Clay minerals and organic matter occur frequently in fault zones. Their structural characteristics and their textural evolution are driven by several formation processes: (1) reaction by metasomatism from circulating fluids; (2)in situevolution by diagenesis; and (3) neoformation due to deformation catalysis. Clay-mineral chemistry and precipitated solid organic matter may be used as indicators of fluid circulation in fault zones and to determine the maximum temperatures in these zones. In the present study, clay-mineral and organic-matter analyses of two major fault zones – the Adams-Tinui and Whakataki faults, Wairarapa, North Island, New Zealand – were investigated. The two faults analysed correspond to the soles of large imbricated thrust sheets formed during the onset of subduction beneath the North Island of New Zealand. The mineralogy of both fault zones is composed mainly of quartz, feldspars, calcite, chabazite and clay minerals such as illite-muscovite, kaolinite, chlorite and mixed-layer minerals such as chlorite-smectite and illite-smectite. The diagenesis and very-low-grade metamorphism of the sedimentary rock is determined by gradual changes of clay mineral ‘crystallinity’ (illite, chlorite, kaolinite), the use of a chlorite geothermometer and the reflectance of organic matter. It is concluded here that: (1) the established thermal grade is diagenesis; (2) tectonic strains affect the clay mineral ‘crystallinity’ in the fault zone; (3) there is a strong correlation between temperature determined by chlorite geothermometry and organic-matter reflectance; and (4) the duration and depth of burial as well as the pore-fluid chemistry are important factors affecting clay-mineral formation.
-Les phénomènes de retrait et de gonflement des argiles induisent chaque année des dégâts sur les structures d'habitation. Leur compréhension et leur quantification constituent donc deux objectifs importants pour réduire la sinistralité. C'est dans ce contexte qu'a été développé, au sein d'un microscope électronique à balayage environnemental (MEBE), un nouveau dispositif pour caractériser le retrait et le gonflement de matériaux argileux. Il permet d'établir la relation qui lie la variation de volume d'un micro-échantillon à sa variation de teneur en eau, très rapidement. Associé à un modèle analytique qui s'appuie sur des profils types de pénétration de la sécheresse dans le sol, ce nouvel essai permet d'estimer l'amplitude du tassement et/ou du gonflement attendu à l'échelle d'un ouvrage. Mots clés : caractérisation des argiles / retrait / gonflement / dispositif expérimental Abstract-A new device to analyse the shrinkage and swelling of clayey soils. Each year, the shrinkage and swelling of clays induce damages on housing structures. Their understanding and their quantifying constitute two important objectives to reduce damages. It is in this context that was developed a new device to characterise the swelling-shrinkage behaviour of these clayey materials within an Environmental Scanning Electron Microscope (ESEM). It allows to construct a relationship between the volume variation of a microsample and its water content variation in a very short time. Associated with an analytical model based on standard profiles of drought penetration in the soil, this new test allows to estimate the amplitude of settlement and/or swelling at the house scale.
Low-and very low-grade metamorphic studies investigating the alteration and reaction progress of clay minerals are powerful tools to decipher the thermal evolution of sedimentary and inverted meta-sedimentary basins. Sheet silicates such as illite and chlorite are very common in sedimentary basin sequences. They can be used to determine the grade of diagenesis and low-temperature metamorphism as measured through the XRD: illite Kübler-Index (KI; illite "crystallinity" in older literature) and the chlorite Árkai-Index (ÁI; chlorite "crystallinity" in older literature), respectively. Although the ÁI method is considered to be slightly less sensitive than the KI method, a reliable correlation between both methods is often observed in metamorphic domains with a uniform heat-flow history and minor tectono-structural complexity. Complementary to these methods, the K-white mica b cell dimension provides a robust estimate of pressure facies reached in very low-to low-grade temperature domains.Here, we present a case-study from the Markstein basin located in the Southern Vosges.The lithostratigraphic units in the basin are characterized by deep marine flysch sequences of Upper Devonian to Upper Visean age and volcano-clastic sediments, respectively. The Markstein basin is surrounded by granitoids with intrusion ages between 340 and 326 Ma. A previous study showed orogenic deformation characterized by regional folding, and a contact metamorphism found in an outer halo of the granitoids up to 1500 m away from the contact (delineated by the occurrence of biotite and andalusite). Here we present a multidisciplinary study combining mineral assemblages, illite and chlorite "crystallinity indices", and K-white mica b cell dimension. Our approach allows to (i) map in (great) detail the areal extent of both regional/burial metamorphic and contact metamorphic domains; (ii) reveal the metamorphic zonation within both domains; and (iii) better constrain regional/burial and contact metamorphic history. The contact metamorphic domain is characterized by the occurrence of biotite and/or actinolite and low K-white mica b cell dimensions, whereas the zone of incipient orogenic metamorphism yields KI and ÁI values of the high-grade diagenesis and anchizone with intermediate K-white b cell dimensions.
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