Fernando Flávio Marques de Almeida

The origin and evolution of the South American Platform

Abstract The South American Platform is defined as the stable continental portion of the South American plate not affected by the Phanerozoic — Caribbean and Andean — orogenic zones. It is surrounded by these orogenic zones and extends to the marginal Atlantic coast. The basement of the platform consists of Archean and Proterozoic continental crusts arranged during three main sets of orogenic events: (1) Trans-Amazonian (Paleoproterozoic), (2) Late Mesoproterozoic and (3) Brasiliano/Pan African. The latter resulted in the consolidation of the youngest mobile belts of the platform basement. It is, by far, the main phenomenon responsible for the overall pattern of tectonic components (cratonic nuclei and fold belts) and the formation of the general structural framework at the time when the platform was a portion of the Gondwana supercontinent. During the Phanerozoic Eon, different cover stages were developed through six main sedimentary cratonic sequences, of which the last one is exclusive to the South American continent. The final individualization stages and their respective post-Paleozoic sequences were accompanied by a series of specific intracratonic processes, both tectonic (rift basins, overprint of new structural styles in previous basins) and magmatic (basaltic and alkaline). The activation processes have generally been attributed to the opening of the Atlantic Ocean on the east and the Andean orogeneses on the north and west. Nevertheless, a minor part of these events may have been caused by sublithospheric actions (mantle-activated processes) beneath the interior of the platform.

DEZ MOTIVOS PARA A INCLUSÃO DE TEMAS DE GEOLOGIA NA EDUCAÇÃO BÁSICA

Education is privileged to form a responsible and consequent citizenship due to the introduction of a broad environmental concern, amongst other concepts. There are at least ten reasons for the insertion of a geologic culture in this framework. The insertion will widely benefit the Brazilian education, as follows: (1) the Science curricula of basic education is fragmentary and too much superficial. (2) the humanist formation, inherent to the practice of Earth Sciences, should induce solidary and humanist attitudes in the new generations, develop critical thinking and the capacity of observation. Geology allows one to think about the rational use of the technological applications and advances of Science; it supplies (3) a broad vision of the Earth Systems, needed to understand the complex dynamics of the planet. The theoretical body of Geology brings a (4) time perspective of the changes that have affected the planet and living organisms. The particular situation of Geology, the unique science which has experienced a scientific revolution in the XX th Century, offers the students an (5) education on the causes of geologic risks and their consequences for the human kind, and allows many examples on (6) participation of Geology in modern discoveries of Science. It introduces the most current discussion of (7) the question of the available resources versus sustainability of life. It also constitutes a (8) preparation and orientation for further studies or critical reflections on the human activities on Earth. More can be learned on the methodological aspects of Science, since Geology favors (9) an education on varied scientific procedures. The Information Society, whose dominant role is felt in all sectors of human activity, constitutes another reason: (10) Geosciences helps to create a planetary perspective. The development of a geologic culture extends beyond the mere domain of the recent advances of Science & Technology; therefore, it allows to bring the real world into the classroom and, overall, it allows to move the classroom towards the real world. Geology concepts help a search for a more practical and efficient education, since it is supported by an existential reality that will allow people to retain such knowledge through life.

MAGMATISMO PÓS-PALEOZÓICO NO NORDESTE ORIENTAL DO BRASIL

Since the Jurassic, the area of Northeastern Brazil was affected by several magmatic events, both intrusive and effusive ones. The ages of the igneous bodies are not perfectly known, but it can be stated that some sedimentary basins accomodated prominent part of the geologic record from the igneous activity. The area north to the 8°S parallel was more intenselly affected, comprising deposits in the Parnaiba, Potiguar, and Lavras da Mangabeira basins. The occurrences are relatively scattered on Precambrian areas, restricted to zones very far from the Sao Francisco Craton. There are three main age groups. The Jure-Cretaceous bodies (pre-Aptian) are composed of tholeiitic basalts normally without olivine. The Albian and Tertiary ones usually exhibit modal olivine and alkaline composition. It seems that a quiet phase occurred comprising the Neocretaceous and the Paleocen. Based on aeromagnetic and geologic maps at a regional scale,it has been possible to show that some areas were strongly affected by igneous activity. Part of them can be followed along hundreds of kilometers, in a shape of igneous allignements. The most expressive are the Rio Ceara-Mirim, the Mecejana-Fernando de Noronha, both trending E-W, and Macau-Queimadas, trending NNW. The former is related to Neocomian, and the others belong to the Tertiary. There are no Quaternary igneous activity in the continental area, but some oceanic islands were evolved until the end of the tertiary.

REVISÃO DO LIMITE DA PLATAFORMA SUL-AMERICANA

According to modern knowledge about the Argentinian tectonics, the meridional limit of the South America Platform is revised. The extreme southern limit is situated under the Claromeco Basin in the Province of Buenos Aires and the southwestern one leaves the Pampean Foldbelt out of the platform.