Nolan Dehler

The Rio Paraiba do Sul structural divergence is a regional scale structure in which the mylonitic foliation delineate a fan geometry, those of the northern limb dipping to SE, and those of the southern limb dipping to NW. On the core of the structure occurs a large dextral ductile transpressional shear belt. The present work deals with the geometry and kinematics of structures in a section cross-cutting the southern limb of this structure. At the base of section, occurs a high-grade banded mylonite granitoids, with mylonitic foliation dipping moderately to NW and stretching lineation plunging to NNE. Northwards, the same geometry of ductile structures is seen in mylonitic S-type granitoids and paragneisses. In this region, however, the foliation steepens and the lineation increases in plunge with a slightly rotation towards the NE. Flattening strains leads to the formation of S and SL high-grade tectonites. At the base of section shear-sense indicators (S-C foliations, s -porphyroclasts and shear-bands) suggest a dextral component in horizontal planes and down-dip normal motion in a monoclinic shear-zone model. Towards the top of section, however, the asymmetry of the structures is not clear, but one oblique shearing with top-to-SW is also present. To the north the foliation have slow dips, and the stretching lineation plunges shallower to NE. In this domain the shearsense indicators suggest top-to-SW sense of shear parallel to the orogenic trend. Field relations suggests that ganitoids sheets were intruded lit-par-lit during this stage of the tectonic movements. In this domain also occur open to tight folds with axis sub-parallel to the stretching lineation in the same outcrop and axial surfaces with high dips preferentially to NW. This folding is interpreted as resulted from a shortening component at high angle to the regional shear-zones during transpressional strain, coeval with transpressive dextral shear-zones and thrust (ductile/ruptile) with top-to-SE sense of shear. The structural and field data suggests that the early movements were parallel and oblique to the belt in this region, and occurred near the peak temperatures of regional metamorphism accompanied by widespread crustal anatexis. This early stage of motion would be interpreted as horizontal and vertical extrusion of a partially molten crust during unconfined transpression.

The S-type granitoids, described as syncollisional in the literature, are now interpretated as late collisional. The generation of these granitoids is related to the partial melting (anatexis) of metasedimentary rocks. Their emplacement was controled by ductile shear zones of low- to high-angle. Peraluminous leucogranites with garnet, muscovite, sillimanite and turmaline predominante. They are foliated and gneissified, and have granitic and granodioritic composition. The enclaves are metasedimentary rocks (calci-silicatic, quartzite, garnet-biotite gneiss). Two tectonic models are discussed for the generation these rocks: one with initial crustal duplication (Alpine-Hymalaian type) and other without significative crustalthickening. The first model is in accord with different tectonic models proposed for the belt. In this model, the peak of metamorphim is reach after compressional tectonic and crustal thickening, for a range of normal physical parameters in the crust, with metamorphi...

A divergência em leque do vale do rio Paraíba do Sul constitui-se numa estrutura regional, aparecendo em ambos os flancos da estrutura uma foliação milonítica: no flanco norte, ela mergulha para SE, e no flanco sul, o mergulho é para NW, associando-se ainda em sua parte central uma zona de cisalhamento de alto ângulo (Zona de Cisalhamento de Além Paraíba), desenvolvida em regime transpressivo dextral. Discute-se neste trabalho dados geométricos e cinemáticos obtidos ao longo de uma seção transversal no flanco sul desta estrutura. Na base da seção, afloram ortognaisses miloníticos de alta temperatura, com mergulhos moderados para NW e lineação (estiramento e mineral) caindo para NNE. Para norte, a geometria da estrutura é persistente, havendo apenas um ligeiro aumento de mergulho e do caimento da lineação, com o giro desta última para NE, e desenvolvimento de tectonitos-SL e S. Os indicadores cinemáticos na base da seção (pares de foliações S-C, porfiroclastos de feldspato tipo-s e b...

Structural analysis carried out on a segment of the Neoproterozoic Ribeira Belt, southeastern Brazil, show that it represents part of the transpressive dextral orogen related to the Central Mantiqueira Province. NNE-trending and steeply dipping regional mylonitic belts form anastomosed geometry, and describe a map-scale, S-C-like structure that is characterized by their deflection towards NE near the Além Paraíba Lineament. Lithological and structural control related to deformation partition were responsible for the formation of felsic mylonitic granulites with S-type granites lenses developed in ductile shear zones, alternated with less deformed intermediate to basic granulites associated with charnockites. The dextral shear sense indicators are consistent with transpressive deformation in the region and are common especially at the border of the main shear zones. The presence of S-type leucogranite may lead to variations of linear and planar relationships, which result in local ex...

The Arrozal Granite, situated in the southwestern region of the State of Rio de Janeiro, has a granitic to granodioritic composition. It contains a strong mylonitic foliation along its border, passing gradually to a well-developed magmatic foliation towards its center. Structural analysis indicates that the Arrozal Granite was emplaced along the Além-Paraíba Shear Zone in a dextral transpressive tectonic regime. A regional shift of the trend along this shear zone from NE-SW to E-W, observed in the area, is interpreted to be casually related to the creation of space for the emplacement of the granite. Our data indicate that releasing bends may have played an important role for space generation during the emplacement of the Arrozal Granite and other plutons.

This paper presents 40Ar/39Ar ages of the rocks from the Serra do Azeite transtensional shear zone in the southern part of the Ribeira belt, between the States of São Paulo and Paraná, and also discusses the regional correlations and the tectonic implications for other parts of the belt. The geochronological data suggest that transtensional deformation was active between 600 and 580 Ma (hornblende and muscovite apparent ages, respectively). This time span is considerably older than previous proposals for the period of activity of these structures (520-480 Ma) in the northern segment of this belt and in the Araçuaí belt. Kinematic analysis of the dated mylonites shows extensional structures with top-down movement to ESE compatible with structures found in other tectonic segments in the eastern portion of the Quadrilátero Ferrífero and in the Rio Doce Valley region. Our ages are situated in the same time interval defined for the alkaline magmatism of the Serra do Mar suite. We suggest...

Shear zones geometry in the Paraíba do Sul belt, southeastern Brazil, delineates a NE-trending fan-like structure. Shear zones dip towards SE in the northern limb, and towards NW in the southern one. This geometry has been interpreted either due to transpression or to late folding of flat-lying thrust surfaces. Stretching lineation plunges to ENE-ESE in the northern limb and towards NNE-NE in the southern one. Structural data in the southern limb of the divergent fan suggest a two stage kinematic evolution in high-temperature conditions: an earlier stage with top-to-SSW/SW sinistral thrusting and orogenic-parallel tangential motion, and a later stage with top-down to NNE/NE transtensional deformation. We propose a heterogeneous deformation model to explain the observed shear reversal, and suggest that the imposed transpressional displacement gradient may change during progressive deformation due to transient rheological inhomogeneities in bulk pure shear strain. In the earlier stage...

Fractured Albian carbonate reservoirs, localized along eastern Brazilian marginal basins, have proven to contain important hydrocarbon reserves. The purpose of this paper is to discuss the structural analysis carried out on a selected oriented core, and its application for exploring and modeling fractured reservoirs. Attributes of fractures and faults measured on the core were correlated with depositional sequences, facies, and other petrophysical properties. We observe that the most deformed facies are mechanically soft, porous laminated mudstones. We suggest that this occurs due to flexural slip. We suggest that a clear, positive correlation between fracture density and some petrophysical parameters occurs when deformation results from wider wavelength structures in relation to thickness, like large folds. Nevertheless, this correlation is not so obvious when restricted fault-related deformation occurs. Like sub-seismic faults, flexural slip zones are virtually undetectable by sei...

An international research cruise named Iatá-Piuna took place on the São Paulo Plateau on May, 2013 in the Campos and Espírito Santo basins, off Brazil. The cruise was carried ou on board the research vessel (R/V) Yokosuka that hosts the human operated vehicle (HOV) SHINKAI 6500. It aimed at finding chemosynthetic communities, composed of organisms capable of generating their own vital energy by metabolizing organic and inorganic compounds related to seeps. Identification of these organisms could provide information for understanding the origin of life, since they may resemble primitive organisms that existed in the initial stages of life on Earth. During Leg 2 (May 10–24, 2013), however, dives on the northern part of the São Paulo Plateau at the Espírito Santo Basin led to the discovery of a giant oil seep. The seep, ca. 3 nautical miles (ca. 5.6 km) in length is located along an outcrop of Eocene rocks on a salt-induced escarpment of the plateau and at a water depth of ca. 2700 m. The 200 m relief of the seafloor suggests that the seep takes place along an active fault system driven by salt diapirism. The oil was analyzed and identified as a severely biodegraded marine oil, generated by carbonate rocks within a minibasin located to the east of the escarpment. This represents valuable exploratory information because it proves that an active petroleum system is present in the context of minibasins associated with salt diapirism in the area.

Uma exsudação de petróleo de grandes proporções foi identificada em águas ultra profundas da Bacia do Espírito Santo, na costa leste brasileira. A descoberta foi feita durante a missão de pesquisas Iatá-Piuna, realizada através de cooperação técnica entre os governos do Brasil e do Japão, tendo como principal objetivo investigar ecossistemas marinhos de grandes profundidades no Oceano Atlântico Sul, associados ao escape de fluidos. Para tal investigação foi utilizado o navio de pesquisas R/V Yokosuka e o submersível tripulado HOV Shinkai 6500, capaz de realizar mergulhos tripulados em lâminas d’água de até 6500 metros. Durante a segunda pernada da missão (Leg 2), realizada entre os dias 10 e 24/05/2013, foi descoberta uma mega-exsudação de petróleo em lâmina d’água de 2728 m, ao longo de uma escarpa com 200 m de desnível no leito marinho, localizada a cerca de trinta quilômetros a oeste do limite crustal continente-oceano (COB) e a cerca de 170 km da cidade de Vitória. Nessa escarpa...

... de Geologia Sedimentar e Ambiental, Instituto de Geociências da Universidade de São Paulo, Rua do Lago 562, Cidade Universitária, São Paulo – SP, CEP ... Campos (Neto, 1983) and (Campagnoli, 1996) suggest that it consists of northwestward ductile thrust-and-fold nappes. ...