Abstract

An aeromagnetic study was conducted over the Oceanographer Fracture Zone on the Mid-Atlantic Ridge between 33° to 37°N and 31° to 39°W. A sea-floor–spreading interpretation of the magnetic anomalies reveals that the ridge crest is formed of short, en echelon segments 40 to 60 km long. These segments are offset by transform fractures. An average spreading rate of about 1.1 cm/yr active over the last 10 m.y. can be fitted to the ridge crest anomalies 2′ through 5. However, positive identification of the outer flank anomalies is not possible. The ridge crest anomalies younger than 7 m.y. old (anomaly 4) show a general trend of N30°E, but anomalies between 9.3 and 17.5 m.y. old (anomaly 5 to 5′) have trends of about N8°E. The oldest flank anomalies (anomaly 6) trend about N35°E. Application of the anomaly trend superposition technique to account for the offset anomaly and fracture-zone pattern has allowed a new calculation of rotation pole parameters for the North American–African plate systems. For anomaly 2′ (2.7 m.y. ago), the finite rotation pole is located south of Iceland at 58.8°N, 17.4°W, with an angular rotation of 1.26°. For anomaly 5 and the older flank anomalies 5′ and 6, the finite rotation poles are located near Svalbárd at 78.6°N, 34.5°E; 80°N, 29.9°E; and 80°N, 46.1°E, with angular rotations of 2.67, 3.84, and 4.64 degrees, respectively. The major change in the pole location between anomalies 2′ and 5 about 7 m.y. ago appears to have been accompanied by the creation of a new transform fracture pattern with old fractures terminating and new ones being formed. Comparison of the two general pole locations deduced here with poles determined by others for the earlier opening history of the North American–African plate system shows that all finite poles lie in either of these locations. This suggests that a bi-stable dynamic equilibrium condition has prevailed throughout the opening history, with the rotation poles being located south of Iceland during the earliest period (200 to 80 m.y. ago) and the latest period (∼7 m.y. ago to the present) of opening. During the intervening period, the poles were located near Svalbard.

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