Rômulo Machado

Reavaliação e novos dados Geocronológicos (Ar/Ar, Rb/Sr e Sm/Nd) do Batólito Pelotas no Rio Grande do Sul: implicações Petrogenéticas e Idade de Reativação das Zonas de Cisalhamento

O Batolito Pelotas e constituido por suites graniticas com idades U/Pb (em zircao, Evaporacao e SHRIMP) e Pb/Pb (em zircao, por evaporacao) situadas no intervalo de 575 a 633 Ma, com um padrao de idade mais antigo entre 625 e 635 Ma (Suite Pinheiro Machado) e outro mais novo entre 575 e 600 Ma (Suites Viamao, Encruzilhada do Sul e Dom Feliciano). As razoes isotopicas Sr87/Sr86 situadas entre 0,7060 a 0,7016, juntamente com valores de µNd fortemente negativos (em geral, entre -5 a -10) e idades modelos T DM (entre 1600 e 2200 Ma), indicam a predominância de processos de reciclagem de materiais de uma crosta provavelmente Paleoproterozoica (Ciclo Transamazonico). Por outro lado, a presenca de magmatismo basico e feicoes de misturas de magmas nas Suites Pinheiro Machado, Viamao, Encruzilhada e Dom Feliciano associados com magmatismo alcalino (Sienito Piquiri), com idade Pb/Pb (em zircao, por evaporacao) entre 610 e 615 Ma, sugerem tambem contribuicao mantelica na constituicao do batolito. As idades Ar/Ar aqui obtidas a partir de rochas miloniticas de zonas de cisalhamento (de baixo e alto ângulo) que afetam o batolito sugerem a existencia de um importante evento tectonico ocorrido entre 540 e 530 Ma. Este evento teria sido responsavel pela reativacao em regime transpressivo de zonas de cisalhamento mais antigas, relacionadas com a colocacao das suites graniticas do batolito, e geracao de estruturas-em-flor positiva em varios dominios do mesmo. Esta tectonica parece se articular no espaco e no tempo com a tectonica extensional relacionada com a instalacao da Bacia do Camaqua.

Kinematics and geometry of structures in the southern limb of the Paraíba do Sul divergent structural fan, SE Brazil: a true transtensional shear

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, the partially molten material could easily accommodate the imposed strain rates, giving rise firstly to the SW-directed shearing. As the thermal disturbance tended to vanish and the convergence increased, the NNE-directed transtensional shearing developed. We propose that the transtensional deformation characterized in this paper could be related to extrusion processes during regional transpressional strain.

Correlation of lineaments (magnetic and topographic) and Phanerozoic brittle structures with Precambrian shear zones from the basement of the Paraná Basin, Santa Catarina State, Brazil

The objective of this article is to characterize the main directions of structures of the Parana Basin, to define the relation of deep structures (magnetic lineaments) with superficial structures [lineaments obtained over Shuttle Radar Topography Mission (SRTM) images] and analyse reactivated structures in the Phanerozoic. The magnetic lineaments at the Eastern edge of the Parana Basin, in the central-southern region of the Santa Catarina State, show a main orientation pattern NE-SW and secondarily NNE-SSW. The NE-SW pattern reflects a framework of the ductile structures of the Santa Catarina Shield that continues in the substratum of the basin. The structural NNE pattern appears in the south of the Major Gercino Shear Zone (MGSZ) and the NE pattern appears in the north of this structure. The data obtained shows that the structural NE-SW pattern of the basin, present in the MGSZ and Itajai-Perimbo Shear Zone as well as in the Tijucas Terrain (Brusque Metamorphic Complex) in Santa Catarina, clearly extends underneath the Gondwanic sequence of the Parana Basin in the region. Among the products obtained by geophysical data processing in this work, what propitiated the best definition and distinction of the magnetic structures was the tilt angle of the total horizontal gradient (TAHG) method. However, the great flight line spacing of one of the geophysical projects (7 km) makes the identification of small magnetic structures difficult, particularly in N-S and E-W directions. Such structural directions are referred to in the previous works and were confirmed through the interpretation of SRTM images and studies of brittle structures in the field.

40Ar/39Ar ages (600-570 Ma) of the Serra do Azeite transtensional shear zone: evidence for syncontractional extension in the Cajati area, southern Ribeira belt

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 Sao Paulo and Parana, 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 Aracuai 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 Quadrilatero Ferrifero 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 that the regional tectonic framework was developed during continental-scale extension. This process has been coeval with convergent strain in the adjacent Neoproterozoic shear zones of the Apiai/Ribeira and Aracuai belts, which make up significant segments of these belts. The available data show that these structures may not be simply related to post-orogenic gravitational collapse, but must involve a more complex process probably related to dynamic balance between crustal thickening and thinning during tectonic convergence, basin formation and exhumation processes.

The inventory of geological heritage of the state of São Paulo, Brazil: Methodological basis, results and perspectives

An inventory of geological sites based on solid and clear criteria is a first step for any geoconservation strategy. This paper describes the method used in the geoheritage inventory of the State of São Paulo, Brazil, and presents its main results. This inventory developed by the geoscientific community aimed to identify geosites with scientific value in the whole state, using a systematic approach. All 142 geosites representative of 11 geological frameworks were characterised and quantitatively evaluated according to their scientific value and risk of degradation, in order to establish priorities for their future management. An online database of the inventory is under construction, which will be available to be easily consulted and updated by the geoscientific community. All data were made available to the State Geological Institute as the backbone for the implementation of a future state geoconservation strategy.

A study of structural lineaments in Pantanal (Brazil) using remote sensing data

This paper presents a study of the structural lineaments of the Pantanal extracted visually from satellite images (CBERS-2B satellite, Wide Field Imager sensor, a free image available in INTERNET) and a comparison with the structural lineaments of Precambrian and Paleozoic rocks surrounding the Cenozoic Pantanal Basin. Using a free software for satellite image analysis, the photointerpretation showed that the NS, NE and NW directions observed on the Pantanal satellite images are the same recorded in the older rocks surrounding the basin, suggesting reactivation of these basement structural directions during the Quaternary. So the Pantanal Basin has an active tectonics and its evolution seems to be linked to changes that occurred during the Andean subduction.

A comparison for a multiscale study of structural lineaments in southern Brazil: LANDSAT-7 ETM+ and shaded relief images from SRTM3-DEM

This paper presents a comparison of descriptive statistics obtained for brittle structural lineaments extracted manually from LANDSAT images and shaded relief images from SRTM 3 DEM at 1:100,000 and 1:500,000 scales. The selected area is located in the southern of Brazil and comprises Precambrian rocks and stratigraphic units of the Parana Basin. The application of this methodology shows that the visual interpretation depends on the kind of remote sensing image. The resulting descriptive statistics obtained for lineaments extracted from the images do not follow the same pattern according to the scale adopted. The main direction obtained for Proterozoic rocks using both image types at a 1:500,000 scale are close to NS±10, whereas at a 1:100,000 scale N45E was obtained for shaded relief images from SRTM 3 DEM and N10W for LANDSAT images. The Paleozoic sediments yielded the best results for the different images and scales (N50W). On the other hand, the Mesozoic igneous rocks showed greatest differences, the shaded relief images from SRTM 3 DEM images highlighting NE structures and the LANDSAT images highlighting NW structures. The accumulated frequency demonstrated high similarity between products for each image type no matter the scale, indicating that they can be used in multiscale studies. Conversely, major differences were found when comparing data obtained using shaded relief images from SRTM 3 DEM and Landsat images at a 1:100,000 scale.

Análise de lineamentos estruturais no Domo de Lages (SC) com uso de imagens de satélite e mapas de relevo sombreado

This work focuses on the study of structural lineaments in the Domo de Lages region, central-south area of Santa Catarina State, and their relationship with the different stratigraphic units of the Parana Basin that are present in this area. The lineaments were interpreted from Landsat and shaded relief images, using geoprocessing techniques. The results show structural lineaments with NW, NNE, NE, ENE and E-W trends. The larger lineaments (4 - 50 km) were interpreted using shaded relief maps, whereas the smaller lineaments were interpreted using satellite images (1 - 3 km). According to field data, these lineaments affect the volcanic rocks of Serra Geral Formation (Parana Basin) and alkaline rocks associated with the dome. In all these structural trends, evidences of transcurrent faults were found. The formation of transcurrent faults with NNE trend has been related to compressive NE-SW stress. Late Cretaceous/Paleogene is the maximum age considered for the development of these structural trends associated with transcurrent faults. The formation of this structural pattern has been linked to the reactivation of older structures present in the basin and its basement.

Transpressive dextral shear in the Italva-Itaperuna section, Northern State of Rio de Janeiro, Brazil

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 extension zones. These elements are consistent with oblique continental collision considering the São Francisco Craton as a stable block.

Pluton emplacement in a releasing bend in a transpressive regime: the arrozal granite in the Paraíba do Sul shear belt, Rio de Janeiro

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.