An empirical relationship between ridge elevation and age of the oceanic crust is presented for the Pacific, Atlantic, and Indian oceans. This relationship is accounted for by the thermal contraction of a cooling lithosphere as it moves away from a center of spreading, and thus is compatible with plate theory. Hence, it is possible to use topographic profiles to predict the age of the ocean floor. Detailed examination of profiles in different areas indicates that slow-spreading ridges (half-rate 3 cm/yr). However, all ridges appear to show a uniform subsidence rate near the ridge crest. This uniform rate makes it possible to use topographic profiles in regions of smooth topography to predict the age of oceanic crust, less than 40 m.y. old, to better than ±2 m.y. Topographic profiles will be particularly useful for predicting age where magnetic anomaly patterns are absent or difficult to interpret. Magnetic, topographic, seismic reflection, twenty-nine new, and all other heat-flow observations across the east Pacific rise and Mathematician and Clipperton seamount chains in the central eastern Pacific are examined. The magnetic and heat-flow observations are too inconclusive to enable a tectonic reconstruction of the area. However, topographic profiles at right angles to the rise between 20°N and the equator show that the Mathematician and Clipperton seamount chains are the old crest of the east Pacific rise. These parts of the rise crest terminated approximately 5 m.y. B.P. by the spreading center jumping 4° to the east. Seismic reflection profiles at right angles to the crest support this conclusion. The topography west of these two chains is used as the basis for a proposed evolution of the central eastern Pacific during the past 20 m.y.

Elevation of ridges and evolution of the central Eastern Pacific.

A simplified model is proposed that can explain the evolution of marginal offsets, ridges, and fracture zones. Submarine features of the equatorial Atlantic margins are examined and are found to be in reasonable agreement with the model. The results reaffirm the suggestion that the South Atlantic opened in two stages beginning about 140 m.y. and 80 m.y. ago. The pole of rotation describing the relative motion of the South American and African plates changed radically when the constraints imposed by adjacent continental blocks were relaxed.

Marginal offsets, fracture zones, and the early opening of the North Atlantic

Kinematics of transform and transcurrent faults

Remote sensing techniques must be employed to determine the nature of the regional geologic and tectonic structure of the Arctic Basin. Magnetic measurements from aircraft are the most commonly used method. Since 1972 the U.S. Navy has been engaged in a long-term program of mapping, in relative detail, the earth's magnetic field over that portion of the Arctic Basin accessible to the P3 aircraft. A description is presented of the results of the 1977 and 1978 field efforts. The description represents a continuation of an investigation reported by Vogt et al. (1979). The efforts currently considered were directed towards understanding the nature, age, and origin of the major physiolographic features of the western Arctic Basin. Particular attention was given to the Canada and Makarov basins (Fletcher Abyssal Plain) and the Alpha Ridge. The aeromagnetic data are interpreted with respect to the theory of origin presented by Carey (1958).

Detailed aeromagnetic investigation of the Arctic Basin

Asthenosphere motion recorded by the ocean floor south of Iceland

Discontinuities in sea-floor spreading

As yet inaccessible to deep-sea drilling and ship-borne surveying, the ice-locked Arctic Basin has been slow to give up details of crustal genesis and later modification. Extensive geophysical surveying both in the Arctic Basin and the North Atlantic confirms sea-floor spreading as the only probable mode of crustal genesis for the Eurasia Basin. Magnetic anomalies, although less clear than elsewhere, suggest spreading rates of approximately 0.5 to 1 cm/yr since 10 m.y.b.p.; the basin was born perhaps 60 m.y.b.p., with the separation of the Lomonosov Ridge from the Eurasian margin. The Eurasia Basin terminates rather abruptly, still 1000 to 2000 km from the plate rotation pole. It is unknown exactly how northern Siberia has accommodated up to 500 km crustal extension in post-Cretaceous times. Other mysteries include the abnormally low magnetic anomaly amplitudes and deep basement levels of the Eurasia Basin; the spreading axis is 0.5 to 1.0 km below the — 2.8 km average for the Mid-Oceanic Ridge. This topographic anomaly seems part of a much larger, regional pattern centered on Iceland, where the crustal elevation is over 3 km above the norm. Crustal, and probably also older basement elevations decrease monotonically northward and southward. Elevations of the adjacent continental margins appear to mirror this pattern. Current speculations link the regional topographic high to an asthenospheric bulge pumped from the lower mantle by a convection plume under Iceland. The abrupt beginning, or rejuvenation, of the Iceland plume about 70 to 60 m.y.b.p. and its subsequent fluctuations may have played a primary role in cracking and then forcing the plates apart.

Tectonic History of the Arctic Basins: Partial Solutions and Unsolved Mysteries

Marine magnetic survey off the southern Bahamas; Project M-15. [By] Dewey R. Bracey and Otis E. Avery

Marine magnetic survey off the southern Bahamas

: The earth's magnetic intensity over a 41,000 square mile area north of the Lesser Antilles has been charted in detail by a shipboard survey. The contoured data show a major magnetic anomaly trending N20 deg E through the center of the survey area.

Shipboard magnetic survey of an area north of the lesser antilles.

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