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The new satellite derived gradient fields for gaining a better understanding of the Paranà Basin
No full textThe Paraná and surrounding regions are of great interest due to the presence of a Large igneous Province (LIP), representing one of the most important continental flood basalt deposits on
Earth. In the last decades many efforts have been spent (e.g. Ernesto et al., 2002, Piccirillo and Melfi, 1988) to understand the evolution of the basalts and alkaline rocks of the Paraná Basin, but the geodynamic processes are not understood yet. One big question regards the geochemical signature of the volcanic rocks and their origin due to existence of mantle heterogeneity or crustal contamination and the presence of underplating).
Here, we analyze the gravity field and gravity gradient tensor in order to formulate a density model that accounts for the surface geology and the deeper lying structures. The gravity field and the gradient tensor are calculated using the recent EGM08 gravity potential expansion into spherical harmonics. Moreover, when available, we analyze the observations of the GOCE satellite mission.
We present the correlation of the geological units defined in existing geological maps, with the Marussi tensor or with quantities derived from it. The geologic structures are seen in the gravity signal when density variations accompany the contact between different geologic units. We find that the gravity gradient tensor is useful to mark different kind of geological structures as fold belts , faults, magmatic deposits. Our goal is to correlate all known geologic structures with the fields in order to check whether there are signals tied to unknown structures. We formulate a model for the different units filling the basin., The sediment and basalt densities are constrained by density values we find in literature. The density model allows to make isostatic
calculations, that considers topographic and intracrustal masses, as the basalts and the sediments. Our goal is to determine to which extent the isostatic model allows to define the amount of underplated material below the crust which should have accompanied the large basalt igneous province.
We use our density model to estimate the expected resolution power of the GOCE-satellite regarding crustal density inhomogeneity. GOCE is the first satellite to measure the gravity gradient on board and we show what new results can be achieved in the Paraná region by analyzing the upcoming GOCE data.
Our study is accomplished in the frame of different projects as the GOCE-Italy project supported by the Italian Space Agency, responsible Prof. F. Sansò, the FAPESP project, responsible prof. I. Vittorello, and is part of the ESA GOCE EO project ID 4323, responsible Prof. C. Braitenberg.
References:
Piccirillo, E.M. and Melfi, A.J. (1988). The Mesozoic flood volcanism from the Paraná basin (Brasil). Petrogenic and geophysical aspects. São Paulo: IAG-USP, 600 pp.
Ernesto M., Marques L.S., Piccirillo E.M., Molinz E.C., Ussami N., Comin-Chiaramonti P., Bellieni G. , (2002). Paraná Magmatic Province–Tristan da Cunha plume system: fixed versus mobile plume, petrogenetic considerations and alternative heat sources. Journal of Volcanology and Geothermical Research 118, 15-36 pp
Unvealing the border of a craton by the gravity gradient with examples of the Parana’ Basin
No full textComputation of the gravity gradient tensor due to topografic masses using tesseroids
No full textThe GOCE satellite mission has the objective of measuring the Earth's gravitational field with an unprecedented accuracy through the measurement of the gravity gradient tensor (GGT). One of the several applications of this new gravity data set is to study the geodynamics of the lithospheric plates, where the flat Earth approximation may not be ideal and the Earth's curvature should be taken into account. In such a case, the Earth could be modeled using tesseroids, also called spherical prisms, instead of the conventional rectangular prisms. The GGT due to a tesseroid is calculated using numerical integration methods, such as the Gauss-Legendre Quadrature (GLQ), as already proposed by Asgharzadeh et al. (2007) and Wild-Pfeiffer (2008). We present a computer program for the direct computation of the GGT caused by a tesseroid using the GLQ. The accuracy of this implementation was evaluated by comparing its results with the result of analytical formulas for the special case of a spherical cap with computation point located at one of the poles. The GGT due to the topographic masses of the Parana basin (SE Brazil) was estimated at 260 km altitude in an attempt to quantify this effect on the GOCE gravity data. The digital elevation model ETOPO1 (Amante and Eakins, 2009) between 40° W and 65° W and 10° S and 35° S, which includes the Paraná Basin, was used to generate a tesseroid model of the topography with grid spacing of 10' x 10' and a constant density of 2670 kg/m3. The largest amplitude observed was on the second vertical derivative component (-0.05 to 1.20 Eötvos) in regions of rough topography, such as that along the eastern Brazilian continental margins. These results indicate that the GGT due to topographic masses may have amplitudes of the same order of magnitude as the GGT due to density anomalies within the crust and mantle. -------------------------------------------------------------------
References: Amante, C., Eakins, B.W., 2009. ETOPO1 1 Arc-Minute Global Relief Model: Procedures, Data Sources and Analysis. NOAA Technical Memorandum NESDIS NGDC-24, p. 19. Asgharzadeh, M.F.; Von Frese, R.R.B.; Kim, H.R.; Leftwich, T.E.; Kim, J.W., 2007. Spherical prism gravity effects by Gauss-Legendre quadrature integration. Geophysics Journal International, v. 169, p. 1 - 11. Wild-Pfeiffer, F., 2008. A comparison of different mass elements for use in gravity gradiometry. Journal of Geodesy, v. 82 (10), p. 637 - 653
Spessore litosferico nel bacino del Parana' dall'analisi dei dati gravimetrici di nuova generazione (GOCE)
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Explaining the thick crust in Parana' basin, Brazil, with satellite GOCE-gravity observations
No full textSeismologic observations in the last decades have shown that the crustal thickness in Paraná basin locally is over 40 km thick, which is a greater value than expected by the simple isostatic model considering the topographic load. The goal of this work is to explain this apparent discrepancy by modeling the internal crustal density anomalies through the gravity field. We use the latest Earth Gravity Model derived from the observations of the GOCE satellite mission, to retrieve the gravity anomaly and correct it for the topographic effects, thus obtaining the Bouguer field. We then model the gravity effect of known stratigraphic units and of the seismological crustal thickness. The large Paraná basin comprises over 3500 m of Paleozoic sedimentary sequences with density between 2400 and 2600 kg/m3. During the Early Cretaceous the same basin was affected by a large amount of igneous activity with a volume of over 0.1 Mkm3. The flood basalt volcanism is known as the Serra Geral Formation, and has a maximum thickness of 1500 m. The stratigraphic units of the basin are topped by post volcanic deposits of the Bauru Group, of about 300 m thickness, located in
the northern part of the basin. The density and thickness of the sedimentary sequencesediments is constrained by sonic logs of drill-holes and exploration seismic. We use the crustal thickness
estimated from the newest seismological results for South America to calculate its gravity effect.
Further we model the isostatic crustal thickness variation, allowing the comparison between a seismological Moho, an isostatic Moho, and a gravity based Moho. We find that there is a clear positive Bouguer residual anomaly located in the northern and southern part of the Paraná basin, indicating the presence of a hidden mass, not considered up to now. We propose a model that explains this mass as magmatic rock, probably gabbro in lower crust, with density contrast of 200 kg/m3 and thickness of more than 10km, thus demonstrating that the flood basalt layer constitutes only a part of the melted material, the rest being emplaced into the lower crust. The presence of the
magmatic material in the crust presumably has altered the thermal state, consequently changing the maturation process of the hydrocarbons in the pre-volcanic and post-volcanic ediments rocks of the Paraná basin
GOCE demonstrate magmatic underplating beneath the Parana' basin
No full textThe Paraná is an intracratonic basin located on the stable South American platform. The shallow stratigraphy of the basin is composed of 3500 m of Paleozoic rocks, 1500 m of Serra Geral Formation and 300 m of Late Cretaceous sediments. The recent seismological studies have shown that the crust-mantle interface below the basin is deep between 40-46 km, depending on the specific model. Thick crust and thick sediments generally generate a strongly negative Bouguer anomaly that is not found in the Paraná basin. Instead of the expected Bouguer minimum, a relative anomaly high along the maximum sediment accumulation is found. During the Early Cretaceous, the same basin was affected by a large amount of basalt deposits of (Serra Geral Formation) that belong to a Large Igneous Province (LIP). The volcanic deposits in the basin are however too thin to explain the relative gravity high. The goal of this work is to explain the apparent discrepancy between crustal thickness and the Bouguer anomaly by modeling the crustal densities of and below the Paraná basin. Our approach integrates the new gravity observations of the GOCE satellite, and the constrains provided by the geophysical and seismological information to define geometry and densities. We reduce the gravity value for these known structures. The final residuals we obtain are interpreted as deviations from the assumption of a contrast density contrast located either in the crust or mantle according to the involved wavelengths of the residual gravity signal. Assuming a fixed density contrast, we estimate the thickness of the underplated body by inverting the gravity residual. The clear positive Bouguer residual anomaly suggests the presence of hidden mass. This hidden mass is located in the mid to lower crust, with a thickness over 10 km, and is probably made of gabbro. This mass is a magmatic material left behind by the ascending basalts, and contributes to isostatic balance. The study of underplating under the LIP is very useful to understand the relationship between the alteration of the thermal gradient of the crust and the hydrocarbon maturation of sediments
GOCE observations for detecting unknown tectonic features
No full textThe global coverage of a satellite allows us to investigate areas that are difficult to access due to remoteness or ruggedness. We intend to use the GOCE gradiometric observations to detect unknown tectonic features. At the present stage of the GOCE mission we take the error levels of the observations to estimate the resolution of known density discontinuities of the Earth crust: the crust-mantle and sediment-basement transition. The spherical harmonic expansion of mass distributions is compared with the estimated degree error curves of the gravity field. We find that the GOCE data will contribute to a better resolution of these discontinuities by one order of magnitude for degree between 52 and 200 compared to the EGM2008 gravity field model. The geodynamic context affects the resolution as it controls the average Moho depth, shallower levels being better resolved. For the basement the dominant resolution parameter is the density contrast across the interface
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