Geophysical data collected by R/V Vema during cruises from 1960 to 1973 in the area south of Greenland were used to establish details of the magnetic lineation pattern and the basement morphology associated with an extinct triple junction. The sequence of magnetic anomalies 20 to 24 (49 to 60 m.y. B.P.) is continuous from the western flank of the Reykjanes Ridge into the Labrador Sea north and south of the triple junction and defines a period of simultaneous sea-floor spreading in the Norwegian Sea, the North Atlantic, and the Labrador Sea. At the time of anomaly 20 (49 m.y. B.P.) a major slowdown in the rate of spreading of the Labrador Sea Ridge occurred, and it met with extinction prior to the time of anomaly 13 (∼40 m.y. B.P.). The magnetic lineation pattern shows the amount of opening in the Labrador Sea between the interval spanned by anomalies 13 to 21 and that spanned by anomalies 21 to 23. An independent estimate of the relative motion between Greenland and North America was obtained from the difference in the opening between Greenland and Europe and that between North America and Europe. This second estimate predicts more opening of the Labrador Sea than is observed. Bounds can nevertheless be placed on the early Tertiary relative motion between Greenland and North America, and this motion compares well with the history of tectonic deformation in the Canadian Arctic islands during the Eurekan orogeny.

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