Eder Cassola Molina

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.

Climatological Analysis: A Fast Non-linear Signal Filtering Approach

This paper proposes a new approach to detect, model and analyze climatological signals solely based on the largest eigenvalue of a signal derived matrix, used as a measure of innovation. The resulting process offers a substantial reduction in computational complexity with respect to presently applied methods. Furthermore, the approach yields much precise and detailed result. A case study illustrates the application of the proposed approach to the identification and analysis of a Nino event, tracing the interactions between different regions in the planet and pointing out perturbation paths otherwise unknown, allowing the prediction of the phenomena months before the climatological development.

Basement Structure of the Santos Basin from Gravity Data

The ocean basement represents the tectonic situation of a specific area and its knowledge is crucial in studies aimed at exploring the seabed. Due to the high sedimentation rate, especially in ocean basins, the topography of the basement is masked and its structures can be buried, inaccessible to direct observation. This paper aims to estimate the depth of the basement in the region of the Santos Basin in Brazil from a combined analysis of gravity data obtained from satellite altimetry and marine gravimetry, bathymetric data and sediment thickness. In the first step of the work the gravity effect of sediments in Santos Basin was calculated and the Crustal Mantle Interface (CMI) was modeled from constrained gravity inversion. Next, the reliability of the models so obtained was tested by flexural analysis. The flexural and gravity CMI proved to be in agreement. The gravity effect of the CMI and the gravity effect of the sediments were then calculated and subtracted from the original Bouguer anomaly. The residual field thus obtained, which is assumed to represent the topographical features of the basement, was inverted in the last step of the work, providing information that shows a basement with features of up to 700 m that appear to be in agreement with tectonic features previously discussed, such as the Avedis volcanic chain. The depth of the basement estimated during this study showed values ranging from 1,500 to 10,500 m, and the deepest region is consistent with the Cabo Frio Fault.

Seismostratigraphy of the Ceará Plateau: Clues to Decipher the Cenozoic Evolution of Brazilian Equatorial Margin

The Ceara Plateau offshore Fortaleza holds some particular characteristics when compared to the other seamounts of the Brazilian Equatorial Margin (BEM). Not only it is the largest and the closest to the continent, it is also located at the boundary between the continental and the oceanic crusts, while all the others seamounts along the BEM are located on oceanic crust. Seismic imaging of the Ceara Plateau shows a “disorganized” interior, probably of volcanic origin, overlain by a series of horizontal seismic reflectors that can be interpreted as pelagic/hemipelagic sediments. As large uncertainties exist about the age of the initial formation of this seamount, three scenarios must be considered. If the age of the volcanic edifice is Coniacian (1), then the overlying pelagic/hemipelagic sedimentary succession can include an almost continuous record of the last ~90 Ma at the Equatorial Atlantic Ocean. In the case that the volcanic edifice is Eocene in age (2), the sedimentary sequence would still encompass the upper Paleogene and all the Neogene. There is also the possibility that the volcanic edifice was built during multiple magmatic events (3). In this case, it is likely that the sediments are interfingered with volcanic rocks at the edge of the structure. Although the age estimation (between Coniacian and Eocene) has an uncertainty of more than 40 Myr, the current interpretation is that it developed initially as a volcanic edifice, formed by a series of magmatic events that occurred between the Santonian and the Eocene. Since then, the topography has been leveled by pelagic/hemipelagic sedimentation. Whichever was the initial age, a continuous and constant sequence of sediments deposited onto the Ceara Plateau, at the same latitude, and thus under the same oceanographic conditions, for the last several tens of million years. This represents a unique opportunity to record a long-term history of the Atlantic Equatorial Margin.

Análise da variação dos elementos do campo de gravidade na região do Aqüifero Guarani a partir dos dados GRACE

This work estimates the variation of the gravity field elements in a period of two years and three months - April 2002 to July 2004 - using the GRACE mission data in the Guarani aquifer region; these changes are the sum of all the gravity changes that are caused by variations from mass distribution of the Earths surface and by the deformation of the solid Earth in response to these mass variations. The solutions have been calculated only from the disturbances of the satellite orbits and do not depend on the gravity data available in the oceanic and continental areas. The calculations of the Earths gravity field monthly solutions by using the GRACE mission data, considering the spherical harmonics, i.e., the Stokes coefficients, for the global gravity field, up to degree and order 150 (February/2003 to July/2004) and 120 (April, August and November 2002), have been transformed in geoidal height and free-air anomaly grids, with the respective associated errors. A linear 3-D inversion using the residual geoid height supplied the three dimensional density distribution, showing that the use of the GRACE data in investigation of the water mass variability in Guarani aquifer is feasible. A Bouguer anomaly profile, calculated by using the GRACE free-air anomaly and a digital topographic model data, was used in order to estimate the water mass variation in the Guarani aquifer region.