APLICACAO DOS METODOS GRAVIMETRICO E MAGNETOTELURICO NA CONTRIBUICAO DO ENTENDIMENTO DAS ESTRUTURAS GEOLOGICAS DA REGI AO CENTRAL DA BACIA DO PARANA, CENTRO-SUL DO BRASIL.

Abstract

This work aims to define a geological-geophysical model for the central region of the Paraná Basin obtained from models of gravity with lateral variation of depth and density, complementing the magnetotelluric studies that are being promoted by the National Petroleum Agency of Brazil - ANP. Studies of the basement structures in sedimentary basin are important to understand how does the tectonic-stratigraphic evolution occurred. The Paraná Basin is a shallow basin with depocenter estimated at 7000 m. Two major regional structures are prominent in the study area: the Ponta Grossa Arch, which is a feature of NW-SE direction; and the Paranapanema gravimetric high, an elongated positive anomaly with NE-SW direction. The region also includes large lava flows represented by basalts of the Serra Geral Formation. Intrusions of diabase dikes and sills, dating from the Cretaceous Period, are common in the region. These intrusions have settled in certain ancient groups of structural lineaments. The Bouguer surface of the study area was represented by the 4th degree polynomial function to estimate the depth of the Moho. The gravimetric final model proposed shows good fit, with an error of around 1.7 mGal and includes the regional topography. Data from six stratigraphic wells and a 1D composed model of electric resistivity measured in a magnetotelluric survey were used to modelate the shallow part of the gravimetric model. The model deepest structures, in particular the depth of the Moho, were linked from studies of seismic refraction, analysis of the receiver function and scattering of superficial waves. Magnetoteluric and gravimetric syntetic tests were criated to support the geophysical-geological model proposed, that presents as most notable features a smoothness of the mantle-crust interface, that ranges around 42 Km. The upper crust was represented by two blocks that corresponds, in the northwest portion, to the Paranapanema Precambrian continental crust and, in the southeastern portion, to the Rio Apiaí Precambrian mobile belt that is characterized by a positive residual anomaly of gravity, in accordance with the tectonic context of the region, already described in previous studies. The limit between the basin and the basement was derived from the MT model that revealed a fractured surface with an average depth of 5.5 Km. The sedimentary portion was differentiated in its well known super-sequences.