Anomalous electrical structure in the northwestern Paraná Basin, Brazil, observed with broadband magnetotellurics Mauricio S. Bologna a, * , Higo O. Nunes a , Antonio L. Padilha b , Ícaro Vitorello b , Marcelo B. Pádua b a Departamento de Geofísica, Universidade de São Paulo (IAG-USP), Rua do Matão 1226, 05508-090 São Paulo, Brazil b Instituto Nacional de Pesquisas Espaciais e INPE, C.P. 515,12201-970 São José dos Campos, Brazil article info Article history: Received 14 October 2011 Accepted 19 July 2012 Keywords: Electrical properties Magnetotellurics Paraná Basin Lithospheric structure abstract The deep structure of the northern part of the Paraná Basin has been investigated through the analysis of a SWeNE magnetotelluric (MT) profile composed of 24 broadband stations with periods from 0.001 to w1000 s that are regularly spaced w10 km apart. The SW portion of the profile crosses a negative Bouguer anomaly with amplitudes of w20 mGal. Our analyses show that there are two lithospheric domains with different gravity and electric conductivity properties. In the regularized two-dimensional MT inversion model the mid-to-lower crust in the SW part of the profile is almost two orders in magnitude more conductive compared to the NE portion. This difference in conductivity extends to the uppermost mantle depths, suggesting the existence of a major lithospheric discontinuity in the north- central Paraná Basin, locally coincident with the position of the Aporé River. We interpret this discon- tinuity as a juxtaposition of terranes with distinct natures and ages, possibly as result of tectonic accretion during the formation of Western Gondwana. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Intracratonic basins are characterized by their large extensions, the origin and development of which are subject to debate, in contrast to continental margin or foreland basins, which are formed by known tensional processes. A number of hypotheses have been suggested for intracratonic basin development, including increase in crustal density via an eclogite phase transformation, rifting associated with the intrusion of a thermal plume, thermal meta- morphism of the lower crust to the greenschist and amphibolite facies, and mechanical subsidence caused by isostatically uncom- pensated excess mass from igneous intrusions (Klein and Hsui, 1987 , and references therein). The lack of geophysical data capable of corroborating the models and the complexity of the processes responsible for the origin and evolution of these basins are the primary reasons for such uncertainty. Much of the South American plate is covered by intracratonic basins. One of them is the Paraná Basin, which is located mainly in south-central Brazil and is composed of a volcano sedimentary sequence up to 6000 m thick, making studies of the tectonic setting of the basement very difficult. The configuration of the crystalline basement under the basin has been largely inferred from analysis of a few rock samples from drill cores penetrating the basement and from geophysical data (seismic reflection and potential methods) collected mainly by Petrobrás in the attempt to find hydrocarbons (see, for example, Marques et al., 1993). Based on radiometric ages from two basement samples, a cratonic nucleus with an age of 2 Ga was proposed (Cordani et al., 1984; Brito-Neves and Cordani, 1991). In recent years, this single-unit model has been supported by the proposition of the Paranapanema block (Mantovani et al., 2005) based on interpretation of gravity data covering the entire basin. However, using information from additional wells that are mainly concentrated in the central and southeastern portion of the basin, Milani and Ramos (1998) suggested a model of a fragmented craton bounded by Neoproterozoic mobile belts. Receiver function anal- yses give support to this model (Julià et al., 2008). Despite considerable progress in recent years through regional studies using seismic tomography and receiver functions (e.g., Feng et al., 2004; Julià et al., 2008), the structure and nature of the deep crust and mantle lithosphere underlying the basin remain rela- tively unknown. This is due largely to the absence of detailed geophysical studies capable of probing lithospheric-scale depths. The magnetotelluric (MT) method has proven to be suitable for determining lithospheric structures beneath sedimentary basins, as shown in studies in Western Canada (Boerner et al., 1995), the Great Basin (Wannamaker et al., 2008) and the Williston Basin (Gowan et al., 2009). MT is sensitive to a physical parameter (electrical resistivity) that varies by many orders of magnitude within the * Corresponding author. Tel.: þ55 11 3091 2776; fax: þ55 11 3091 5034. E-mail addresses: mauricio@iag.usp.br, mbologna@usp.br (M.S. Bologna). Contents lists available at SciVerse ScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames 0895-9811/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jsames.2012.07.006 Journal of South American Earth Sciences 42 (2013) 74e82