GLACIAL GEOLOGIC CONFIRMATION OF AN INTRAPLATE BOUNDARY IN THE PARANABASIN OF BRAZIL

The predrift fit of South America and Africa remains problematic given the inability to match all parts of the coastlines simultaneously. Ea rly opening of the South Atlantic Ocean may have been facilitated by i ntraplate deformation in either South America or Africa, but the locat ion of such deformation is controversial and poorly constrained. Broad -scale remote sensing has suggested an intraplate boundary in South Am erica, stretching from the Cochabamba bend in the Andes to the Rio Gra nde-Walvis rise, but the inferred boundary crosses the Parana basin of Brazil where any evidence of deformation is obscured by Late Jurassic flood basalts. A subsurface study of the upper Paleozoic glaciogenic infill of the Parana basin (the 1300-m-thick Itarare Group; ca. 300-26 0 Ma) identifies repeated episodes of intracratonic rifting on either side of an accommodation zone crossing the basin along the trend of th e inferred intraplate boundary. Renewed influence of this zone during the opening of the South Atlantic is suggested by the asymmetric distr ibution of successive magma units within the Serra Geral lava pile and by the trend of dike swarms. The intraplate boundary identified withi n the Parana basin may extend westward along the northern margin of th e Chaco basin in Bolivia.