Naomi Ussami

Paraná Magmatic Province-Tristan da Cunha plume system: fixed versus mobile plume, petrogenetic considerations and alternative heat sources

Abstract Paleomagnetic reconstructions demonstrate that the Tristan da Cunha (TC) plume, which is usually related to the genesis of the high- and low-Ti flood tholeiites of the Parana Magmatic Province (PMP), was located ∼1000 km south of the Parana Province at the time of the magma eruptions. Assuming plume mobility, and considering the low-velocity zone identified in the northern portion of the PMP as the TC ‘fossil’ plume (∼20° from the present TC position), the plume migrated southward from 133–132 (main volcanic phase) to 80 Ma at a rate of about 40 mm/yr. From 80 Ma to Present the plume remained virtually fixed, leaving a track (Walvis Ridge) compatible with the African plate movement. However, geochemical and Sr–Nd–Pb isotopic data do not support that the tholeiites from Walvis Ridge, Rio Grande Rise and Parana can result from mixing dominated by the TC plume and mid-ocean ridge basalt components. The similarity among the high-Ti basalts from Rio Grande Rise, part of Walvis Ridge (525A) and the Parana Province suggests that delaminated subcontinental lithospheric mantle must be considered in their genesis. Regional thermal anomalies in deep mantle mapped by geoid and seismic tomography data offer an alternative non-plume-related heat source for the generation of intracontinental magmatic provinces.

Basement reactivation in a sub‐Andean foreland flexural bulge: The Pantanal wetland, SW Brazil

The role of sub-Andean foreland peripheral bulges in controlling sedimentary sequence deposition and Quaternary geomorphology is discussed in the case of the Pantanal wetland, SW Brazil. This shallow (0.5 km) and wide (>200 km) depression was formed because of uplift and flexural extension in the brittle upper crust which mechanically reactivated the Neoproterozoic Paraguai fold/thrust belt. Depth to the basement and structural information within the Pantanal wetland were obtained from gravity, reflection seismics, drill hole, and digital topography data. The upper part of the basin is filled with Pliocene-Quaternary unlithified sands. The age of the oldest sediments is unknown, and normal faults are mostly observed on the western border of the basin. A two-dimensional flexural model composed of a semi-infinite thin elastic plate for the Brazilian shield lithosphere with an effective elastic thickness of 125–150 km, its western edge at 67.5° W under the Altiplano and loaded on a free end by the Andes topography, predicts the forebulge to be presently located over the Pantanal wetland. Flexural modeling was constrained using gravity data extending from the Pacific Ocean to the east of the Pantanal wetland. The width of the bulge is 700 km, and its amplitude is ∼310 m. The total amount of extension and subsidence observed in the Pantanal basin is interpreted as due to a cumulative strain since late Oligocene when the Brazilian shield started its westward migration, together with compression and uplift of the Central Andes. The main phase of the wetland subsidence is very likely related to the last compressional pulse in the Andes, during the upper Pliocene-lower Pleistocene (∼2.5 Ma), as inferred from field study of fault kinematics of the Central Andes.

Gravity map of Brazil: 2. Regional and residual isostatic anomalies and their correlation with major tectonic provinces

An isostatic residual gravity map of Brazil has been computed by removing from a 0.5° × 0.5° Bouguer anomaly grid a regional gravity field calculated for compensating masses of surface topography. The coherence function, a statistical measure of the correlation between Bouguer anomaly and topography, was first computed in order to constrain the compensation mechanism within Brazil. Similar to results for North America and Australia, the coherence function of South America has a broad transition between high and low coherence values, suggesting a combination of tectonic provinces with different flexural rigidities and/or loading processes. In view of this result, we have considered, as a first approximation, a model in which the surface topography is the only load acting on a nonrigid lithosphere. A regional gravity field has been computed assuming Airy-Heiskanen isostasy with compensation at the crust-mantle boundary. The residual gravity map, which was obtained by removing the computed regional gravity field from the observed Bouguer anomaly, shows a long-wavelength N-S trending negative anomaly over most of Brazil. This gravity feature of approximately 3000 km width is the southern continuation of the western North Atlantic negative geoid/gravity anomaly and reaches at least ∼15 mGal in the northern portion of Brazil. Using the upward continued isostatic residual gravity field at 300 km, this long-wavelength component, which may be dynamically induced, has been removed to first approximation. The final isostatic residual gravity anomaly map depicts anomalies with wavelengths between 100 and 1000 km which correlate with major tectonic provinces. Negative anomalies occur mainly over Paleozoic intracratonic and Cretaceous rift-type sedimentary basins, and granitic intrusions and along Proterozoic thrust belts. Positive residual anomalies are generally observed over regions affected by igneous activity and volcanism such as in the Amazon basin and the Parana flood basalt province. Positive anomalies are also associated with overthrust crustal plates which define a suture zone in central Brazil and over sub-Andean Tertiary foreland basins.

Gravity map of Brazil: 1. Representation of free‐air and Bouguer Anomalies

Gravity data of Brazil were processed to obtain continental scale free-air and Bouguer anomaly maps for a large segment of the South American plate. Gravity measurements are referred to the International Gravity Standardization Net 1971 and the anomalies are calculated using Geodetic Reference System 1967 and 2.67 g cm−3 for Bouguer reduction. In the calculation of the gravity anomalies, information used included geopotential model, topography, geoidal heights, and astrogeodetic deviation of the vertical. Depending on the availability and distribution of land gravity data, various regions have been classified as complete, incomplete, or empty. In complete regions, the gravity field is represented by 0.5° × 0.5° mean free-air and Bouguer anomalies which were computed from observed gravity data only. In incomplete regions, mean anomalies in empty 0.5° × 0.5° blocks were computed from nearest known mean anomalies, by least squares collocation. Regions totally devoided of gravity data had the gravity field calculated by least squares collocation using all available geodetic data. The gravity map obtained using this approach allows to identify the major gravity anomalies with dimensions greater than 100 km. Such anomalies are mainly caused by intracrustal density variations of geological and tectonic interest.

The geoid in southeastern Brazil and adjacent regions: new constraints on density distribution and thermal state of the lithosphere

Abstract Least-squares collocation technique was used to process regional gravity data of the SE South American lithospheric plate in order to map intermediate (10–2000 km) wavelength geoid anomalies. The area between 35–10° S and 60–25° W includes the Parana CFB Province, the Southern Sao Francisco Craton and its marginal fold/thrust belts, the Brazilian continental margin and oceanic basins. The main features in the geoid anomaly map are: (a) Parana CFB Province is characterized by a 1000 km long and 500 km wide, NE-trending, 9 m-amplitude negative anomaly which correlates with the distribution of sediments and basalts within the Parana basin. (b) A circular (600–800 km in diameter) positive, 8 m-amplitude geoid anomaly is located in the southern S. Francisco craton and extends into the northeastern border of the Parana CFB Province. This anomaly partially correlates with Alto Paranaiba Igneous Province (APIP), where alkalic volcanism and tholeiitic dikes of ages younger than 80 Ma are found and where a low-velocity zone in the mantle has been mapped using seismic tomography. This positive geoid anomaly extends towards the continental margin at latitude 21° S and joins a linear sequence of short wavelength positive geoid anomalies associated with Vitoria–Trindade seamounts. (c) A NE-trending, 1000 km long and 800 km wide, 4 m-amplitude, positive geoid anomaly, which is located along the southeastern coast of Brazil, from latitude 24 to 35° S. The northern part of this anomaly correlates with the Ponta Grossa Arch and Florianopolis dyke swarm provinces. The age of this intrusive volcanism is 130–120 Ma. (d) A circular positive anomaly with 9 m of amplitude, located over the Rio Grande and Uruguay shields and offshore Pelotas basin. Few alkaline intrusives with ages between 65 and 80 Ma are found in the region and apatite fission track ages in basement rocks indicates cooling at around 30 Ma. A semi-quantitative analysis of the observed geoid anomalies using isostatic considerations suggests that the mechanism which generated Parana CFB Province did not change, in a significant manner, the lithospheric thermal structure, since the same geoid pattern observed within this province continues northward over the Neoproterozoic fold/thrust belts systems separating the Sao Francisco and Amazon cratons. Therefore, this observation favours Anderson’s idea of rapid basaltic outpouring through a pull-apart mechanism along a major suture zone. A thermal component may still be present in the Southern Sao Francisco Craton and in the Rio Grande Shield and contiguous continental margins, sites of Tertiary thermal and magmatic reactivations.

Origin of the Rio Grande Rise–Walvis Ridge reviewed integrating palaeogeographic reconstruction, isotope geochemistry and flexural modelling

Abstract A palaeogeographical reconstruction of the South American and African continents back to anomaly C34 (84 Ma) brings together the Rio Grande Rise (RGR) and the central portion of the Walvis Ridge (WR), thus the RGR–WR aseismic ridges may have a common origin. If the construction of the RGR–WR basaltic plateau took place mainly between 89 and 78 Ma, as indicated by the ages of the basalts sampled by DSDP wells, then the basaltic magmas are the result of an ‘on-ridge’ volcanism. Once separated, the normal sea-floor spreading and thermal subsidence of the RGR and WR ridges continued until approximately 47 Ma when an Eocene magmatism took place in the RGR. In the WR, a younger volcanism is observed in the Guyot Province. The available geochemical and isotope data of the WR–RGR basalts do not indicate the participation of the continental crust melting component. Incompatible trace element ratios and isotope signatures of the basalts from the RGR–WR ridges are distinct from the present-day Tristan da Cunha alkaline rocks, and are nearly identical to the high-Ti Paraná Magmatic Province (PMP) tholeiites (133–132 Ma). Both the high-Ti PMP and the WR–RGR basalts are characterized by moderate initial 87Sr/86Sr and low 206Pb/204Pb isotope ratios [Enriched Mantle I (EMI) mantle component], suggesting melting from a common source, with significant participation of sub-continental lithospheric mantle (SCLM). A three-dimensional (3D) flexural modelling of the RGR and WR was conducted using ETOPO1 digital topography/bathymetry and EGM2008-derived free-air anomalies as a constraint. The best fit between the observed and calculated free-air anomalies was obtained for an elastic plate with elastic plate thickness (Te) of less than 5 km, consistent with an ‘on-ridge’ initial construction of the RGR–WR. The modelling of the crust–mantle interface depths indicates a total crustal thickness of up to 30 km in the RGR–WR. Flexural analysis reinforces the geological evidence that RGR was constructed during two main magmatic episodes, the tholeiitic basalts in the Santonian–Conician times and the alkaline magmatism in the Eocene. Geochemical and geophysical evidence, which rules out the classical deep-mantle plume model in explaining the generation of basalts of these volcanic provinces, is presented. Finally, three models to explain the geochemical and isotope signatures of RGR–WR basalts are reviewed: (1) thermal erosion of SCLM owing to edge-driven convection; (2) melting of fragmented or detached SCLM and lower crust; and (3) thermal erosion at the base of the SCLM with lateral transport of enriched components by mantle flow.

Density and P‐wave velocity structure beneath the Paraná Magmatic Province: Refertilization of an ancient lithospheric mantle

We estimate density and P-wave velocity perturbations in the mantle beneath the southeastern South America plate from geoid anomalies and P-wave traveltime residuals to constrain the structure of the lithosphere underneath the Parana Magmatic Province (PMP) and conterminous geological provinces. Our analysis shows a consistent correlation between density and velocity anomalies. The P-wave speed and density are 1% and 15 kg/m3 lower, respectively, in the upper mantle under the Late Cretaceous to Cenozoic alkaline provinces, except beneath the Goias Alkaline Province (GAP), where density (+20 kg/m3) and velocity (+0.5%) are relatively high. Underneath the PMP, the density is higher by about 50 kg/m3 in the north and 25 kg/m3 in the south, to a depth of 250 − 300 km. These values correlate with high-velocity perturbations of +0.5% and +0.3%, respectively. Profiles of density perturbation versus depth in the upper mantle are different for the PMP and the adjacent Archean Sao Francisco (SFC) and Amazonian (AC) cratons. The Paleoproterozoic PMP basement has a high-density root. The density is relatively low in the SFC and AC lithospheres. A reduction of density is a typical characteristic of chemically depleted Archean cratons. A more fertile Proterozoic and Phanerozoic subcontinental lithospheric mantle has a higher density, as deduced from density estimates of mantle xenoliths of different ages and composition. In conjunction with Re-Os isotopic studies of the PMP basalts, chemical and isotopic analyses of peridodite xenoliths from the GAP in the northern PMP, and electromagnetic induction experiments of the PMP lithosphere, our density and P-wave speed models suggest that the densification of the PMP lithosphere and flood basalt generation are related to mantle refertilization. Metasomatic refertilization resulted from the introduction of asthenospheric components from the mantle wedge above Proterozoic subduction zones, which surrounded the Parana lithosphere. The high-density PMP lithosphere is presently gravitationally unstable and prone to delamination.

Gravity survey of the pantanal wetland: data acquisition and processing

A gravity survey was conducted between 1988 and 1993, mostly during the dry season (july to october) in the Pantanal Wetland and surrounding highlands, aiming to obtain a first regional gravity coverage. The Pantanal Wetland is an important geomorphological unit extending from -54o to -59.o, east-west, and from 14o to 23o , north-south, in SW Brazil. The topography ranges from 80 m to 200 m above sea level within the plain. A total of 3,662 gravity stations were collected. Because of the difficulty in accessing the flood plain, apart from conventional ground survey, fluvial and air routes were used in the field data acquisition. The data acquisition procedure is described in detail and a discussion on the final gravity anomaly accuracy is presented. From the total number of stations, 7 stations belong to the Fundamental Brazilian Gravity Network, 1,886 gravity stations were obtained on bench marks, 1,505 stations were set up together with barometric levelling or the altitude was estimated from topographic maps in places with smooth topography (horizontal gradient < 0.5 m/km), 237 stations were collected along the main rivers and 33 stations using aircraft.

Aquisição e interpretação de anomalias gravimétricas do Quadrilátero Ferrífero, SE do Cráton São Francisco

A new gravity survey was conducted between coordinates 39o to 49oW and 17o to 23oS in SE Sao Francisco Craton. A total of 176 new gravity stations was obtained. The position of each gravity station was determined using GPS (Global Positioning System) relative positioning method. The new data was added to the gravity data-bank of IAG (Instituto de Astronomia, Geofisica e Ciencias Atmosfericas)/USP (Universidade de Sao Paulo). A total of 12,339 stations was processed and interpreted. The geology of this area is represented by Archean and Proterozoic metamorphic terrains of SE Sao Francisco Craton, including the Aracuai and Ribeira fold/thrust belts. In order to separate shallow from deep sources of gravity field, a 50 km upward continued regional field was removed from Bouguer anomaly map. The residual gravity field from the Quadrilatero Ferrifero was correlated with the exposed geological units. The main gravity anomalies were forward modeled in order to map the contact between the main metamorphic complexes. Metamorphic complexes in this area have distinct gravity signatures. Bacao and Caete metamorphic complexes have negative gravity anomalies with similar amplitude. Bonfim metamorphic complex has positive gravity anomalies and its limits extend beyond the geologically mapped area, under younger sedimentary sequences. Forward gravity modeling constrained by observed geological contacts allowed to estimate the directions and dips of two major tectonic contacts. The contact between Barbacena and Bonfim metamorphic complexes dips 21o northward, whereas Belo Horizonte complex dips 31o westward under the Rio das Velhas supergroup.

Linear inversion of a negative gravity anomaly in se Rio Grande cone: a graben on oceanic crust?

We detect, for the first time, a negative free-air gravity anomaly of 23 mGal amplitude over a region in the South Atlantic Ocean centered at 48oW and 35oS. To this end, we used the integration of conventional shipborne gravity data and gravity data derived from GEOSAT/ERM satellite altimetry. The north bound of this anomaly coincides with the Chui Lineament and the south bound indicates another lineament, which is the extension of the Meteor Fracture Zone. The anomaly trend is NE-SW, its width is 400 km and its length is 600 km. Two-dimensional linear inversion with relative and absolute equality constraints was used to calculate the density distribution along three profiles perpendicular to the main axis of the anomaly. The result suggests that the sediment thickness in the deepest part of the basin is at least 3.0 km where the ocean bathymetry is 4,800 m. This tectonic feature, an asymmetric half-graben formed between two lineaments, probably lies over an oceanic crust. The estimated volume of sediments in this basin is approximately 50% of the post-Miocene sediments volume deposited in the Rio Grande Cone where gas-hydrates were found.