Contrasting stress fields on correlating margins of the South Atlantic

The “passiveness” of passive continental margins across the globe is currently under debate since several studies have shown that these margins may experience a variety of stress states and undergo significant vertical movement post-breakup. Of special interest is the South Atlantic, because the bounding continents have very different recent geological histories, with Africa experiencing continental rifting whereas South America is influenced by subduction on the Pacific side. It is not clear to what extent the Atlantic continental margins are subject to the same stresses and vertical motions as the main continents. To address this problem, we performed a paleostress analysis of two originally adjacent areas, i.e. NW Namibia and SE/S Brazil. Both areas are covered by the ~ 133-Ma-old Paraná-Etendeka extrusives that were emplaced shortly before or during the onset of the Atlantic rifting. Thus, the volcanics serve as a time marker for syn- or post-rift deformation. Collected fault slip data in the volcanics reveal remarkable differences between the two correlating areas. NW Namibia was dominated by extension in ENE-WSW and SW-NE directions, and by minor strike-slip movement with NW-SE directed compression. SE/S Brazil was mostly affected by strike-slip faulting, with compression oriented E-W and SW-NE. Similar fault systems appear widespread across SE Brazil and may be the combined result of flexural margin bending and the Nazca plate subduction. The results of NW Namibia differ from known compressional stress tensors in western South Africa, post-dating 90 Ma. The south-western African continental margin may thus have experienced a spatially variable stress history. Our results show that the tectonic evolution of the continental margins of the South Atlantic is not passive and that both margins vary significantly in structural style and stress fields, indicating that variable plate boundary forces play a major role in margin evolution

New assessment of basement units in the fitz roy area: implications for an early mesozoic tectono-metamorphic event at southern patagonia

Among the complex collage of metamorphic belts exposed in the Patagonian Andes, the Triassic metamorphic rocks are poorlyrepresented. They only outcrop at the southern Chilean coast within the Chonos Metamorphic Complex (CMC), a subductioncomplex (Hervé et al. 2008), which witness of an active subduction during the Late Triassic (Hervé and Fanning 2001). Recently,U-Pb dating in detrital zircons performed in metasedimentary rocks from the Nunatak Viedma, which crops out west of the Fitz Royarea, within the Southern Patagonian Ice Field, revealed grain-ages youngest at Late Triassic (Suárez et al. 2018). We proposedto define the Nunatak Viedma Unit (NVU) in the Southern Patagonian Andes (SPA; Suárez et al. 2018) on the basis of these newobservations. The rocks of the NVU exhibit a main phase of ductile to brittle-ductile deformation tentatively correlated with theLate Triassic-Early Jurassic Chonide deformation event (Suárez et al. 2018) identified in the southern Chilean coast (Thomson andHervé 2002; Hervé et al. 2008). However, the age of the NVU?s tectono-metamorphic event, as well as its tectonic significance, arestill unknown. New geological mapping carried out in the Fitz Roy region (Fig. 1) reveals that outcrops previously mapped either asBahía de la Lancha Formation (BLF) or as volcanic rocks of the El Quemado Complex, shows lithological and structural similaritieswith the rocks of the NVU (Fig. 1). The preliminary structural results presented in this contribution aim to better constrain thedeformation phase of the NVU and to discuss its age and tectonic significance.Fil: Suárez, Rodrigo Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Ghiglione, Matias. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Sue, Christian. Universite de Franche-Comte; FranciaFil: Roy, Sandrine. Université de Savoie Mont-Blanc; FranciaXV Congreso Geológico ChilenoConcepciónChileUniversidad de ConcepciónColegio de Geólogos de ChileSociedad Geológica de Chil

Extent and Mechanism of Footwall Shear Adjacent to the Ruby\u27s Inn Thrust Fault, Southern Utah

The Ruby’s Inn Thrust, located in the Bryce Canyon region, is an uncharacteristic demonstration of a south-directed shortening episode located near the predominately east-directed contractional structures of the Sevier Orogeny. The Paleocene to Eocene Claron Formation in the footwall of the Ruby’s Inn Thrust contains conjugate shear structures and vertical fault planes with slickensides and slickenlines, indicating complex multidirectional shearing. We determined the north-south extent of a broad shear zone along a traverse immediately west of Bryce Canyon National Park, and shearing intensifies slowly from the main thrust at the northern end of our traverse to a maximum intensity at 13 kilometers south of the thrust, where it then gradually diminishes until an abrupt end approximately 29 kilometers south of the thrust. No evidence of conjugate shear structures in the hanging wall of the thrust was observed. The footwall outcrops adjacent to the thrust and at the southern portion of the traverse contained the structures, but they were more difficult to visually recognize, whereas the structures within the outcrops of the central region were obvious. The conjugate shear structures crosscut bedding and vary from small scale (a few centimeters) to large scale (tens of meters) throughout each outcrop, and are best observed parallel to their east-west strike. The conjugate shear structures contain distinct structural planar surfaces that include very well developed slickensides and slickenlines. This research supports the idea that the deformation structures were a significant contributor in the formation of hoodoos found in the Claron Formation

Three-dimensional physical models of sedimentary basins as a resource for teaching-learning of geology

sem InformaçãoThree-dimensional modeling connects several fields of knowledge, both basic and applied. 3D models are relevant in educa-tional research because the manipulation of 3D objects favors students' acquisition of spatial vision, but in the Geosciences, there a144379384sem Informaçãosem Informaçãosem Informaçã

Azimuthfinder: ferramenta para a extração de dados e apoio na análise estrutural

Apresenta-se uma ferramenta computacional denominada de AzimuthFinder, para funcionamento dentro do conjunto de programas do ArcGis®. O programa desenvolvido na linguagem Python tem por finalidade facilitar e otimizar a geração de tabelas de dados azimutais, a partir de lineamentos estruturais previamente traçados em imagens de satélite, fotografias aéreas ou mapas geológicos em meio digital. As características do AzimuthFinder permitem que, a critério do usuário, sejam geradas diferentes tabelas para o mesmo objeto de análise, dependendo do peso relativo atribuído aos lineamentos, por exemplo dando maior peso àqueles de traçado mais longo. Outra característica da ferramenta é a exportação dos dados no formato TXT, que é universal para vários tipos de programas, com formatação específica para softwares como o Stereo32, Win Tensor, OpenStereo e StereoNet 7, que por sua vez são softwares livres preparados para a confecção de diagramas de rosetas. Os testes com a ferramenta demonstraram que é bastante eficiente e rápida para a geração das tabelas azimutais, facilitando de maneira eficaz a confecção de diagramas de rosetas necessários à análise estrutural em áreas essencialmente submetidas à deformação frágil.Here is presented a computational tool named AzimuthFinder, for functioning inside the set of programs of ArcGis®. The developed program is intended to facilitate and optimize the generation of azimuth data tables, using structural lineaments previously traced in maps that are being worked on that automatic information system. The characteristics of AzimuthFinder allow that, upon the user's choice, different tables get generated for the same analysis object, depending on the relative weight attributed to the lineaments, giving for example greater weight to those of greater extension. Another characteristic of the tool is the file exportation in TXT format, which is universal to several types of programs, with specific formatting to one the software between Stereo32, Win Tensor, OpenStereo and StereoNet7, which are all free software prepared to the confection of rose diagrams. Tests with the program showed that it is very efficient and fast for generating the azimuth data tables, effectively allowing easier confection of rose diagrams, which are necessary for the structural analysis in areas submitted to fragile deformation

Analysis of the Hite Fault Group, Southeast Utah: Insights into Fluid Flow Properties in a Reservoir Analog

In the subsurface faults can act as both barriers and conduits for fluids or gases such as CO2, hydrocarbons, or water. It is often thought that faults in porous rocks such as sandstone are barriers to fluid flow. In this study we show that this is not always the case. In sandstones like the Cedar Mesa Sandstone it is very important to understand the relationships between this history of fault slip and fluid flow. Better understanding of how fluids migrate through faults and the damaged areas surrounding these faults has strong significance to the oil and gas industry. In this study we examine a group of faults and their surrounding damage zones near Hite, Utah. We analyze three of these small-scale faults in more detail. In doing so we give insights into how these faults and their damage zones can effect fluid migration as well as the porosity and permeability in the Cedar Mesa Sandstone. Whole rock geochemistry, X-ray diffraction mineralogy, permeability data, petrography, ultraviolet photography, and outcrop observations were used to gain insights into cross-cutting relationships, past fluid compositions, and fault characteristics. From the data that was collected from these faults we have begun to describe a series of structural and fluid flow events. This series allows us to say that small-scale faults and fractures are features by which fluids can migrate preferentially. In this series of events we isolate two separate phases of movement. The first phase of movement being has a component of shear in which the edges of the fractures are not moving directly apart. This event is accompanied by a fluid flow event the emplaced iron oxide in the fractures and the surrounding formation. The second event is a phase when the faults become reactivated by a stress that created open mode fractures. This second is accompanied by a fluid flow event that has high calcium content and emplaces calcite in the fractures. Throughout this study we give evidence to support this series of movement and fluid events

Geology of the Northern Apennines nappe stack on eastern Elba (Italy): new insights on the Neogene orogenic evolution of the Northern Tyrrhenian Sea

We document the tectonic and metamorphic evolution of thrust nappes of the eastern island of Elba. The area exposes a natural cross section of the Northern Apennines hinterland, from the metamorphic basement units to the overlying continent- and ocean-derived nappes. We integrated mapping, analysis of structures and microstructures, and the interpretation of drill core logs with lithostratigraphic, metamorphic, and geochronological constraints, producing a novel geological map of eastern Elba (1:5’000 scale). We show that the area experienced polyphase Oligocene–Pliocene contractional tectonics marked by in-sequence and out-of-sequence thrusting accompanied by folding and overprinted by faulting in the Pliocene. Magmatism occurred during contraction with post-magmatic thrusting ultimately coupling HP-LT and LP-HT units. Drill core logs allow for the first time the reconstruction of the N-dipping character of the Zuccale Fault, which represents the youngest (late Miocene–early Pliocene) large-scale structure in the area

Filling Materials in Brittle Structures as Indicator of Cenozoic Tectonic Events in Southeastern Brazil

The filling materials in brittle structures can provide useful information about the Cenozoic evolution developed over proterozoic terrains. When these materials are affected by faults, they record deformation phases that can be determined chronologically and, in the occurrence of lateritic materials, it is possible to infer the paleoenvironmental conditions during the mineral formation. This work aimed to identify crystalline phases of brittle structure filling materials and to propose evolutionary interpretations for Cenozoic tectonic reactivation based on literature data. The study area is located in the Southern part of the Espírito Santo State, near the Brazilian Southeastern Continental Margin, where proterozoic geological structures have been reactivated since the mesozoic rift phase, up to the Holocene. The mineral assemblage found in the filling materials includes primary minerals such as quartz, muscovite, microcline, rutile, titanite, and bannisterite; and the weathering minerals such as kaolinite, illite, hematite, goethite, hydrobiotite, lithiophorite and, birnessite. The mineralogical association found in the filling materials denotes the action of fluid phases with mineral precipitation at the brittle discontinuities during the weathering processes that occurred during the Cenozoic, probably between the Miocene and the Pleistocene. The faults, which striations are marked on the filling materials, originated after (in the case of the manganese oxides) or during (in the case of the illite) the mineral formation, indicating that the maximum age of these faults is in the Miocene. The origin of the brittle structures that affected the filling materials studied here is linked to the uplifting of the Continental Brazilian Margin, when ancient geological structures were reactivated as normal faults due to the local action of an extensional regime.The filling materials in brittle structures can provide useful information about the Cenozoic evolution developed over proterozoic terrains. When these materials are affected by faults, they record deformation phases that can be determined chronologically and, in the occurrence of lateritic materials, it is possible to infer the paleoenvironmental conditions during the mineral formation. This work aimed to identify crystalline phases of brittle structure filling materials and to propose evolutionary interpretations for Cenozoic tectonic reactivation based on literature data. The study area is located in the Southern part of the Espírito Santo State, near the Brazilian Southeastern Continental Margin, where proterozoic geological structures have been reactivated since the mesozoic rift phase, up to the Holocene. The mineral assemblage found in the filling materials includes primary minerals such as quartz, muscovite, microcline, rutile, titanite, and bannisterite; and the weathering minerals such as kaolinite, illite, hematite, goethite, hydrobiotite, lithiophorite and, birnessite. The mineralogical association found in the filling materials denotes the action of fluid phases with mineral precipitation at the brittle discontinuities during the weathering processes that occurred during the Cenozoic, probably between the Miocene and the Pleistocene. The faults, which striations are marked on the filling materials, originated after (in the case of the manganese oxides) or during (in the case of the illite) the mineral formation, indicating that the maximum age of these faults is in the Miocene. The origin of the brittle structures that affected the filling materials studied here is linked to the uplifting of the Continental Brazilian Margin, when ancient geological structures were reactivated asnormal faults due to the local action of an extensional regime

Reconstructing fluvial bar surfaces from compound cross-strata and the interpretation of bar accretion direction in large river deposits

The interpretation of fluvial styles from the rock record is based for a significant part on the identification of different types of fluvial bars, characterized by the geometric relationship between structures indicative of palaeocurrent and surfaces interpreted as indicative of bar form and bar accretion direction. These surfaces of bar accretion are the boundaries of flood-related bar increment elements, which are typically less abundant in outcrops than what would be desirable, particularly in large river deposits in which each flood mobilizes large volumes of sediment, causing flood-increment boundary surfaces to be widely spaced. Cross-strata set boundaries, on the other hand, are abundant and indirectly reflect the process of unit bar accretion, inclined due to the combined effect of the unit bar surface inclination and the individual bedform climbing angle, in turn controlled by changes in flow structure caused by local bar-scale morphology. This work presents a new method to deduce the geometry of unit bar surfaces from measured pairs of cross-strata and cross-strata set boundaries. The method can be used in the absence of abundant flood-increment bounding surfaces; the study of real cases shows that, for both downstream and laterally accreting bars, the reconstructed planes are very similar to measured bar increment surfaces.Sao Paulo Research Foundation (FAPESP)CAPESCNPqLiliane JanikianUniv Sao Paulo, Inst Energia & Ambiente, Av Prof Luciano Gualberto 1289,Cidade Univ, BR-05508900 Sao Paulo, SP, BrazilUniv Sao Paulo, Inst Geociencias, Rua Lago 562,Cidade Univ, BR-05508900 Sao Paulo, SP, Brazil|Univ Fed Itajuba, Inst Recursos Nat, Av BPS 1303, BR-37500903 Itajuba, MG, BrazilCPRM Geol Survey Brazil, Rua Costa 55, BR-01304010 Sao Paulo, SP, BrazilUniv Fed Sergipe, Dept Geol, Av Marechal Rondom S-N, BR-49100000 Sao Cristov, SE, BrazilUniv Fed Sao Paulo, Campus Baixada Santista, BR-11030400 Santos, SP, BrazilUniv Fed Sao Paulo, Campus Baixada Santista, BR-11030400 Santos, SP, BrazilFAPESP: 2009/53363-8FAPESP: 2009/52807-0FAPESP: 2009/51766-8FAPESP: 2010/51103-6FAPESP: 2010/51559-0FAPESP: 2013/01825-3FAPESP: 2014/16739-8CAPES: PROEX-558/2011CNPq: 301774/2012-9Liliane Janikian: 301775/2012-5Web of Scienc