Processus de déformation et diagenèse dans les zones de subduction : impact sur les propriétés mécaniques des roches : Approche expérimentale

The shallow portion of subduction zones (0-10 km depth) has long been considered as unable to store and release seismic energy. However, the detection of very-low frequency earthquakes in this zone, as well as the propagation of the coseismic rupture to the trench during the Tohoku-Oki earthquake, question this hypothesis. The difficulty to assess the seismogenic potential of this shallow portion lies principally in the complexity of the processes that occur in this zone, combining deformation and diagenesis (especially the smectite-to-illite transition), and hence not easily reproducible in laboratory. In order to analyse the mechanical properties of the shallow portion of subduction zones, triaxial tests have been performed with smectitic and illitic samples, under confining pressure between 50 and 200 MPa and at temperature of 20 and 300°C. The aim of these experiments was to identify the deformation modes of such sedimentary material and to determine the effects of deformation and diagenesis on rheology of these materials and on the rock potential to exhibit instable failure. In the experiments, deformation operates under the same pattern with a progressive localisation from shear band to fracturation. Even if the deformation style does not differ much between experiments, the rheology of the samples tested at 20°C and at 300°C contrasts drastically. Indeed, while the samples (smectitic and illitic) tested at 20°C show exclusively a strengthening behaviour (i.e. stable), the smectitic samples tested at 300°C exhibit a rheology that systematically evolves from strengthening to stick-slip behaviour (i.e. unstable). These results indicate that the chemical reactivity of smectite under diagenetic conditions (diagenesis is activated in the experiments conducted at 300°C) constitutes a weakening mechanism promoting unstable sliding. Finally, we propose that, at the subduction zone scale, the chemical metastability of smectite could promote the propagation of the coseismic rupture to the very shallow portion of accretionary prisms.La partie superficielle des zones de subduction (0-10 km de profondeur) a longtemps été considérée comme asismique. Cependant la découverte de séismes très basses fréquences dans cette zone, ainsi que la propagation très superficielle de la rupture cosismique lors du séisme de Tohoku-Oki (Japon) remettent en question cette hypothèse jusqu’alors largement admise. L’une des raisons pour lesquelles le potentiel sismogénique de cette zone est mal contraint réside dans le fait que les processus qui y règnent sont complexes, mêlant déformation et diagenèse (principalement la transformation des argiles de type transition smectite vers illite), et ainsi difficilement reproductibles en laboratoire. Au cours de cette thèse, des expérimentations en presse triaxiale sur échantillons smectitiques (représentatifs des matériaux accrétés dans les prismes d’accrétion) et illitiques ont été réalisées sous différentes conditions de pression de confinement (de 50 à 200 MPa) et de température (20°C et 300°C). Ces expériences ont été menées afin d’identifier les modes de déformation de ces échantillons, et de déterminer les effets couplés de cette déformation et de la diagenèse sur la rhéologie de ces roches et notamment leur potentiel à générer des instabilités de glissement. Dans ces expériences, le style de déformation est à chaque fois similaire, avec, tout d’abord, une localisation progressive de la déformation le long d’une zone de cisaillement, puis formation d’une fracture. Malgré cette constance dans le style de déformation, le comportement rhéologique des échantillons, lui, est drastiquement opposé entre les expériences à 20 et à 300°C, avec un comportement exclusivement durcissant à 20°C (i.e. stable) quel que soit la minéralogie, et un comportement qui évolue systématiquement vers du stick-slip (i.e. instable) à 300°C (pour les échantillons smectitiques). Ces résultats montrent que la réactivité chimique des smectites au cours de la diagenèse (activée dans les expériences à 300°C) conditionne la formation d’instabilités de glissement. Nous proposons que la réactivité chimique des smectites dans les zones de subduction pourrait promouvoir la propagation de la rupture cosismique vers la surface

Long-term evolution of an accretionary prism: The case study of the Shimanto Belt, Kyushu, Japan

International audienceThe Shimanto Belt in SW Japan is commonly described as a paleo-accretionary prism, whose structure is explained by continuous accretion like in modern accretionary prisms such as Nankai. We carried out a structural study of the Cretaceous to Miocene part of the Shimanto Belt on Kyushu to test this hypothesis of continuous accretion. Most deformation structures observed on the field are top-to-the-SE thrusts, fitting well the scheme of accretionary wedge growth by frontal accretion or underplating. In particular, the tectonic mélange at the top of the Hyuga Group records a penetrative deformation reflecting burial within the subduction channel. In contrast, we documented two stages of extension that require modifying the traditional model of the Belt as a "simple" giant accretionary wedge. The first one, in the early Middle Eocene, is mostly ductile and localized in the foliated bases of the Morotsuka and Kitagawa Groups. The second one, postdating the Middle Miocene, is a brittle deformation spread over the whole belt on Kyushu. Integrating these new tectonic features to existing data, we propose 2-D reconstructions of the belt evolution, leading to the following conclusions: (1) Erosion and extension of the margin in the early Middle Eocene resulted from the subduction of a trench-parallel ridge. (2) The Late Eocene to Early Miocene evolution is characterized by rapid growth of the prism, followed by a Middle Miocene stage where large displacements occurred along low-angle out-of-sequence thrusts such as the Nobeoka Tectonic Line. (3) From middle Miocene, the strain regime was extensional

Processes of deformation and diagenesis in subduction zones : Impact on the mechanical properties of the rocks

La partie superficielle des zones de subduction (0-10 km de profondeur) a longtemps été considérée comme asismique. Cependant la découverte de séismes très basses fréquences dans cette zone, ainsi que la propagation très superficielle de la rupture cosismique lors du séisme de Tohoku-Oki (Japon) remettent en question cette hypothèse jusqu’alors largement admise. L’une des raisons pour lesquelles le potentiel sismogénique de cette zone est mal contraint réside dans le fait que les processus qui y règnent sont complexes, mêlant déformation et diagenèse (principalement la transformation des argiles de type transition smectite vers illite), et ainsi difficilement reproductibles en laboratoire. Au cours de cette thèse, des expérimentations en presse triaxiale sur échantillons smectitiques (représentatifs des matériaux accrétés dans les prismes d’accrétion) et illitiques ont été réalisées sous différentes conditions de pression de confinement (de 50 à 200 MPa) et de température (20°C et 300°C). Ces expériences ont été menées afin d’identifier les modes de déformation de ces échantillons, et de déterminer les effets couplés de cette déformation et de la diagenèse sur la rhéologie de ces roches et notamment leur potentiel à générer des instabilités de glissement. Dans ces expériences, le style de déformation est à chaque fois similaire, avec, tout d’abord, une localisation progressive de la déformation le long d’une zone de cisaillement, puis formation d’une fracture. Malgré cette constance dans le style de déformation, le comportement rhéologique des échantillons, lui, est drastiquement opposé entre les expériences à 20 et à 300°C, avec un comportement exclusivement durcissant à 20°C (i.e. stable) quel que soit la minéralogie, et un comportement qui évolue systématiquement vers du stick-slip (i.e. instable) à 300°C (pour les échantillons smectitiques). Ces résultats montrent que la réactivité chimique des smectites au cours de la diagenèse (activée dans les expériences à 300°C) conditionne la formation d’instabilités de glissement. Nous proposons que la réactivité chimique des smectites dans les zones de subduction pourrait promouvoir la propagation de la rupture cosismique vers la surface.The shallow portion of subduction zones (0-10 km depth) has long been considered as unable to store and release seismic energy. However, the detection of very-low frequency earthquakes in this zone, as well as the propagation of the coseismic rupture to the trench during the Tohoku-Oki earthquake, question this hypothesis. The difficulty to assess the seismogenic potential of this shallow portion lies principally in the complexity of the processes that occur in this zone, combining deformation and diagenesis (especially the smectite-to-illite transition), and hence not easily reproducible in laboratory. In order to analyse the mechanical properties of the shallow portion of subduction zones, triaxial tests have been performed with smectitic and illitic samples, under confining pressure between 50 and 200 MPa and at temperature of 20 and 300°C. The aim of these experiments was to identify the deformation modes of such sedimentary material and to determine the effects of deformation and diagenesis on rheology of these materials and on the rock potential to exhibit instable failure. In the experiments, deformation operates under the same pattern with a progressive localisation from shear band to fracturation. Even if the deformation style does not differ much between experiments, the rheology of the samples tested at 20°C and at 300°C contrasts drastically. Indeed, while the samples (smectitic and illitic) tested at 20°C show exclusively a strengthening behaviour (i.e. stable), the smectitic samples tested at 300°C exhibit a rheology that systematically evolves from strengthening to stick-slip behaviour (i.e. unstable). These results indicate that the chemical reactivity of smectite under diagenetic conditions (diagenesis is activated in the experiments conducted at 300°C) constitutes a weakening mechanism promoting unstable sliding. Finally, we propose that, at the subduction zone scale, the chemical metastability of smectite could promote the propagation of the coseismic rupture to the very shallow portion of accretionary prisms

Associations between organisations’ motivated workforce and environmental performance

Purpose – This study aims to investigate the association between a motivated and prepared workforce and environmental performance. Design/methodology/approach – Self-administered surveys were used to collect data for the study from 300 organisations operating in Australia. Confirmatory factor analyses were used to test the robustness of the various measurement models, and structural equation modelling was used to test the two propositions for this study. Findings – The results identify significant associations between affective commitment, employee performance process and training and enhanced environmental performance, which is mediated through the value-creating processes, work practices, process improvement and innovation process. Also, there is a set of sequential associations between work practices and process improvement and well as process improvement and innovation process. Practical implications – The study has identified specific management practices that enhance environmental performance. The findings add to the body of knowledge because previous studies focused on general conceptual rather than actual management practices. The implications for practice are that organisations should enhance organisational affective commitment to their environmental strategy, tailor employee performance processes and provide regular, specific training to employees to improve processes that lead to better environmental performance. Originality/value – Results, mentioned in findings above, provide some specificity to associations that had been illustrated and explained previously in an abstract or conceptual framework. A framework of identified associations provides a basis for future research and for practical application to assist with organisational environmental performance as part of a corporate sustainability strategy

Smectite reactions and slip instabilities in subduction zones

International audienceThough it is of prime importance in terms of seismic and tsunami risk, the mechanical behavior of the shallow(z<5km) domains of accretionary prisms is not well understood. The concomitant progress of mechanicalcompaction and diagenetic reactions results in the transformation of a soft sediment into a hard sedimentary rock,which modifies the rock potential to localize deformation and be involved in slip instabilities.While it is the major control on diagenetic reactions, the effect of temperature on the mechanical behavioris not well constrained experimentally. To address this question, we have designed triaxial deformation experimentsin the Paterson rig either at ambient temperature or at 300 C. The tested material includes siltstonesfrom the Boso Peninsula in Japan (corresponding to the shallow domain of a paleo-accretionary prism), either ascylindrical cores or as ground powders as well as powders composed principally of smectite. For this material, themain consequence of the high temperature conditions is to trigger the smectite-to-illite reaction or the smectiteinterlayer space collapse.The first result is that at 300 C, all tested samples show slip instabilities. These instabilities are apparentas a sudden ( 4s) and large (10 to 45 MPa depending on the starting material and the confining pressure) stressdrop in the macroscopic stress-strain curve, in some cases followed by a rapid restrengthening of the material. Incontrast, no instability was observed for the experiments at ambient temperature. As slip instabilities are activatedby the temperature and occur as well in smectite powders, we attribute these instabilities to the diagenetic reactionsof smectite.An additional experiment on a powder of smectite where the smectite-to-illite reaction has been inhibitedby cationic exchanges does not show instabilities upon deformation at 300 C. We propose therefore that catastrophicdehydration of smectite associated with the smectite-to-illite reaction may be responsible for triggering theinstabilities. This catastrophic dehydration is potentially a major control on the genesis of instabilities in naturalconditions, as (1) smectite is a major component of subducted sediments and (2) its transformation into illite mayoccur over a large depth range

How far the postorogenic extensional tectonics in the Aegean domain is symmetric?

International audienceCurrent models to explain the finite geometry of the Aegean domain and the exhumation of the eclogite-bearing HP rocks in the Aegean domain rely on the recognition of a synorogenic exhumation regime followed by a postorogenic back-arc extension. A first generation of low-angle normal faults (LANF) developed during the subduction and the formation of the Hellenic Eocene orogenic wedge contemporaneously with the overthrusting of the Cycladic Blueschists over basement units. Other LANF were formed subsequently and accompanied the development of Metamorphic Core Complexes (MCC) during the collapse of the internal zones in the back-arc domain. Most of the stretching during postorogenic extension was accommodated by N-dipping LANF with N- to NE-sense of shear pertaining to the North Cycladic Detachment System (NCDS). More recently, the description of the top-to-the-south West Cycladic Detachment System (WCDS) resulted in a more symmetrical character of the postorogenic extension. However, the transition in time and space between these major structures, particularly the NCDS and the WCDS, which is a key question to understand the dynamics of the back-arc extension, remains widely unclear. The Folegandros-Sikinos area, whose tectonometamorphic evolution appears poorly constrained, is however quite exemplary of this discussion. Located to the south of the Cycladic Archipelago, at short distance from the WCDS last outcrop in Serifos Island, this area offers the opportunity to study both the synorogenic deformation preserved at the vicinity of the Cycladic basement and the structural transition between areas characterised by top-to-the-North and top-to-the-South kinematics during the postorogenic history. Based on an extensive field survey making the link between kinematics of the noncoaxial deformation and changing metamorphic conditions, we show that (1) a E-W syn-blueschists facies deformation is preserved within the HP lenses; (2) a penetrative, post-blueschists facies top-to-the-North sense of shear is present throughout the study area and (3) deformation that concentrates in the vicinity of the contact with the Cycladic basement is only characterized by a top-to-the-North kinematics. These results imply (4) the activity of a major postorogenic LANF roofing the Cycladic Blueschists (5) and the complete overprint of initial top-to-the-South thrust kinematics by top-to-the-North ones over the contact with the Cycladic basement that are consistent with a strong asymmetry of the deformation. Only a late, localized ductile-brittle deformation is recognized and thus ascribed to the activity of the WCDS to the West of the study area. LANF dipping in opposite directions with opposite sense of shear are then restricted to the west of Sifnos Island and separated from the central Cyclades by a sharp transition, highlighting a highly non-cylindrical deformation. Asymmetry of the postorogenic extension could be correlated with the amount of stretching. Rather symmetric deformation seems to occur in marginal areas of the domain while asymmetric deformation is encountered within highly-extended areas where lower crustal material is exhumed within MCCs

Conditions et mécanismes de genèse d’instabilités de glissement dans une zone de subduction

National audienceLa compaction, la déformation et la diagenèse sont autant de processusqui modifient la rhéologie des sédiments accrétés au prisme d’accrétionou enfouis en subduction. A l’image de ces modifications rhéologiquesmajeures, les mécanismes de déformation dans la partie superficielle(<40 km de profondeur) d’une zone de subduction sont multiples, duglissement asismique aux mécanismes de déformation procédant d’instabilitéscomme les mégaséismes ou les séismes lents.Les relations entre les propriétés du matériau (porosité, lithologie, microstructure,pression de fluide) et son comportement mécanique sontencore mal comprises. De ce fait les mécanismes contrôlant la genèsed’instabilités sont encore débattus.Notre étude, qui consiste en une approche expérimentale, vise à explorerde manière couplée les effets de la déformation et de la diagenèse(transformation smectite-illite) sur la genèse d’instabilités de glissementet d’en déterminer les mécanismes déclencheurs.Pour cela nous avons réalisé des tests triaxiaux en compression sur dessiltstones peu déformés provenant du paléo prisme d’accrétion de Boso(Japon). Trois séries de tests ont été réalisées pour des pressions deconfinement allant de 50 à 200 MPa :(1) À température ambiante sur échantillons naturels - pour identifier lesmodes de déformation et la rhéologie des échantillons naturels de départ(2) À 300°C sur échantillons naturels - pour évaluer les effets des réactionsdiagénétiques en cours sur la rhéologie(3) À température ambiante sur échantillons naturels préalablement illitisésexpérimentalement en autoclave - pour estimer l’effet d’un enrichissementen illite sur la rhéologie de ces roches Les résultats d’expériencesmontrent que les instabilités sont uniquement générées dansles expériences conduites à 300°C (comportement de type stick-slip).Ceci indique donc que les réactions diagénétiques en cours (transitionsmectite-illite, déshydratation des smectites) pourraient constituer undes mécanismes source d’instabilité