Electrical structure of the lithosphere from Rio de la Plata craton to Paraná basin: amalgamation of cratonic and refertilized lithospheres in SW Gondwanaland

We conducted a magnetotelluric (MT) study from Paleoproterozoic Rio de la Plata Craton, in Uruguay, toward Paleozoic-Mesozoic Paraná Basin, in Brazil. The 850-km-long MT transect comprises 35 evenly spaced broadband electromagnetic soundings sites. In the Paraná Basin, 11 additional long-period measurements were acquired to extend the maximum depth of investigation. All data were inverted using two- and three-dimensional approaches obtaining the electrical resistivity structure from the surface down to 200 km. The Rio de la Plata Craton is>200-km thick and resistive (~2,000Ωm). Its northern limit is electrically defined by a lithosphere scale lateral transition and lower crust conductive anomalies (1–10Ωm) interpreted as a Paleoproterozoic suture at the southern edge of Rivera-Taquarembó Block. The latter is characterized by an approximately 100-km thick and moderate resistive (>500Ωm) upper mantle. The Ibaré shear zone is another suture where an ocean-ocean subduction generated the 120-km thick and resistive (>1,000Ωm) São Gabriel juvenile arc. Proceeding northward, a 70- to 80-km thick, 150-km wide, and inclined resistive zone is imaged. This zone could be remnant of an oceanic lithosphere or island arcs accreted at the southern border of Paraná Basin. The MT transect terminates within the southern Paraná Basin where a 150- to 200-km-thick less resistive lithosphere (<1,000Ωm) may indicate refertilization processes during plate subduction and ocean closure in Neoproterozoic-Cambrian time. Our MT data support a tectonic model of NNE– SSW convergence for this segment of SW Gondwanaland

Mudanças hidrológicas no pantanal associadas a processos erosivos e tectônicos na Bacia do Rio Taquari, MS.

Nas últimas décadas, grandes áreas de Cerrados de Mato Grosso do Sul foram convertidas em sistemas produtivos agrícolas e pastoris, sem a devida observância do potencial de utilização das terras. Com o tempo, algumas regiões, como a Bacia do Rio Taquari, passaram a apresentar problemas ambientais de grandes proporções em resposta a processos erosivos, cuja evolução progressiva sugere ineficiência dos sistemas de manejo visando à conservação ambiental. Os processos erosivos, estabelecidos na região de planaltos da alta Bacia do Rio Taquari, manifestam-se em escalas temporal e espacial ainda não inteiramente compreendidas e consideradas. Há falta de estudos sobre a erosão fluvial, principalmente em cabeceiras de drenagem, que acumulam grandes quantidades de matérias nas calhas dos rios da alta Bacia, cujos depósitos passam a funcionar como fonte de materiais disponíveis para transporte para o Pantanal. Mudanças hidrológicas na região baixa do lique aluvial do Taquari, com inundações permanentes em grandes áreas, são uma resposta ambiental de larga escala, cuja causa está associada a processos de intensificação da erosão na região de planaltos da alta Bacia do Rio taquari e aumento da taxa de transporte de materiais arenosos para a planície pantaneira. O transporte e a deposição de sedimentos no leito do rio Taquari, ou seja, os assoreamentos, forçam o ajuste morfológico da calha do rio incluindo a abertura de canais divergentes, os "arrombados", que permitem inundações permanentes em grandes áreas que eram de utilização pela pecuária pantaneira. Entretanto, estudos geofísicos e de sensoriamento remoto recentes indicam que essas mudanças hidrológicas, verificadas no baixo curso do rio Taquari, apresentam um padrão espacial de ocorrência e podem estar associadas ao neotectronismo, ou seja, a movimentação do embasamento no segmento do rio onde se concetram os arrombamentos de margens. O presente trabalho tem como objetivo apresentar novos dados que sugerem que, além de processos antropogênicos, processos geológicos naturais contribuem para aumentar a instabilidade dos ecossistemas da região de Cerrados e Pantanal em Mato Grosso do Sul. Um melhor conhecimento da importância relativa desses processos é fundamental no planejamento do uso e manejo de solos na Bacia do Rio Taquari.bitstream/item/104773/1/Mudanca-hidrologicas-no-pantanal.pd

Reappraisal of the effective elastic thickness for the sub-Andes using 3-D finite element flexural modelling, gravity and geological constraints

P>Estimates of effective elastic thickness (T(e)) for the western portion of the South American Plate using, independently, forward flexural modelling and coherence analysis, suggest different thermomechanical properties for the same continental lithosphere. We present a review of these T(e) estimates and carry out a critical reappraisal using a common methodology of 3-D finite element method to solve a differential equation for the bending of a thin elastic plate. The finite element flexural model incorporates lateral variations of T(e) and the Andes topography as the load. Three T(e) maps for the entire Andes were analysed: Stewart & Watts (1997), Tassara et al. (2007) and Perez-Gussinye et al. (2007). The predicted flexural deformation obtained for each T(e) map was compared with the depth to the base of the foreland basin sequence. Likewise, the gravity effect of flexurally induced crust-mantle deformation was compared with the observed Bouguer gravity. T(e) estimates using forward flexural modelling by Stewart & Watts (1997) better predict the geological and gravity data for most of the Andean system, particularly in the Central Andes, where T(e) ranges from greater than 70 km in the sub-Andes to less than 15 km under the Andes Cordillera. The misfit between the calculated and observed foreland basin subsidence and the gravity anomaly for the Maranon basin in Peru and the Bermejo basin in Argentina, regardless of the assumed T(e) map, may be due to a dynamic topography component associated with the shallow subduction of the Nazca Plate beneath the Andes at these latitudes.ANPFAPESP[06/01211-1]CNPq[300736/2005-3

Computation of the gravity gradient tensor due to topografic masses using tesseroids

The 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\ub0 W and 65\ub0 W and 10\ub0 S and 35\ub0 S, which includes the Paran\ue1 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 E\uf6tvos) 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

Explaining the thick crust in Parana' basin, Brazil, with satellite GOCE-gravity observations

Seismologic observations in the last decades have shown that the crustal thickness in Paran\ue1 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 Paran\ue1 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 Paran\ue1 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 Paran\ue1 basin

The new satellite derived gradient fields for gaining a better understanding of the Paran\ue0 Basin

The Paran\ue1 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 Paran\ue1 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 Paran\ue1 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. Sans\uf2, 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 Paran\ue1 basin (Brasil). Petrogenic and geophysical aspects. S\ue3o Paulo: IAG-USP, 600 pp. Ernesto M., Marques L.S., Piccirillo E.M., Molinz E.C., Ussami N., Comin-Chiaramonti P., Bellieni G. , (2002). Paran\ue1 Magmatic Province\u2013Tristan da Cunha plume system: fixed versus mobile plume, petrogenetic considerations and alternative heat sources. Journal of Volcanology and Geothermical Research 118, 15-36 pp

GOCE data evidence underplating beneath the Paran\ue1 basin

The Paran\ue1 basin in the stable South American Platform accumulated a thick sediment layer during the Paleozoic, with more than 3500 m thickness. In Early Cretaceous the same area experienced a flood basalt volcanism which produced a LIP-(Large Igneous Province, Bryan & Ernst, 2008). We present here a new approach that integrates the newest gravity data of the satellite mission GOCE (Gravity Ocean Circulation Explorer) and the seismologic and geophysical drilling information to determine the Paran\ue1 basin lithospheric structure. The latest seismological investigations in South America (Assump\ue7\ue3o et al., 2012) reveal a deep (> 40 km) Moho under the Paran\ue1 basin. These observations do not agree with the gravity data that evidence a relative gravity high trending NE-SW in the central portion of the basin and over the greatest thickness of the sediment layers. Further constraint for the gravity modeling are the geophysical data obtained from a drilling survey in search for hydrocarbons (Melfi et al., 1987). Isostatic modeling show us, that this relative gravity high cannot be explained by the volcanic deposits because they are less in volume with respect to the light pre-volcanic alluvional layer. The gravity high certainly cannot be explained by crustal thinning, as the seismological data suggest normal to thick crust. In our work we calculate the effect of underplating related to magmatic effusion. This magmatism is supposed to be more than 10 time the volume of superficial flood volcanism (Bryan & Ernst, 2008). We calculate the missing mass using isostatic modeling: we first reduce the Bouguer field by the gravity effect of crustal thickness using the seismological Moho, and the effect of sediment. We test 2 different density contrasts between crust and mantle: -0.5 and -0.3 Mg/m3, the first takes into account a light, and the second a heavy crustal density. After that we use the spectral methodology for gravity inversion, and obtain a crustal body that explains our missing mass. To test the different possible patterns, we adopted several models, changing the reference depth, and the density contrast. Finally we simplify the unknown body with a cut off cone, and we estimate the volume of intruded basaltic material. We see that the geometry is in agreement with petrologic considerations. We conclude that the deep Moho under Paran\ue1 basin can be explained for example by a thick layer of gabbro with a contrast density of 0.2 Mg/m3 with respect to a normal crust, located at 40 km, with a volume of about 1.66 7 106 km3 (Assump\ue7\ue3o et al., 2012) when the total volume of the superficial LIP is 0.46 7 106 km3