Abstract
The plate tectonics theory, which explains the tectonic and physiographic aspects of the Earth, has found a good reception among contemporary geophysicists and geologists. The South American plate may be divided in two parts: the Guianas Shield and the Brazilian Shield, separated by the Amazon basin. The seismological station of Brasilia, with its array system (SAAS), has detected an anomaly in the amplitude of seismic events occuring to the North of the Amazon basin, and appears as a loss of energy somewhere on their wave path. Earthquakes with the same magnitude and epicentral distance, occuring in the Chilean or Peruvian Andes, show a larger amplitude at SAAS than the ones occuring in the Colombian or Venezuelan Andes. We therefore think that there is a discontinuity in the wave path of events occuring north of the Amazon basin. The Guianas and Brazilian shields are considered by some authors as individual blocks, based mainly on the Mesozoic and Cenozoic sediments of the Amazon basin separating them, and based also on the different trend of the structural lineaments of both shields. The seismic anomaly detected by SAAS could therefore represent a deeper separation of both shields to such a depth that would cause a discontinuity in these wave paths. This therefore could represent a tectonic anomaly under the Amazon basin.
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