Opening Of South Atlantic Ocean Research Articles

Volcanoclastic and epiclastic diagenesis of sandstones associated with volcano-sedimentary deposits from the upper Jurassic, Lower cretaceous, Paraná Basin, southern Brazil

The opening of South Atlantic Ocean produced an immense volume of lava that covered an active aeolian system during Lower Cretaceous. The contact of Botucatu Formation sandstones with the Serra Geral volcanic flows generated a variety of volcano-sedimentary features and deposits. During volcanism, the temperature contrast between lava and wet or water-saturated sediments formed volcanoclastic features and related sedimentary deposits. In moments of magmatic quiescence, epiclastic deposits were generated due to the return of sedimentation. Volcanoclastic deposits include volcanic breccias with sandy matrix, and epiclastic deposits include conglomerates and conglomeratic sandstones with intraclasts. Quantitative petrography of the sandy matrix samples of these deposits was analyzed, in order to discuss the paragenesis of volcano-sedimentary interactions. Regular burial diagenetic processes were discussed for epiclastic sandstones and contact diagenesis processes for volcanoclastic sandstones. The original composition of volcanoclastic and epiclastic sandstones is heterogeneous. However, the main difference between them is the presence of textural fluidization indicators found in the volcanoclastic sandstones, along the lava-sediment interface. These textural characteristics indicate that the unconsolidated sediments were probably wet or saturated in water during the interaction with the volcanic flow. Epiclastic sandstones have infiltrated clays and neoformed smectites as their main diagenetic constituent due to alteration of available volcanic lithoclasts. For volcanoclastic sandstones, the formation of opal, chalcedony and megaquartz cements through burial is unlikely, due to the intergranular volume occupied by these cements, indicating near-surface early diagenetic conditions. Therefore, the mechanisms of dissolution and repreciptation of the siliceous fluids, incorporated in the system, formed mainly the chalcedony cementation. Furthermore, zeolite cementation, which occurs along the lava-sediment contact, is another important indicator of contact diagenesis for volcanoclastic sandstones. During telodiagenesis, the formation of secondary porosity by dissolution of volcanic lithoclasts and detrital K-feldspar grains was of great importance in volcanoclastic and epiclastic sandstones.

Origin and evolution of a CO2-Rich gas reservoir offshore Angola: Insights from the Gas Composition and isotope analysis

The pre-salt sections of the offshore Kwanza Basin, Angola are potentially petroliferous reservoirs like their Brazilian counterparts on the other side of the South Atlantic Ocean. In this study, a pre-salt reservoir of the offshore Kwanza Basin was found to contain solid bitumen and gas of high portions of CO2 (ca. 83%) and methane (ca. 16%). The chemical and isotopic (C, H and He) compositions of the gas indicate that the CO2 is primarily mantle-derived while the gaseous hydrocarbons are mainly late mature thermogenic gas. The Barremian-Aptian Red Cuvo Formation with sapropelic and humic organic matter and an average TOC content of 1.7% could be the principal source of the bitumen and gaseous hydrocarbons in the reservoir. Rapid oil generation since the source rock sedimentation could have occurred as a result of the fast and thick salt deposition or the high thermal gradient created by the crustal thinning and magmatism during the South Atlantic Ocean opening. The δ13C of gaseous hydrocarbons also indicates their mixed origins of sapropelic and humic organic matters and possible abiotic processes.This reservoir is similar in the origin of CO2 and geological background to some pre-salt reservoirs in the Brazilian basins. Such similarities with prolific petroleum systems may imply great petroleum potential in the pre-salt sections of the offshore Kwanza Basin, Angola.

Tectonic and stratigraphic evolution of the Santos Basin rift phase: New insights from seismic interpretation on Tupi oil field area

Most recent published work aims to understand pre-salt carbonates interval development and depositional environment conditions. Previous contributions which debate about rift extension usually consist of regional analysis regarding the South Atlantic Ocean opening and keep the debate about syn-rift and sag stage limits. This work uses 3D seismic volume analysis, structural contour maps and isopach maps analysis in order to understand and establish rift extension development in Santos Basin pre-salt interval. Our analysis concludes that the pre-salt interval is inserted into the syn-rift context, divided into 3 phases: Lower Rift, Upper Rift and Later Rift. Fault analysis shows that the 3 main fault directions act as a structural control for the rift phases. Lower and Upper Rifts are controlled by SW-NE and N–S trend faults, while Later Rift is controlled by SE-NW trend faults. Seismic interpretation and well log analysis indicates that, at Santos Basin, Lower Rift includes Camboriú and Piçarras Formation, Upper Rift comprehend Itapema and Lower Barra Velha Formation, while Later Rift contains Upper Barra Velha Formation, as sag-type basin.

Exhumation of the passive margin of the DR Congo during pre- and post- Gondwana breakup: Evidence from low-temperature thermochronology, geology and geomorphology

The Lower Congo region encompasses the South Atlantic passive margin of the Democratic Republic of Congo (Central Africa). It is host to the lowermost course of the Congo River, cutting through the Central African Atlantic Swell (CAAS). The region is underlain by litho-structural units of the Pan-African West Congo Belt, which consists of different tectono-metamorphic domains. The Precambrian basement is covered to the west by marine deposits of the South Atlantic Ocean and to the east by continental deposits of the Congo Basin. In this study we aim to constrain the timing of uplift and exhumation of the CAAS by using apatite fission track (AFT) thermochronology in combination with an updated overview of the geology and geomorphology of the Lower Congo region. AFT ages vary widely between 108 and 312 Ma. Short track lengths (11–12 μm) and broad, complex track length distributions indicate mixed ages between multiple thermal events. We derive the timing of exhumation from inverse thermal history models, that show that the Lower Congo region experienced a first exhumation event before Gondwana breakup during the Carboniferous to Middle Jurassic. This event is probably related to compressive forces at the boundaries of Gondwana. Both rifting and subsequent opening of the South Atlantic Ocean do not seem to have had a pronounced effect on the CAAS region. During the Late Cretaceous to Palaeogene, a slight reheating is suggested and could be due to subsidence and consequential modest reburial of the Lower Congo rocks. A second phase of exhumation initiated around the Palaeogene–Neogene and eventually emplaced the sampled rocks at surface temperatures. The multi-phased differential denudation results from reactivation of fault-bounded tectono-metamorphic blocks of the Precambrian basement, controlled by the combination of two systems of faults related to the Cretaceous South Atlantic Ocean opening and Pan-African orogeny respectively. Differential denudation of the Lower Congo region is also well-marked by independent qualitative indicators of the present-day geomorphology including distinct knickpoints and steep valleys along the course of the Lower Congo River, reworking of erosion surfaces and associated laterite crust and contrasting karst morphology.

An exotic Cretaceous kimberlite linked to metasomatized lithospheric mantle beneath the southwestern margin of the São Francisco Craton, Brazil

We present major and trace element compositions of mineral concentrates comprising garnet xenocrysts, ilmenite, phlogopite, spinel, zircon, and uncommon minerals (titanite, calzirtite, anatase, baddeleyite and pyrochlore) of a newly discovered Late Cretaceous kimberlite (U-Pb zircon age 90.0 ± 1.3 Ma; 2σ) named Osvaldo França 1, located in the Alto Paranaíba Igneous Province (APIP), southeastern Brazil. Pyrope grains are lherzolitic (Lherz-1, Lherz-2 and Lherz-3), harzburgitic (Harz-3) and wehrlitic (Wehr-2). The pyrope xenocrysts cover a wide mantle column in the subcratonic lithosphere (66–143 km; 20–43 kbar) at relatively low temperatures (811–875 °C). The shallowest part of this mantle is represented by Lherz-1 pyropes (20–32 kbar), which have low-Cr (Cr2O3 = 1.74–6.89 wt.%) and fractionated middle to heavy rare earth elements (MREE-HREE) pattern. The deepest samples are represented by Lherz-2, Lherz-3, Harz-3, and Wehr-2 pyropes (36–43 kbar). They contain high-Cr contents (Cr2O3 = 7.36–11.19 wt.%) and are characterized by sinusoidal (Lherz-2 and Wehr-2) and spoon-like (Lherz-3 and Harz-3) REE patterns. According to their REE and trace elements, pyrope xenocrysts have enriched nature (e.g., Ce and Yb vs. Cr2O3), indicating that the cratonic lithosphere has been affected by a silicate melt with subalkaline/tholeiite composition due to their low Zr, Ti and Y concentrations. Besides minerals with typical kimberlitic signatures, such as ilmenite and zircon, the exotic compositions of phlogopite and ulvöspinel suggest an enriched component in the magma source. The formation of rare mineral phases with strong enrichment of light-REE (LREE) and high field strength elements (HFSE) is attributed to the late-stage kimberlitic melt. We propose a tectonic model where a thermal anomaly, represented by the low-velocity seismic anomaly observed in P-wave seismic tomography images, supplied heat to activate the alkaline magmatism from a metasomatized cratonic mantle source during the late-stages of Gondwana fragmentation and consequent South Atlantic Ocean opening. The metasomatism recorded by mineral phases is a product of long-lived recycling of subducted oceanic plates since the Neoproterozoic (Brasiliano Orogeny) or even older collisional events, contributing to the exotic character of the Osvaldo França 1 kimberlite, as well as to the cratonic lithospheric mantle.

Geophysical-structural framework of the Campo Alegre basin (Santa Catarina State, southern Brazil)

Numerous syn- to post-collisional basins originated as a response to crust stabilization processes after the collisions between Apiaí, Curitiba and Luis Alves Cratons during the Neoproterozoic in Brazil. The late Neoproterozoic Campo Alegre Basin represents one of these so called transitional-stage basin in the southern Brazil. The structural record of this basin is an important clue for understanding the regional geological development after the Western Gondwana supercontinent agglutination. An integrated study of airborne geophysical data, ground gravimetry and structural analysis of the basin and its basement outlined its magnetic-structural framework and its tectonic evolution. Thus, it was possible to present a basin 3-event tectonic evolution history. The first event (E1) is marked by WNW and ENE normal faults as a response to conjugated extension stress fields (σ3 and σ2). The normal faults generated in (E1) conditioned the deposition of the coarse-grained basal deposits close to basin borders. A second event (E2) was recognized through EW normal faults and ENE-WSW right-lateral strike-slip faults on basin intermediate formations. Hydrothermalism from volcanism final fluids is associated with these faults, especially on the south portion. The last tectonic event (E3) that affected basin rocks is characterized by the reactivation of E1 normal faults as inverse and occurred in Mesozoic Era associated with South Atlantic Ocean opening. This event is also marked by NNW e EW strike-slips faults. Besides the gravimetric model and the magnetic interpretation, field data allow us to conclude that the nucleation of Campo Alegre Basin began as a conjugated rift system.

Emplacement of dike swarms from the island of Ilhabela (SE Brazil) and its relationship with the South Atlantic Ocean opening revealed by magnetic fabrics

Magnetic fabric and rock-magnetism studies were performed on 153 dikes from diabase, lamprophyre and alkaline dike swarms that outcrop on the island of Ilhabela, NE São Paulo State. The dikes crosscut Archean and Proterozoic polymetamorphic rocks. Their thicknesses range from a few centimeters to 4 or 5 m. They trend predominantly N30-60E with a very steep (~60°) to vertical dip. Magnetic fabrics were determined using anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Rock-magnetism measurements reveal that the magnetic mineral of the three swarms is magnetite mainly with grains in the range of 2–5 μm. For a few dikes, these grains are the carriers of bulk magnetic susceptibility but are not responsible for the AMS, which is due to Fe-bearing minerals, as shown by AARM. The main AMS fabric recognized in the swarms is due to magma flow, in which the Kmax–Kint plane is parallel to the dike's plane, and the magnetic foliation pole (Kmin) is perpendicular to it. The analysis of the Kmax inclination showed that the dikes were fed by horizontal to vertical flows. However, for the minority of the dikes, the AMS and AARM tensors are not coaxial. The AARMmax is oriented N30–60 W, approximately perpendicular to the Kmax. The AARMmax orientation is similar to the direction of a fault system located mainly in the Santos marginal basin which, was formed in the Cretaceous rifting during the South Atlantic opening. The AARM fabric is tectonic in origin, and is parallel with the extensional paleostress responsible for the Gondwana break-up. The comparison of AMS and AARM fabrics suggests that diabase and lamprophyre dikes were emplaced in three distinct events in the earliest stages of the South Atlantic opening. Later, the alkaline dikes were emplaced in the final stages of the Atlantic opening.

Shallow necking depth and differential denudation linked to post‐rift continental reactivation: The origin of the Cenozoic basins in southeastern Brazil

AbstractThe southeastern Brazilian margin presents post‐breakup Cenozoic tectonism that created a series of grabens and small sedimentary basins, known as the Continental Rift of Southeastern Brazil. The formation of this rift occurred long after the South Atlantic ocean opening and has been attributed to different mechanisms like regional uplift induced by hotspot activity, pulses of Andean orogeny and reactivation of pre‐existing faults. However, the proposed models lack an analytical or numerical verification from a geodynamic point of view. Based on finite element modelling of the lithospheric stress field evolution we conclude that a shallow necking depth, consistent with the hyperextended southeastern Brazilian margin, combined with differential denudation of the continent, resulted in an extensional stress field in the upper crust that induced the observed Cenozoic tectonism.

Geochemical characterization of black shales from the Río Mayer Formation (Early Cretaceous), Austral-Magallanes Basin, Argentina: Provenance response during Gondwana break-up

During the Early Cretaceous important changes occurred in South-western Gondwana related to its fragmentation, and South Atlantic Ocean opening. Then, two basins developed in southernmost Patagonia: the Austral-Magallanes Basin and the Rocas Verdes-Magallanes Basin. The Río Mayer Formation corresponds to the initial sedimentary stages of the Austral Basin including black shales, marls and fine-grained sandstones deposited in an outer shelf environment. We studied and sampled three detailed sedimentary logs at Río Guanaco Locality (S 49° 57′ 11"; W 72° 04′ 56″), dividing this unit into three informal sections (lower, middle and upper). Whole rock geochemical analyses were carried out over the three sections with the focus on trace elements, in particular, rare earth elements (REE). While, the lower section of the unit shows an affinity to acid volcanic source areas, the middle and upper sections show an affinity to intermediate source areas. These changes through the Río Mayer Formation can be correlated with the initial magmatic activity hosted in the Southern Patagonian Batholith during the Neocomian. Considering the REE patterns, the lower section of the Río Mayer Formation shows an affinity to the felsic compositions of the Tobífera Formation, while the upper section would be related to the erosion of the Punta Barrosa/Cerro Toro formations of the Rocas Verdes-Magallanes Basin. The clear distinction between the REE patterns of the Río Mayer and Zapata formations is considered evidence of the compartmentalization of these basins during the Early Cretaceous. Finally, the absence of correlation between the geochemistry of the studied unit and the Jurassic volcanic rocks of the Deseado Massif indicates that the north-eastern sector of the basin was not a source area for the Austral Basin, at least during the deposition of the Río Mayer Formation in the study area.

Testing Early Cretaceous Africa–South America fits with new palaeomagnetic data from the Etendeka Magmatic Province (Namibia)

We provide an independent test for the proposed pre-breakup fits between the South American and African continental margins based on new palaeomagnetic data. In the course of modelling the opening of the South Atlantic Ocean, several reconstructions have been proposed, which correspond to different estimates of the amount of pre-drift extension in the South American and African continental margins, and amounts of displacement along intracontinental accommodation zones. The Paraná–Etendeka Large Igneous Province erupted during a brief time interval between ~136 and ~131Ma, just prior to opening of the South Atlantic, and has magmatic products on both the African (Etendeka lavas) and South American (Paraná lavas) plates. Here we provide new palaeomagnetic constraints on the Etendeka volcanic rocks. These yield a declination of D±∆Dx=314.4±7.0° and inclination of I±∆Ix=−42.7±8.3° (Plat=47.5, PLon=88.9, K=26.1, A95=6.3, n=21). When our results are combined with another recently published dataset, we obtain an Etendeka palaeopole at 49.1° S, 87.6° E (D±∆Dx=315.9±3.8°, I±∆Ix=−43.9±4.4°, K=23.4, A95=3.5, n=75). We compare three recent models for South Atlantic Ocean opening, discuss which of the models best fits the palaeomagnetic data, and present a revised reconstruction for West Gondwana during the Early Cretaceous.

Magnetic fabrics of the Cretaceous dike swarms from São Paulo coastline (SE Brazil): Its relationship with South Atlantic Ocean opening

Magnetic fabric and rock magnetism studies were performed on 91 dikes from Cretaceous diabase and lamprophyre dike swarms that outcrop side by side on the beaches of NE São Paulo State coastline. The dikes crosscut Archean and Proterozoic poly-metamorphic rocks of the Costeiro Complex. Their thicknesses range from a few centimeters to 2m for lamprophyre and up to about 10m for the diabase. They trend predominantly N30°–60°E with vertical dip. Magnetic fabrics were determined using anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Rock-magnetism measurements reveal that magnetite grains in the range of 2–5μm are the magnetic mineral of both swarms. For most dikes, these grains are the carriers of bulk magnetic susceptibility but, surprisingly, are not responsible for the AMS which is carried by Fe-bearing minerals as shown by AARM. The main AMS fabric recognized in the swarms is due to magma flow, in which the Kmax-Kint plane is parallel to the dike's plane, and the magnetic foliation pole (Kmin) is perpendicular to it. The analysis of the Kmax inclination showed that the dikes were fed by horizontal to vertical flows. However, for the majority of the dikes the AMS and AARM tensors are not coaxial. The AARM lineation (AARMmax) is oriented N30–60W, approximately perpendicular to AMS lineation (Kmax) suggesting that magnetite grains were rotated approximately 90° anticlockwise from the dike plane. The AARMmax orientation is similar to the direction of a fault system mainly in the Santos marginal basin which was formed in the Cretaceous rifting during the South Atlantic opening. Therefore the AARM fabric is tectonic in origin, and the comparison of AMS and AARM fabrics suggests that lamprophyre and diabase dikes were emplaced in three distinct events in the earliest stages of the South Atlantic opening.

Geochronology (Ar/Ar and K–Ar) of the South Atlantic post-break-up magmatism

Abstract This work integrates the available geological information and geochronology data for the Cretaceous–Recent magmatism in the South Atlantic, represented by onshore and offshore magmatic events, including the oceanic islands along the transform faults and near the mid-ocean ridge. The analysis of the igneous rocks and their tectonic settings allows new insights into the evolution of the African and Brazilian continental margins during the South Atlantic opening. Following the abundant volcanism in the Early Cretaceous, the magmatic quiescence during the Aptian–Albian times is a common characteristic of almost all Brazilian and West African marginal basins. However, rocks ascribed to the Cabo Granite (104 Ma) are observed in NE Brazil. In West Africa, sparse Aptian–Albian ages are observed in a few coastal igneous centres. In the SE Brazilian margin, an east–west alkaline magmatic trend is observed from Poços de Caldas to Cabo Frio, comprising igneous intrusions dated from 87 to 64 Ma. Mafic dyke swarms trending NW also occur in the region extending from the Cabo Frio Province towards the Central Brazilian Craton. On the West African side, Early Cretaceous–Recent volcanism is observed in the Walvis Ridge (139 Ma), the St Helena Ridge (81 Ma) and the Cameroon Volcanic Line (Early Tertiary–Recent). Volcanic islands such as Ascencion (1.0–0.65 Ma), Tristão da Cunha (2.5–0.13 Ma) and the St Helena islands (12 Ma) most probably correspond to mantle plumes or hot spots presently located near the mid-Atlantic spreading centre. Within the South America platform and deep oceanic regions, the following volcanic islands are observed: the Rio Grande Rise (88–86 Ma), Abrolhos (54–44 Ma), the Vitória–Trindade Chain (no age), Trindade (2.8–1.2 Ma) and Fernando de Noronha (12–1.5 Ma). There are several volcanic features along the NW–SE-trending Cruzeiro do Sul Lineament from Cabo Frio to the Rio Grande Rise, but they have not been dated. The only known occurrence of serpentinized mantle rocks in the South Atlantic margin is associated with the Saint Peter and Saint Paul Rocks located along the São Paulo Fracture Zone. The Cameroon Volcanic Line in NW Africa is related to the magmatism that started in the Late Cretaceous and shows local manifestations up to the Present. The compilation of all available magmatic ages suggests an asymmetrical evolution between the African and South America platforms with more pre-break-up and post-break-up magmatism observed in the Brazilian margin. This is most likely to have resulted from the different geological processes operating during the South Atlantic Ocean opening, shifts in the spreading centre, and, possibly, the rising and waning of mantle plumes. Supplementary material: A complete table with radiometric dates that have been obtained by universities, government agencies and research groups is available at: www.geolsoc.org.uk/SUP18596

Integração de métodos potenciais (gravimetria e magnetometria) na caracterização do embasamento da região sudoeste da bacia de pelotas, Sul do Brasil

Por meio da aplicacao integrada dos metodos potenciais de gravimetria e magnetometria, visou-se contribuir com a caracterizacao do arcabouco estrutural e composicional do embasamento da Bacia de Pelotas. A area selecionada para este estudo e a regiao sul da Provincia Costeira do Rio Grande do Sul, entre as latitudes 31o45' e 34oS e as longitudes 52o e 53o30'W. A interpretacao integrada dos mapas gerados atraves dos metodos potenciais, em um Sistema de Informacoes Geograficas (SIG), permitiu verificar uma complexa estruturacao e a heterogeneidade composicional do embasamento. Essa estruturacao esta relacionada com a evolucao do Escudo Uruguaio-Sul-Rio-Grandense e com a abertura do Oceano Atlântico Sul. Uma anomalia gravimetrica negativa, na regiao do Banhado do Taim, foi interpretada como uma calha no embasamento. Nessa regiao, trabalhos anteriores identificaram uma segmentacao nos sistemas deposicionais pleistocenicos da Planicie Costeira do Rio Grande do Sul. A presenca desta calha na mesma posicao da segmentacao identificada indica que o registro estratigrafico da regiao foi condicionado pelo embasamento, sugerindo a existencia de um controle da heranca geologica na area. Esse controle e possivelmente relacionado com a reativacao de falhas e com a subsidencia diferencial devido a heterogeneidade das litologias presentes no embasamento. Estudos futuros com a aquisicao de novos dados em regioes onde nao foramrealizadas medidas (lagoas e banhados) e a integracao com outros metodos possibilitarao melhorar e testar as interpretacoes realizadas.

Padrão de fraturas da região da Serra dos Órgãos (RJ) e sua relação com a tectônica mesozóico-cenozóica do sudeste do Brasil

The Serra dos Órgãos range, between Petrópolis and Teresópolis cities, Rio de Janeiro State, is one of the most remarkable physiographic features of southeastern Brazilian coast. This feature, part of the Serra do Mar coastal range, with summits over 2.200 m.a.s.l, follow Precambrian basement structures constituted of metamorphic rocks intruded by several generations of granitoid rocks, included in the Oriental Terrane of the central segment of the Ribeira folded belt. After a long period of tectonic quiescence, the present-day southeastern Brazilian coastal region was subjected to an important process of tectonic reactivation, manifested since Jurassic times, which culminated with the opening of South Atlantic Ocean. Analysis of the fracture pattern of the Serra dos Órgãos range in the region of the Serra dos Órgãos National Park (Parque Nacional da Serra dos Órgãos - PARNASO) indicated the existence of five tectonic deformation phases, active from Neoproterozoic-Cambrian until the Recent times, in part correlatable with previously recognized deformations in southeastern Brazil: (1) Neoproterozoic-Cambrian left-lateral E-W-trending strike-slip; (2) and (3) Early Cretaceous and Late Cretaceous to Paleocene left-lateral E-W-trending strike-slip; (4) Eocene NW-SE-trending extension; and (5) Holocene E-W-trending compression.

Padrão de fraturas da região da Serra dos Órgãos (RJ) e sua relação com a tectônica mesozóico-cenozóica do sudeste do Brasil

The Serra dos Orgaos range, between Petropolis and Teresopolis cities, Rio de Janeiro State, is one of the most remarkable physiographic features of southeastern Brazilian coast. This feature, part of the Serra do Mar coastal range, with summits over 2.200 m.a.s.l, follow Precambrian basement structures constituted of metamorphic rocks intruded by several generations of granitoid rocks, included in the Oriental Terrane of the central segment of the Ribeira folded belt. After a long period of tectonic quiescence, the present-day southeastern Brazilian coastal region was subjected to an important process of tectonic reactivation, manifested since Jurassic times, which culminated with the opening of South Atlantic Ocean. Analysis of the fracture pattern of the Serra dos Orgaos range in the region of the Serra dos Orgaos National Park (Parque Nacional da Serra dos Orgaos - PARNASO) indicated the existence of five tectonic deformation phases, active from Neoproterozoic-Cambrian until the Recent times, in part correlatable with previously recognized deformations in southeastern Brazil: (1) Neoproterozoic-Cambrian left-lateral E-W-trending strike-slip; (2) and (3) Early Cretaceous and Late Cretaceous to Paleocene left-lateral E-W-trending strike-slip; (4) Eocene NW-SE-trending extension; and (5) Holocene E-W-trending compression.

Ages of the Magmatism and the Opening of the South Atlantic Ocean

The analysis of published and unpublished 368 K/Ar radiometric ages of basic, intermediate and alkaline volcanic rocks, related to the post-Paleozoic magmatism linked to the opening of the South Atlantic Ocean, yields some important evidence concerning the break up of the Gondwana supercontinent. At the Brazilian Equatorial margin, the Gondwana break up started in the Permo-Triassic, when the opening of the Equatorial South Atlantic Ocean began and spread out south-eastward up to the present day Amazon River mouth. During the middle Jurassic/lower Cretaceous (pre-Aptian), the continuity of this separation, towards the Potiguar Basin, was coeval with the northward opening of the south-east Brazilian margin, up to the Espírito Santo State latitude. The relationship between large volcanic events in the basins and the resistance to the rifting process development offered by the cratonic area was shown by the trend of the magmatic age. Along the equatorial margin, the fragmentation resistance caused by the São Luis / West African craton is manifested by a large basic magmatism described in the Tacutu, Acre, Solimões, Amazonas and Parnaíba basins. A similar mechanism along the south-east margin, is proposed for the magmatism described in the Paraná Basin which is associated with the fracturing resistance offered by the São Francisco/Congo cratonic area. The integration of geochronological, micropalentological, sedimentological and geochemical data from the basins of the east Brazilian continental margin supports a model to explain the final disruption between South America and Africa during Cenonian/Turonian time. This model implies that 90 Ma basic magmatic rocks, related to the oceanic crust formation, probably occur offshore from the present-day eastern Brazilian coast line.