Contribution of studies of sub-seismic fracture populations to paleo-hydrological reconstructions (Bighorn Basin, USA)

This work reports on the reconstruction of the paleo-hydrological history of the Bighorn Basin (Wyoming, USA) and illustrates the advantages and drawbacks of using sub-seismic diffuse fracture populations (i.e., micrometric to metric joints and veins forming heterogeneous networks), rather than fault zones, to characterize paleo-fluid systems at both fold and basin scales. Because sub-seismic fractures reliably record the successive steps of deformation of folded rocks, the analysis of the geochemical signatures of fluids that precipitated in these fractures reveals the paleo-fluid history not only during, but also before and after, folding. The present study also points out the need for considering pre-existing fluid systems and basin-scale fluid migrations to reliably constrain the evolution of fluid systems in individual folds

Fracture controlled paleohydrogeology in a basement-cored, fault-related fold: Sheep Mountain Anticline, Wyoming, United States

International audienceNew geochemical and microstructural data constrain the origins and pathways of paleofluids during the growth of Sheep Mountain Anticline, Wyoming, United States. Oxygen, carbon, and strontium isotope studies were performed on prefolding and fold-related calcite veins and their sedimentary host rocks and combined to fluid inclusion microthermometry results. We show that most of the cements precipitated from Paleogene meteoric fluid. Stable isotopes and fluid inclusion homogenization temperatures further indicate that most veins were mineralized from upward moving fluids after these fluids were heated at depth (T > 110°C). This implies that fluids migrated along the basement thrust underlying the fold and/or at the base of the cover. Above the fault tip, the fluids circulated rapidly in the diffuse synfolding (and early folding) fracture network. The zone of preferential migration of the warm fluids is currently located in the backlimb of the fold, which supports some of the previously published structural interpretation of the subsurface. This study also highlights the potential of combined fracture analysis and geochemical analyses of paleofluid flows in fractures to constrain both the deformation history and the fluid flow during basement-involved shortening in Laramide-style forelands

Paleostress magnitudes in folded sedimentary rocks

International audienceUsing Sheep Mountain Anticline (Wyoming, USA) as a case study, we propose a new approach to quantify effective paleo-principal stress magnitudes in the uppermost crust. The proposed mechanical scenario relies on a well-documented kinematic and chronological sequence of development of faults, fractures and microstructures in the folded strata. Paleostress orientations and regimes as well as differential stress magnitudes based on calcite twinning paleopiezometry are combined with rock mechanics data in a Mohr construction to derive principal stress magnitudes related to the successive steps of layer-parallel shortening and to late stage fold tightening. Such quantification also provides original insights into the evolution of the fluid (over)pressure and amount of syn-folding erosion

Stress and strain patterns, kinematics and deformation mechanisms in a basement-cored anticline: Sheep Mountain Anticline, Wyoming

International audienceIn order to characterize and compare the stress-strain record prior to, during, and just after folding at the macroscopic and the microscopic scales and to provide insights into stress levels sustained by folded rocks, we investigate the relationship between the stress-strain distribution in folded strata derived from fractures, striated microfaults, and calcite twins and the development of the Laramide, basement-cored Sheep Mountain Anticline, Wyoming. Tectonic data were mainly collected in Lower Carboniferous to Permian carbonates and sandstones. In both rock matrix and veins, calcite twins recorded three different tectonic stages: the first stage is a pre-Laramide (Sevier) layer-parallel shortening (LPS) parallel to fold axis, the second one is a Laramide LPS perpendicular to the fold axis, and the third stage corresponds to Laramide late fold tightening with compression also perpendicular to the fold axis. Stress and strain orientations and regimes at the microscale agree with the polyphase stress evolution revealed by populations of fractures and striated microfaults, testifying for the homogeneity of stress record at different scales through time. Calcite twin analysis additionally reveals significant variations of differential stress magnitudes between fold limbs. Our results especially point to an increase of differential stress magnitudes related to Laramide LPS from the backlimb to the forelimb of the fold possibly in relation with motion of an underlying basement thrust fault that likely induced stress concentrations at its upper tip. This result is confirmed by a simple numerical model. Beyond regional implications, this study highlights the potential of calcite twin analyses to yield a representative quantitative picture of stress and strain patterns related to folding

Continental break-up history of a deep magma-poor margin based on seismic reflection data (northeastern Gulf of Aden margin, offshore Oman)

International audienceRifting between Arabia and Somalia started around 35 Ma followed by spreading at 17.6 Ma in the eastern part of the Gulf of Aden. The first-order segment between Alula-Fartak and Socotra-Hadbeen fracture zones is divided into three second-order segments with different structure and morphology. Seismic reflection data were collected during the Encens Cruise in 2006 on the northeastern margin. In this study, we present the results of Pre-Stack Depth Migration of the multichannel seismic data from the western segment, which allows us to propose a tectono-stratigraphic model of the evolution of this segment of the margin from rifting to the present day. The chronological interpretation of the sedimentary sequences is mapped out within relation to the onshore observations and existing dating. After a major development of syn-rift grabens and horsts, the deformation localized where the crust is the thinnest. This deformation occurred in the distal margin graben (DIM) at the northern boundary of the ocean-continent transition (OCT) represented by the OCT ridge. At the onset of the OCT formation differential uplift induced a submarine landslide on top of the deepest tilted block and the crustal deformation was restricted to the southern part of the DIM graben, where the continental break-up finally occurred. Initial seafloor spreading was followed by post-rift magmatic events (flows, sills and volcano-sedimentary wedge), whose timing is constrained by the analysis of the sedimentary cover of the OCT ridge, correlated with onshore stratigraphy. The OCT ridge may represent exhumed serpentinized mantle intruded by post-rift magmatic material, which modified the OCT after its emplacement

Oblique rifting and segmentation of the NE Gulf of Aden passive margin

The Gulf of Aden is a young, obliquely opening, oceanic basin where tectonic structures can easily be followed and correlated from the passive margins to the active mid-oceanic ridge. It is an ideal laboratory for studies of continental lithosphere breakup from rifting to spreading. The northeastern margin of the Gulf of Aden offers the opportunity to study on land the deformation associated with oblique rifting over a wide area encompassing two segments of the passive margin, on either side of the Socotra fracture zone, exhibiting distinct morphologic, stratigraphic, and structural features. The western segment is characterized by an elevated rift shoulder and large grabens filled with thick synrift series, whereas the eastern segment exhibits low elevation and is devoid of major extensional structures and typical synrift deposits. Though the morphostructural features of the margin segments are different, the stress field analysis provides coherent results all along the margin. Four directions of extension have been recognized and are considered to be representative of two tensional stress fields with permutations of the horizontal principal stresses s2 and s3. The two dominant directions of extension, N150 E and N20 E, are perpendicular to the mean trend of the Gulf of Aden (N75 E) and parallel to its opening direction (N20 E-N30 E), respectively. Unlike another study in the western part of the gulf, our data suggest that the N150 E extension stage is older than the N20 E extension stage. These conflicting chronologies, which are nowhere unambiguously established, suggest that the two extensions coexisted during the rifting. On-land data are compared with offshore data and are interpreted with reference to oblique rifting. The passive margin segmentation represents a local accommodation of the extensional deformation in a homogeneous regional stress field, which reveals the asymmetry of the rifting process. The first-order segmentation of the Sheba Ridge is inherited from the prior segmentation of the passive margin

Stress rotations and the long-term weakness of the Median Tectonic Line and the Rokko-Awaji Segment

International audienceWe used a field analysis of rock deformation microstructures and mesostructures to reconstructthe long-term orientation of stresses around two major active fault systems in Japan, the Median TectonicLine and the Rokko-Awaji Segment. Our study reveals that the dextral slip of the two fault systems, activesince the Plio-Quaternary, was preceded by fault normal extension in the Miocene and sinistral wrenching inthe Paleogene. The two fault systems deviated the regional stress field at the kilometer scale in their vicinityduring each of the three tectonic regimes. The largest deviation, found in the Plio-Quaternary, is a more faultnormal rotation of the maximum horizontal stress to an angle of 79° with the fault strands, suggesting anextremely low shear stress on the Median Tectonic Line and the Rokko-Awaji Segment. Possible causes of thislong-term stress perturbation include a nearly total release of shear stress during earthquakes, a low staticfriction coefficient, or lowelastic properties of the fault zones comparedwith the country rock. Independently ofthe preferred interpretation, the nearly fault normal orientation of the direction of maximum compressionsuggests that the mechanical properties of the fault zones are inadequate for the buildup of a pore fluidpressure sufficiently elevated to activate slip. The long-term weakness of the Median Tectonic Line and theRokko-Awaji Segment may reside in low-friction/low-elasticity materials or dynamic weakening rather than inpreearthquake fluid overpressures

Croissance des failles normales et des rifts continentaux : développement du Golfe d'Aden et dynamique de la plaque Arabe

The deformation of the eastern margin of the North Gulf of Aden are first studied in this thesis from data acquired during a field mission and campaign at Sea (Incense Sheba). The field data show that the faults are highly segmented and widely scattered directions (N60 ° E to N120 ° E to major faults). To understand these features, analog models were performed to better understand the mechanisms involved in the creation of these fault networks: the influence of levels ductile reactivation of inherited faults and boundary conditions responsible for the extension. The influence of viscous levels on the growth of normal fault networks is studied using analog and digital models. The presence of viscous levels and their resistance control the geometry of major faults. A low level of viscous resistance induces large faults farther apart, limiting blocks slightly deformed. The only small flaws that are initiated are then located near major faults and are widely scattered directions. To account for the geometry of fault system in the Gulf of Aden, it is necessary to take into account the obliquity of the direction of extension of the rift and fault networks inherited. Analogue models of oblique reactivation were performed. They are well aware of the geometries observed in the Gulf of Aden, as in other rifts like the Gulf of Suez rift Lake Tanganyika or the Viking Graben. Reactivation of these phenomena occur on the edges of the rift or late extension direction (N160 ° E) is recorded on the ground. To better define the boundary conditions leading to the formation of networks of faults observed on land and at sea, it is necessary to understand the mechanisms responsible for the formation of the Gulf of Aden. To do this, models of analog system lithosphere-upper mantle explore the role of the subduction of the Tethys in the north. The collision of Africa-Arabia comes at an early stage in the Northwest while subduction is still active in the east. In models, this phenomenon causes the intraplate deformation in North-Eastern Africa, whose geometry, in the presence of a weak representative of the Afar hot spot, is very similar to that of Afro-Arab rifts. In this configuration, an area of ​​oblique extension (similar to the Gulf of Aden) is generated, without weakness or preexisting lithospheric rifting propagation. This result implies a particular model of oblique rifting may explain the presence of N160 ° E extension, late on the edges of the rift, perpendicular to the rift that locates the direction of the Gulf today.Les déformations de la marge orientale Nord du Golfe d'Aden sont tout d'abord étudiées dans cette thèse à partir de données acquises lors d'une mission de terrain et d'une campagne en Mer (Encens Sheba). Les données de terrain montrent que les failles sont fortement segmentées et leurs orientations très dispersées (de N60°E à N120°E pour les failles majeures). Afin de comprendre ces caractéristiques, des modèles analogiques ont été réalisés pour mieux cerner les mécanismes intervenant dans la création de ces réseaux de failles: l'influence de niveaux ductiles, la réactivation de failles héritées et les conditions aux limites responsables de l'extension. L'influence des niveaux visqueux sur la croissance des réseaux de failles normales est étudiée à partir de modèles analogiques et numériques. La présence des niveaux visqueux ainsi que leur résistance contrôlent la géométrie des failles majeures. Une faible résistance du niveau visqueux induit des grandes failles plus espacées, limitant des blocs peu déformés. Les seules petites failles qui s'initient sont alors localisées à proximité des grandes failles et ont des directions très dispersées. Pour rendre compte de la géométrie du réseau de failles du Golfe d'Aden, il est nécessaire de prendre en compte l'obliquité de la direction d'extension sur le rift et les réseaux de failles héritées. Des modèles analogiques de réactivation oblique ont été réalisés. Ils rendent bien compte des géométries observées dans le Golfe d'Aden, comme dans d'autres rifts tels que le Golfe de Suez, le rift du Lac Tanganyika ou le Viking Graben. Ces phénomènes de réactivation ont lieu sur les bords du rift où une direction d'extension tardive (N160°E) est enregistrée sur le terrain. Afin de mieux définir les conditions aux limites conduisant à la formation des réseaux de failles observés à terre et en mer, il est nécessaire de comprendre les mécanismes responsables de la formation du Golfe d'Aden. Pour ce faire, des modèles analogiques du système lithosphère-manteau supérieur explorent le rôle de la subduction de la Téthys au Nord. La collision de l'Afrique-Arabie intervient de manière précoce au Nord-Ouest tandis que la subduction est encore active à l'Est. Dans les modèles, ce phénomène provoque les déformations intraplaques dans le Nord-Est de l'Afrique, dont la géométrie, en présence d'une faiblesse représentant le point chaud des Afars, est très similaire à celle des rifts Afro-Arabes. Dans cette configuration, une zone d'extension oblique (similaire au Golfe d'Aden) est générée, sans faiblesse lithosphérique préexistante ni propagation du rifting. Ce résultat implique un modèle particulier de rifting oblique qui peut expliquer la présence de l'extension N160°E, tardive sur les bords du rift, perpendiculaire au rift qui se localise suivant la direction du golfe actuel