Abstract
Information about the onset of spreading in the South Atlantic and the rate of motion of the South American plate can be developed using Brazilian alkalic and basaltic rocks and the stratigraphy of the coastal basins. The first basalts are 147 m.y. old and herald the start of general domal uplifts. With continued uplift, fracturing parallel to the coast led to diabase dike filling and the Jacupiranga and Anitápolis nodes of alkalic activity, 122 to 138 m.y. ago. This may be the time the first triple junctions developed in the area: two arms of each became sites of plate movement and outline the present coast; the third arm failed and was the locus of basaltic or alkalic activity. Normal faulting and deposition of typical rift-filling sediments occurred at this time.
After a Barremian hiatus, both alkalic and basaltic activity was renewed about 110 m.y. ago. The first marine sediments and thick evaporite sequences were laid down, so plate accretion must have begun. The Minas Gerais–Goiás belt of alkalic rocks was emplaced in a restricted zone within a foldbelt formed during the Pre-cambrian Brazilian cycle in a phase lasting from 91 m.y. until about 65 m.y. ago. A hot-spot–South America relative motion approximately 1.4 ± 0.5 cm/yr in a northwest-southeast direction is suggested for this time.
The littoral belt of alkalic rocks developed from São Sebastião to Cabo Frio starting about 82 m.y. ago and lasting until about 59 m.y. ago. This was about the same time as a northerly shift in the pole describing Africa–South America motion and accompanied an increase in the half–spreading rate to about 2.0 cm/yr. The South America–hot-spot relative motion for the past 59 m.y. is inferred from the track of seamounts leading to Trindade Island. If it is the same hot spot or a similar one to that which was on the coast, then the rate has been about 2.3 cm/yr.
