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| Author(s): |
Luiz Gustavo Rodrigues Pinto
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Instituto Astronômico e Geofísico (IAG/SBD) |
| Defense date: | 2009-03-27 |
| Examining board members: |
Naomi Ussami;
Irineu Figueiredo;
Wladimir Shukowsky;
Adalene Moreira Silva;
Wilson Teixeira
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| Advisor: | Naomi Ussami |
| Abstract | |
This thesis comprises of two parts in which two geophysical studies are described improving the knowledge of the lithosphere of the south São Francisco craton. In the first part, we propose an alternative method for defining the limits of lithosphere plate using gravity data. We used the first vertical derivative of Bouguer anomaly upwarded to 100 km height. Crustal results along a deep refraction seismic profile in the Goiás state together with a 2-D gravity model along a profile between Minas Gerais and São Paulo states were used to calibrate the vertical derivative results. Recent geochemical, petrological data and seismic topography data reinforce the gravity results that the southern and western parts of the São Francisco plate extends under parts the Brasília Belt and parts of the Paraná basin. In the second part, the interpretation of a positive Bouguer gravity anomaly, circular shaped and + 25 mGal amplitude gravity anomaly is presented. This gravity anomaly coincides with a positive, circular shaped, 7 m amplitude geoid anomaly with 250 km of radius. Receiver function results do not show a crustal thinning, excluding this possibility as the cause of gravity high. Deep magnetotelluric soundings along two profiles radial to gravity anomaly indicate a region in the mantle with lower electrical resistivity (< 100 .m) from the depth of 200 km and extending upward and reaching the lower crust. The residual Bouguer anomalies are inverted using a 3-D algorithm and indicate that the denser lower crust reaches 12 km of thickness for + 50 kg/m3 of density contrast. Coincident with a denser lower crust, magnetotelluric soundings reveal a low electrical resistivity crust, reinforcing the presence of magmatic underplating. At the surface, the contamination of magmatic material in the lower crust is supported by the presence of basaltic tholeiitic dike swarms similar, in composition, to the basalts of northern Paraná basin (high TiO2) of Early Cretaceous age. Tholeiitic dikes swarms are distributed throughout the southern region of the craton. Lithospheric mantle with lower electrical resistivity reaches 200 km of depth, indicating that the lithospheric mantle in the southern São Francisco craton underwent compositional rejuvenation accompanied by a mild increase in density (+ 50 to + 70 kg/m3) responsible for the positive geoid anomaly. The decrease of electrical resistivity in the mantle at depths beyond 100 km is easier explained by the presence of carbonatitic melts in the upper mantle, derived from a low degree of partial melt of a metasomatized deeper mantle. The carbonatitic magmatism was prior to tholeiitic magmatism since petrological and geochemical data show that the Early Cretaceous Espinhaço tholeiitic dikes contain 5 to 10% of carbonatites in their composition. The circular shaped erosional limit of the Bambui Group may have been produced by lithosphere uplift at the time of carbonatitic magmatism. (AU) | |