Interpretação de dados gravimétricos e eletromagnéticos do sul do cráton São Francisco: novos modelos crustais e litosféricos

Abstract

Interpretação de dados gravimétricos e eletromagnéticos do sul do cráton São Francisco: novos modelos crustais e litosféricos ABSTRACT PINTO, L. G. R., 2009. Interpretação de dados gravimétricos do sul do cráton São Francisco: novos modelos crustais e litosféricos. Thesis (Doctor) -Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, São Paulo. 145pp.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 Ω.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/m 3 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 TiO 2 ) 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

Composition and P-T conditions of the lithospheric mantle beneath the Azimuth 125º Lineament, Northern and Southern Brazil: constraints from peridotite xenoliths enclosed in diamond-bearing kimberlites

“…Mantle xenoliths hosted by kimberlites and kamafugites are otherwise more reported in papers from the APIP (Meyer and Svisero 1991;Leonardos et al 1993;Carlson et al 2007;Almeida et al 2014;Nannini 2015;Fernandes et al 2021). Overall, the geophysical, isotopic and microstructural data on peridotite samples provide evidence for the presence of remnants of an Archean cratonic root beneath the Brasília Belt in the region of the APIP (Carlson et al 2007;Pinto 2009;Fernandes et al 2021).…”

Composition and P-T conditions of the lithospheric mantle beneath the Azimuth 125º Lineament, Northern and Southeastern Brazil: constraints from peridotite xenoliths enclosed in diamond-bearing kimberlites

“…Mantle xenoliths hosted by kimberlites and kamafugites are otherwise more reported in papers from the APIP (Meyer and Svisero 1991;Leonardos et al 1993;Carlson et al 2007;Almeida et al 2014;Nannini 2015;Fernandes et al 2021). Overall, the geophysical, isotopic and microstructural data on peridotite samples provide evidence for the presence of remnants of an Archean cratonic root beneath the Brasília Belt in the region of the APIP (Carlson et al 2007;Pinto 2009;Fernandes et al 2021).…”