1,721,235 research outputs found
Archéosismicité de l'Égée : étude des failles actives de la Thessalie
No full textCaputo Riccardo, Helly Bruno. Archéosismicité de l'Égée : étude des failles actives de la Thessalie. In: Bulletin de correspondance hellénique. Volume 124, livraison 2, 2000. pp. 560-588
The puzzling regmatic system of Eastern Dolomites, Italy
No full textFollowing the mesoscale structural investigation of the Eastern Dolomites leading to the partial unravelling of the tectonic stratigraphy of the area, of which the results are presented in a companion paper, the study of the macroscale structures, and particularly their pattern, geometry and kinematic behaviour, was performed. A detailed structural map of the region was prepared (scale 1:50 000). The geometry of most fracture planes was traced according to standard techniques of photo-interpretation, while systematic field work was carried out to observe the real kinematics of the tectonic features, as well as to recognise the low- to middle-angle fault planes which are otherwise almost undetectable from aerial views. A second map, which shows the distribution of the outcropping lithologies, qualitatively grouped in four major rheological categories, was also prepared. According to the geometry and mostly the kinematic behaviour, three principal types of tectonic structures were distinguished: i) transcurrent faults, ii) normal and transtensional faults and iii) low- to middle-angle contractional faults. The first type is certainly the most represented one, forming dense sets of parallel planes which locally characterise entire sectors of the study area. In contrast, the normal and transtensional features are developed uniquely in the northeastern sector and are represented by both negative flower structures and graben. The low- to middle-angle contractional faults are represented by both north- and south-verging thrusts, among which the Ansiei Thrust System is certainly the most important structure being more than 30 km long. Finally, the general tectonic evolution of the puzzling regmatic system was tentatively reconstructed with the aid of geological sections
A comparison between joints and faults as brittle structures used for evaluating the stress field
No full textA simple genetic model is proposed for the formation of extensional joints and thus relationships between joints and the three principal axes of a stress field. The basic assumption is that joints are purely extensional and have no shear component on them. Consequently they can form only parallel to principal planes. The model also explains why joints usually create two roughly orthogonal sets. The model is tested by comparison with associated fault systems. Using well-tested methods of structural analysis for faults and a recently proposed one for joints, some case studies are considered, both at the scale of a whole site and at a smaller scale. In both cases the results confirm the model. In conclusion, a close relationship between faults and joints has been brought into focus and the importance of joint systems as brittle mesostructures useful for evaluating a stress field is emphasized
Morphotectonics and kinematics along the Tyrnavos Fault, northern Larissa Plain, mainland Greece
No full textThe study of the Tirnavos Fault, recently recognized as one of the Late Pleistocene-Holocene faults of the Northern Larissa Plain (Thessaly, Central Greece), is presented. It was formed with an E-W direction during the last and still active tectonic regime activated by a N-S extension. It shows conclusive indications of extremely recent reactivations. In the tectonic frame of the Aegean Region, the Tirnavos Fault is a typical example of active fault few kilometres long (10-12), with small co-seismic displacements (10-20 cm) and characterized by low magnitude seismicity. Accordingly, the investigating approach must differ from the classical palaeoseismological studies performed, for example, in the Basin and Range Province by U.S.-workers. The paper is an attempt to apply both palaeoseismological and kinematic approaches to the study of the Tirnavos Fault in order to determine which is more applicable to this setting. Although emphasis on the kinematic analysis of all the brittle structures and on bedrock scarps has been given, topographic scarps in unconsolidated deposits have been also observed and a detailed morphotectonic mapping carried out. Besides the description of the fault-generated morphologies, other fault-related features such as the damming of a valley, the truncation of a scree fan and the differential erosion of a fan uplifted by the fault, are described and discussed. Due to the complete lack of new chronological data, was not possible to determine the age of any palaeoseismic event. In contrast, it is demonstrated how understanding the kinematics of a fault and of the regional tectonic framework can constrain the timing of fault activity
Sea level curves: perplexities of an end-user in morphotectonic applications
No full textSeveral sea-level curves, from the classical ones to some of the most recently published, are analysed and compared. Although this short paper is not a review of the original data and analytical methods used by the different authors, similarities and mainly differences between published sea-level curves are emphasised especially as concern the suggested height and age of the highstand sea-level peaks during the last 450 ka. The perplexities of an end-user when attempting to exploit different curves for investigating uplifted marine terraces are discussed and recommendations are suggested
JF3 (v. 2.0): a programme for quantitative estimates of the joints to faults ratio.
No full textIt is a commonplace field observation that extension fractures are more abundant than shear fractures. The questions of how much more abundant, and why, are posed in this paper and qualitative estimates of their ratio within a rock volume are made on the basis of field observations and mechanical considerations. A conceptual model is also proposed to explain the common range of ratios between extension and shear fractures, here called the j/f ratio. The model considers three major genetic stress components originated from overburden, pore-fluid pressure and tectonics and assumes that some of the remote genetic stress components vary with time (i.e. stress-ates are included). Other important assumptions of the numerical model are that: i) the strength of the sub-volumes is randomly attributed following a Weibull probabilistic distribution, ii) all fractures heal after a given time, thus simulating the cementation process, and therefore iii) both extensional jointing and shear fracturing could be recurrent events within the same sub-volume. As a direct consequence of these assumptions, the stress tensor at any point varies continuously in time and these variations are caused by both remote stresses and local stress drops associated with in-situ and neighbouring fracturing events. The conceptual model is implemented in a computer program to simulate layered carbonate rock bodies undergoing brittle deformation. The numerical results are obtained by varying the principal parameters, like depth (viz. confining pressure), tensile strength, pore-fluid pressure and shape of the Weibull distribution function, in a wide range of values, therefore simulating a broad spectrum of possible mechanical and lithological conditions. The quantitative estimates of the j/f ratio confirm the general predominance of extensional failure events during brittle deformation in shallow crustal rocks and provide useful insights for better understanding the role played by the different parameters. For example, as a general trend it is observed that the j/f ratio is inversely proportional to depth (viz. confining pressure) and directly proportional to pore-fluid pressure, while the stronger is the rock, the wider is the range of depths showing a finite value of the j/f ratio and in general the deeper are the conditions where extension fractures can form. Moreover, the wider is the strength variability of rocks (i.e. the lower is the m parameter of the Weibull probabilistic distribution function), the wider is the depth range where both fractures can form providing a finite value of the j/f ratio. Natural case studies from different geological and tectonic settings are also used to test the conceptual model and the numerical results showing a good agreement between measured and predicted j/f ratios
Stress variability and brittle tectonic structures
No full textThe problem of stress variability, both in space and time, during brittle deformation of rocks is faced in this note. Several possible causes of the temporal and spatial variations of the stress tensor are reviewed and discussed. A genetic partitioning of the stress tensor is proposed, showing that any stress field within a crustal volume can be seen as the sum of relatively simple stress tensors associated to specific dgeneticT components. The genetic components discussed in this note are the gravitational, the tectonic, the fluid, the thermal and the diagenetic one. Each genetic component is a function of several parameters, but for all of them the possible important role played by the variable time is emphasised. According to numerous researches, based on theoretical and experimental grounds, brittle deformation in a rock mass occurs when some critical conditions are reached starting from initial equilibrium conditions and leading to even different stress conditions as a consequence of failure. Field examples are presented and data from literature are discussed to exhibit evidences from complex tectonic structures and to argue about small- and large-scale spatial variations as well as about short- and long-term temporal variations of the stress trajectories and of the stress magnitudes that continuously occur during a brittle deformational event. The relationships between local and temporal stress variations and what is commonly defined the regional stress field are discussed adding recommendations for future researches
The polyphase tectonics of Eastern Dolomites, Italy
No full textThe detailed structural analysis of a large sector of the Eastern Dolomites has been carried out. The field work based on existing unpublished geological maps, has been concentrated on the reconstruction of the major tectonic elements as well as on the unravelling of the complex regmatic system affecting the area. Mesostructural data have been collected throughout the area and analysed using well tested computer aided methodologies. From each station of measurements the three principal stress axes have been obtained. These results permit to reconstruct the tectonic stratigraphy of the area. According to the superposition of striae and similar relationships criteria, four compressional events have been recognised. The average directions of the sigma1 axes are N58°, N26°, N346° and N307°, from older to younger, respectively. Unfortunately, due to the age of the affected rocks (Permian-Triassic), only the relative chronology of the phases has been established. However, by comparison with the results of a similar study carried out in other sectors of the Southern Alps, these phases can be tentatively dated as pre-Oligocene, Chattian-Burdigalian, Serravalian-Tortonian and Late Messinian-Early Pliocene, respectively. The results also allow some conclusions concerning the temporal evolution of the stress ellipsoid whose shape changed from triaxial and purely compressional (subvertical sigma3), to triaxial and transcurrent (subvertical sigma2) via an intermediate stage with uniaxial geometry (sigma2 = sigma3). The possible plate-wide significance of the results is also discussed
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