3rd International Congress of the Brazilian Geophysical Society

The Brazilian Lithosphere Seismic Project is part of a multinational geological and geophysical program to investigate the structure, tectonics, and geologic evolution of the continental lithosphere beneath SE Brazil. The project, which will run for a period of at least 18 months, is a collaborative effort by the Carnegie Institution of Washington and the Institute of Astronomy and Geophysics of the University of Sao Paulo. The broad band seismic studies are aimed at mapping the fine structure of the continental crust and lithospheric mantle beneath Brazil and interpreting it in terms of the origin, evolution, and composition of the Brazilian shield. The first nine stations of a planned 14 station portable array of digital REFTEK dataloggers and the broad band threecomponent Streckeisen STS-2 seismometers have been installed, or are in the process of being installed, in southeast Brazil (see Figure 1). The seismic array traverses the four major tectonic provinces of the region --- the Archean Sao Francisco craton, the surrounding late Proterozoic BrasilianolPan-African mobile belts, the intracratonic Parana Basin, and the coastal Ribeira belt, a complex transcurrent shear system. Stations are configured about 50-100 km apart in an approximately east-west saw-tooth array along 200 S latitude, with an E-W aperture of approximately 800 km. The station configuration is designed to capitalize on the abundance of high-quality seismic sources, particularly Andean, that surround and illuminate the Brazilian shield. Digital data are recorded continuously at 10 slsec and in triggered mode at 50 slsec. Timing is via GPS clocks and is accurate to better than 1 msec absolute. The matched STS-2 three component recording covers the frequency band 0.0083 to 20 Hz. Preliminary results from receiver function analysis indicate that crustal thicknesses beneath the Sao Francisco craton and surrounding Archean crust are about 40 to 42 km. Crustal velocities in these regions increase approximately linearly with depth and there is no evidence of major midcrustal discontinuities. Results from receiver functions and from two station surface wave phase velocity inversion indicate that mantle shear wave velocities are high, about 4.8 to 4.9 krn/sec as in other cratonic areas, in the uppermost mantle beneath the Sao Francisco craton and surrounding area and to at least 130 km depth beneath the Parana Basin.

O Enxame de Diques Rio Ceará Mirim (EDCM) situa-se no nordeste do Nordeste Oriental do Brasil e constitui uma entidade alongada E-W em forma de leque, que se extende por mais de 500km longitudinalmente, e que tem, na sua porçăo ocidental, uma largura de mais de 100km. Os diques do EDCM distribuem-se desde o oeste de Natal (RN) até o centro do Estado do Ceará (figura 1).

This paper describes, in a preliminary form, the new, extensive program of deep seismic reflection lines using special acquisition equipment and processing to 18 s TWT (seconds, two-way travel time) using state-of-the art seismic procedures. This program started in 1989 and aims at comparing crustal structures of different basins along the continental margin of northern and northeastern Brazil, a region with contrasting styles of deformation. The rifting process that resulted in the break-up of Pangea originated both Atlantic-type passive margin basins and sheared passive margin basins, with different crustal geometries and subsidence patterns. The regional seismic lines will also help in the evaluation of geological features that are difficult to characterize using standard petroleum industry seismic surveys. Besides tectonic studies that will focus on the geological interpretation and understanding of basinforming processes, the program will also provide a more reliable visualization of rift-phase structures In the lower part of the sedimentary sequence, both in shallow and deep water regions, and the regional characterization of oceanIc fracture zones, intraplate magmatism and halokinetic/lutokinetic structures.

La Sa. De Pie de Palo pertenece al sistema de Sas. Pampeanas, es un afloramiento aislado con forma de escudo elongado en dirección aproximada N-S, de unos 30 Km de ancho por 78 Km de largo. Se extiende desde los 31° hasta los 31°42' de latitud S, y desde los 67°42' hasta los 68°8' de longitud W (Fig. 1).

Se analiza en este trabajo una nueva sección gravimétrica en 32° Sur (Figura 1) preparada con datos de distintas fuentes: a - Universidad Nacional de San Juan, Universidad Nacional de Rosario e Instituto Geográfico Militar Argentino, para el sector Argentino. b - Para el sector Chileno se extrajeron de Introcaso-Pacino (1988), en el cual para el tramo oceánico, las anomalías de Aire Libre fueron transformadas en anomalías de Bouguer sustituyendo el agua y los sedimentos por masas de igual volumen y densidad 2,8 g/cc.

Using the three interface Coherence Function model of Bechtel et al. (1987), the effective elastic plate thickness, Te of the shield area in southern Brazil-Uruguay has been determined. The Tevalue of 114 km corresponds closely with values obtained for similar tectonic provinces. Intercrustal loads are largely the legacy of the Brasiliano cycle shield geology, whereas the eastwest Moho topography is best explained in terms of both flexural buckling origins of the Chaco-Parana Basin and crustal thinning related to the rifting of the South Atlantic.

Em geral os estudos de temperatura do solo săo feitos com metodologias voltadas para os problemas agricolas e se limitam até um metro de profundidade (de Vries. 1975; Ribeiro e Santos, 1975; Ghuman and Lal, 1982; Santos et al., 1988). Por outro lado os estudos de sondagens convencionais visam a determinaçăo do fluxo geotermal profundo e portanto evitam a camada influenciada pelo aquecimento solar, isto é, as perfilagens se iniciam a partir de 30 a 40 metros de profundidade.

O fenômeno da ressurgęncia costeira que ocorre mais intensamente nos meses de verăo na regiăo de Cabo Frio manifesta-se pela ocorręncia de águas superficiais provenientes das camadas profundas, mais frias e ricas em nutrientes. Este fenômeno constitue o suporte físico de uma cadeia biológica: nutrientes-fitoplancton-hervívoros, etc., de alta produtividade. O mecanismo básico da ressurgęncia foi proposto inicialmente por Thorade (1909), como uma aplicaçăo da teoriade Ekman (1905). Este modelo propőe como agente causal, o esforço produzido pela componente de vento paralela a costa (componente leste do vento para o caso de Cabo Frio). O desenvolvimento do mecanismo de Ekman está associado ŕ ocorręncia de uma corrente superficial paralela ŕ costa a favor do vento.

As camadas mais superficiais do subsolo amazônico săo predominantemente constituidas por latosolo argiloso amarelo. Apesar do seu alto teor de argila o subsolo é poroso, o que permite a rapida percolaçăo o da precipitaçăo pluviométrica na superficie. Por outro lado, em regiőes desmatadas observa-se uma compactaçăo deste latosolo (Silva et al, 1992), aumento do escoamento das águas pluviais e elevaçăo de cerca de 5° Celsius da temperatura média á superficie (Souza et al., 1993). Esse processo de aquecimento, rebaixamento do nivel freático e perda de porosidade do solo e subsolo amazônicos constitui um processo de intemperismo que leva a formaçăo de blocos de rocha lateritica que pode inutiliza-lo para agricultura e outros objetivos econômicos.