Interpretation of the gravity anomalies of Bahia state Brazil

This thesis describes the gravity interpretation of Bahia State, Brazil, which comprises the northern Sao Francisco craton, the Upper Proterozoic fold belts and the basins adjacent to the continental margin. The study centres on the isostatic analysis of the region and on the interpretation of large and high amplitude negative anomalies which occur over the Precambrian and the sedimentary basins. The isostatic analysis of the northern Sao Francisco craton was carried out using the isostatic response function technique. Taking into account subsurface loads, an elastic plate with a minimum effective thickness of 20-40 km explains the observed isostatic response function. The subsurface loads are (l) a slight thickening of the crust under the Espinhaco Fold System and (2) five to ten kilometres of low density rocks in the upper crust. A large and high amplitude ( - 50 mGal ) negative anomaly of shallow origin, centred near the western border of the Paramirim complex and parallel to the Espinhaco fold belt, is interpreted as caused by a large and mainly unexposed granite batholith. The granite substantially underlies the fold belt and extends towards the centre of the Paramirim complex. The minimum density contrast between the granite and the country rocks is estimated to be -0.06 g cm(^-3). The thickness of the granite is 8 to 13 km for density contrasts of -0.15 g cm(^-3) to -0.10 g cm(^-3). A series of high amplitude negative anomalies (50 to 100 rrGal), without flanking positive anomalies, characterizes the onshore Reconcavo, Tucano and Jatoba basins, which were ail formed in connection with the South Atlantic opening. The gravity interpretation indicates up to 7 km of sediments infilling these basins and no significant Mo ho upwarp beneath. In contrast, the gravity anomalies over the offshore Jacuipe and Sergipe-Alagoas basins are explained by a thick accumulation of sediments on a strongly attenuated crust. The onshore basins show short-lived subsidence ( < 25 Ma) with little, if any thermal subsidence. Syn-rift and post-rift (thermal) sedimentation is observed only in the continental margin basins. A mechanism in which upper crustal extension in one region (onshore basins) is compensated and balanced against lower extension in another region (offshore basins), through a detachment fault, may explain the way these basins formed

Integraltion of Chaco-Paraná and Paraná basins terrestrial gravity data using GOCE geopotential model: a Major Proterozoic to Cambrian suture revealed

The Chaco-Paraná basin (CPB) is located mostly in the north-eastern Argentina, covering an area of approximately 700,000 km2 in a lowland region (~100 m a.s.l.), known as Chacopampean plain. The average sediment thickness of the basin is 4,000 m, and along the Las Breñas fault zone the sediment thickness may reach more than 6,000 m. The Paraná basin (PB) is located in south Brazil, covering an area of over 1,000,000 km2 and its cumulative sediment and basalt thickness reaches up to 7,000 m. The topography over the PB is more irregular and has an average altitude of 700 m around its borders and of 300 m along the Paraná River. (...)CAPES - Mincyt Project 234/13CNPqSECIT

Utilização de tesseróides na modelagem de dados de gradiometria gravimétrica

Poster apresentado no XIII Simpósio de Iniciação Científica do IAG-USP (2008).<br><br><b>Resumo</b> <p>A ESA (European Space Agency) planeja lançar no outono de 2008 a missão GOCE (Gravity field and steady-state Ocean Circulation Explorer). A missão foi planejada para medir o campo gravitacional da Terra com acurácia e resolução sem precedentes. Para isso, fará uso de um gradiômetro de gravidade eletrostático que consiste de três pares de acelerômetros idênticos mutuamente ortogonais. O GOCE fornecerá dados do tensor gradiente da gravidade (TGG) a uma altitude de órbita de aproximadamente 250 km.</p> <p>Está sendo desenvolvido um programa computacional para analisar dados do TGG sobre as bacias sedimentares brasileiras. O programa utilizará o método da Quadratura Gauss-Legendre para efetuar a modelagem direta do TGG gerado por feições ou corpos geológicos com geometria esférica. A modelagem será feita discretizando o corpo por tesseróides, também denominados prismas esféricos. Os tesseróides são segmentos de uma casca esférica de espessura finita limitados por linhas de grade geográficas. A geometria dos tesseróides possibilita a construção de modelos levando em conta a curvatura da Terra. Isto se torna importante na modelagem de corpos geológicos com grande extensão lateral, como por exemplo, a bacia do Paraná. Será criado um modelo de densidade desta bacia a partir de dados de poços e dados sísmicos e utilizaremos o programa desenvolvido para obter estimativas do TGG. As estimativas serão comparadas com os futuros dados do GOCE na tentativa de separar o componente gravimétrico associado às variações de densidade na parte mais profunda da bacia.</p

Cálculo do tensor gradiente gravimétrico utilizando tesseroides

<p>Thesis for my Bachelor's degree in Geophysics from the Universidade de São Paulo.</p> <p><strong>Abstract</strong></p> <p>The GOCE satellite mission has the objective of measuring the Earths gravitational field with an unprecedented accuracy through the measurement of the gravity gradient tensor (GGT). The data provided by this mission could be used to study large areas, where the flat Earth approximation can have its  limitations. In these cases the modeling could be done with tesseroids, also called spherical prisms, in order to take the Earths curvature into account. The GGT caused by a tesseroid can be calculated with numerical integration methods, such as the Gauss-Legendre Quadrature (GLQ). In the current project, a computer program was developed for the direct calculation of the GGT 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. Next, the developed program was used to calculate the differences in the GGT caused by the flat Earth approximation. These differences reach are up to 30% in the Tzz component for a 50 deg x 50 deg x 10 km model. Finally, the computer program was used to calculate the effect caused by the topographic masses on the GGT at 250 km altitude for the Paraná basin region. In regions of large topographical variations, the components of the GGT due to the topographic masses have amplitudes of the same order of magnitude as the GGT components due to density anomalies in the interior of the crust and mantle.</p> <p> </p

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

<p>Slides for the oral presentation "Computation of the gravity gradient tensor due to topographic masses using tesseroids" given at the 2010 AGU Meeting of the Americas at Foz do Iguaçu, Brazil.</p

Western Paraná suture/shear zone and the limits of Rio Apa, Rio Tebicuary and Rio de la Plata cratons from gravity data

We present a new gravity map between 45°?70° W and 5°?40° S integrating open source terrestrial gravity data of Argentina with the South American Gravity Model 2004 (SAGM04), a 5 min-arc resolution gravity model. The Bouguer anomaly map reveals a 2000 km long linear gravity feature from 15° S to 30° S at longitude 55° W, with a steep horizontal gradient separating two gravity domains. The eastern domain is the Paraná basin, with NE-SW trending Bouguer anomalies of −80 mGal in average. The western domain comprises the Chaco-Paraná, Chaco-Tarija and Pantanal basins, with circular positive anomalies of up to 20 mGal in amplitude. Previous seismic studies mapped a thinner crust of less than 35 km in the western domain and the present gravity models indicate a 10?20 kg/m3 denser crust. On the other hand, the eastern domain has a thicker crust of more than 40 km. Seismic tomography models also show P- and S-wave velocity reduction in the western domain whereas high-velocity characterises the Paraná basin. These geophysical data indicate that the gravity gradient marks a transition between two distinct lithospheres. The gravity gradient is associated with a tectonic feature referred to as the Western Paraná suture/shear zone. Granites of 530?570 Ma ages, located parallel or over the gravity gradient, suggest a Neoproterozoic to Early Cambrian age suture/shear zone, thus approximately synchronous and parallel to the Pampean belt. Sediment corrected residual gravity map and its vertical derivative allow us to define the limits of the Rio Apa, Rio de la Plata and Rio Tebicuary cratons. Their eastern and western limits are the Western Paraná suture and the Pampean belt, respectively. This study unravels Precambrian tectonic elements concealed by the Phanerozoic sedimentary basins adding new constraints for the amalgamation history of SW Gondwana.Fil: Dragone, Gabriel Negrucci. Universidade de Sao Paulo; BrasilFil: Ussami, Naomi. Universidade de Sao Paulo; BrasilFil: Gimenez, Mario Ernesto. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto Geofísico Sismológico Volponi; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Lince Klinger, Federico Gustavo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto Geofísico Sismológico Volponi; ArgentinaFil: Chaves, Carlos Alberto Moreno. Universidade de Sao Paulo; Brasi

Magnetotelluric deep soundings, gravity and geoid in the south São Francisco craton: Geophysical indicators of cratonic lithosphere rejuvenation and crustal underplating

In the south São Francisco craton a circular and 8-m amplitude geoid anomaly coincides with the outcropping terrain of an ArcheanPaleoproterozoic basement. Broadband magnetotelluric (MT) data inversions of two radial profiles within the positive geoid and Bouguer gravity anomaly yield geo-electrical crustal sections, whereby the lower crust is locally more conductive (10 to 100 &#937;m) in spatial coincidence with a denser lower crust modeled by the gravity data. This anomalous lower crust may have resulted from magmatic underplating, associated with Mesoarchean and Proterozoic episodes of tholeiitic dike intrusion. Long-period MT soundings reveal a low electrical resistivity mantle (20 to 200 &#937;m) from depths beyond 120 km. Forward geoid modeling, using the scope of the low electrical resistivity region within the mantle as a constraint, entails a density increase (40 to 50 kg/m3) possibly due to Fe enrichment of mantle minerals. However, this factor alone does not explain the observed resistivity. A supplemented presence of small amounts of percolated carbonatite melting (~0.005 vol.%), dissolved water and enhanced oxygen fugacity within the peridotitic mantle are viable agents that could explain the less resistive upper mantle. We propose that metasomatic processes confined in the sub-continental lithospheric mantle foster the conditions for a low degree melting with variable CO2, H2O and Fe content. Even though the precise age of this metasomatism is unknown it might be older than the Early Cretaceous based on the evidence that a highdegree of melting in a lithospheric mantle impregnated with carbonatites originated the tholeiitic dike intrusions dispersed from the southeastern border of the São Francisco craton, during the onset of the lithosphere extension and break-up of the western Gondwana. The proxies are the NE Paraná and Espinhaço (130 Ma, Ar/Ar ages) tholeiitic dikes, which contain (~3%) carbonatites in their composition. The occurrence of a positive geoid anomaly (+10 m) and pre-tholeiites (ageN138 Ma), carbonatites and kimberlites along the west African continental margin (Angola and Namibia) reinforces the presumed age of the São FranciscoCongo craton rejuvenation to be prior to its fragmentation in the Lower Cretaceous.Pages: 423-43

Magnetotelluric data across Uruguay and Southern Brazil

&lt;p&gt;This dataset presents .edi files from 35 magnetotelluric broadband and 11 long period soundings across central Uruguay and southern Brazil. This data collection was financially supported by FAPESP Thematic Projects 2012/06082-6 and 2009/50493-8&lt;/p&gt