Asymmetric continental deformation during South Atlantic rifting along southern Brazil and Namibia

Plate restoration of South America and Africa to their pre-breakup position faces the problem of gaps and overlaps between the continents, an issue commonly solved with implementing intra-plate deformation zones within South America. One of these zones is often positioned at the latitude of SE/S Brazil. However, geological evidence for the existence of a distinct zone in this region is lacking, which is why it remains controversial and is not included in all modeling studies. In order to solve this problem we present a study of multiple geological aspects of both parts of the margin, SE/S Brazil and its conjugate part NW Namibia at the time of continental breakup. Our study highlights pronounced differences between these regions with respect to Paraná-Etendeka lava distribution, magmatic dyke emplacement, basement reactivation, and fault patterns. In Namibia, faults and dykes reactivated the rift-parallel Neoproterozoic basement structure, whereas such reactivation was scarce in SE/S Brazil. Instead, most dykes, accompanied by small-scale grabens, are oriented margin-perpendicular along the margin from northern Uruguay to São Paulo. We propose that these differences are rooted in large-scale plate movement and suggest a clockwise rotation of southern South America away from a stable northern South America and Africa, in a similar way as proposed by others for a Patagonian continental section just prior to South Atlantic rifting. This rotation would produce margin-parallel extension in SE/S Brazil forming margin-perpendicular pathways for lava extrusion and leading to the asymmetric distribution of the Paraná-Etendeka lavas. NW Namibia instead remained relatively stable and was only influenced by extension due to rifting, hot spot activity, and mantle upwelling. Our study argues for significant margin-parallel extension in SE/S Brazil, however not confined to a single distinct deformation zone, but distributed across ~ 1000 km along the margin

Dynamic development of hydrofracture

Many natural examples of complex joint and vein networks in layered sedimentary rocks are hydrofractures that form by a combination of pore fluid overpressure and tectonic stresses. In this paper, a two-dimensional hybrid hydro-mechanical formulation is proposed to model the dynamic development of natural hydrofractures. The numerical scheme combines a discrete element model (DEM) framework that represents a porous solid medium with a supplementary Darcy based pore-pressure diffusion as continuum description for the fluid. This combination yields a porosity controlled coupling between an evolving fracture network and the associated hydraulic field. The model is tested on some basic cases of hydro-driven fracturing commonly found in nature, e.g., fracturing due to local fluid overpressure in rocks subjected to hydrostatic and nonhydrostatic tectonic loadings. In our models we find that seepage forces created by hydraulic pressure gradients together with poroelastic feedback upon discrete fracturing play a significant role in subsurface rock deformation. These forces manipulate the growth and geometry of hydrofractures in addition to tectonic stresses and the mechanical properties of the porous rocks. Our results show characteristic failure patterns that reflect different tectonic and lithological conditions and are qualitatively consistent with existing analogue and numerical studies as well as field observations. The applied scheme is numerically efficient, can be applied at various scales and is computational cost effective with the least involvement of sophisticated mathematical computation of hydrodynamic flow between the solid grains

Deformation and fluid flow in the Huab Basin and Etendeka Plateau, NW Namibia

The Lower Cretaceous Twyfelfontein sandstone formation in the Huab Basin in NW Namibia shows the effects of volcanic activity on a potential reservoir rock. The formation was covered by the Paraná-Etendeka Large Igneous Province shortly before or during the onset of South-Atlantic rifting. Deformation bands found in the sandstone trend mostly parallel to the continental passive margin and must have formed during the extrusion of the overlying volcanic rocks, indicating that their formation is related to South-Atlantic rifting. 2D-image porosity analysis of deformation bands reveals significant porosity reduction from host rock to band of up to 70 %. Cementation of the sandstone, linked to advective hydrothermal flow during volcanic activity, contributes an equal amount to porosity reduction from host rock to band when compared to initial grain crushing. Veins within the basaltic cover provide evidence for hot fluid percolation, indicated by spallation of wall rock and colloform quartz growth, and for a later low-temperature fluid circulation at low pressures indicated by stilbite growth sealing cavities. Sandstone samples and veins in the overlying volcanic rocks show that diagenesis of the Twyfelfontein sandstone is linked to Atlantic rifting and was affected by both hydrothermal and low-thermal fluid circulation

Die dynamische Entwicklung von Styloliten

Stylolite sind raue Lösungssäume, die sich in einer großen Zahl von natürlichen Gesteinen bilden. Sie werden häufig verwendet um die Kompaktion von sedimentären Becken abzuschätzen und die Hauptspannungsrichtung zu finden. Allerdings sind die meisten Beschreibungen von Styloliten qualitativ und wir wissen nicht viel über ihre dynamische Entwicklung und den Einfluss der Spannung auf die Entwicklung der Rauhigkeit. Wir präsentieren numerische Simulationen mit deren Hilfe wir die dynamische Entwicklung von Styloliten untersuchen und erforschen, welchen Einfluss Heterogenitäten, Oberflächenenergien und Spannungen auf die Entwicklung der Rauhigkeit haben...conferenc

Fluid inclusions associated with the generation of pseudotachylyte and ultramylonite in the French Pyrénées

In the Saint-Barthélemy Massif, French Pyrenees, narrow ultramylonite and pseudotachylyte bands were generated in a major ductile shear zone at the end of a retrogressive mylonitic event. Ultramylonite bands formed partly by grain size reduction and recrystallization mechanisms and partly by ductile deformation of pseudotachylyte. Microthermometry shows that fluid inclusions in the major shear zone contain pure CO2, and are never aqueous. The low water content of the rock probably hampered hydrolitic weakening during mylonitic deformation. As a result, ultramylonite and pseudotachylyte bands were formed at 2-3.5 kbar an 450-550 °C The relatively high temperature of the host rock is probably the main cause for ductile deformation of pseudotachylyte. Fluid inclusion study can be an important method to detect depth of pseudotachylyte generation.Dans le Massif du Saint-Barthélémy, des rubans d'ultramylonite et de pseudotachylite se sont développés dans une zone de cisaillement importante à la fin d'une phase de mylonitisation à caractère rétromorphique. Certains rubans ultramylonitiques ont été partiellement formés par des mécanismes de recristallisation et de décroissance de la taille des grains, d'autres par une déformation ductile de la pseudotachylite. Des mesures microthermométriques indiquent que les inclusions fluides dans la zone de cisaillement contiennent du CO2 pur. Le manque d'eau dans la phase fluide a probablement entraîné le durcissement de la roche pendant la déformation mylonitique. L'ultramylonite et la pseudotachylite ont été formées à 2-3,5 kbar et 450-550 °C. La température relativement élevée de la roche-hôte est probablement la cause principale de la déformation ductile de la pseudotachylite. Des recherches sur les inclusions fluides autour des rubans de pseudotachylite peuvent être importantes pour déterminer les conditions de formation de tels rubans.Passchier Cees W. Fluid inclusions associated with the generation of pseudotachylyte and ultramylonite in the French Pyrénées. In: Bulletin de Minéralogie, volume 107, 2, 1984. Inclusions fluides

Sediment Supply and Transport Directions in the Gulf of Gdańsk as Observed from SEM Analysis of Quartz Grain Surface Textures

Quartz grains were examined under a Scanning Electron Microscope (SEM) to determine the geographic distribution of sediments from different sources in the western part of the Gulf of Gdańsk. Main source areas for sediment supply in the Gulf of Gdańsk are coastal cliffs bordering the western part of the Gulf, and the Vistula river entering the Gulf at the southern coastal border. Twelve samples of sediments from the Gulf of Gdańsk sea floor, five samples of Tertiary and Pleistocene deposits from eroded cliffs and three samples of sands from the Vistula river mouth were analyzed. All samples were sieved over 63 microns and 35 quartz grains of each sample were selected and analyzed usinga checklist of 40 grain parameters, including grain surface textures and grain shape. Frequencies of observed checklist variables were statistically analyzed and used to determine the transport history of the grains in each sample. The cliff samples and the river samples served as two end members in the analysis. The core samples were separated into different textural groups related to one of the two main source areas. The distribution of Vistula fluvial material and material eroded from cliffs in the western part of the Gulf of Gdańsk was determined. Results show that transport of sandy fraction material is not very strong within the gulf. Despite long shore currents sedimentation near the Vistula river mouth is dominated by the input of fluvial material. In the westernmost part of the gulf material eroded from the cliffs with admixtures of material of other origin predominates. It is evident that transport of sediments by waves and currents is responsible for the admixture of grains exhibiting subaqueous surface features. Sediments from several sources may have been redistributed during storms by waves and currents and on ice floes during heavy winters, however a strong local component is present

Preuves du nettoyage des concrétions calcaires de l’aqueduc antique de Divona-Cahors

International audienc

Brittle reactivation of ductile shear zones in NW Namibia in relation to South Atlantic rifting

Rifting has occurred worldwide along preexisting mobile belts, which are therefore thought to control rift orientation on a large scale. On a smaller scale, shear zones within mobile belts are reactivated as rift faults. In NW Namibia, shear zones of the Neoproterozoic Kaoko Belt run sub-parallel to the present-day continental passive margin and are inferred to have been reactivated during opening of the South Atlantic ocean. However, the extent of this reactivation and the influence of the reactivated shear zones on South Atlantic rifting is largely unknown. A combined remote sensing and field study was conducted to quantify offsets that are a direct function of shear zone reactivation. The shear zones of the Kaoko Belt are partly overlain by the Paraná-Etendeka volcanic rocks, which were emplaced shortly before or simultaneously to the Atlantic rifting. Faulting within these volcanic rocks can be linked to syn- or post-rift movements. Along the shear zones, down-faulting of the basalts is widespread along listric faults where half-graben form in the hanging wall. At three sites we could determine vertical offsets of ~1180 m, ~470 m, and ~70 m. Although many shear zones were reactivated as faults, these are isolated, and offsets are small, suggesting that reactivation occurred only as a side effect of the rifting and that the Kaoko Belt shear zones have not exerted a significant influence on the rift orientation