Geophysical studies made in the southern Labrador Sea and related to the drilling of deep holes there, have given evidence of the change from a two-plate to a three-plate spreading geometry in the evolution of the North Atlantic and Labrador Sea. The change is dated at about 60-65 m.y. ago and can be recognized by the magnetic anomaly pattern and the basement topography.

By use of magnetic anomalies and fracture zones, the stages in the evolution of the North Atlantic can be described as follows.

1. 180 m.y. African plate started to separate from North American plate accompanied by shearing between Africa and Europe.

2. Between 180 m.y. and 80 m.y. Separation of European and North American plates along Greenland-Spain fracture zone gave rise to primitive Iceland basin and Rockall trough and separated Spain from the Grand Banks. Orphan Knoll, Porcupine Bank, Flemish Cap, and Galicia Bank were detached and displaced.

3. 150 m.y. to 80 m.y. Spain rotated counterclockwise about rotation pole in Paris to open Bay of Biscay.

4. 80 m.y. to 60 m.y. Spreading occurred between North American plate and plate comprising Greenland, Rockall plateau, and northwest Europe, to create Labrador Sea and North Atlantic.

5. 60 m.y. Rockall plateau separated from Greenland along new spreading axis on east side of primitive Iceland basin. Triple junction developed, and spreading axes and fracture zones shifted to accommodate new geometry.

6. 60 m.y. to 47 m.y. Simultaneous opening of Labrador Sea, Reykjanes Ridge, and North Atlantic.

7. 47 m.y. Greenland virtually stopped moving relative to North America, and Labrador Sea growth finished.

8. 47 m.y. to present. Reykjanes Ridge and North Atlantic grew as European plate separated from North America-Greenland plate.

Paleogeographic reconstructions have been made and their validity tested against the data obtained from drill sites on Leg 12 of the Deep Sea Drilling Project.

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