Geophys. J. Int. (2005) 162, 357–370 doi: 10.1111/j.1365-246X.2005.02643.x Geoelectric crustal structures off the SW border of the Sao˜ Francisco craton, central Brazil, as inferred from a magnetotelluric survey Maur´ıcio S. Bologna, Antonio L. Padilha and Icaro´ Vitorello Instituto Nacional de Pesquisas Espaciais - INPE, C.P. 515, 12201-970 S˜ao Jos´e dos Campos, Brazil. E-mails: [email protected] (MSB); [email protected] (ALP); [email protected] (IV) Accepted 2005 March 17. Received 2005 January 21; in original form 2004 June 4 SUMMARY A magnetotelluric (MT) survey of the crust beneath sedimentary basins and thrust sheets along the southwestern edge of the S˜ao Francisco craton, central Brazil, reveals intricate electrical characteristics that are interpreted in the context of Proterozoic collision tectonics and hori- zontal transport of allochthonous rock units, emplacement of Cretaceous hypabyssals, lavas and diatremes of ultrapotassic–mafic composition, and occurrence of induced seismicity. The data exhibit strong distortions represented by 3-D induction effects and galvanic disturbances resulting from shallow structures, frequency and site dependence of electrical strike, and in- homogeneous anisotropic layers with smoothly varying phase split, conductance and azimuth of the highly conductive direction. Geoelectrical and orthogonal phase difference directions, GJI Geomagnetism, rock magnetism and palaeomagnetism 2-D inversion, and forward modelling characterize three distinct subhorizontal sections show- ing two anisotropic conductors within a highly resistive crust, laterally segmented into unique blocks. The model for the uppermost crust section has E–W geoelectric directions and a 15–30 S anisotropic zone at a depth of approximately 1–2 km along the entire profile. This conducting layer is interpreted to represent a brine-filled fracture layer possibly controlled by the present-day state of crustal stresses, as disclosed from reservoir-triggered quakes. The mid-crust section presents a deeper conducting zone located at depths below 10 km. It is de- fined by stronger MT responses having phase split directions oscillating from WNW–ESE to E–W beneath sites in the central-western area (Paran´a basin and allochthonous cover units) and NNE–SSW in the northeastern region (autochthonous platform units and Sanfranciscana basin). Anisotropy is greater than an order of magnitude in the highly conductive direction, with conductance in the range of 250–400 S. Conjecturally, the source of this anomalous fea- ture would come from interconnected grain-boundary phases and hypersaline fluids, exsolved and precipitated from upwelling Cretaceous magma. In the central-western area, favourable trapping of conductors was constrained along a nearly E–W direction, feasibly associated with relic structures inherited from Brasiliano/Pan African continental collisions. Along the northeast, however, the coincidence with superficial NNE–SSW structural directions suggests a localized direct causal relationship with the trend of extension related to magma emplace- ment. The lower crust has a highly resistive quasi–1-D section along the entire profile that prevails also at uppermost mantle depths. Thus, whereas the brittle crust would have recon- ciled subhorizontal strain with fluid percolation related to uplift and magma emplacement, a mechanically coupled and stronger lower crust/upper mantle would have controlled the deep magma generation during Cretaceous distention pulses. Key words: Alto Parana´ıba igneous province, central Brazil, crustal structure, electrical anisotropy, magnetotellurics, S˜ao Francisco craton. general, MT models require bulk rock masses with a much lower 1INTRODUCTION conductivity than expected from the solid-state conduction of the Interpretations of magnetotelluric (MT) data for studies of geody- most common silicates and carbonates. In the crust, except for the namic processes are often hindered by the poor understanding about particular cases of exotic carbon- and sulfide-rich lithologies, the ob- the origin and agents of conductivity in deep-Earth conditions. In servation of enhanced conductivity at depth is frequently attributed C 2005 RAS 357 358 M. S. Bologna, A. L. Padilha and I.´ Vitorello still controversial delimitation of the hidden western border of the S˜ao Francisco Plate, the extent of lithosphere remobilization by the Amazonas Craton -12˚ Brasiliano orogenies, the nature and physical state of the rocks un- Sa˜o Francisco derlying the thrust sheets and cratonic cover units, and the search Craton 1 for either a deep lithospheric root that could explain the occurrence of Cretaceous kimberlites and lamproites, or traces of a root-erasing DF process linked to the upwelling mantle material accountable for the -16˚ 2 multicompositional Cretaceous volcanic complexes. Suitable deep geophysical information, which could contribute to the solution of 3 such problems, is scarce and only regional potential-field and local- BH ized seismographic analyses are presently available (Bosum 1973; -80˚ -40˚ -20˚ S´a et al. 1993; James & Assump¸c˜ao 1996; Assump¸c˜ao et al. 2001). Parana´ Basin A long-term MT programme is under way in the region to sup- 0˚ plement the above techniques with a large-scale reconnaissance of 4 BRAZIL SP major conducting geostructures that could have persisted as records -24˚ of past episodes. 5 cean We discuss here geoelectrical signatures of the crust observed along an MT profile aligned in a NE–SW direction, from the south- tlantic O A -40˚ ern margin of the Phanerozoic Sanfranciscana basin over the S˜ao -28˚ Francisco craton, across the Neoproterozoic Bras´ılia fold belt and into the northeastern margin of the Paran´a basin, which probably rests on a different cratonic basement (site 3 of Fig. 1, blown up in -60˚ -56˚ -52˚ -48˚ -44˚ -40˚ Fig. 2). At most of the MT soundings, the data period ranges from Figure 1. Schematic outline of the major geological provinces of central- 0.0008 to 13 653 s, which allows the vertical imaging of geoelectric southern Brazil and main alkaline-carbonatite occurrences (modified from structures from the near surface (tens of metres) to great depths into Ulbrich & Gomes 1981): 1, Paranatinga/Poxor´eo; 2, Ipor´a; 3, Alto Parana´ıba the upper mantle (more than 150 km). The model to be presented is igneous province (APIP); 4, Serra do Mar; 5, Ponta Grossa. Locations of however restricted to discussion of the data in the interval limited some major cities are indicated as: DF, Bras´ılia; BH, Belo Horizonte; SP, by periods shorter than 53 s. This interval has an inductive scale S˜ao Paulo. The square box enclosing site 3 refers to the study area. of the same order of the regional crustal thickness, estimated from seismic data to be approximately 40 km (Berrocal et al. 2004). to proportionally minute amounts of high conductors in intercon- nected grain-boundary arrangements (Wannamaker 2000). Given 2GEOLOGICAL CONTEXT the appropriate environmental constraints (temperature, pressure, permeability, geochemistry), the commonly mentioned conductors The South American platform is composed of a Precambrian central are saline fluid, graphite, metallic and partial melt material (Jones core bordered by active orogens to the west and northwest, and of 1992). At higher mantle temperatures, solid-state conductivity of a Mesozoic–Cenozoic passive continental margin to the northeast olivine enhanced by hydrogen diffusion is another likely candidate and east. The core is formed by several Archean to Mesoprotero- (Karato 1990). zoic blocks amalgamated during the Neoproterozoic Brasiliano/Pan To make the interpretative link from the present-day measured African orogeny in the final assembly of Gondwana (Almeida et al. conductivity to past geological environments, it becomes necessary 1981). In central Brazil, the S˜ao Francisco craton, which was contin- to elucidate the origin of the conductor and of a required intercon- uous with the African Congo craton prior to the opening of the South nectivity, and how both were preserved or modified by processes Atlantic, presently encompasses a substantial part of the southeast- operating throughout the intervening geological time. This is bet- ern Brazilian highlands. Brasiliano fold-and-thrust belts fringe all ter accomplished by confronting the electromagnetic induction data sides of the craton with strong vergence towards its interior (Campos with available geological and geophysical data from the study area Neto 2000). in order to diminish the inherent ambiguity arising from the many Flanking the western margin of the S˜ao Francisco craton, the plausible causes of conductivity. Bras´ılia belt represents the external portion of a large Neoprotero- In central Brazil (Fig. 1), a complex history of multifaceted ge- zoic orogen in central Brazil (the Tocantins province; Fig. 1), which ological events, marked by Proterozoic continental collisions and developed in response to the convergence between the Amazon Cretaceous mantle magmatism, characterizes the southwestern mar- and the S˜ao Francisco–Congo cratons and another continental block gin of the Archean–Proterozoic S˜ao Francisco craton. The deep crust presently covered by the Phanerozoic sedimentary and volcanic is hidden by thrust sheets, cratonic cover units and basin rocks, mak- rocks of the Paran´a basin (Pimentel et al. 2000). Proterozoic meta- ing it an ideal region to test the MT potential to unravel underlying morphic rock units of varied nature and age constitute most of structural and tectonic