Plate Tectonics, Paleomagnetism, and the Opening of the Atlantic

Finite rotations of the African, European, and North and South American plates about the poles of Bullard and others (1965) have been employed to reconstruct the ancient configuration of the Atlantic Ocean. The rates and duration-times for the motions have been estimated from sea-floor spreading interpretations of marine magnetic profiles and deep-sea drilling results across the Mid-Atlantic Ridge. The central Atlantic, Caribbean, and Gulf of Mexico began to form in Triassic time about 200 m.y. ago, with the drift of Africa and South America away from North America. The South Atlantic first opened about 150 m.y. ago as South America separated from Africa. The formation of the North Atlantic was somewhat more complex. Initial opening began in the Rockall Trough between 200 and 150 m.y. ago. The Bay of Biscay formed during the period 150 to 80 m.y. ago. The Labrador Sea opened between 80 and 45 m.y. ago. The present spreading regime along the Reykjanes Ridge and Mid-Atlantic Ridge north of the Azores began about 65 m.y. ago. Paleomagnetic poles for the respective continental plates for various geologic time intervals have also been subjected to the same finite rotations as the continents. A world mean polar wander path reconstructed from the rotated paleomagnetic poles reveals that the entire Atlantic plate system has gradually shifted northward toward the present spin axis as opening progressed.