PALEOMAGNETISM OF MESOZOIC IGNEOUS ROCKS FROM THE MARANHAO BASIN, BRAZIL, AND THE TIME OF OPENING OF THE SOUTH-ATLANTIC

From Middle-Upper Jurassic volcanics at the western margin of the Maranhão Basin (6.4°S, 47.4°W) 15 sites (121 samples) have a mean magnetization direction D = 3.9°, I = -17.9° with α95 = 9.3°, k = 17.9 after AF cleaning (all sites have normal polarity). This yields a pole (named SAJ2) at 85.3°N, 82.5°E (A95 = 6.9°) which is near to the other known Middle Jurassic South American pole. For 21 sites (190 samples) from Lower Cretaceous basalt intrusions from the eastern part of the Maranhão Basin (6.5°S, 42°W) the mean direction is D = 174.7°, I = +6.0° with α95 = 2.8°, k = 122 (all sites have reversed polarity) yielding a pole (SAK9) at 83.6°N, 261°E (A95 = 1.9°) in agreement with other Lower Cretaceous pole positions for South America. Comparing Mesozoic pole positions for South America and Africa in the pre-drift configuration after Bullard et al. [13] one finds a significant difference (with more than 95% probability) for the Lower Cretaceous and Middle Jurassic poles and also a probable difference for the mean Triassic poles indicating a small but probably stationary separation of the two continents from the predrift position in the Mesozoic until Lower Cretaceous time which may be due to an early rifting event. © 1979.