Geoelectric Crustal Structures Off the SW Border of the S˜Ao Francisco

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 horizontal 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 showing 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 defined by stronger MT responses having phase split directions oscillating from WNW–ESE to E–W beneath sites in the central-western area (Paraná 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 feature 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 emplacement. 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, São 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 São Francisco Plate, the extent of lithosphere remobilization by the Amazonas Craton -12˚ Brasiliano orogenies, the nature and physical state of the rocks un- Saõ 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˚ Sá et al. 1993; James & Assump¸cão 1996; Assump¸cão 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 São -28˚ Francisco craton, across the Neoproterozoic Bras´ılia fold belt and into the northeastern margin of the Paraná 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/Poxoréo; 2, Iporá; 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 São 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 interconnected 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 São 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 São 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 São Francisco–Congo cratons and another continental block gin of the Archean–Proterozoic São 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 Paraná 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 information. the Bras´ılia belt, comprising passive margin sequences of the São Specific problems related to the tectonic evolution of this re- Francisco continent (Paranoá, Canastra and Vazante groups), gion have been addressed by several studies focused on spatial backarc and forearc basin sequences (Araxá and Ibiágroups), and and temporal field associations of geochronological, isotopic and ayounger post-inversion platform sequence (Bambu´ıgroup) de- petrological data from cropping out metasediments and granites posited in a foreland basin of the São Francisco craton (Pimentel (see for instance Pimentel et al. 1999, for a comprehensive re- et al. 2001). view). However, because of the blanketing effect of the superficial In the southern segment of the Bras´ılia belt, the area of the units, many geodynamic issues are still controversial and others represent study, the Brasiliano deformation involved overthrust sheets main unsolved. Particularly in the study area, questions reside in the (nappes), transported eastwards at least 150 km towards the

C 2005 RAS, GJI, 162, 357–370 MT survey across the APIP,central Brazil 359

-48˚00' -47˚30' -47˚00' -46˚30' -18˚00'

-18˚30'

-19˚00'

LEGEND

123 4 5 6 7 8 9 10

11 12 19 13

Figure 2. Generalized geological map of the study area (after Schobbenhaus et al. 1984). Main provinces are: Paraná basin (represented by: 1, Cretaceous Bauru group; 2, Jurassic–Cretaceous Serra Geral formation); nappes of the Bras´ılia belt (represented by Proterozoic groups: 3, Araxá; 4, Ibiá; 5, Canastra; 6, Vazante), and the sedimentary cover of the São Francisco craton (7, Proterozoic Bambu´ıgroup; 8, Cretaceous Areado group; 9, Cretaceous Mata da Corda group). Other symbols refer to: 10, Cretaceous alkaline/carbonatite complexes; 11, thrust faults; 12, rivers; 13, MT sites.

platform of the São Francisco craton, and subsequently stacked by during the opening of the South Atlantic ocean and subsequent west- thrust faults over the Bambu´ı pelitic and carbonatic sequence (Fuck ward drift of the continent (Thompson et al. 1998). et al. 1994). From the bottom to the top, the cropping out sequence The Alto Parana´ıba igneous province (APIP), crossed by the MT of imbricated sheets is composed of the Canastra, Ibiá and Araxá profile, is composed of ultrapotassic/potassic and ultramafic/mafic groups. This pile probably overlies the Paranoá rocks and under- diatremes, lavas and hypabyssal intrusions, mainly kimberlites and lies the Paraná basin. A granulitic complex, the Anápolis-Itau¸cu, madupitic olivine lamproites, and kamafugite carbonatite rocks cropping out along a NW–SE belt to the northwest of the area, was (Sgarbi et al. 1998). The former rocks are believed to generally reworked and tectonically emplaced adjacent to lower grade rocks derive from thick and cold lithosphere, whereas the latter rocks are during the Brasiliano event (Pimentel et al. 2000). In the study from shallower depths. area (Fig. 2), the Bras´ılia belt forms a complex structural pattern, The complex and voluminous magmatism occurred either in a the Araxá synform, locally characterized by foliations associated shorter interval from 90 to 80 Ma, according to Gibson et al. (1995), with the thrust sheets and long lineaments corresponding to sub- or in a longer period from 120 to 85 Ma, after Bizzi et al. (1994). vertical lateral ramps or wrench faults originated from differential Diamonds have been explored in this region mostly from con- displacement of the thrust wedges (Seer & Dardenne 2000). Part glomerates of Precambrian and Phanerozoic ages, as well as from of the synform is located in the central-western portion of the MT Cenozoic to recent fanglomerates, colluvial and alluvial deposits, profile. because most of the identified diatremes have not yielded economi- To the east and centre-north, the study area is covered by the cally satisfactory results (Bizzi 2001). Large alkaline-carbonatite Proterozoic foreland basin sequence, predominantly pelites and car- complexes occur to the northwest (Catalão I and Catalão II) bonates of the Bambu´ı and Vazante groups. Cretaceous siliciclastic and southeast (Serra Negra and Salitre) of the profile (Fig. 2). rocks are found in the southwestern corner (Bauru group of the Diatremes of kimberlitic and lamproitic affiliations are concentrated Paraná basin) and in the eastern portion (Mata da Corda and Areado in and around the central region of the profile, whereas kamafugitic groups of the Sanfranciscana basin). associations predominantly occur eastwards, around the limits of Extensive Cretaceous magmatism is also observed in the south- the Mata da Corda outcrops, and northwestwards, in the border of ern portion of the Tocantins province and northern Paraná basin the Paraná basin (Iporá and Poxoréo, Fig. 1). (see Fig. 1 for location). They are referred to in the literature as Alto A remarkable NW–SE continental trend crossing all the distinct Parana´ıba, Iporá and Poxoréo igneous provinces. The magmatism terrains seems to bind the igneous occurrence and other crustal is possibly related to several other Cretaceous alkaline-carbonatite features in the direction of extensive straight lineaments, mostly ge- provinces that evolved contemporaneously around the Paraná basin omorphological and others defined by aeromagnetic data. This is in

C 2005 RAS, GJI, 162, 357–370 360 M. S. Bologna, A. L. Padilha and I.´ Vitorello agreement with the general tectonic grain of the Araxá synform and Hz) and the orthogonal horizontal components of the induced and the Anápolis-Itau¸cu complex. As indicated by geological and electric field (Ex and Ey)toobtain the distribution of the electric geophysical evidence (Hasui et al. 1975; Almeida et al. 1980), the conductivity in the interior of the Earth. In this study, the five electro- APIP is located in a well-defined regional topographic high, approx- magnetic components were recorded at 25 stations along a 180-km imately 100 km wide and 300 km long, along this NW–SE direction. profile, in a NE–SW direction, roughly perpendicular to some of the This high has separated the Paraná and Sanfranciscana basins at least major surface features, such as the direction of the uplifted APIP, since the Palaeozoic, but was more conspicuous in the Cretaceous. It the nappe kinetics and related features (foliation, lateral ramp and appears to have imposed a strong control on the sharp linear edge of thrust faults), alignment of volcanic plugs, and aeromagnetic and the northeastern Paraná basin, as deduced from tapering isopachs, geomorphologic lineaments (see Fig. 2). erosion of the Paraná basin basalts, and provenance of siliciclastic– A commercial single-station wide-band MT system (Metronix volcanoclastic sedimentation to both the Bauru group and Mata da GMS05) was used at every site in a coordinate system with one Corda and Areado groups during the Cretaceous (Hasui & Haralyi of the horizontal axes aligned with the magnetic meridian (20◦W). 1991). The strong uplift of the area seems to be contemporaneous The telluric field variations were measured with 100-m dipoles, with the Cretaceous magmatism, as indicated by fission tracks in in a cross-configuration, with non-polarizable cadmium—cadmium apatite (Amaral et al. 1997). chloride electrodes, whereas the magnetic fields were measured with To the northeast of the study area, the cratonic cover units of induction coils for the two horizontal and one vertical components. the Vazante and Bambu´ıgroups show general structural directions The period range of this equipment is of 0.0008–1024 s and record- around N–S, in concordance with the basement structures of the sub- ing time was typically of at least 24 hr. At 14 of the above sta- basin enclosing the Cretaceous Mata da Corda and Areado groups tions, other commercial remote-referenced long-period MT systems in the Sanfranciscana basin. This direction also agrees with the front (Phoenix LRMT) were operated in the period range of 20 to 13 653 s of the thrust sheets and of a strong high–low gravity pair formed by a with recording time up to 2 weeks under the same geomagnetic horizontal gradient that runs, along the central part of the Tocantins coordinate system. The horizontal telluric fields were acquired province, from north to south and splits into two branches, to the with lead—lead chloride electrodes in a cross-configuration with southeast and to the southwest, underneath the northern border of 150-m lengths, while the three components of the magnetic field the Paraná basin. It has been interpreted as a continental collision were acquired with high-sensitivity ring-core fluxgate sensors. suture (Hasui & Haralyi 1991) and locally related to an ophiolitic melange sequence (Strieder & Nilson 1992). The gravity anomaly is roughly located 200 km west of the MT profile and continues along 3.1 Data quality the southwest of the study area. Evidence of crustal remobilization The first step in data processing involved estimation of the complex- underneath the study area by the Brasiliano orogeny is unknown, but valued MT tensor elements and geomagnetic transfer functions us- it could be minimal if the Palaeoproterozoic basement, cropping out ing the technique of Egbert & Booker (1986). Fig. 3 exemplifies south and west and strongly affected by the Transamazonian orogeny our MT results at two representative sites displaying different data (Teixeira et al. 1989), would extend northwards and eastwards under quality. The combined sounding curves from wide-band and long- the study area. period MT systems are shown in unrotated coordinates for both orthogonal directions. Site 78 is within the realms of the Paraná basin. 3MTDATAACQUISITION Apart from some scattering at specific short periods, associated with AND PROCESSING 60-Hz transmission lines, and larger error bars at the longest periods, The MT method uses simultaneous measurements of natural time attributable to the limits of the recording time, the data appear to variations in the magnetic field components of the Earth (Hx,Hy be of excellent quality. The apparent resistivity curves from both

5 5 78 72 .m) .m) 4 4 Ω Ω ( ( a a ρ ρ 3 3 Log Log

2 2

1 1

90 90

(˚) 45 45 (˚) φ φ

0 0 -3 -2 -1 0 1 2 3 4 5 -3 -2 -1 0 1 2 3 4 5 Log Period (s) Log Period (s)

Figure 3. Unrotated MT results from soundings over the sediments of the Paran´a basin (site 78) and over the metasediments of the Bras´ılia belt (site 72). Apparent resistivities and phases from electric dipole oriented at 20◦NW (triangles) and 70◦NE (squares) directions are shown with error bars representing one standard deviation. Open symbols are data from broad-band MT system, closed symbols are data from long-period MT system.

C 2005 RAS, GJI, 162, 357–370 MT survey across the APIP,central Brazil 361

directions are identical at the shortest period range (0.001 to the text). At periods longer than 32 s (bands 6 to 9), where the fields 0.1 s) and coincident with the results of previous geoelectric mea- are probably sampling the lower crust and beyond, the strike rotates surements directly over the sediments of the Bauru group (Padilha further clockwise, to approximately 45◦W (or 45◦E). et al. 1989). Also, in spite of a small difference in electrode posi- As observed in these rose diagrams, changes in strike direction tion for each MT system as a result of distinct dipole lengths, the occur at transitions from bands 3 to 4 and from 5 to 6, limiting apparent resistivity curve is continuous along the entire data set, an intermediate period interval between 1.3 and 21 s, mostly in varying smoothly with period. We thus conclude that this sounding the mid-crust. The decision to separate the short-period from the is relatively free of near-surface static shift effects. However, the long-period data at 53 s, within band 6, and thus to constrain our phase values at the shortest periods are higher than 45◦,asymp- analyses to the crust has considered the observed transitions from tom that these periods are not short enough to map the transition the upper crust to the mid-crust and the one from the mid-crust to the between the unsaturated and saturated Bauru sediments close to the lower crust. This interval also encompasses the inferred anisotropy surface (clearly detected in the audimagnetotelluric band, Padilha centred at 10 s. A similar approach was previously used on MT data et al. 1989). Phase and apparent resistivity curves in orthogonal from the Southern Canadian Cordillera (Marquis et al. 1995) and on directions show a small split in the 0.01–100 s period interval, in- theoretical studies of the limitations of 2-D interpretations of 3-D dicative of a departure from the 1-D condition in the crust. data (Ledo et al. 2002). Data quality at site 72, in the Bras´ılia belt, is much less satisfac- The vertical to horizontal magnetic transfer functions, normally tory, showing large scattering at short periods as a result of cultural represented as induction arrows, are also robust indicators of re- noise, a mismatch in apparent resistivity curves from the two differ- gional conductivity structures and, under certain conditions, can ent MT systems and biased estimates of apparent resistivities from help define an unambiguous 2-D strike angle. Reversed real induc- the magnetic north–south channel at periods around 10 s. Severe tion arrows, pointing towards zones of enhanced conductivity, were distortions at short periods indicate that cultural noise spreads over calculated for several periods representing distinct levels of the crust large distances and is consequently stronger in the resistive terrains and upper mantle (Fig. 5). Excepting for an anomaly mainly at 32 s of the study area. Discontinuity in apparent resistivity is associ- located close to the Parana´ıba river, their magnitudes are generally ated with near-surface heterogeneities that generate static shifted very small, which implies that no meaningful regional vertical field data. Biased results around 10 s indicate that the data at this sta- is present for most of the area. The near absence of a regional ver- tion are probably affected by polarized signals in the South Atlantic tical magnetic field implies that no significant lateral conductivity magnetic anomaly during magnetically disturbed periods (Padilha variation might occur at the subsurface below or that the measure- 1995). This last effect was minimized during inversion by empha- ments were carried out just above the centre of a regional conduct- sizing the unbiased phase estimates. Besides the inhomogeneity of ing feature, which in the present case could be attributed to crustal the upper crust, as indicated by the phase split at the shortest pe- conductors. riods, the presence of another anomalous structure in the crust is At 32 s, however, a number of arrows at the northeastern part of evidenced by the strong phase split in the 0.3–100 s period band and the profile reach magnitudes around 0.5, indicative of a conductivity the large difference in apparent resistivity at all periods longer than anomaly in this region. The presence of anisotropic layers within the 0.01 s. crust (to be discussed in a later section) may disturb both amplitude and direction of the arrows (Pek & Verner 1997), thus making it difficult to define the location of the high-conductivity zone. As the period increases, the arrows tend to become aligned in a SW 3.2 Distortion corrections, geoelectric strike direction, indicating a gradient at lower crust and beyond, towards directions and induction arrows the Paraná basin. The MT response functions were subsequently analysed to correct for local distortion effects caused by local near-surface inhomo- geneities and to determine the appropriate geoelectric strike di- 3.3 Mid-crustal anisotropy rection, using the Groom–Bailey matrix decomposition technique (Groom & Bailey 1989). The data set for periods between 0.01 and Station 72 (Fig. 3) shows a large phase difference between the two 13 653 s was subdivided into nine bands, roughly equally spaced orthogonal directions in the period range 0.3–100 s. It exempli- in a log scale. Data at periods shorter than 0.01 s were discarded fies the results that are observed from most of the sites with the because the decomposition model was inadequate for such periods maximum difference in the orthogonal phases being found at a pe- (model of distortion fits the data with large mean chi-squared mis- riod of approximately 10 s. To represent the overall pattern of this fit). Within each band, the best-fitting period-independent twist and phase split, we followed the phase-difference technique proposed by shear parameters and regional 2-D strike were determined. Kellett et al. (1992). Fig. 6 shows the variations in azimuth and am- Fig. 4 displays the unconstrained Groom–Bailey strike directions, plitude of the phase split, calculated at periods around 10 s for each in the geographical coordinate system, determined from all periods sounding. The azimuth at each site indicates the most conductive at all 25 sites grouped into the nine period bands. The first three direction and the length of the bar represents the amplitude, propor- bands, grouping periods shorter than 1 s, which correspond to signal tionally to the phase difference between the most conductive and its penetration down to approximately 6–8 km into the upper crust, orthogonal direction. present relatively uniform geoelectric strike trending E–W (or N–S It can be observed that the azimuth and the amplitude of the because of the 90◦ ambiguity in the determination of the strike). At enhanced conductivity vary strongly over horizontal distances of the longer periods, from 1.3 to 21 s (bands 4 and 5), the fields are sensing order of a few tens of kilometres. In the southwestern stations, over mainly the middle crust. The strike is somewhat more diffuse, cleft the sediments of the Paraná basin, the high-conductivity structure into two distinct directions and slightly rotated clockwise from the has a WNW–ESE direction. No direction has been plotted for site above (from 0◦ to 30◦). Such a result is affected by the behaviour 70 because the large noise in the dead band has prevented the correct of a mid-crustal anisotropy centred at 10 s (to be discussed later in determination of the phase split. In the stations away from the basin,

C 2005 RAS, GJI, 162, 357–370 362 M. S. Bologna, A. L. Padilha and I.´ Vitorello

0.01 - 0.04 s 0.06 - 0.25 s 0.33 - 1 s

N N N

W E W E W E

S S S

1.3 - 5 s 8 - 21 s 32 - 106 s

N N N

W E W E W E

S S S

160 - 640 s 853 - 3413 s 5120 - 13653 s

N N N

W E W E W E

S S S

Figure 4. Rose diagrams giving the variation of the best-fitting Groom–Bailey regional azimuth at nine different period bands between 0.01 and 13 653 s. The ambiguity of 90◦ in the determination of the strike angles is also represented. the strikes change anticlockwise, getting to be almost E–W, and in a particular area and their vertical magnetic field is close to the magnitudes gradually increase towards the centre of the profile zero. (station 16, with a phase split of 45◦). To the northeast of this station, To perform our analysis, the stations are subdivided into three the magnitudes remain large but the azimuth changes clockwise main areas: (i) within the Paraná basin in the southwest; (ii) the to NNE–SSW, at stations to the northeast of the Parana´ıba river. Bras´ılia belt in the centre; and (iii) the exposed metasediments of Finally, another slight variation occurs close to the northeastern end the São Francisco craton in the northeast. Within each of these areas, of the profile, with the maximum conductivity trending N–S and the strike and the amount of phase splitting presented in Fig. 6 can a significant decrease in magnitude being observed at station 85. be considered to be very similar at every site, making the anisotropy At the easternmost station (84), no anisotropy is observed in the hypothesis more likely than a localized conductivity anomaly. Also, cratonic platform. the very small magnitude of the vertical geomagnetic field at most Models incorporating electrical anisotropy within conducting lay- stations favours the anisotropy model. On the other hand, the major ers are being increasingly reported in recent years for several re- switch in the phase split directions at stations close to the Parana´ıba gions of the world (Eisel & Haak 1999, and references therein). The river, also characterized by longer local induction arrows, points to possible models that can explain the above phenomena are a local an anomalous contribution from a likely conductor body. conductivity anomaly associated with discrete conductor bodies, an It appears that an electrically anisotropic mid-crust controls the intrinsic electrical anisotropy in the crust, or the occurrence of both behaviour of most of the sites in this study. The orientation of in the study area. Tests can be performed to distinguish the predom- the highly conductive structure varies smoothly from site to site inance of either of the two main models by evaluating the phase along most of the profile. Noticeable changes seem to be correlated differences occurring at different sites and the behaviour of the ver- with the boundary between the basement under the Paraná basin tical geomagnetic field (Bahr et al. 2000). In general, anisotropy is and under the Bras´ılia belt, in the southwestern part of the profile, the favoured model if the phase splitting is similar at most sites and mainly between the crust under the Bras´ılia belt and the São

C 2005 RAS, GJI, 162, 357–370 MT survey across the APIP,central Brazil 363

T = 11 s T = 0.13 s T = 5.3 s

-18˚

´ba river ranai Pa

-19˚ 0.5

T = 32 s T = 130 s T = 520 s

-18˚

-19˚

-48˚ -47˚ -48˚ -47˚ -48˚ -47˚

Figure 5. Real induction arrows from single-station transfer functions for six periods. Arrows have been reversed to point towards induced internal currents.

-48˚00' -47˚30' -47˚00' -46˚30' 4FORWARD AND INVERSE MODELLING 40˚ The lateral variability in anisotropy directions indicates a 3-D crustal complexity, yet the available data are too sparse for a full 3-D modelling. On the other hand, detailed testing has shown that it is im- -18˚00' possible to ﬁnd a regional 2-D strike direction if stations located 84 to the northeast of station 16 are included (Bologna 2001). In fact,

86 38 regional decomposition models are inadequate in the region of the 76 85 16 profile where the large-scale switch in the major anisotropy direction 31 11 72 36 82 37 occurs. Consequently, these stations have been excluded from the 15 14 following quantitative analyses, which include only stations in the 21 18 -18˚30' 74 22 central and the western side of the profile. Sites within the Paraná 19 81 basin are only minimally affected if a slightly different strike direc- 70 tion is chosen to decompose the data. Most of the included stations 79 75 24 show a common east–west strike direction (Fig. 4). The strike an- 78 25 Pa gle that best fits all these sites, over the 0.01–53 s period range, r anai -19˚00' ◦ ´ba river is 82 W, in close agreement with the main direction of the crustal anisotropy. km In a more rigorous test of the 2-D characteristics of the impedance 0 10 20 tensor, the seven-parameter Groom–Bailey distortion model, with a strike parameter fixed at 82◦W, and the twist and shear parameters LEGEND frequency invariant, was fitted to the data of the selected stations. Examples of apparent resistivity and phase data corresponding to Phanerozoic cover Carbonatite complexes Proterozoic rocks electromagnetic induction in orientation parallel and perpendicular thrust fault river 19 MT site to the strike direction are shown in Fig. 7 at sites 78 (Paraná basin) and 81 (Bras´ılia belt). It can be seen that even after rotation, phase Figure 6. Variation of azimuth and amplitude of the mid-crustal anisotropy splitting is still not significant at long periods. at the sounding sites, represented by the phase difference between orthogonal On the other hand, curve splitting at short periods, associated with estimates of the electric and magnetic fields (see text for explanation). The data are for a period of 10 s where the maximum phase split occurs. near-surface anisotropy, were corrected at each site by shifting apparent resistivity data of both polarization modes to their geometric mean at periods shorter than 0.03 s. Finally, the absolute levels of Francisco craton, in the northeastern end. The anisotropic azimuth the apparent resistivity curves at each site (static shifts) were esti- variation can, thus, explain the strike directions and small scat- mated as part of the 2-D inversion procedure. The approach used was tering in the interval of 1.3–21 s representing the mid-crust in to fit the phase data from all stations and the apparent resistivities Fig. 4. only from stations within the Paraná basin under well-constrained

C 2005 RAS, GJI, 162, 357–370 364 M. S. Bologna, A. L. Padilha and I.´ Vitorello

5 5 78 81

.m) 4 4 .m) Ω Ω ( ( a a ρ ρ 3 3 Log Log

2 2

1 1

90 90

(˚) 45 45 (˚) φ φ

0 0 -3 -2 -1 0 1 2 3 4 5 -3 -2 -1 0 1 2 3 4 5 Log Period (s) Log Period (s)

Figure 7. Rotated apparent resistivities and phases at sites over the Paran´a basin (78) and the Bras´ılia belt (81). TE mode is represented by triangles and TM mode by squares.

SITE 79 SITE 19

4 0 4 0

3 3 .m) .m) Ω Ω ( ( a a 2 10 2 10 ρ ρ

Log 1 Log 1 Depth (km) Depth (km) 0 20 0 20

90 90

(˚) 45 (˚) 45 φ φ

0 30 0 30 -2 -1 0 1 2 0 1 2 3 4 5 -2 -1 0 1 2 0 1 2 3 4 5 Log Period (s) Log Resistivity (Ω.m) Log Period (s) Log Resistivity (Ω.m)

Figure 8. 1-D layered-earth models obtained by inverting the apparent resistivity and phase curves from stations 79 and 19. Data and models are presented in the minor (squares and dashed lines) and major (triangles and solid lines) axes of anisotropy, which, for 2-D structure, correspond to the TM and TE polarization modes, respectively. conditions, and to allow a larger misfit of the apparent resistiv- layered-earth models that fit the responses with the smallest number ity data from the other stations. A small phase misfit ensures that of uniform layers. The error floor for the 1-D inversion was assumed the modelled apparent resistivity curves present the same shape as to be 1◦ in phase and 3.5 per cent in apparent resistivity. the observed curves, yet allows a frequency independent shift. The The result from station 79 is typical of the Paraná basin stations, model then reflects the regional trends of the apparent resistivity where superficial low-resistivity Palaeozoic sediments extend to a data without distortions from local features caused by static shifts. depth of some hundreds of metres. Below this, the resistivity in- Despite being very small, the static shift values estimated in this creases by 2 orders of magnitude or more until a depth around 3 km. waywere used to correct each site prior to 1-D modelling. A relatively thin zone of low resistivity appears at this depth beyond which structures begin to depart from the 1-D behaviour. In fact, a resistive layer starting at approximately 4 km shows quite different 4.1 1-D inversion resistivities in orthogonal directions. The depth of the anisotropic A simple approach to obtain a model of the conductivity structure layer structure under this station is 10 km with a confidence limit that causes splitting in the sounding curves is to perform a 1-D of less than 1 km. Its conductance (depth integrated conductivity) inversion of each polarization at each site (e.g. Kellett et al. 1992; is 194 S in the high-conductivity direction and less than 15 S in the Eisel & Haak 1999). Fig. 8 shows two examples of inverted apparent perpendicular direction. Thus, the conductance ratio between both resistivities and phases in the period range of 0.01–53 s, assuming directions is 13 or larger.

C 2005 RAS, GJI, 162, 357–370 MT survey across the APIP,central Brazil 365

Site 19 lies in a more resistive region, directly over the metased- SW NE iments of the Bras´ılia belt. The main difference from station 79 is the absence of the conductive layer associated with the sedimentary 0 78 79 75 81 19 22 21 15 11 16 package at the surface. Again, the structures are not 1-D below the conductive layer located at 3-km depth. The anisotropic layer is observed at a deeper level (around 14 km) and the conductances are 10 215 S in the 82◦W direction and less than 18 S in the perpendicular direction. The conductance ratio at this station is at least 12. Similar calculations were performed for all stations. It was ob- 20 served that, in most cases, the representative values of the anisotropic Depth (km) layer are nearly the same, with a maximum conductance around 30 200 S and the conductance ratio in the range of 12–13. The only exception was observed at stations close to the centre of the profile in Fig. 2, coincident with the occurrence of the largest amplitude of 40 the phase split (Fig. 6). At station 16, for instance, the conductance 0 20 40 60 80 100 along the high-conductivity direction is 500 S and the conductance Distance (km) ratio of the perpendicular directions is 22. However, these quantitative 1-D results must be treated with caution as the data are obviously affected by the varying strike in the 0.01–53 s period range (Fig. 4) ohm-m 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 and the close proximity of regional 2-D structures. Log[ ρ ] 4.2 2-D inversion Figure 9. 2-D resistivity cross-section obtained by REBOCC inversion us- To determine whether the 1-D inversions represent well the 2-D ing both TE and TM modes distortion-decomposed apparent resistivity and earth, only data from the western side of the APIP profile were in- phase data of selected stations in the central-western side of the APIP profile. verted using 2-D algorithms. As there are no MT inversion codes available for anisotropic data, we used a 2-D inversion for isotropic Some shallow conductivity features above the 3-km depth in the models to recover the anisotropy following the procedure described 2-D model of Fig. 9 are clearly associated with geological structures by Heise & Pous (2001). An algorithm that seeks to estimate identified at the surface. To the southwest, there is a 3–40 m con- static shifts and generate a smooth model simultaneously (REBOCC, ducting layer that reaches a thickness around 1 km under station 78 Siripunvaraporn & Egbert 2000) was used, and 10 sites were chosen but seems to lessen under station 79. Such a feature would corre- for modelling based on their stability, quality and minimal scatter. spond to the sediment layer of the Paraná basin. The occurrence of At these sites, data were taken at 12 periods between 0.01 and 53 a 1000–2000 m continuous layer to the northeast of station 79, s, equally spaced in logarithm scale, in both polarization modes with a thickness up to 2–3 km, agrees with the estimated resistivity (transverse electric (TE) and transverse magnetic (TM), with cur- for the Neoproterozoic metasediments and their expected thickness rents parallel and perpendicular to the electrical strike, respectively). (Campos Neto 1984). The induction arrows have not been interpreted together with MT There are also two zones of enhanced conductivity in the model responses because their typical errors are as large as the arrows at greater depths. The upper zone lies at depths between 2–5 km, themselves. Also, theoretical studies have shown that the arrows are has resistivities of 40–150 m and extends along the whole pro- severely affected when anisotropic structures are present (Heise & file. The mean conductance of this zone is around 15–30 S and it Pous 2001). can be related either to an anisotropic layer at shallow depths or As previously explained, we preferentially fitted the phase data to isolated conductive pockets within the resistive basement. The from all the selected sites and the apparent resistivity from the sites middle crust anisotropic layer is mapped mainly at depths of 10 to within the Paraná basin that seemed to be little affected by static 20 km, and appears in the model as a horizontal sequence of sec- shifts. Accordingly, error tolerances for apparent resistivity from tioned conductors and non-conductors. It must be considered that sites outside the basin were set at 24 per cent, whereas an error floor 2-D inversion algorithms usually give the smoothest model that fits of 3 per cent was imposed on the apparent resistivity of sites within the data, which means the largest width of the smallest number of the basin and on phases from all sites. structures. Therefore, for our profile only two conductors alternating The final 2-D resistivity model derived from the REBOCC algo- with resistive structures are sufficient to explain the data. A more rithm is shown in Fig. 9. The model responses fit the apparent re- realistic anisotropic model with several narrower alternating struc- sistivity and phase data with a rms of 2.78. A comparison between tures would also fit the data, as discussed by Eisel & Haak (1999). the data and model responses is shown in Fig. 10 through pseudo- Using their formulae to calculate an approximation of the intrinsic sections for the phases and apparent resistivities of both polarization anisotropy ratio from the average width and resistivities observed modes. There are excellent agreements between the calculated and in the model, we estimated a ratio of 12.3, in close agreement with observed data, with larger misfits being observed in the TE apparent the results from 1-D inversions. resistivities at sites located on the Bras´ılia belt, probably as a result The pseudo-sections of apparent resistivity and phase shown in of static shift effects. The robustness of the model was tested in dif- Fig. 10 illustrate also the non-isotropic structure of the study area. ferent inversions, by adding and removing sites, changing the inver- Noteworthy is the higher conductivity of the TE data, mainly at sion parameters, and using a different inversion code (RRI; Smith & 10 s, also highlighted by the separated inversions of both polariza- Booker 1991). The main features presented in Fig. 9 were, however, tion modes (not shown here). The alternating resistive–conductive required to fit the data. A more detailed model sensitivity analysis clustering at the middle crust in the TM–TE joint inversion (Fig. 9) was performed during the forward modelling to be described in the is remarkably similar to the 2-D inversion obtained by Heise & Pous next section. (2001) for a synthetic anisotropic model.

C 2005 RAS, GJI, 162, 357–370 366 M. S. Bologna, A. L. Padilha and I.´ Vitorello

Measured Calculated

ohm-m

3.5 -1 Log( 3.0 ρ

2.5 a

0 ) [TM] 2.0 1 1.5 Log[ periodo (s) ] 1.0 ohm-m

3.5 -1 Log( 3.0

2.5 ρ 0 a ) [TE] 2.0 1 1.5 Log[ periodo (s) ] 1.0 degrees 90

-1

60 φ [TM] 0 30 1 Log[ period (s) ]

0 degrees 90

-1

60 φ [TE] 0 30 1 Log[ period (s) ]

0 0 30 60 90 0 30 60 90 Distance (km) Distance (km)

Figure 10. Comparison of ﬁeld and model apparent resistivity and phase data for the model illustrated in Fig. 9. The left column gives the ﬁeld data, whereas the right column gives the modelled data. The top two pseudo-sections are the apparent resistivity responses, whereas the bottom two are the phases.

model for the TE and TM modes of the 10 selected sites. A good 4.3 Forward modelling and sensitivity tests data fit was achieved for the phases with the major discrepancies The sensitivity of the measured data to the basic features of the inver- in apparent resistivities being probably related to limitations of the sion model was tested through 2-D numerical forward modelling. In model to represent near-surface conductivity variations. order to examine whether the presence of crustal anisotropic layers At the southwestern end of the profile, the superficial volcanic– is meaningful, the modelling was carried out using the forward fi- sedimentary layers of the Paraná basin are modelled with a thickness nite code developed by Pek & Verner (1997). This code can be used of some 100 m and overall resistivity of 20–50 m (not shown in the to simulate 2-D anisotropic conductivity structures with arbitrary model of Fig. 11). As expected, the conductance is higher towards horizontal directions, allowing to represent any orientation of the the centre of the basin. Resistivity (1500 m) and thickness of the anisotropy axes, different from the regional model strike. metasedimentary rocks cropping out to the northeast of the basin The half-space of the earth was represented by a grid of blocks, were fixed to the values derived from the inversion. In the model, defined by 34 columns and 27 rows (plus 10 air layers). The starting they are prolonged under the basin because tests indicated that a model was based on the 2-D inverse model and the responses were fit resistive layer must be present between the basin and the underlying to the data by a trial-and-error approach. The final model is shown in anisotropic layer. Fig. 11 and is nearly the same as in Fig. 9 but includes two anisotropic Tests demonstrated that the data were most sensitive to the depth layers. The shallower layer has principal resistivities of 60/200/ and the total conductance of both anisotropic layers, whereas the ρ /ρ 60 m(ρ ξξ/ρ ηη/ρ zz in the notation of Pek & Verner 1997), while ξξ ηη anisotropy ratio was more critical in the deeper layer. This the principal resistivities of the deeper layer are 30/600/30 m. For was probably as a result of the low ratio of the shallow layer. Both both layers, the anisotropic strike was kept the same as the structural layers are underlain by very resistive regions, with resistivity values strike (α = 0◦). Fig. 11 also shows a comparison of apparent resis- poorly constrained and strongly dependent on the resistivity within tivity and phase between the data and the responses from the final the anisotropic layers above them. The high resistivities extend deep

C 2005 RAS, GJI, 162, 357–370 MT survey across the APIP,central Brazil 367

5 78 79 75 81 19 4 .m) Ω (

a 3 ρ 2 Log 1 90

(˚) 45 φ 0

5 22 21 15 11 16 4 .m) Ω (

a 3 ρ 2 Log 1 90

(˚) 45 φ 0 -2 -1 0 1 2 -2 -1 0 1 2 -2 -1 0 1 2 -2 -1 0 1 2 -2 -1 0 1 2

Log Period (s) Log Period (s) Log Period (s) Log Period (s) Log Period (s)

78 79 75 81 19 22 21 15 11 16 0 1500 Ωm 60/200/60 Ωm (α = 0˚)

<105 Ωm 10

30/600/30 Ωm (α = 0˚) 20 Depth (km)

30 <104 Ωm

40 0 20 40 60 80 100 SW Distance (km) NE

Figure 11. 2-D forward modelling with the anisotropic algorithm of Pek & Verner (1997). Lower panel shows the ﬁnal anisotropic model, in which anisotropy strikes are at the same direction as the structural strike (α = 0◦) and the conductivity tensors of the anisotropic layers are given by the principal resistivities ρ ξξ/ρ ηη/ρ zz. Upper panel shows the apparent resistivity and phase of data (triangles for the TE mode, circles for the TM mode) and model responses (lines) for the 10 sites along the proﬁle.

into the upper mantle to depths around 80–100 km. Lower resistiv- and mid-crustal anisotropic layers with the high-conductivity zone ities were required beyond these depths by the higher phase values that generates the anomaly. at periods of 100–10 000 s (see Fig. 7). To fit the TE-mode data at periods in the range 1–10 s at the three 5DISCUSSION AND southwestern most sites, a deep conductor beyond the end of the pro- INTERPRETATION file was included under the Paraná basin (not shown in Fig. 11). Such a conductor could be possibly related to some major anomaly under The results indicate distinct geoelectric behaviour within three hori- the basin, in agreement with the magnitudes and WSW direction zontal sections of the crust and also in three lateral areas transversal of the real arrows at the southwestern sites for periods longer than to the MT profile. 130 s (Fig. 5). On the other hand, anomalous geoelectrical structures The upper 10 km (periods up to 1 s) are dominated by a relatively possibly present close to the very complex northeastern end of the homogeneous highly resistive layer (larger than 10 000 m), except profile, which generate large induction arrows at periods around 32 s for a more conducting subhorizontal anisotropic zone of approxi- (Fig. 5) and the major change in the mid-crustal anisotropy direction mately 15–30 S with a top at approximately 1–2 km (periods around (Fig. 6), do not appear to affect the MT data at the sites along the pro- 0.1 s). The conductivity of this anisotropic zone tends to be more file. This is another aspect to be considered in the interpretation of enhanced towards the northeast (Figs 9 and 11). 1-D inversions of the induction arrows observed in that region. A more complete study stations excluded from the 2-D modelling show that this conducting will be necessary, including data acquisition at other sites and 3-D layer would also extend to the northeast of the modelled profile. Be- modelling, to interpret possible coupling effects between the upper- cause it extends beyond the eastern limits of the Bras´ılia belt, this

C 2005 RAS, GJI, 162, 357–370 368 M. S. Bologna, A. L. Padilha and I.´ Vitorello conductor is not likely related to a cataclastically fractured fault zone required to generate the anisotropy. On the contrary, a network of that would be formed at the sliding base of the transported units, a fluid-filled, partially aligned cracks would probably generate signif- likely possibility if brine-filled pores would be preserved. Also, the icant mid-crustal seismic reflectors. basal detachment of the nappes exposed in the region is dominated The complex behaviour of the above conductive structure is by mylonites, which present less porosity as a result of the higher probably related to the major magmatic and deformational events deformation intensity. As a result of its lateral extension of more recorded in the study area and thus can provide an important marker than 100 km, and a lack of geological and geophysical evidence horizon to unravel the regional tectonic history. The main regional- for the presence of a near-surface extensive mineralized zone in the scale tectonic event in this region was related to the continental region, it is also unlikely there is an association of this conducting convergence and collision in the final stages of amalgamation of the layer with metallic conduction. western Gondwana. Precise ages and circumstances of the collision A more viable alternative for the conductor is supported by are still disputed. Metamorphic peaks around 600 Ma have been geophysical evidence. Reservoir-induced seismicity, reported by linked to the E–W collision between southern São Francisco craton Assump¸cão et al. (2002) for the general area of this study, indicates a and the cratonic block beneath the Paraná basin involving west- stress-related seismogenic layer within the resistive upper few kilo- ward continental subduction (Campos Neto & Caby 2000). How- metres of the crust. Particularly, seismicity occurring around two ever, geochronological studies indicate a much more complex early reservoirs (Nova Ponte and Miranda) near the southernmost site of Brasiliano (ca 790 Ma) collision event in the southern part of the the MT profile defines a nearly E–W transpressional fault plane. This Bras´ılia belt (Pimentel et al. 1999). seismic evidence supports the hypothesis of electrolytic conduction Residual fluids and graphite evolved from this large-scale Pro- through interconnected brines in a subhorizontal layer within the terozoic orogeny would be dissipated during the long time interval crystalline basement and maintained by regional stress. since the event (Wannamaker 2000) or loose the original connectiv- In terms of strike directions, there is an agreement of the results of ity by brittle deformations (Mareschal et al. 1994), especially during Fig. 4 with the above mentioned seismicity and other geophysical the Cretaceous uplift. However, petrological studies (Ribeiro et al. data, such as MT soundings at a contiguous area (Padilha et al. 2001) indicate that the widespread Mesozoic magmatism yielded 1992) and gravity (Ussami 1999), indicating the predominance of large amounts of CO2-H2Ovolatiles, a substantial portion of which the E–W electrical strike for the upper crust. This is slightly different could have remained trapped as hypersaline fluids or precipitated from the directions of the main geological structures observed at the as solid carbon at depths of sudden drops in pressure and temper- surface of the study area, which include a system of WNW–ESE ature, provided the oxygen fugacity was not too high for graphite. sinistral strike-slip faults and dykes. It can then be suggested that It is envisioned that favourable percolation conditions would have such structural features do not provide the primary control on the been generated by the simultaneous decompression during the up- upper conductive layer in the APIP. lift, opening cracks and fractures especially along weak subparallel The mid-crust, from 10 to approximately 20 km depth (periods of planes of relic structures. Thus, a network of graphite and/or fluid- 1–20 s), is characterized by an anomalous conductive section, which filled cracks and fractures aligned with the tectonic grain could is an apparently common feature found in many studies around the have generated the observed mid-crustal conductor, as suggested by world (e.g. Jones 1992). Generically, the tops of these conductors Shankland et al. (1997). seem to be located at depths greater than 10 km and their conduc- Lateral differences in the Mesozoic intrusive and extrusive mag- tance would vary apparently as a function of several parameters (e.g. matism at APIP and neighbouring regions would have affected the tectonic characteristics, age of structures). An implied causal rela- Proterozoic mid-crustal structures in different ways along the MT tionship between conducting layers and the brittle–ductile transition profile. In the APIP itself, around the central region of the MT as proposed by several authors (e.g. Gough 1986; Jones 1987), but profile, the magmatism is mainly kimberlitic and lamproitic. These contested by Simpson (1999), is discarded for the study area based are explosive types of magmatism with a relatively rapid journey on the unlikely presence of a warm geotherm needed for the transi- from the mantle to the surface, which is unlikely to form subsurface tion depth at only 10 km, considering heat-flow data (Vitorello et al. magma chambers. It is hypothesized that such volcanism would 1980) and the tomography of anomalous positive seismic velocity not necessarily rearrange the mid-crustal structures and the relic for the lithosphere (Feng et al. 2004). Furthermore, the proposals E–W direction would be preserved. However, to the southwestern given below are constrained by the estimated electrical strikes, the and northeastern termini of the profile, quite different situations depths of the conductors and the anisotropic orientation, associated are observed. To the southwest, the Palaeozoic sediments of the with the geological evidence. Paraná basin were covered during the Early Cretaceous by the Serra Bahr et al. (2000) have argued that the occurrence of electrical Geral formation, enclosing an important event of continental flood anisotropy is a more common feature rather than an exception for basaltic volcanism (Fig. 2). Locally in the study area, aeromagnetic models that link the presence of crustal conductors to direction- data show the existence of a prominent system of NW–SE trend- dependent crack geometries. In the present study, interconnected ing linear anomalies crossing the southwestern end of the MT pro- brines and carbon, in the form of graphite, are considered to be the file, which have been interpreted as associated with deep fractures major source of the 250–400 S enhancement of conductivity at mid- filled by basic dykes probably related to the Paraná basin volcanism crustal depths, in concordance with conditions documented by the (Bosum 1973; Hasui et al. 1975). The close parallelism between crustal deep hole in Europe (ELEKTB group 1997). Saline fluids the electric anisotropy and the magnetic lineaments is a strong alone would be sufficient to explain the results, yet in a voluminous constraint for macro-anisotropy in this region, with the disconti- amount, whereas only a very small fraction of the conductive solid nuities provided by the dykes controlling the electric current flow phase would be required to generate a conductance of hundreds of at mid-crust. To the northeast, the Cretaceous volcanic and sedi- Siemens. A distinction between these two hypotheses could be pos- mentary sequences of the Mata da Corda and Areado groups were sible if detailed seismic reflection data were available. The carbon affected by large-scale synvolcanic deep normal faults (Campos & model would not probably create important seismic reflectors be- Dardenne 1997). Furthermore, there is evidence, both in the field cause only a very small fraction of the conductive phase would be and satellite imagery, for tectonic controls of the extrusives along

C 2005 RAS, GJI, 162, 357–370 MT survey across the APIP,central Brazil 369

major rift-related NNE–SSW structures (Bizzi et al. 1994, 1995). southwestern end, indicates a probable influence of Early Cretaceous It is proposed that magma-derived brines and carbon concentrated basaltic dykes related to the Paraná igneous province. along these structures are responsible for the mainly NNE–SSW Finally, it must be remembered that MT data models are always mid-crustal anisotropy to the north of the Parana´ıba river. Moreover, non-unique and modelling becomes particularly complicated by the it is speculated that the pervasive magmatism would also extend fur- presence of many other structures outside the profile. The proposed ther to the southwest of the river, concentrating in the pre-existing model incorporates and is compatible with all available geophysical fractured hinges and then enhancing the conductance in that region. and geological information, but many unanswered questions remain On the other hand, disappearance of the anisotropic layer beneath that will need additional MT array data and other complementary the São Francisco craton is only registered by a single station and geophysical studies. still needs to be corroborated by additional data. The lower crust, at depths larger than 20 km (periods between 20 ACKNOWLEDGMENTS and 53 s), has resistivities in the range 1000–10 000 m throughout the profile. The main strike direction (NW–SE or SW–NE) indicated This study was supported by research grants and fellowships from by the rose diagrams of Fig. 4 is not significant because the MT data Funda¸cão de Ampora a Pesquisa do Estado de São Paulo (95/0687- 4, 00/00806-5 and 01/02848-0) and Conselho Nacional de Desen- along the profile are nearly 1-D at long periods (see Figs 3 and 7). c These results are valid to depths of at least 80 km, into the uppermost volvimento Cientifico e Technologico (142617/97-0, 350683/94-8 mantle (Bologna 2001). The resistivity values for the lower crust and 351398/94-5). Relevant logistical support from De Beers Brasil are consistent with laboratory studies on silicic dry rocks at tem- is also acknowledged. The authors are grateful to Sérgio Fontes, peratures appropriate for the depths involved (Kariya & Shankland for loaning the long-period MT systems of Observatório Nacional, 1983). For the uppermost mantle, the results agree with laboratory Marcelo Pádua, for assistance in fieldwork and data processing, measurements of dry olivine under upper-mantle conditions, which Josef Pek, for providing the anisotropic code, and reviewers Alexan- predict not only a resistive mantle but also a mantle with no signif- der Gatzemeier and Heinrich Brasse, for the suggestions and useful icant anisotropy (Constable et al. 1992). Thus, our results suggest comments that helped us to improve the manuscript. that the deformational effects of the Brasiliano orogeny in the study area were more intensely felt within the brittle upper and interme- REFERENCES diate crust because it appears not able to imprint a geoelectrical Almeida, F.F.M., Hasui, Y., Davino, A. & Haralyi, N.L.E., 1980. Informa¸cões signature in the deeper parts of the lithosphere. geof´ısicas sobre o oeste mineiro e seu significado geotectônico, An. Acad. Bras. 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