Determination of the electrical resistivity structure of the crust and lithospheric mantle between the Goiás massIf and the Amazonian Craton

Abstract

Electrically, the southern and southeastern margins of Amazonian craton are characterized by the existence of one of the most important anomalies of the South American Platform named Paraguay-Araguaia Conductivity Anomaly (PACA). It is an elongated feature of high electrical conductivity with more than 1200 km of extension, approximately parallel to the geological contact between the Amazonian craton and the Paraguay and Araguaia belts. Its origin is possibly related to Neoproterozoic/Cambrian convergence processes, and may indicate the closure of a large ocean basin. Unlike in the south of the craton, where its geometry and position were well resolved due to the availability of magnetotelluric data, in the southeast margin the PACA is much less known since it was inferred exclusively with basis on geomagnetic data. In order to solve this deficiency of information, we propose here the realization of a magnetotelluric (MT) profile of about 300 km in length crossing orthogonally the northern segment of the PACA. The east end of the profile is located in the Goiás Massif, near the city of Niquelândia. Towards the west, the profile crosses the Goiás Magmatic Arc and the Araguaia Belt, ending on the exposed rocks of the southeastern Amazonian craton. Its position approximately coincides with an existing profile of deep seismic refraction and constitutes an extension of a previous MT profile between the São Francisco craton and the Goiás Massif. The proposed MT profile will be composed of about 27 stations separated by 12 km. In all stations, five-component broadband MT systems will be used, operating in the frequency range from thousandths of hertz to some thousand of seconds. This frequency range should allow us to investigate from hundreds of meters to several tens of kilometers deep. In addition, long-period systems will extend the period range in some of the stations to increase the depth of investigation, ensuring that upper mantle structures will be sampled. For this study, the IAG/USP will provide two broadband MT systems. Other broadband as well as some long-period systems will be borrowed from the Geophysical Equipment Pool of Brazil, maintained at the National Observatory and conceived within the scope of the Petrobras Geotectonic Studies Network. A set of two-dimensional inversion and robust processing techniques will be applied to the MT data. In interpreting the MT results we will integrate magnetic and gravimetric data available in the region. The resources for collecting MT data from this proposal will come from the ongoing FAPESP thematic project, process 2016 / 06114-6. Our project fits within the scope of the ongoing Thematic project, more specifically in Work Package 4, by addressing the deep structure of the main Neoproterozoic tectonic boundary of Brazil between the Amazonian craton, the Goiás Massif and the São Francisco craton. (AU)