Academic literature on the topic 'Paleopiezometry'

Abstract. Paleopiezometry and paleowattometry studies are essential to validate models of lithospheric deformation and therefore increasingly common in structural geology. These studies require a single measure of dynamically recrystallized grain size in natural mylonites to estimate the magnitude of differential paleostress (or the rate of mechanical work). This contribution tests the various measures of grain size used in the literature and proposes the frequency peak of a grain size distribution as the most robust estimator for paleopiezometry or paleowattometry studies. The novelty of the approach resides in the use of the Gaussian kernel density estimator as an alternative to the classical histograms, which improves reproducibility. A free, open-source, easy-to-handle script named GrainSizeTools ( http://www.TEOS-10.org) was developed with the aim of facilitating the adoption of this measure of grain size in paleopiezometry or paleowattometry studies. The major advantage of the script over other programs is that by using the Gaussian kernel density estimator and by avoiding manual steps in the estimation of the frequency peak, the reproducibility of results is improved.

Electron backscatter diffraction (EBSD) was used to determine the orientation of mechanically twinned grains in Carrara marble experimentally deformed to a small strain (≤4%) at room temperature and at a moderate confining pressure (225 MPa). The thicknesses of deformation twins were mostly too small to permit determination of their orientation by EBSD but it proved possible to measure their orientations by calculating possible twin orientations from host grain orientation, then comparing calculated traces to the observed twin traces. The validity of the Turner & Weiss method for principal stress orientations was confirmed, particularly when based on calculation of resolved shear stress. Methods of paleopiezometry based on twinned volume fraction were rejected but a practical approach is explored based on twin density. However, although twin density correlates positively with resolved shear stress, there is intrinsic variability due to unconstrained variables such as non-uniform availability of twin nucleation sites around grain boundaries that imposes a limit on the achievable accuracy of this approach.

Unravelling the burial-deformation history of sedimentary rocks is prerequisite information to understand the regional tectonic, sedimentary, thermal, and fluid-flow evolution of foreland basins. We use a combination of microstructural analysis, stylolites paleopiezometry, and paleofluid geochemistry to reconstruct the burial-deformation history of the Meso-Cenozoic carbonate sequence of the Cingoli Anticline (Northern Apennines, central Italy). Four major sets of mesostructures were linked to the regional deformation sequence: (i) pre-folding foreland flexure/forebulge; (ii) fold-scale layer-parallel shortening under a N045 σ1; (iii) syn-folding curvature of which the variable trend between the north and the south of the anticline is consistent with the arcuate shape of the anticline; (iv) the late stage of fold tightening. The maximum depth experienced by the strata prior to contraction, up to 1850 m, was quantified by sedimentary stylolite paleopiezometry and projected on the reconstructed burial curve to assess the timing of the contraction. As isotope geochemistry points towards fluid precipitation at thermal equilibrium, the carbonate clumped isotope thermometry (Δ47) considered for each fracture set yields the absolute timing of the development and exhumation of the Cingoli Anticline: layer-parallel shortening occurred from ~6.3 to 5.8 Ma, followed by fold growth that lasted from ~5.8 to 3.9 Ma.

Mechanical twins are common microstructures in deformed calcite. Calcite twins have been used for a long time as indicators of stress/strain orientations and magnitudes. Developments during the last decade point toward significant improvements of existing techniques as well as new applications of calcite twin analysis in tectonic studies. This review summarises the recent progress in the understanding of twin formation, including nucleation and growth of twins, and discusses the concept of CRSS and its dependence on several factors such as strain, temperature and grain size. Classical and recent calcite twin measurement techniques are also presented and their pros and cons are discussed. The newly proposed inversion techniques allowing for the use of calcite twins as indicators of orientations and/or magnitudes of stress and strain are summarized. Benefits for tectonic studies are illustrated through the presentation of several applications, from the scale of the individual tectonic structure to the continental scale. The classical use of calcite twin morphology (e.g., thickness) as a straightforward geothermometer is critically discussed in the light of recent observations that thick twins do not always reflect deformation temperature above 170–200 °C. This review also presents how the age of twinning events in natural rocks can be constrained while individual twins cannot be dated yet. Finally, the review addresses the recent technical and conceptual progress in calcite twinning paleopiezometry, together with the promising combination of this paleopiezometer with mechanical analysis of fractures or stylolite roughness.

Abstract. Paleopiezometry and paleowattometry studies, required to validate models of lithospheric deformation, are increasingly common in structural geology. These studies require a numeric parameter to characterize and compare the dynamically recrystallized grain size of natural mylonites with those obtained in rocks deformed under controlled conditions in the laboratory. We introduce a new tool, a script named GrainSizeTools, to obtain a single numeric value representative of the dynamically recrystallized grain size from the measurement of grain sectional areas (2-D data). For this, it is used an estimate of the most likely grain size of the grain size population, using an alternative tool to the classical histograms and bar plots: the peak of the Gaussian kernel density estimation. The results are comparable to those that can be obtained by other stereological software available, such as the StripStar and CSDCorrections, but with the advantage that the script is specifically developed to produce a single and reproducible value avoiding manual steps in the estimation, which penalizes reproducibility.

We present results from a natural deformed shear zone in the Turon de Técouère massif of the French Pyrenees that directly addresses the processes involved in strain localization, a topic that has been investigated for the last 40 years by structural geologists. Paleopiezometry indicates that differential stresses are variable both spatially across the zone, and temporally during exhumation. We have, however, also calculated strain rate, which remains constant despite changes in stress. This result appears to be at odds with recent experimental deformation on monophase (olivine) rocks, which indicate that strain localization occurs dominantly as a result of constant stress. We hypothesize that in the Turon de Técouère massif—and many natural shear zones—strain localization occurs as a result of reactions, which decrease the grain size and promote the activation of grain size sensitive deformation mechanisms. From a tectonics perspective, this study indicates that the deformation rate in a particular plate boundary is relatively uniform. Stress, however, varies to accommodate this deformation. This viewpoint is consistent with deformation at a plate boundary, but it is not the typical way in which we interpret strain localization.

La présente thèse a pour objectif d'éprouver des méthodologies permettant de reconstituer l'évolution des propriétés réservoirs et de la pression subit par un réservoir. Ce manuscrit présente (1) une caractérisation pétrologique des roches réservoirs avec une estimation du calendrier de l'évolution de la porosité, (2) les résultats une étude paléopiézométrique, et (3) une évolution des contraintes effectives et potentielles (sur)pressions de fluides ayant prévalu dans le réservoir carbonaté au cours de son évolution. Ces éléments permettent de reconstituer l'histoire d'enfouissement et les paléocontraintes subies par des réservoirs carbonatés dans un contexte de marge passive. Pour cela ce travail s'est basé sur une approche couplée de deux outils paléopiézométrique complémentaires combinant l'inversion des macles de la calcite et l'inversion de la rugosité des stylolites. Cette approche a été combinée avec des analyses pétrographiques, géochimiques, géomécaniques et géochronologiques pour bien caractériser le réservoir. Le matériel étudié lors de cette thèse consiste en des carottes de forage offshore récupérées de puits profonds fournis par TotalEnergies situés dans les bassins du Bas Congo et de Kwanza sur la marge ouest africaine de l'océan Atlantique Sud. Ces bassins ont subi un épisode de rifting au Crétacé inférieur (145.5 - 112 Ma). L'étude de la destruction de la porosité des réservoirs pré-sel de la formation TOCA syn-rift d'âge Barrémien (130-125 Ma) à partir de carottes prélevées dans le bassin du Bas Congo a révélé que la porosité initiale avait été réduite à sa valeur actuelle de 4-8% au cours des 35 premiers Ma de son histoire d'enfouissement, atteignant ~10% après seulement 10 Ma, i.e. dans les 400-500 premiers mètres d'enfouissement et que la porosité actuelle n'avait pas évolué de manière significative depuis 95 Ma (fin de la stylolitisation). Cette étude a ainsi montré que les propriétés des réservoirs dans les formations carbonatées bioclastiques telles que la formation TOCA peuvent être largement contrôlés par des processus diagénétiques précoces et très superficiels plutôt que par des réactions mésogénétiques qui se produisent ultérieurement en profondeur dans l'histoire de l'enfouissement. Une étude paléopiézométrique a été réalisée dans le but de reconstituer l'histoire d'enfouissement et des paléocontraintes des formations carbonatées de la TOCA (syn-rift) du Barrémien et Sendji (post-rift) de l'Albien sur la marge ouest africaine. La paléopiézométrie basée sur l'inversion de la rugosité des stylolites et l'inversion des macles de calcite a été combinée à l'analyse de la fracturation, à la datation géochronologique U-Pb du ciment calcitique et à la modélisation de l'enfouissement des deux formations pour reconstruire les orientations et les grandeurs des contraintes horizontales et verticales ayant affecté les deux formations au cours du temps. L'inversion des macles de la calcite réalisée sur des ciments précoces a révélé que les carbonates syn-rift et post-rift avaient enregistré une histoire des paléocontraintes polyphasée, incluant (1) des régimes de contraintes extensifs liés à l'ouverture l'océan Atlantique Sud (145.5-112 Ma), avec un σ3 orienté NE-SW à E-W, et à l'échelle du bassin des failles normales orientées N-S et NE-SW. Cette phase d'extension n'est observée que dans les formations syn-rift. (2) des régimes de contraintes extensifs associés à une tectonique salifère locale affectant les formations post sel, trouvés uniquement dans la formation Sendji (101 à 80 Ma) avec un σ3 orienté ~N-S et ~E-W. (3) Des régimes de contraintes compressifs et décrochants avec un σ1 horizontal orienté ~N-S à NE-SW probablement liés à la collision Afrique-Eurasie. (4) Des régimes de contraintes compressifs et décrochants avec un σ1 horizontal orienté ~E-W que nous avons proposé d’attribuer à la poussée à la ride médio-Atlantique et qui prévalent depuis ~15 - 10 Ma
This thesis aims to test methodologies allowing the reconstruction of the evolution of reservoir properties and their pressure. This manuscript presents (1) a petrological characterization of the reservoir rocks with an estimation of the timing of porosity evolution, (2) the results of a paleopiezometric study, and (3) an evolution of effective stresses and potential fluid (over)pressures that prevailed in the carbonate reservoir during its evolution. These tools allow the reconstruction of the burial history and the paleostresses experienced by carbonate reservoirs in a passive margin. For this purpose, this work was based on a coupled approach of two complementary paleopiezometric tools combining the inversion of calcite twin data and stylolite roughness data. This approach was combined with petrographic, geochemical, geomechanical and geochronological analyses to fully characterize the reservoir. The studied material during this thesis consists of offshore cores recovered from deep wells provided by TotalEnergies located in the Lower Congo and Kwanza basins on the West African margin of the South Atlantic Ocean. These basins underwent a rifting event in the early Cretaceous times (145.5 - 112 Ma). The study of the porosity destruction of the pre-salt reservoirs of the syn-rift TOCA formation of Barremian age (130-125 Ma) from offshore core located in the Lower Congo basin revealed that the initial porosity was reduced to its current value of 4-8% during the first 35 Ma of its burial history, reaching ~10% after only 10 Ma, i.e. in the first 400-500 meters of burial and that the current porosity has not evolved significantly since 95 Ma (end of stylolitization). This study has shown that the outcome of reservoir properties in bioclastic carbonate formations such as the TOCA formation may be largely controlled by early and very shallow diagenetic processes rather than by mesogenetic reactions that occur later in the burial history. A paleopiezometric study was carried out in order to reconstruct the burial and paleostress history of the TOCA (syn-rift) Barremian and Sendji (post-rift) Albian carbonate formations on the West African margin. Paleopiezometry based on stylolite roughness inversion and calcite twins inversion was combined with fracture analysis, U-Pb geochronological dating of the calcite cement and burial modelling of the both formations to unravel the orientations and magnitudes of horizontal and vertical stresses that affected the TOCA and Sendji formations over time. The inversion of calcite macles on early diagenetic cements revealed that syn-rift and post-rift carbonates recorded a complex, polyphase paleostress history, (1) extensional stress regimes related to the opening of the South Atlantic ocean (145.5-112 Ma), with a σ3 oriented NE-SW to E-W, and at the basin scale N-S and NE-SW oriented normal faults. This extensional phase is only recorded by the TOCA syn-rift formation. (2) Extensional stress regimes associated with local salt tectonics that affect the post-salt formations, only found in the Sendji formation (101 to 80 Ma) with a σ3 oriented ~N-S and ~E-W. (3) Compressional and strike-slip stress regimes with horizontal σ1 oriented ~N-S to NE-SW probably related to the Africa-Eurasia collision at ~67-60 Ma. (4) Compressional stress regimes with horizontal σ1 oriented ~E-W that we proposed to attribute to the mid-Atlantic ridge push and that have prevailed since 15-10 Ma onwards