Regional gravity modelling and geohistory of the Parnaiba Basin (N.E. Brazil)

Abstract

The Parn8.1ba Basin is one of the three large Palaeozoic intracratonic basins found in Brazil, the others being the Amazon and Parana. Parnaiba is an ovalshaped basin situated in NE Brazil and its area is about 600.000 km2 • Gravity data have been collected mainly along accessible roads crossing the Parn81ba Basin and merged with existing data bases of several Brazilian Institutions. The collection of all available geological and geophysical data included several gravity profiles crossing the eastern half of the basin. These profiles are part of a much larger data set forming polygons over a large proportion of the Brazilian territory. The whole gravity network has been internally adjusted and referred to the International Gravity Standardization Net 71. The net has been found precise to ±0.041 mGal. Although the distribution of gravity stations is not ideal, Bouguer and free-Air anomaly maps have been produced, these being the first gravity maps for the whole basin. Contrary to the basin physiography, the Bouguer map unexpectedly shows elongated gravity lows with NE-SW and NNW -SSE directions. These are parallel to the 'l'ransbrasiliano Lineament and subparallel to the Araguaia Fold Belt, respectively. A first attempt at interpreting the gravity anomalies resulted in the proposal of an anomalous, denser zone at lower crustal depths. The tectonic subsidence in the basinal area was estimated through systematic backstripping using 22 boreholes which reached the metamorphic or sedimentary basement. A non-uniform lithospheric stretching model was used as a first approximation for the modelling of the tectonic regime. This procedure was suggested by the presence of several grabenlike structures, as confirmed by shallow seismic sections, exploratory boreholes and gravity maps. Assuming an elastic rheology for the lithosphere, a regional W-E tectonic subsidence profile could be reproduced employing an axisymmetric subsurface load and a flexural ridigity of 0.2 x 1024 N m. Results of the present study are consistent with the apparent bimodality of the flexural ridigity of the continental lithosphere.