Sea-Floor Spreading in the North Atlantic
The magnetic anomaly lineation pattern in the North Atlantic Ocean (between the latitudes of 15° N. and 63° N.) has been examined in ligln of the hypotheses of sea-floor spreading and plate tectonic . There is no evidence of significant subduction or deformation along the,margins of the. Atlantic since the Late Triassiez asd thus the sea-floor spreading that has occurred since that, time has resulted in, continental drift only.
The rate and direction of drift between Europe and North America and between Africa and North America have differed at all times since the Late Triassic. Although Eurasia may have been rifted from North America in the Jurassic, the major phase of drift did not begin until the Late Cretaceous. Separation varied from 5.0 to 4.0 cm/yr (at a latitude of 45° N.) from the Cretaceous until 53 m.y. ago. The rate of separation slowed about 53 m.y. ago. The average rate was slightly less than 2 cm/yr for ths intervals from 53 m.y. to 38 m.y. ago and from 38 m.y. to 9 m.y. ago. The sediment , discontinuity found by others at about the location of anomaly 5 on both flanks of the Mid-Atlantic Ridge, north of the Azores, thus cannot be explained by a discontinuity or drastic slowing in the rate of spreading. From 9 m.y. to the preset, separation has been at a rate somewhat greater than 2.0 cm/yr.
The initiation of rifting between Africa and North America may have occurred 200 m.y. ago. However, we have assumed that the active ,phase of drift did not begin until 180 m.y. ago. The separation proceeded at an avenge rate .jof 4:0 cm/yr from 180 m.y. to 81 m.y. ago; 3.4 cm/yr from 81m.y. to 63 m.y. ago; 2.4 cm/yr from 63 m.y. to 39 m.y. ago; 2.0 cm/yr frdm f 38 m:y. to 9 nj.y. ago; and 2;8 cm/yr 9 m.y. ago to the present (the rates are computed for a latitude of 36° N.).
We have fitted together lineations of the same age but from opposite, sides of the ridge axis in the same fashion that previous workers have fitted together continental margins. Each fit is described by a pole and angle of rotation about the pole. Each fit gives the paleogeographic relations of the respective continents ana oceanic plates for the particular age of the lineation.
We conclude from these paleogeographic reconstructions that there was probably no Late Cretaceous (81 m.y. to 63 m.y. ago) sea-floor spreading, in tlfe Arctic, but that the relative motion between Erurasia and Nopth America in the Arctic region was compres stonal fturing this interval. This compression may have been accommodated by subduction at Bowers Ridge (which appears to be an inactive island-arc trench system) and subduttion in eastern Siberia. It also may have been accommodated by compressional deformation in the Brooks Range, the Verkhoyansk Mountains, and the Sverdrup Basin (in central northern Canada).
All the spreading in the Arctic region that has occurred since the Late Cretaceous has taken.place in the last 63 m.y. The locus of this spreading had been the Mid-Arctic Ridge which lies between the Lomonosov Ridge and the Eurasian continental shelf. The effect of this spreading has been jto separate the pre-existing Lomorjosov Ridge from the , Eurasian ccontinental shelf. The Alpha Cordillera has not been the locus of sea-floor spreading in the Cenozoic.
The exact pattern of the separation of Greenland from North America is not known. There may have been minor rifting in The Labrador Sea durine the Jurassic. However, the major phase of drift .occurred from the Late Cretaceous to the late pocene. The final separation of Eurasia (Spitsbergen), Greenland, and North America did not occur until the middle Eocene.
The pattern of magnetic lineations suggests that the well documented counterclockwise rotation of the Iberian Peninsula occurred between the Late Triassic and the Late Cretaceous, and that there has been little if any counterclockwise rotation subsequent to time
We have used the derived poles and the angular rates of rotation to compute isochrons Which give the age of the. basement in the North atlantic. The basement ages agree wellwith other data such as those obtained as the result of JOIDES-drilling. The isochrons sometimes give greater ages which can be reconciled with the, drilltig / results by.involving subsequent volcanism, but in, no case do the isochrons give smaller ages. The Keathey sequence of magnetic anomalies which lie just seaward bf the quiet zone and southwest of Bermuda in.the western Atlantic atjd north-west of Dakar in the eastern Atlantic, has been gjven an age of about 130 to 155 m.y.
Comparison of the isochrons with, the magnetic lineations indicate that two important shifts of the ridge axis may have occurred The first,, in the region South ,of the New England Seamounts ana the Canary Islands was a 200- km eastward jump or migration that took place prior to 155 m.y. ago; the second in the region north-, of the New England Seamounts and Canary Islands but south of the Azores was a more complex westward . Shift of 150 km maximum extent that occurred between 135(?) m.y. ana 72 m.y. ago.
We have also computed a pattern .of synthetic fracture zones or flow lines. Previous Workers have proposed that the South Atlas fault, the western Canary Islands, and the.NewEngland Seamounts lie along a fundamental fault or fracture zone. We note that these features are approximately. parallel one of these synthetic flow lines. The seaward escarpmentbounding the southern Bahamas as well is several well-surveyed fracture zones and other bathymetric-features are parallel to the synthetic fracture zones.
Figures & Tables
Edited by Peter A. Roma and published in 1976, Mid-Atlantic Ridge contains a collection of related articles reprinted from other Geological Society of America publications as well as a brief review of exploration of the Mid-Atlantic Ridge from 1960 to 1975.