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Variable coupling across weak oceanic transform fault: Flateyjarskagi, Iceland (2000)

Angelier, J. ; Bergerat, F. ; Homberg, C.

Oxford, UK : Blackwell Science Ltd

Terra nova 12 (2000), S. 0

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ISSN: 1365-3121

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: 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 Tjörnes 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.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1046/j.1365-3121.2000.123279.x

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Estimation des paléo-contraintes liées à la formation de décrochements dans la plate-forme d'Europe du Nord (1985)

Bergerat, F. ; Bergues, J. ; Geyssant, J.

Springer

International journal of earth sciences 74 (1985), S. 311-320

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ISSN: 1437-3262

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Description / Table of Contents: Abstract Further to the analysis of paleo-stress fields in the platform of Southern Germany, we try to calculate the values of tectonic stresses responsible for strike-slip faulting. Compressional laboratory experiments were made on limestones from Basel, Swabian and Franconian Jura and on sandstones from Black Forest. They allow to draw rupture curves and cracking curves. We choice to assimilate the necessary stresses to initiation of cracking to those responsible for natural, long-termed ruptures. The values of these tectonic paleo-stresses are comprised between 40 and 130 MPa, for lithostatic stresses of 5 to 20 MPa.

Abstract: Résumé A la suite de l'étude des paléochamps de contrainte dans la plateforme de l'Allemagne du Sud, nous avons tenté une estimation des valeurs des contraintes tectoniques tertiaires à l'origine des systèmes de décrochements. Des essais de compression en laboratoire ont été réalisés sur des calcaires du Jura bâlois, franconien et souabe et sur des grès de la Forêt Noire; ils ont permis de construire les courbes intrinsèques à la rupture et à la fissuration. Nous avons choisi d'assimiler les contraintes nécessaires à l'initiation de la fissuration à celle responsable des ruptures naturelles à long terme. Les valeurs de ces paléo-contraintes tectoniques sont comprises entre 40 et 130 MPa, pour des contraintes lithostatiques de 5 à 20 MPa.

Notes: Zusammenfassung Nach der Darstellung der alten Spannungsfelder der süddeutschen Scholle werden die Spannungswerte für die tertiäre Tektonik an den verschiedenen Störungssystemen ermittelt. Im Labor wird die Festigkeit der Jurakalke aus der Gegend von Basel und der Fränkisch-Schwäbischen Alb und der Schwarzwälder Sandsteinen bestimmt. Aus diesen Daten können die Kurven der Bruchfestigkeit aufgestellt werden. Vergleicht man die im Labor gemessenen Spannungswerte mit den im Gelände ermittelten tektonischen Daten findet man Werte zwischen 40 und 130 MPa für die tektonischen Spannungen und 5–20 MPa für die Scherspannungen.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF01824898

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Brittle deformation, palaeostress field reconstruction and tectonic evolution of the Eastern Balkanides (Bulgaria) during Mesozoic and Cenozoic times (2010)

Bergerat, F. ; Vangelov, D. ; Dimov, D.

In: Geological Society Special Publication 340: 77-111.

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Publication Date: 2010-09-14

Description: Brittle tectonics analysis and stress tensor reconstructions allow us to better depict the Mesozoic and Cenozoic geodynamical evolution of the Eastern Balkanides which is characterized by a series of overimposed basin-systems. The Late Permian-Triassic corresponds to a wide carbonate platform with local embryonic troughs. During the Jurassic-Early Cretaceous period, the area, limited by regional unconformities, was at first dominated by the existence of a deep basin until the latest middle Jurassic, then by its gradual closure during the Late Jurassic-Early Cretaceous. Traces of these tectonic events are found as a result of brittle tectonic data analysis, especially in the Strandja Zone where NE-SW extension and ENE-WSW compression may be related to the Early Alpine phases of rifting and shortening, respectively. The Late Cretaceous-middle Eocene period was governed by the back-arc basin/island-arc system of East Balkan/Srednogorie zones. The inversion of these extensional zones occurred after the Maastrichtian, then important shearing and thrusting developed during the major shortening in the latest Middle Eocene. These Late Alpine tectonic phases were well characterized by brittle tectonics and the analysis of faulting in terms of stress tensors allows us to identify the main directions of extension of the rifting stage which is north-south to NNE-SSW, and the directions of compression of the Laramian and Illyrian phases, which are NE-SW and north-south, respectively. The relationships between folding and reverse and strike-slip faults are established as well as the occurrence of post-folding normal faults assigned to collapse process following the shortening.

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Fracture patterns and their relations to mountain building in a fold-thrust belt: A case study in NW Taiwan (2013)

Chu, H.-T., Lee, J.-C., Bergerat, F., Hu, J.-C., Liang, S.-H., Lu, C.-Y., Lee, T.-Q.

Societe Geologique de France (SGF)

In: Bulletin de la Societe Geologique de France

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Publication Date: 2013-09-20

Description: 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 1, 2, 3, were calculated by means of inversion of fault slip data at each site. The ratio that defines the shape of stress ellipsoid is generally small, indicating that the value of the maximum principal stress axe 1 is much larger compared to that of 2 and 3, 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.

Print ISSN: 0037-9409

Electronic ISSN: 0037-9409

Topics: Geosciences

Published by Societe Geologique de France (SGF)

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Scale effect and impact of discontinuities in the dynamic elastic constants of the Campanian chalk of Bougival (France) (2013)

Porjesz, R., Bergerat, F.

Societe Geologique de France (SGF)

In: Bulletin de la Societe Geologique de France

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Publication Date: 2013-09-20

Description: 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.

Print ISSN: 0037-9409

Electronic ISSN: 0037-9409

Topics: Geosciences

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Faults, stresses and mechanics of the upper crust: a tribute to Jacques Angelier (2013)

Lacombe, O., Bergerat, F.

Societe Geologique de France (SGF)

In: Bulletin de la Societe Geologique de France

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Publication Date: 2013-09-20

Print ISSN: 0037-9409

Electronic ISSN: 0037-9409

Topics: Geosciences

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Structure and activity of the geothermal field of Hvalfjorthur (Iceland) from brittle tectonic, geothermal and paleostress analysis (2013)

Bergerat, F., Saemundsson, K., Fourel, L., Angelier, J.

Societe Geologique de France (SGF)

In: Bulletin de la Societe Geologique de France

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Publication Date: 2013-09-20

Description: This paper presents the results of brittle tectonic, palaeostress inversion, and hydrothermal mineralisation studies of the Hvalfjörður low-temperature geothermal field in Southwest Iceland. This geothermal field (including two pronounced thermal anomalies) is located in the highly altered core area of an extinct and deeply eroded Tertiary central volcano. Most of the geothermal water appears to be conducted by vertical extension fractures. Palaeostress analysis indicates a rather complex stress history, with four major trends of extension involving normal and strike-slip faulting modes as well as dyke injection. Analysis of the data on the relative chronology indicates that these four regimes were closely intricate in time and space. The most important regime is a NW-SE, rift-perpendicular extension related to the oceanic rifting in Iceland. This trend partly controls the past (Pliocene-Early Pleistocene) hydrothermal activity in the Hvalfjörður area; however, an E-W rift-oblique extension also occurred with a N-S trending fracture system including normal faults, dykes and veins that show higher levels of hydrothermal mineralisation. Currently, there is strong hot-water convection, producing a thermal anomaly, in this N-S-trending fracture system. Our study highlights the paleostress evolution and the development of fracture systems in Hvalfjörður, including the past geothermal history; nevertheless the most efficient tool in geothermal prospection in such complex area remains the shallow geothermal survey. It has proved successful in many localities in Early Pleistocene to Miocene rocks where no surface indication of geothermal activity exists.

Print ISSN: 0037-9409

Electronic ISSN: 0037-9409

Topics: Geosciences

Published by Societe Geologique de France (SGF)

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