Sphalerite composition in relation to the stress distribution of a boudinage

The content of FeS in sphalerites from different parts of a boudinage is compared with the theoretical stress pattern. The calculated differences in stress correspond to less than one mole percent FeS and are often masked by the small variations in the fugacity of sulfur.     

Lithospheric boudinage in the Western Mediterranean back-arc basin

The western Mediterranean is composed of irregular troughs formed as back-arc basins in the hanging wall of the W-directed Apenninic subduction which retreated eastward during Neogene and Quaternary times. The basins are progressively younger toward the east, ageing from late Oligocene–early Miocene (Valencia, Provençal, Alboran and Algerian basins), to middle Miocene–Pleistocene (Tyrrhenian Sea). The basins isolated boudins of continental lithosphere, the Sardinia–Corsica block representing the largest. The boudinage has a wavelength of 100–400 km and facilitated stretching of the continental lithosphere with formation of new oceanic crust in the Provençal, Algerian, Vavilov and Marsili basins. The boudins developed both in the earlier Alpine–Betic orogen (Alboran basin) and in its foreland (Provençal and Valencia troughs). The extension appears clearly asymmetric due to its eastward polarity, accommodated by E-dipping master low-angle normal faults. Moreover the thinning shows variations in boudinage wavelength and is characterized by several along-strike transfer zones and heterogeneities. The western Mediterranean back-arc setting is comparable with Atlantic and western Pacific back-arc basins associated with W-directed subduction zones that show similar large-scale lithospheric boudinage.     

Geochemical trends in the Belgian Ardennes

The paper summarizes the results of a regional geochemical survey in the Belgian Ardennes. Various geochemical trends are defined and tentatively interpreted; a possible relation between geochemical and metallogenic zoning is discussed.     

A stress-transfer model for the development of extension fracture boudinage

The term boudinage is used to describe a wide variety of extensional structures in deformed rocks. This paper is mainly concerned with boudinage resulting from through-layer extension fractures followed by separation of the layer segments, thus forming boudins with more or less rectangular cross-sections. In principle, this process is similar to the break up of fibres in fibre-reinforced composite materials extended parallel to the fibre direction. Both processes are controlled by the transfer of stress from the matrix to the fibre (or layer) and a mathematical model for fibre-matrix stress transfer (the ‘fibre-loading’ model) is well established. We have used this as a basis for developing a stress transfer model for boudinage. The only difference in the basic mathematical formulation results from geometric differences between the two systems; the geometric expressions in the fibre-loading model have, therefore, been rederived for the layer-matrix case.Stress-transfer theory predicts that the tensile stress in a layer segment rises from a minimum at the end of a segment to a maximum at the centre. This behaviour, which is clearly shown by finite-element models of boudinage structure, suggests that extension fracture boudinage develops by successive ‘mid-point’ fracturing. According to stress-transfer theory, the process will continue until a layer is reduced to segments (boudins) all of which are shorter than some critical length (for which the tensile fracture strength of the layer is equal to the tensile stress at the mid-point). In practice, successive fracturing will be influenced by two other factors: (1) in nature the controlling material properties (tensile fracture strength, elastic moduli) will not be single-valued but will have a distribution reflecting local variations in lithology and microstructure and (2) major pre-deformation flaws may be present in a layer which will control the ‘starting length’ of layer segments. These factors are incorporated with the stress-transfer theory into a statistical (Monte Carlo) model for extension fracture boudinage which results in a prediction of boudin aspect ratios. The predicted distribution compares very closely with the observed distribution of 91 quartzite boudins within Lower Carboniferous slates at Tintagel, Cornwall.The stress-transfer model implies that boudin-defining fractures occur sequentially so that inter-boudin gap lengths will be unequal. Strain estimates based on boudinage structure will vary according to which part and how much of a layer is sampled. A much improved strain estimate is possible based on sequentially closing the inter-boudin gaps. The stress-transfer theory also leads to the possibility of estimating palaeostress from boudinage structure and is the only model available which predicts an aspect ratio distribution of boudins formed by extension fracture. Other than our own, we know of no published data on boudin aspect ratio distributions. Hence, further elaboration of the model is not possible until more field data is available. We hope that our work will encourage the systematic measurement of boudinage as well as the development of alternative models.     

Factors affecting the kinematic interpretation of asymmetric boudinage in shear zones

Based on work along a major mylonite zone in the northern Appalachians and scale model studies, a new mechanism for the origin of asymmetric boudins in shear zones is proposed. Along the Honey Hill Fault in southern Connecticut, granitic sills intruded into calc-silicate gneisses and schists were oblique to the boundaries of the mylonite zone and experienced the following sequence in the production of sigmoidal boudins: (1) as the calc-silicate schists experienced mylonitization and flow, the more competent, coarse-grained granitic sills deformed by extensional fracturing and quartz veining; (2) continued extension of granitic sills was accommodated by ‘normal’ shear on early-formed quartz veins; (3) continuing extension of sills and ductile modification of the corners of boudins resulted in granitic ‘fish’ with tails which stream from the top of the boudin in the ‘down-dip’ direction and from the bottom in the ‘up-dip’ direction. Based on a variety of kinematic indicators, the sense of asymmetry of the tails is identical to that expected for recrystallization tails on sheared augen (σ structure). Models composed of silicone putty and Plasticine were created to investigate the effect of pre-shearing geometry on boudin evolution, and were deformed in a simple shear device. The models reproduce the kinematics deduced from field relations and suggest that one of the primary factors in controlling the amount of extension is the angle which early veins make with the shear zone boundary. Varying the angular relationships in the models suggests that other pre-shearing geometric factors may affect the geometry of boudins formed in this way. Boudins formed through this mechanism appear very similar to Hanmer's type II asymmetric boudins. Because the pre-shearing geometry can exert a control on boudin asymmetry, caution should be used when attempting to deduce shear sense or shear strain values from asymmetric boudinage.     

Coeval folding and boudinage under plane strain with the axis of no change perpendicular to the layer

Plane-strain coaxial deformation of a competent plasticine layer embedded in an incompetent plasticine matrix was carried out to improve our understanding about the evolution of folds and boudins if the layer is oriented perpendicular to the Y-axis of the finite strain ellipsoid. The rock analogues used were Beck’s green plasticine (matrix) and Beck’s black plasticine (competent layer), both of which are strain-rate softening modelling materials with a stress exponent n=ca. 8. The effective viscosity η of the matrix plasticine was changed by adding different amounts of oil to the original plasticine. At a strain rate of 10⁻³ s⁻¹ and a finite strain e of 10%, the effective viscosity of the matrix ranges from 1.2×10⁶ to 7.2×10⁶ Pa s. The effective viscosity of the competent layer has been determined as 4.2×10⁷ Pa s. If the viscosity ratio is large (ca. 20) and the initial thickness of the competent layer is small, both folds and boudins develop simultaneously. Although the growth rate of the folds seems to be higher than the growth rate of the boudins, the wavelength of both structures is approximately the same as is suggested by analytical solutions. A further unexpected, but characteristic, aspect of the deformed competent layer is a significant increase in thickness, which can be used to distinguish plane-strain folds and boudins from constrictional folds and boudins.An erratum to this article can be found at     

Crustal-scale boudinage and migmatization of gneiss during their exhumation in the UHP Province of Western Norway

ABSTRACT A field study in the coesite province, the deepest unit of the Norwegian Caledonides, gives new constraints on the rheological behaviour of the continental crust during exhumation. Lithological heterogeneities and differential retrogression led to crustal-scale boudinage during the late-orogenic intense E–W stretching event in the footwall of the Nordfjord-Sogn Detachment. The main gneissic lithologies display a modest but widespread syn-exhumation migmatization. Textural criteria allow estimation of a 30% fusion rate. Partial melting mostly post-dates eclogitization and is synchronous with ductile stretching and top-to-west shearing. Presented observations suggest that the melt reactions and migmatization resulted in a soft rheology. During subduction to ~ 100 km depth and subsequent exhumation, crustal viscosity can be reduced by up to four orders of magnitude. Models are discussed that consider a transition from a small internal strain of the crust to viscous flow during exhumation.     

Tension fractures and boudinage oblique to the maximum extension direction: an analogy with lüders' bands

A study has been made of the orientation of boudin necklines and extension fractures with respect to the maximum extension direction X determined by infilling fibre growth. Several localities, within the Variscan belt of Western Europe, the Pyrenees and the Alps, have been investigated in detail. It has been generally accepted that boudinage and extension fracturing occur perpendicular to X in the rock at the time of their formation; however, we have shown that is not the case: boudin necklines and extension fractures occur at an angle between 45°–90° to X, with the most frequent orientations between 65°–70° and 80°–85°. Angles of exactly 90° are rare.In order to explain · this obliquity, an analogy is made with the phenomenon of Lüders' bands (localised regions of plastic deformation) developed in thin metal plates under tensile testing. Because the thickness of a layer is negligible compared with its lateral extent, we will only be concerned here with the case of thin metal plates.     

湖北银洞沟银金矿床石香肠构造样式及形成机制

该矿床中以平行陡倾的含矿石英脉为特征的矿体,在构造变形过程中常产生石香肠化.由于局部应力环境的差异,所形成的石香肠构造样式也各不相同,并控制了矿体形态及有用矿物组分的分布.以不同样式石香肠构造在矿区的分布特点进行了构造分区,并对其形成机理作初步探讨.     

Regional geochemical reconnaissance in the Belgian Ardennes: Secondary dispersion patterns in stream sediments

Two geochemical stream sediment surveys were conducted in the Belgian Ardennes for Fe, Cu, Pb, Zn, Co, Ni and Mn. The geological setting is of quartzo-pelitic rocks of Early Devonian and Cambro-Ordovician age. One survey covers the Ourthe and Salm valleys, the other the Lesse and Lomme drainage basins. Results of these surveys show the geochemical similarity of the two regions. Anomalies have been ascribed to four factors: human activity, Fe-Mn enrichment, known mineralization and potential mineralization. Mean values of metal concentration and statistical dispersion are identical. This results from similar physico-chemical conditions which induce the same types of surficial metallic association. The stream environment is characterized by two main phases, an oxidizing phase where Fe-Mn oxides trap trace metals, and a reducing phase where organic matter and secondary sulphides are bound with Pb, Zn and Cu. In oxidizing conditions, Cu and Ni show a greater affinity for the Fe-oxides while Zn and especially Co are related to the Mn-oxides. Pb has only a weak relationship with the Fe-Mn oxides but is also associated with the organic matter and the secondary sulphides in the reducing environment. Consequently, one may propose the following scale of mobility: Zn > Ni-Mn > Cu-Co > Pb > Fe.     

Summer Field Meeting in the Ardennes and Vosges: 6–20 September 1969

Six days were spent in the field in the Ardennes and four in the Vosges. In the Ardennes, the party followed the section along the River Meuse from Charleville-Mézières to Huy, studying the Lower Palaeozoic of the Givonne, Rocroi and Brabant massifs, the Devonian and Carboniferous of the Dinant and Namur synclinoria and the Grande Faille du Midi. One day was spent on the Devonian biostromes and bioherms near Couvin and Frasnes. The Stavelot massif was examined en route for the Vosges.In the Vosges, the party visited the classic contact metamorphic aureole of the Barr-Andlau granite, saw a granite differentiation sequence in the Champ-du-Feu complex, Permian volcanics at Nideck Falls, ultrametamorphics at the Col des Bagenelles and the glacial lakes near Gerardmer. The Ballon d'Alsace granite and the Lower Carboniferous volcanics forming its country rock were examined.The party returned to London via Lièvin in the North French Coalfield, where they collected early Devonian faunas, stopping at a Holocene river channel en route.     

Textural relations of porphyroblasts in the Salm phyllites from Vielsalm and Salm-Chateau,Ardennes, Belgium

The effects of two deformation phases can be seen in the rocks of Vielsalm and Salm-Chateau; the first deformation produced the principal schistosity and the second a crenulation cleavage. The main metamorphism and the, formation of chloritoid, kaolinite and possible garnet porphyroblasts are later than the development of the first schistosity but earlier than the crenulation. It is very probable that the schistosity in both these places represents the same period of deformation. This deformation may be Hercynian, but it cannot be definitely ascertained. If this were true, the metamorphism would be post-tectonic with respect to the principal Hercynian deformation.     

Rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones

Emphasized in this paper are the deformation processes and rheologies of rocks at high temperatures and high effective pressures, conditions that are presumably appropriate to the lower crust and upper mantle in continental collision zones. Much recent progress has been made in understanding the flexure of the oceanic lithosphere using rock-mechanics-based yield criteria for the inelastic deformations at the top and base. At mid-plate depths, stresses are likely to be supported elastically because bending strains and elastic stresses are low. The collisional tectonic regime, however, is far more complex because very large permanent strains are sustained at mid-plate depths and this requires us to include the broad transition between brittle and ductile flow. Moreover, important changes in the ductile flow mechanisms occur at the intermediate temperatures found at mid-plate depths.Two specific contributions of laboratory rock rheology research are considered in this paper. First, the high-temperature steady-state flow mechanisms and rheology of mafic and ultramafic rocks are reviewed with special emphasis on olivine and crystalline rocks. Rock strength decreases very markedly with increases in temperature and it is the onset of flow by high temperature ductile mechanisms that defines the base of the lithosphere. The thickness of the continental lithosphere can therefore be defined by the depth to a particular isotherm T_c above which (at geologic strain rates) the high-temperature ductile strength falls below some arbitrary strength isobar (e.g., 100 MPa). For olivine T_c is about 700°–800°C but for other crustal silicates, T_c may be as low as 400°–600°C, suggesting that substantial decoupling may take place within thick continental crust and that strength may increase with depth at the Moho, as suggested by a number of workers on independent grounds. Put another way, the Moho is a rheological discontinuity. A second class of laboratory observations pertains to the general phenomenon of ductile faulting in which ductile strains are localized into shear zones. Ductile faults have been produced in experiments of five different rock types and is generally expressed as strain softening in constant-strain-rate tests or as an accelerating-creep-rate stage at constant differential stress. A number of physical mechanisms have been identified that may be responsible for ductile faulting, including the onset of dynamic recrystallization, phase changes, hydrothermal alteration and hydrolytic weakening. Microscopic evidence for these processes as well as larger-scale geological and geophysical observations suggest that ductile faulting in the middle to lower crust and upper mantle may greatly influence the distribution and magnitudes of differential stresses and the style of deformation in the overlying upper continental lithosphere.     

Shape and amount of the Quaternary uplift of the western Rhenish shield and the Ardennes (western Europe)

A good evaluation of the Quaternary uplift of the Rhenish shield is a key element for the understanding of the Cenozoic geodynamics of the western European platform in front of the alpine arc. Previous maps of the massif uplift relied on fluvial incision data since the time of the rivers' Younger Main Terrace to infer a maximum post-0.73 Ma uplift of ~ 290 m in the SE Eifel. Here, we propose a new interpretation of the incision data of the intra-massif streams, where anomalies in the terrace profiles would result from knickpoint retreat in the tributaries of the main rivers rather than from tectonic deformation. We also use additional geomorphological data referring to (1) deformed Tertiary planation surfaces, (2) the history of stream piracy that severely affected the Meuse basin in the last 1 Ma, and (3) incision data outside the Rhenish shield. A new map of the post-0.73 Ma uplift of the Rhenish shield is drawn on the basis of this enlarged dataset. It reduces the maximum amount of tectonic uplift in the SE Eifel to ~ 140 m and modifies the general shape of the uplift, namely straightening its E–W profile. It is also suggested that an uplift wave migrated across the massif, starting from its southern margin in the early Pleistocene and currently showing the highest intensity of uplift in the northern Ardennes and Eifel. These features seem to favour an uplift mechanism chiefly related to lithospheric folding and minimize the impact on the topography of a more local Eifel plume.     

应用重磁资料对贵州中东部槽台分布区划分的地质效果初探

贵州中东部为一典型的地槽区向地台区的过渡地带，本文试以1：20万布格重力异常和深部剩余重力异常的空间展布规律和变化特征为研究点，并结合航磁、物性、地质等资料，初步论证和说明这一特殊现象的地质效果。通过重磁资料所透露的地质信息，对区内地质构造单元进行划分及分区，旨在初步揭示本区隐伏的深部构造特征和岩性差异情况，为研究大地构造、探索基底属性、寻找盖层岩体矿产提供间接服务。     

A miscibility gap in trioctahedral Mn-Mg-Fe chlorites: Evidence from the Lienne Valley manganese deposit,Ardennes, Belgium

Low-temperature veinlets crosscutting low-grade manganiferous ironstones of Ordovician age contain four texturally distinct types of chlorites with nearly constant Al/Si-ratios that form two separate populations regarding their Mn/Mg/Fe-ratios: One with low iron contents (<1.5 w.% FeO) and molar Mg/Mn-ratios just below unity (magnesian pennantites), the other with higher iron (7–11 w.% FeO) and Mg/Mn?4 (manganoan clinochlores). The two populations, which can be distinguished readily by their characteristic optical elongation and dispersion colors, are intimately intergrown and have formed partly during consecutive stages of a chlorite crystallization sequence, partly by simultaneous growth and possibly even as exsolution products of a pre-existing homogeneous chlorite phase of intermediate composition. These findings indicate a miscibility gap in the chlorite solid solutions beginning along the binary Mg-Mn series and extending into the ternary system. There may be a solvus relationship with the miscibility gap closing at higher temperatures (400° C?). One very intensely colored chlorite type of the pennantite population may contain Fe³⁺ or Mn³⁺ or both. Additional minerals in the veinlets are spessartine, kutnahorite, quartz, and an allanite-piemontite phase. Crystallization began near the centers of the present veins with Mn-rich minerals and continued towards their edges and into the extremely thin ends of the developing fractures with the deposition of the more Fe-rich chlorites that are in equilibrium with the ironstone.     

A review of coal properties pertinent to carbon dioxide sequestration in coal seams: with special reference to Victorian brown coals

This paper presents reviews of studies on properties of coal pertinent to carbon dioxide (CO₂) sequestration in coal with specific reference to Victorian brown coals. The coal basins in Victoria, Australia have been identified as one of the largest brown coal resources in the world and so far few studies have been conducted on CO₂ sequestration in this particular type of coals. The feasibility of CO₂ sequestration depends on three main factors: (1) coal mass properties (chemical, physical and microscopic properties), (2) seam permeability, and (3) gas sorption properties of the coal. Firstly, the coal mass properties of Victorian brown coal are presented, and then the general variations of the coal mass properties with rank, for all types of coal, are discussed. Subsequently, coal gas permeability and gas sorption are considered, and the physical factors which affect them are examined. In addition, existing models for coal gas permeability and gas sorption in coal are reviewed and the possibilities of further development of these models are discussed. According to the previous studies, coal mass properties and permeability and gas sorption characteristics of coals are different for different ranks: lignite to medium volatile bituminous coals and medium volatile bituminous to anthracite coals. This is important for the development of mathematical models for gas permeability and sorption behavior. Furthermore, the models have to take into account volume effect which can be significant under high pressure and temperature conditions. Also, the viscosity and density of supercritical CO₂ close to the critical point can undergo large and rapid changes. To date, few studies have been conducted on CO₂ sequestration in Victorian brown coal, and for all types of coal, very few studies have been conducted on CO₂ sequestration under high pressure and temperature conditions.     

The geometry of the joint pattern and its relation with fold structures in the Aywaille area (Ardennes,Belgium)

The geometry of the joint pattern in the Aywaille area (Ardennes, Belgium) is analysed with statistical methods. The orientations of intersecting joint-planes were recorded together with the bedding-plane. Four principal sets have been distinguished. These sets are related orthogonally to the bedding-plane and geometrically to the regional fold structure. All sets are considered to represent tensional structures, formed in a single period after the folding. The joint pattern is compared with a model given byPrice (1966). It is concluded that this model, describing the generation of joints in relation with folding, cannot explain completely the formation of the joint-sets under consideration here.

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Zusammenfassung Die Geometrie des Klutgefüges im Aywaille Gebiet (Ardennen, Belgien) wurde mit statistischen Methoden analysiert. Die Orientierungen sich schneidender Kluften und der Schichtflächen wurden erfaßt und vier Hauptkluftscharen unterschieden. Diese Scharen durchsetzen die Schichtflächen im rechten Winkel und in Symmetrie zur Faltenachse. Alle Klüfte werden als gleichzeitig nach der Faltung entstandene Zugklüfte gedeutet, Das Kluftgefüge wird mit einem vonPrice (1966) angegebenen Modell für das Entstehen von Klüften nach der Faltung verglichen. Es ergibt sich, daß dieses Modell nicht zur vollständigen Erklärung der betrachten Kluftscharen ausreicht.

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Résumé La géométrie du régime de fractures dans la région d'Aywaille (Ardennes, Belgique) est analysée par des méthodes statistiques. Les orientations des intersections des plans de fractures ont été enregistrées en inculant le plan de stratification. On a distingué quatre ensembles principaux. Ces ensembles sont reliés orthogonalement au plan de stratification et géométriquement au plissement régional. On considére que tous ces ensembles représentent des structures de traction, formées au cours d'une mème période postérieure au plissement. Ce type de fractures peut-être comparé au modèle donné par Price (1966). On peut en conclure que ce modèle, décrivant la formation des fractures associées au plissement, ne peut pas expliquer complètement le développement des ensembles de fractures étudiés ici.

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, 4 ( Aywaille, Ardenne, Belgium). . , . Price, (1966) . .

     

Granite emplacement by crustal boudinage: example of the Calmayo and El Hongo plutons (Córdoba,Argentina)

This study deals with the structure and emplacement of the Calmayo and El Hongo trondhjemite plutons (Famatinian belt of Córdoba, Argentina). It provides structural data from the granites and the country rocks and a study of the magnetic fabric in the plutons. New U/Pb geochronological data yield intrusion ages of 512.1 ± 3.4 Ma and 500.6 ± 4.5 Ma for the Calmayo and El Hongo plutons respectively. The El Hongo massif and the southern part of the Calmayo trondhjemite preserve magmatic structures, whereas the northern domain of Calmayo shows the imprint of solid‐state deformation. The main foliation in the country rocks outlines a boudin‐like pattern at the map scale and the granites are located along boudin necks, suggesting that the emplacement of these trondhjemite plutons was linked to large‐scale boudinage of the country rocks.