Reactivation history of the Carajas and Cinzento strike-slip systems, Amazon, Brazil.

The Carajás-Cinzento fault system is centred within the Itacaiunas Shear Zone, the northern tectonic margin of the Archaean Sul do Pará Granite Greenstone Terrain of the Amazonian Craton, Brazil. The regional tectonostratigraphy can be subdivided based on the geological relationship of units to the main phase of ductile movements along the Itacaiúnas Shear Zone. A Basement Assemblage includes an older group of orthogneisses, migmatites, and granitic to dioritic plutons (Xingu Complex) and a later volcano-sedimentary sequence of ironstones, quartzites. amphibolites and schists (Igarapé Salobo Group). Intense ductile shearing and high grade metamorphism along the Itacaiúnas Shear Zone has led to widespread tectonic interleaving of the gneisses and volcano-sedimentary rocks and has obliterated all traces of the original unconformity between these units. A Cover Assemblage is represented by very low-grade volcanic and sedimentary rocks that are inferred to rest unconformably on the Basement Assemblage rocks deformed within the Itacaiúnas Shear Zone. Older clastic, volcanic and ironstone sequences (Igarapé Pojuca & Grao Pará Groups; ca.2.7 Ga) are overlain by a sequence of shallow-water marine to fluvial clastic deposits (Águas Claras Formation). Both Cover and Basement assemblages are intruded by ca.1.8 Ga A-type granitic plutons and basic dykes. All units are unconformably overlain by a thin, localised sequence of polymictic conglomerates (? Gorotire Formation).During the formation of the upper amphibolite facies within the Itacaiúnas Shear Zone, sub-vertical mylonitic fabrics, generally E-W-trending, were variably developed in the Basement Assemblage rocks. They preserve widespread sinistrally transpressional kinematic indicators. Radiometric dating suggests that the metamorphism and deformation in the shear zones occurred toward the end of the Archaean (ca.2.8 Ga). At least three cycles of brittle-ductile strike-slip reactivation at low metamorphic grades appear to post-date the development of the Itacaiúnas Shear Zone, leading to the formation of the Carajás and Cinzento faults. There is no stratigraphic, structural or sedimentological evidence to suggest that these faults were active during the deposition of the Cover Assemblage sequences. However, as most outcrops of Cover Assemblage rocks are presently localised within bends, branches and offsets of these fault systems it appears that, following their deposition, they were faulted down into dilational jogs formed during an initial phase of brittle dextral movements. The effects of a later episode of brittle-ductile sinistral transpression are widely preserved in both Basement and Cover assemblages, with intense deformation localised in the region of the major fault strands. The 1.8 Ga granites and dykes appear to relate to a regional extensional or dextral transtensional episode recognised in the Middle Proterozoic throughout the Amazon region. There is some circumstantial evidence for further minor fault reactivation during the Phanerozoic, and the region appears to be tectonically active in the present day, as illustrated by the occurrence of recent small-scale earthquakes and hot springs centred along the major fault traces. The influence of the basement architecture and the intensity of later reactivations appears to wane after a time of ca.1.0 Ga following the existence of a weakening effect on a lithospheric-scale with a finite life span, possibly originating in the underlying lower crust and mantle. There is widespread evidence that the mylonitic fabrics of the Itacaiúnas Shear Zone have controlled the orientation of later structures, including the Carajás and Cinzento fault systems. Long-term fault zone weakening mechanisms are recognised in the region. Brittle fracturing processes have caused increases in fault zone permeabilties allowing extensive ingress of fluids, some of which have caused e.g. gold and copper mineralisation

Fusão de imagens altimétricas e aeromagnetométricas como ferramenta de interpretação geológica, exemplo da Província Mineral de Carajás (PA)

A Província Mineral de Carajás localiza-se na parte sudeste do Cráton Amazônico, estado do Pará. É composta por dois domínios tectônicos principais, representados pelos sistemas transcorrentes Carajás e Cinzento. Parte significativa das ocorrências minerais da referida província está vinculada geologicamente a estruturas que compõem esses dois sistemas de falhas. Este trabalho apresenta os resultados obtidos da interpretação realizada a partir da fusão de dados magnetométricos e altimétricos, como ferramenta auxiliar na compreensão do quadro estrutural relacionado à Falha Carajás. Foram utilizadas as imagens orbitais do modelo digital de elevação, gerado a partir dos dados altimétricos orbitais da Shuttle Radar Topographic Mission (SRTM), e dos dados aeromagnetométricos do Projeto Geofísico Brasil-Canadá (PGBC). Para atingir os objetivos propostos foi desenvolvido um método de processamento e fusão dos dados provenientes dos dois levantamentos, geofísico e altimétrico, a partir de técnicas de processamento geofísico e de imagens digitais de sensoriamento remoto. Os resultados obtidos permitiram estabelecer a relação das feições interpretadas a partir da imagem de fusão com os principais domínios geomorfológicos e magnéticos reconhecidos na região. Estes resultados demonstram o potencial de aplicação desses métodos na análise de outras áreas com características metalogenéticas similares às da Amazônia, como forma de facilitar e orientar programas regionais de exploração mineral.The Carajás Mineral Province is located in the eastern portion of the Amazonian Craton, in the Pará state, north of Brazil. It comprises two main tectonic domains: Carajás and Cinzento strike-slip systems. A significant proportion of the mineral deposits and occurrences of this Province is geologically related to structures belonging to these two fault systems. This paper presents the results obtained by the interpretation of merged altimetric and magnetometric data, as an auxiliary tool for understanding the structural framework of the Carajás Fault. Satellite digital elevation data acquired by the Shuttle Radar Topographic Mission (SRTM) were used in combination with aeromagnetometric data acquired by the Brazil-Canada Geophysical Project (PGBC). A method was developed for digital processing and merging the data acquired by these two surveys, based on geophysical and remote sensing processing methods. The results obtained allowed to establish the relationship between features interpreted from the merged image, with the main geomorphologic and magnetic domains known in the region. These results demonstrate the potential of applying these methods for assessing areas with similar metallogenetic characteristics located elsewhere in Amazon, thus facilitating and guiding regional exploration programs

Deformation of paleoproterozoic siliciclastic rocks of the Araí Group as an example of basement fault reactivations, Serra do Tepequém, Roraima, Northern Brazil

Serra do Tepequém region comprises paleoproterozoic siliciclastic rocks from Araí and Suapi Groups, which are part of Roraima Super-group. Field data indicate that bedding is dipping towards SE and NW in different domains limited by NE-SW sinistral oblique faults with both normal and also reverse displacements. The structural setting is formed by regional scale forced folds represented by kilometre scale kink bands and chevron folds. The proposed framework is compatible with upper-to-middle crustal level. The studied faults were controlled by reactivation of early ductile basement fabric. These findings differ from previous regional models, based on folding under ductile conditions related to colisional tectonics. Results evidence the importance of Guiana Shield early basement structures in controlling the geometry of the brittle structures, which were seen on the cover rocks of Serra do Tepequém.As rochas siliciclásticas da Serra do Tepequém são correlacionadas aos Grupos Araí e Suapi, pertencentes ao Supergrupo Roraima de idade paleoproterozoica. A análise estrutural destas rochas revelou que o acamamento exibe arranjos com mergulhos preferencialmente para SE e NW, individualizados em domínios limitados por zonas de falhas oblíquas sinistrais com rejeitos normais e inversos, com direção NE-SW. Essa estruturação é formada por dobras forçadas quilométricas do tipo kink bands e chevrons. O novo arcabouço geométrico observado é característico de um ambiente de deformação de nível crustal raso a médio. A história tectônica é controlada por reativações dos planos de fraqueza preexistentes nas tramas antigas, dúcteis, do embasamento. Esse modelo diverge dos regionais prévios para a região, os quais consideram as dobras existentes como produtos de ambiente dúctil sob tectônica colisional. Os resultados evidenciam a importância da presença de estruturas antigas do embasamento, relacionado ao Escudo das Guianas, como controladoras da geometria das rochas da Serra do Tepequém, em ambiente rúptil