Marine magnetic profiles over the Red Sea between 18° N and 25° N latitudes confirm previous hypotheses that strongly magnetic rocks underlie the axial trough. The symmetrical nature of the anomalies and their close correspondence to seafloor spreading magnetic models support a rifting origin for the trough. The dominant magnetic anomaly trends strike about N 35° W in the northern and southern parts of the trough. In the central region between 20° and 22° N the trend direction is about N 60° E. Geometrical considerations of possible spreading mechanisms suggest that the true separation direction of Africa away from Arabia near 21° N latitude is in either a N 10° E or N 60° E direction. The separation rates then are 3.2 cm a -1 and 2.0 cm a -1 , respectively.
Research Article| June 01, 1972 Plate Tectonics, Paleomagnetism, and the Opening of the Atlantic J. D PHILLIPS; J. D PHILLIPS Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 Search for other works by this author on: GSW Google Scholar D FORSYTH D FORSYTH Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1972) 83 (6): 1579–1600. https://doi.org/10.1130/0016-7606(1972)83[1579:PTPATO]2.0.CO;2 Article history received: 04 Aug 1971 rev-recd: 12 Jan 1972 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J. D PHILLIPS, D FORSYTH; Plate Tectonics, Paleomagnetism, and the Opening of the Atlantic. GSA Bulletin 1972;; 83 (6): 1579–1600. doi: https://doi.org/10.1130/0016-7606(1972)83[1579:PTPATO]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
DNAG Transect E-4. Part of GSA's DNAG Continent-Ocean Transect Series, this transect contains all or most of the following: free-air gravity and magnetic anomaly profiles, heat flow measurements, geologic cross section with no vertical exaggeration, multi-channel seismic reflection profiles, tectonic kindred cross section with vertical exaggeration, geologic map, stratigraphic diagram, and an index map. All transects are on a scale of 1:500,000.
A detailed aeromagnetic survey centered on the Mid-Atlantic Ridge was processed by using a new algorithm to reduce navigational errors. Small areas within the survey were selected for multidirectional stacking to reduce noise and determine best-fitting azimuths. This analysis allowed identification of sea floor spreading anomalies out to anomaly 9. Anomaly azimuths indicate two periods of oblique spreading with a left lateral sense: from 25 to 14 m.y. ago and from 9 m.y. ago to the present. Oblique spreading may have resulted from locking of transform faults each time the African plate began to spread in a more northerly direction away from the American plate.
Ten magnetic profiles across the mid-Atlantic ridge near 27 degrees N show trends that are parallel to the ridge axis and symmetrical about the ridge axis. The configuration of magnetic bodies that could account for the pattern supports the Vine and Matthews hypothesis for the origin of magnetic anomalies over oceanic ridges. A polarity-reversal time scale inferred from models for sea-floor spreading in the Pacific-Antarctic ridge and radiometrically dated reversals of the geomagnetic field indicates a spreading rate of 1.25 centimeters per year during the last 6 million years and a rate of 1.65 centimeters per year between 6 and 10 million years ago. A similar analysis of more limited data over the mid-Atlantic ridge near 22 degrees N also indicates a change in the spreading rate. Here a rate of 1.4 centimeters per year appears to have been in effect during the last 5 million years; between 5 and 9 million years ago, an increased rate of 1.7 centimeters per year is indicated. The time of occurrence and relative magnitude of these changes in the spreading rate, about 5 to 6 million years ago and 18 to 27 percent, respectively, accords with the spreading rate change implied for the Juan de Fuca ridge in the northeast Pacific.
