Wladimir Shukowsky

Mantle plumes and flood-basalt stratigraphy in the Paraná, South America

Geochemical studies of the Parana continental flood basalts in Brazil have led to the recognition of distinct magma types, which have been used to infer the internal stratigraphy of the lava pile. The overstepping of stratigraphic units toward the north and a similar compositional change in sills within the underlying Parana basin sedimentary rocks imply that the site of magnatism migrated ∼750 km toward the north during this volcanic event. This has important implications for recent plume-related models that have argued whether fithospheric rifting must accompany the presence of a mantle plume in order to generate continental flood-basalt magmatism. It is difficult to ascribe the magnitude and direction of the shift in the locus of the Parana magmatism to movement of the Brazilian lithosphere relative to the underlying Tristan mantle plume. Instead, it is suggested that the observed lava distribution and internal structure of the Parana-Etendeka flood-basalt province were imposed by the intracacies of the rifting process during the initial opening of the South Atlantic Ocean.

Tidal gravity anomalies as a tool to measure rheological properties of the continental lithosphere: application to the South American Plate

Characterization of the bulk physical properties of individual tectonic units is important for understanding the dynamics of continental evolution. One important parameter is effective elastic thickness (Te), a measurement of the flexural strength of the lithosphere. Te is traditionally estimated by examination of the transfer function between gravity and topography. Sparse gravity data coverage limits the application of this method in South America. Instead, we use an empirical correlation (as defined by data from Australia and by the world databank) between tidal gravity anomalies and Te to estimate Te for tectonic units of South America. Our results are consistent with independent determinations of Te in several sub-regions of South America. Although the empirical correlation appears to be quite strong, further research needs to be done to develop a physical theory for the connection between gravity tide anomalies (which sample an essentially instantaneous rheological response of the Earth) and Te (which measures the rheological response of the Earth at geological time scales).

Spatial variability of tidal gravity anomalies and its correlation with the effective elastic thickness of the lithosphere

Abstract Associations of the Earth tidal gravity response to physical properties of the lithosphere have been attempted at least for the last four decades. Although experimental data suggest this association, rigorous models have not yet been proposed. In this work, statistical tests are performed on the available World Gravity Earth Tides data set. Autocorrelation analysis shows that the M2 tidal gravity anomalies (TGAs) are significantly correlated up to a distance of about 500 km, with an approximately exponential correlation decay. The analysis of the latitudinal dependence of the anomalies shows that the anomaly variance, estimated inside of different latitude bands, follows a cos 4 ϕ curve within the ±45° latitude interval and defines the noise level for the M2 gravity anomaly data set. The regression analysis between M2 TGA and the lithosphere effective elastic thickness (EET) estimates shows that these quantities are significantly correlated, with a correlation coefficient of −0.82. The wide range of TGA and EET values, combined with a good global distribution of the data used in the regression analysis, makes the regression equation suitable to be used as a predictor for EET values in areas where M2 TGA data exist and meet the required quality criteria.

Magnetização remanescente: um parâmetro crucial para a interpretação e modelamento de anomalias magnéticas em território brasileiro

Remanent magnetization is often neglected in data interpretation, probably for the difficulty on working with it. In most of the cases, both in academic works and professional models in mining and oil industry, prevails the assumption that remanence is irrelevant or inexistent, and the induced effect is the only one considered. The present paper shows that using this parameter is particularly important on Brazilian anomalies, where magnetic latitudes are low, and try to provide tools for the use of the remanence information. In the article we discuss the use of common geophysical techniques, analytic signal and reduction to the pole in Brazilian anomalies due to rocks with and without remanence. Then we presented a method to estimate the total magnetic direction from a magnetic anomaly, from which it is possible to extract the total magnetization parameter, and consequently the resultant sum vector after induced and remanent parameters. Finally, we present a methodology for the use of remanent information to determine the age of the source rocks.

Tectonic pattern of South America inferred from tidal gravity anomalies

Abstract The knowledge of several physical parameters that characterize a tectonic unit is important to evaluate its dynamic evolution and, therefore, its role in the continental evolution. Several methodologies have been used to measure the elastic properties of tectonic units using: seismic waves, the level of coherence between the wavelength pattern of the topography and gravity fields, borehole deformation rate, mineral fabric, etc. In this study, the Earths dynamic elastic response to gravity tides is applied for measuring the lithosphere effective elastic thickness. This methodology proved to be a good alternative to the conventional coherence method. It was applied over the continent, taking into account the poor gravity coverage in South America and the restricted area of some tectonic units. Results are presented as lateral distribution of the effective elastic thickness in South America.

Interpretation of Long Wavelength Magnetic Anomalies

The Earth’s magnetic field (emf) measured at or above the Earth’s surface has two main components of internal origin: a primary (or main) field which is originated in the outer core and appears as large scale features at the Earth’s surface, and a secondary field which is due to both induced and remanent magnetization of high susceptibility ferrimagnetic minerals occurring in crustal rocks. The latter field is much weaker than the main field and it is locally variable according to the nature of the underlying crustal rocks. These two components of the emf appear clearly in a power spectrum analysis of the emf intensity along a world encircling profile (Alldredge et al., 1963). The energy of the spectrum is concentrated at wavelengths greater than about 2,000 km and smaller than 400–500 km (Fig.1). In the representation of the emf as an infinite series of spherical harmonic functions, the main field therefore is described by the harmonics up to order and degree about (13, 13) and the field of crustal origin by harmonics higher than (40, 40).

Resultados geofísicos integrados de um corpo com geometria 3D sem manifestação superficial

Airborne geophysical data of the SP-RJ survey carried out by the Brazilian Geological Survey (CPRM) reveal a magnetic anomaly without geological expression at the surface. The anomalous features are similar to those associated with alkaline complexes that outcrop in the same region. The absence of surface elements motivated the use of different geophysical methods to characterize the magnetic anomaly source. Among these methods, satellite images for different spectral bands were used, seismic sounding, gravity and ground magnetic surveys along a profile crossing the anomaly were performed, and density and susceptibility measurements of different rock types in the area were made. The obtained results were used to constrain 2½D simultaneous gravity and magnetic modeling and 3D airborne magnetic modeling. Both results indicate the presence of a body of about 3.3 x 10(9) metric tons, having its top at 40 meters from the surface and extending to a maximum depth of 1 km.