Geophysical studies and tectonic model of the central and south-east sectors of the Tocantins province, central Brazil

Abstract

The present project refers to a joint study which will be developed by the Institute of Astronomy and Geophysics of the University of São Paulo (IAG/USP) and the Seismological Observatory of the University of Brasilia (LG/UnB). Its main objective is to carry out geophysical studies in the Central and South-east sectors of the Tocantins Province, according to the definition adopted by Marini et al., 1984, aiming for a better understanding of the tectonic structuring of the region and its current dynamic. Despite having a reasonably well-known geology (Marini et al.,1984; Pimentel and Fuck, 1992; Fuck et al., 1993; Fuck et al., 1994; Hasui et al., 1994; Trompette, 1994), in addition to a good gravimetric (Marangoni et al., 1995) and aeromagmetic (DNPM, 1984) redeposition, the tectonic structuring and evolution of this province is still the subject of controversy (Trompette, 1994, Hasui et al., 1994; Fuck et al., 1993, 1994; Fuck 1994). The region presents, also, a significant level of seismicity, which characterizes its current dynamic, still not known in detail. The proposal in this study consists of using different geophysical methods (gravimetric, precision geodesic, deep seismic refraction and auscultation of local and teleseismic shocks) to study the structure of the crust and of the upper mantle and its relationship with current seismic activity. The studies that are being proposed in this project, unique in Brazil, seek to use in a combined manner, geophysical and geological tools with a view to better understanding of geotectonic compartmentation of an extensive Neoproterozoic orogenetic area. This research aims to carry out geophysical surveys (gravimetric, seismic refraction and seismological, of local and teleseismic events, and tectonic monitoring using GPS), which interpreted together with existing geological data on the Central and South-east sectors of Tocantins Province, will make it possible to study the tectonic structuring and current dynamic of this region. The goal of these studies will be, initially, to obtain a tectonic model of the velocity of the seismic waves and the density of the geological structures, to be obtained through gravimetric and seismic refraction surveys, which will enable hypocentral localizations of good quality. The current tectonic activity in this region, represented mainly by a moderate level of seismicity, will be ausculted by temporary local seismographic networks, which in addition to precise hypocenters, will provide solutions of focal mechanism, complemented by a high precision geodesic network for tectonic monitoring using GPS, for control of the relative and slow movement, which can exist between the different geological structures of the region. These determinations will be complemented by seismological studies with data from broadband stations, which will make it possible, through signals emanating from distant seismisms (teleseismisms), to estimate the thickness of the crust under the stations and through the recording of phases of the SKS and SKKS type, to measure the anisotropy of the upper mantle under the region. The joint interpretation of the results obtained from these studies and the existing geological data or data being gathered in ongoing projects, principally by the LMNB, will result in a realistic model and one fairly solidly based on the tectonic compartmentation, of the region which covers the Central and South-east portions of the Tocantins Province. Another important result of this project is the gravimetric map of the State of Goiás, in the mold of the gravimetric map of the states of Bahia and São Paulo, which in addition to demonstrating the principal tectonic structures on a regional level will make it possible to identify geological structures of interest for mineral prospection. (AU)