (Bajocian to Tithonian) in the Boulonnais area (NW France) unconformably overlie a structured Palaeozoic basement. The Jurassic rocks are composed of three litho-structural units: 1) the Bajocian and Bathonian carbonates form a thin brittle unit; 2) the Callovian and Oxfordian clays form a thick, weakly folded unit; 3) the Kimmeridgian and Titho- nian layers of alternately competent and incompetent rocks form both a faulted and a folded unit. Each unit deforms specifically. The mapping of the Jurassic rocks in the Boulonnais area was completed in the submarine western prolongation from the interpretation of seismic profiles. The structural pattern of the Boulonnais is marked by faults striking N110-120, N090 and N030, and by folds with axes trending N090 and N120. The submarine structure is marked by numerous E-W faults and folds. The analysis in the field and the paleostress computation reveal both brittle deformation and folding, from which we interpret a relative chronology of tectonic events. The comparison between the Jurassic and Palaeozoic deformations makes the similarity of their major directions obvious. A structural inheritance of the Palaeozoic basement is thus noteworthy when the Jurassic cover deforms, shown by various types of folding. Nevertheless, not all of the paleozoic faults were reactivated during Mesozoic and Cenozoic tectonics.

International audience; Volcano-tectonic processes in the central part of Iceland, covered by the Vatnajökull glacier, are investigated by inversion of focal mechanisms. Working on a large catalog of focal mechanisms determined by the Icelandic Meteorological Office (IMO), we used a damped regional-scale stress inversion method to obtain an insight of kilometric variations of the stress field. To evaluate the resolution and the stability of this stress field solution, we computed checkerboard tests, stress field models and error propagation tests. Stress field models showed a continuous stress regime between normal and strike-slip faulting, associated with a high stress shape ratio (i.e., σ 1 ≈ σ 2). Two main directions of σ hmin were evidenced: the first one was in agreement with the regional spreading direction of Iceland and the second one was deviated, being almost perpendicular to the first one. The deviated stress direction is sustained through the 20 year time-span of the study around the Bárðarbunga and Grimsvötn central volcanoes while the spreading direction remains predominant around the Hamarinn volcano. This result supports the hypothesis that this volcano lacks collapse caldera and shares a fissure swarm with the larger Bárðarbunga volcano. On a smaller temporal scale, during the 1996 volcanic crisis, a bimodal distribution of σ hmin showed two opposite strike-slip regimes where the deviated direction dominated. Because these two states of stress T1 and T2 show stress regimes away from the Andersonian positions, P, B, and T axes, the rapid flip between these two regimes may be associated with the progressive melt intrusion of a dyke.

The geometry of fracture systems in selected areas of the active Krafla fissure swarm, mid-Atlantic ridge, northeastern Iceland, is analysed. Based on geodetic analysis of the present-day topography at the top of Holocene basaltic lava flows which fill the axial rift zone, the deformation of this initially horizontal surface can be reconstructed. Extensional deformation is localised at all scales and block tilting, though present, remains minor. Using simple models of the surface expression of normal faults, the geometrical characteristics of the topographic features related to active deformation during tectonic-volcanic events are quantitatively analysed. At crustal depths of about 1 km, normal faults are present and have an average 70 ° dip. Comparison with the dip data of older normal faults observed in the uplifted and eroded shoulders of the rift zone, at palaeodepths of 1–2 km, indicates that this dip determination is valid. Comparisons between the local case study and structural analyses of active fissure swarms on a larger scale suggest that normal faulting plays a major role in the middle section of the thin, newly formed brittle crust of the rift zone. In the axial oceanic rift zone of NE Iceland, the extensional deformation in the upper crust is dominated by horizontal tension and shear of normal sense, their relative importance depending on depth. Absolute tension dominates in the uppermost several hundred metres of the crust, resulting in the development of fissure swarms. Effective tension plays an important role at a deeper level (2–5 km), because of the presence of magmatic fluid pressure from magma chambers which feed dyke injections. At crustal depths of about 1 km, normal shear prevails along fault planes which dip 60 °–75 °. This importance of normal shear at moderate depth, between upper and lower crustal levels where tension prevails, is pointed out. Within the extensional context of rifting, these variations of tectonic behaviour with depth are controlled by both the lithostatic pressure and the effective tension induced by the presence of magmatic fluid pressure.

18 pages; International audience; The Oolithe Blanche Formation was studied in three quarries, located at the south-eastern edge of the Paris Basin (France). Heterogeneities in reservoir properties were assessed through a sedimentological, diagenetic and petrophysical study. The relationships between depositional settings, diagenesis and petrophysical properties were analysed using detailed petrographic studies, image analysis, Nano CTscans and petrophysical measurements. The carbonate reservoir pore network is mainly controlled by intraparticle microporosity which ensures the connectivity with interparticle meso- and macroporosity. Early cementation vs. early compaction processes (mainly grain interpenetration) may have considerable influence on fluid-flow properties and parameters such as permeability, acoustic velocities and tortuosity. Better reservoir properties are found when compaction processes begin before cementation. From statistical analyses, e.g. Principal Component Analysis and Linear Discriminant Analysis, a sedimentological/diagenetic and petrophysical model is proposed that is in a good agreement with the geological model developed from field work.

The tectonic effects of the Thulean mantle plume on the opening of the North Atlantic Ocean is still poorly understood. An analysis of the brittle deformation affecting the Late Cretaceous Chalk and Lower Tertiary igneous formations cropping out in Ulster (Northern Ireland), part of the Thulean Province, leads to the recognition of two tectonic phases. Each of these phases is characterized by different stress regimes with similar trends of the horizontal maximum principal stress σH. The first phase, syn-magmatic and dominated by NE–SW to ENE–WSW extension, occurred during the Palaeocene. It is followed by a second post-magmatic phase, characterized initially by a probably Eocene strike-slip to compressional palaeo-stress regime with σ1 (=σH) trending NE–SW to NNE–SSW associated with the partial reactivation (as reverse faults) of normal faults formed during the first phase NE–SW extension. This episode is postdated by an Oligocene extension, with σH (=σ2) still striking NNE–SSW/NE–SW, which reactivated Eocene strike-slip faults as nearly vertical dip-slip normal faults. This Palaeogene tectonic evolution is consistent with the tectonic evolution of similar age in western Scotland and in the Faeroe Islands. In particular, the post-magmatic NE–SW compression is here related to the ‘Faeroe compressive event’, which is related to the earliest stages of drift of the Greenland plate.

The Vestfirdir peninsula of Northwest Iceland consists mainly of gently dipping basaltic lava flows 8–14 Ma old. Detailed studies of its principal tectonic elements (basaltic dykes, normal faults and minor faults) and observations of the trends of the major fjords and valleys make it possible to reconstruct the evolution of the associated stress field. The results show that a kink-shaped (curved) rift zone existed in this area until about 5 Ma ago. In the northern part of the peninsula the rift zone trended NNE, but ENE in the southern part. It is proposed that the tectonic elements of the Vestfirdir peninsula were generated in two stress fields, one operating inside the rift zone, the other operating outside it. The rift-zone stress field was responsible for the formation of most normal faults, dykes and the main extension direction inferred from the minor faults. Conversely, the off-rift stress field was responsible for the formation of several Holocene normal faults in the southern part of the peninsula, the direction of the secondary extension (inferred from the minor faults) as well as for most major fjords and valleys in the area. Boundary-element studies suggest that the off-rift stress field generated on the concave side of a curved rift zone is different from that generated on its convex side. Absolute tensile stresses that encourage the formation of valleys and fjords concentrate very near to, or at, the convex margin, but at a considerable distance from the concave margin. Applying these model results to Iceland as a whole, it is concluded that the valley formation in Northwest Iceland occurred at a large distance form the associated rift zone, whereas in East Iceland the valleys formed near to the margin of the rift zone. Furthermore, the model predicts that zones of relative tension may propagate radially from the convex side of the rift zone and act as magma channels for volcanoes. The location and orientation of major fjords and valleys in East and Northwest Iceland, as well as the existence of several major off-rift Holocene volcanoes, support the model predictions.

The main purpose of this study is to analyse striated micro-faults and other types of fractures (including tensile and shear joints, and veins), in order to elucidate their relationships with regional folds and thrusts and regional tectonic stress. We take the fold-thrust belt (i.e., the foothills and the Hsuehshan range) in NW Taiwan as a case study, which is a product of the Plio-Pleistocene arc-continent collision. A total of about 760 and 1700 faults and other fractures, respectively, were collected at 41 sites in the field. We have identified four sets of bed-perpendicular joints in the study area. The observation of joints and bedding at each site indicates that most of the penetrative joint sets developed in the earlier tectonic stage of the pre-folding/pre-tilting event, illustrating the fact that the intersection of joint sets lies along the line perpendicular to the bedding plane. We thus interpret these sets as tectonic fractures under deep-seated tectonic stress. We used the regional fold axes as reference to define the four fracture sets. However, we found that complexity in the study area makes this rather tentative. Principal stress axes σ1, σ2, σ3, were calculated by means of inversion of fault slip data at each site. The ratio Φ that defines the shape of stress ellipsoid is generally small, indicating that the value of the maximum principal stress axe σ1 is much larger compared to that of σ2 and σ3, which are approximately equal. The paleostress regime was characterized by a combination of thrust and strike-slip tectonic regimes. Based on their geometric relationships with tilted bedding, we found most of striated micro-faults were strongly related to the regional folding and can be categorized as early-, during, and late-folding stages. We characterized two major directions for the compressive event, oriented N110–120°E and N150–160°E respectively, which provide additional evidence to delineate the debates about paleostress changes in the Taiwan mountain building process.

International audience; The Vogar Fissure Swarm is one of four en-echelon fracture swarms that connect the Reykjanes Ridge to the South Iceland Seismic Zone and the Western Volcanic Zone. Occurring in an area of flat topography, this fissure swarm is clearly visible at the surface, where it can be seen to affect recent postglacial lavas. Using remote sensing methods to identify and measure all the faults and fractures in the swarm, combined with additional field observations and measurements, we measured 478 individual fractures, 33% of them being faults and 67% being fissures. The fracture lengths show roughly log-normal distributions. Most of the individual fractures belong to 68 main composite fractures, seven of which are longer than 2500 m and correspond to the main fault scarps of the fissure swarm. We showed that these main faults are distributed along five, equally spaced zones, ~500 m apart and a few kilometers long. We drawn 71 across-strike profiles to characterize the shape of the fault scarps, and 5 along-strike profiles to characterize the evolution of vertical throw along the main faults. Each fault consists of a coalescence of individual segments of approximately equal length. Fault throws are never larger than 10 m and are smallest at the junctions between individual segments. Analyses of along-strike throw profiles allowed us to determine the early stages of growth after coalescence. The earliest stage is characterized by an increase in the throw of the central parts of segments. This is followed by a second stage during which the throw increases at the junctions between segments, progressively erasing these small- throw zones.

International audience; Comparison of the eroded off-rift zone left inactive by plate motion with the inner active seismic rift zone allows us to constrain the fracturing mechanisms. In eastern off-rift zone, we measured 423 fault slips (including normal and strike-slip faults). Inversion of fault slip data reveals the parallelism of the minimum stress (σ3) computed for the normal and for the strike-slip faulting, and consistency with the direction of plate divergence. North of the Vatnajökull, in the active rift zone, we analysed 14,250 earthquakes recorded by the Icelandic Meteorological Office (IMO) between 2004 and 2009, especially northeast of the Askja volcano. Two main average focal depths were determined at 5 km (Herðubreið table mountain) and 15 km (Upptyppingar hyaloclastite ridge). The double couple focal mechanisms determined by IMO revealed that more than half of the mechanisms are strike-slip. Faulting type both in active and off-rift zone reveals an unusual importance of strike-slip regime in such an extensional tectonic context. This can be explained by stress permutations (σ1/σ2). Similarities in terms of stress orientations and type of faulting are observed both in the old and present-day rift zones. We assume that the seismic events may be generated by rapid deep magma intrusion, also associated with shallower hydrothermal activities. We thus propose that the presence of fluids, shear failure (double couple focal mechanism) and stress permutations in both active and off-rift zones are closely linked. Assuming a Drucker-Prager failure criterion, we evaluated analytically that a state near of lithostatic pore pressure is a necessary condition for shear failure at shallow (5 km) and deep (15 km) depth, in a simple context of crustal extension that allows for stress permutations. However, processes favouring stress permutations cannot be further constrained from our observations since the dynamics of fluid, materiel heterogeneity and post-glacial rebound can also play a significant role

Whereas most of the stress inversion methods using fault slip data only minimize the angle between the measured striation and a computed shear stress to find the best fitting reduced stress tensor, Angelier (1990) proposed an alternative method named INVD that also takes into account the relative shear stress magnitude which allows the fault to move. Using artificial datasets and particular fault geometries we compare this method with one of the classical methods based on the minimization of the shear-slip angles (R4DT; Angelier, 1984) and we show that in most cases the new method has improved the quality of the results. Furthermore, as proposed by Angelier, we point out that the quality of the stress inversion primarily depends on the quality of the field data. We give advice and warn about some pitfalls concerning determination of sense of slip on fault planes, recognition of successive faulting events and their chronology, drawer (or wedge) faults, stress permutations, faults in vertical bedding. We also argue that, in case of tilted sequences, fault diagrams should not be presented without bedding planes. But we show that stress inversions, when realized with caution and with the correct method, can havemuch more applications than reconstructing stress fields, like for determining: the paleo-horizontal, the nature and the sense of motion of large faults, the chronology and age of large structures.

In Southeast Iceland, comparison between the deeply eroded off-rift and flank zones, on the one hand, and the active rift zone, on the other hand, allows us to characterize the geometry and mechanisms of faulting and fracturing since the Upper Pliocene to Holocene. We used different approaches based on the inversion of fault-slip data and of focal mechanisms of earthquakes, as well as on study of aerial photographs by means of photogrammetry. Our study reveals that about half of the past and current stress states are strike-slip whereas surface deformations are large normal faults and extensional fractures. The parallelism between the directions of extension (σ 3 ) in both extensional and strike-slip regimes, implies a σ 1 /σ 2 stress permutations probably due to uniaxial extensional stress (σ 1 = σ 2 > σ 3 ) and fluid overpressure. In detail, the deviation of σ 3 trajectories, probably relates to the presence of central volcanoes, which may locally induce perturbations of the general stress field.

Instability structures, synsedimentary faults and turbidites have been studied in the Lower Pliensbachian succession of Saint-Michel-en-Beaumont, belonging to the Taillefer block, an ancient half-graben emplaced during the Liassic Tethyan rifting. Geometrical and mechanical analyses demonstrate that the instability structures occurred thanks to movements along spineless synsedimentary normal faults, when the turbiditic and limestone layers were already case-hardened and partly fractured by tension gashes even when the mudstones were still unlithified. Both the tension gashes and the synsedimentary faults are homogeneous in strike with the major regional faults and are in good agreement with the regional direction of extension for this period. The characters of the turbiditic beds, with erosive base, graded bedding, and incomplete Bouma sequence, are in favour of a seismic origin. Instability structures, spineless synsedimentary faults and turbiditic inflows are thus considered as seismites and interpreted as the result of high seismicity periods including some events with M > 5 in the general extensive ambiance of the Liassic Tethyan rifting. The analysis of the geometrical relationships between all these sedimentary features allows to distinguish the successive stage of occurrence of an instability structure, from the sedimentation of alternating marls and limestones, and sudden turbiditic inflows, then early case-hardening of the turbidites, until the important seismotectonic event generating the spineless normal faults, themselves triggering the fall of indurated blocks and locally the forming of breccias. The Ornon Fault, which constitutes the border of the Taillefer block, 15 km eastward, played a major role during the Liassic sedimentation and may represent the major seismic fault related to the seismites occurrence in the Beaumont basin.

[1] Transform motion along oceanic transforms generally occurs along narrow faults zones. Another class of oceanic transforms exists where the plate boundary is quite large (∼100 km) and includes several subparallel faults. Using a 2-D numerical modeling, we simulate the slip distribution and the crustal stress field geometry within such broad oceanic transforms (BOTs). We examine the possible configurations and evolution of such BOTs, where the plate boundary includes one, two, or three faults. Our experiments show that at any time during the development of the plate boundary, the plate motion is not distributed along each of the plate boundary faults but mainly occurs along a single master fault. The finite width of a BOT results from slip transfer through time with locking of early faults, not from a permanent distribution of deformation over a wide area. Because of fault interaction, the stress field geometry within the BOTs is more complex than that along classical oceanic transforms and includes stress deflections close to but also away from the major faults. Application of this modeling to the 100 km wide Tjornes Fracture Zone (TFZ) in North Iceland, a major BOT of the Mid-Atlantic Ridge that includes three main faults, suggests that the Dalvik Fault and the Husavik-Flatey Fault developed first, the Grismsey Fault being the latest active structure. Since initiation of the TFZ, the Husavik-Flatey Fault accommodated most of the plate motion and probably persists until now as the main plate structure.

International audience; Iceland brings exceptional opportunity for analysing extension related to rifting of the Mid-Atlantic ridge, especially revealing fresh structural patterns in active fissure swarms. Post-glacial fracture systems of the Thingvellir rift segment of the West Volcanic Zone (WVZ) and interaction with holocene lava flow overlapping are analysed in detail in this paper. We mapped 5390 fractures at metric to kilometric scales in order to realise a precise structural map, a representative fault length distribution analysis and some statistical calculations in terms of fault length/number growth rates from Holocene to recent time. Mapping and 3-D geometrical analysis of faults and fissures are based on use of photogrammetric techniques, GPS positioning at ground control points and validation from geological field work. This approach allowed us to measure the vertical throw distribution along 52 faults with a precision around 0.5-1 m. Most of these faults have symmetric serrated fault-displacement profiles; however some of them have profiles offset to the north or south. Fault vertical offset as a function of the age of the hosting lava flows are presented too. Finally, from the study of 70 transverse topographic profiles and the fault offset analysis, we propose a propagation model for Holocene fissure development, partly controlled by Pleistocene tectonic inheritance. Our model takes into special account alternating volcanic events and faulting. Simple fissure zones with small hangingwall monocline or more complex scarp zones with graben and larger hangingwall monocline developed. Because of lava flow accumulation during the rift extension, estimating the amount of extension based on the present-day morphology would have led to severe under-evaluation.

A synthesis of existing borehole data and seismic profiles has been conducted in the Artois area (northern France), along the northern border of the Paris basin, in order to explore the possible control exerted at depth by the Upper Carboniferous Variscan thrust front on the distribution of Late Paleozoic-Mesozoic depositional centers and their subsequent uplift in Tertiary times. Such control was demonstrated recently in the Weald-Boulonnais basin (Eastern Channel area) that forms the western prolongation of the area under study but was so far poorly constrained in the Artois area. Presented data provide evidence for the topography of the Artois hills and the altitude of sedimentary layers to be controlled by the activity of a network of relaying WNW-ESE striking faults inducing the systematic uplift of the southern fault blocks. Those steeply S-dipping faults branch downward onto the ramp of the Variscan thrusts forming listric faults that locally limit to the north buried half-graben structures, filled with fan-shaped fluviatile Stephanian-Permian deposits. Such clear syn-rift geometry shows that the ramp of the main Variscan frontal thrust (the Midi thrust) has been reactivated as a normal fault in Stephanian-Permian times thus forming a very demonstrative example of a negative inversion process. The reverse offset of the transgressive Middle Cretaceous-Lower Eocene layers covering unconformably the Paleozoic substratum argue for a Tertiary (Middle Eocene-Late Oligocene?) contractional reactivation of the fault network thereby documenting a repeated inversion process along the Artois Variscan thrust front. The Variscan frontal thrust zone is thus shown here to represent a prominent crustal-scale mechanical discontinuity that localized deformation in the Artois-Boulonnais area since Upper Paleozoic times.

In order to (1) understand the tectonic significance of jointing, and (2) reconstruct the regional palaeostress history, we have studied macrofractures, especially joints, in the Colorado Plateau. Nine major joint set trends have been identified in the Colorado Plateau. Each set includes joints that may differ in significance and in age. The joints are commonly extensional joints but shear joints were also found. The main tectonic events reconstructed from macrofractures analysis include:(1) a pre-Laramide compressional event with σ1 N 45°E;(2) three Laramide compressional events with σ1 N 65°E, N 95°E and N 115°E successively; and (3) three Neogene extensional events with σ3 N 65°E, N 85°E and N 120°E successively. These events are correlated with major tectonic events described in neighbouring areas of the Western American Cordillera.