Augustinho Rigoti

2-D geoelectrical model for the Parnaiba Basin conductivity anomaly of northeast Brazil and tectonic implications

Abstract A magnetometer array study in the north-northeast of Brazil has revealed a roughly NE–SW-trending conductive structure in the southeastern part of the intracratonic Parnaiba Basin. The magnetovariational response functions of this structure are numerically modelled to constrain its geometry to facilitate its geological and tectonic interpretation. The 2-D numerical model that incorporates the ocean effect and can account for the spatial and period dependence of the observed response locates the source regions of enhanced conductivity in a graben structure in the basement as well as in a block confined to the central part of the basin with an embedded resistive body. The anomalous electrical character of the sediments in the central part of the basin is consistent with the magnetotelluric data, the graben structure in the basement is corroborated by the aeromagnetic data. The formation of the graben structure is considered to be a manifestation of the extensional tectonics associated either with the Brasiliano orogeny or with the Jurassic–Cretaceous magmatic events. The diabase dikes intruded in the basin in association with the Jurassic–Cretaceous magmatic activity are shown to be accountable for the mapped resistive body entrapped in the conducting Paleozoic sediments. The thermal effects associated with magmatic activities are invoked to produce enhanced conductivity by the generation of carbon through the pyrolysis of hydrocarbon-saturated sediments.

Structural styles of the intracratonic reactivation of the Perimbó fault zone, Paraná basin, Brazil

Abstract The style and origin of intracratonic deformation along the Perimbo fault zone (PFZ) in the Parana basin, Santa Catarina State, southern Brazil, is defined by the integration of outcrop, borehole, aerial photography, and digital terrain modeling data. Typical structures are high-angle strike-slip and oblique-slip normal faults in the Permian sedimentary cover that propagate upward from medium-angle reverse faults in the underlying Precambrian basement. Regional and minor structures suggest blind transtensional faulting and tilting of the overlying strata controlled by the basement heritage. A hypothesis linking deformation between the cover and the basement is proposed on the basis of a structural analysis of a branched fault pattern striking N40–50E and N70–80E. Semi-detailed scale mapping shows that the PFZ has a complex history of polyphase reactivation and is characterized as a plate margin fault in the Proterozoic, evolving to an intracratonic fault in the Phanerozoic, with a main period of reactivation in Permian or Permian–Triassic transition times. The reported data imply that fault reactivation is characterized by normal to left-lateral strike-slip faulting produced by strain propagation from the La Ventana orogenic belt toward the continental interior.

Effects of the equatorial electrojet on aeromagnetic data acquisition

In recent years, considerable advances have taken place in aeromagnetic surveying. These improvements involved data acquisition (instruments and survey design), processing, and interpretation. In addition to improved spatial resolution, the high‐resolution aeromagnetics, as applied to oil exploration, attempts to resolve very low amplitude (1 nT or even subnanotesla) magnetic features (Paterson and Reeves, 1985). These features are caused by weak intra‐sedimentary magnetic sources of magnetite and pyrrhotite, which could have been formed as a result of hydrocarbon seepage (e.g., Reynolds et al., 1990, 1991). For such small spatial variations to be meaningful, it is required that similar temporal and spatial variations due to external sources be corrected accordingly.

Geological Interpretation of the Parnaíba Basin Magnetovariational Anomaly Using a 2D Sheet Current Inversion

The magnetovariational response of a 2D electrical structure in the southeastern part of the Parnaíba basin, north-northeast Brazil, has been inverted in terms of laterally variable sheet current. The resulting inverted current distribution helps to visualize the overall basement configuration as a graben-horst-graben structure or alternatively indicates the presence of a resistive intrusive within a broad graben infilled with conductive sediments. Carbon bearing sediments are suggested as an alternative to hydrated sediments to account for the high conductivity of the mapped anomalies. Heating effects associated with the Cretaceous magmatic activity may be responsible in producing carbon through the pyrolysis of the hydrocarbon-saturated Paleozoic sediments or through the coalification of terrestrial plants.