1.2.5.2 Rates of sea-floor spreading

The width of a magnetic lineation (or stripe) depends on two factors: the speed with which the oceanic crust moves away from a spreading center, and the length of time that geomagnetic polarity is constantly normal or reversed. The distance between the edges of magnetized crustal stripes can be measured from magnetic surveys at the ocean surface, while the ages of the reversals can be obtained by correlating the oceanic magnetic record with the radiometrically dated reversal sequence determined in subaerial lavas for about the last 4 Ma. When the dis­tance of a given polarity reversal from the spreading axis is plotted against the age of the reversal, a nearly linear relationship is obtained (Fig. 1.15). The slope of the best fitting straight line gives the average half-rate of spreading at the ridge. These are of the order of 10 mm yr ' in the North Atlantic ocean and 40-60 mm yr^-1 in the Pacific ocean. The calculation applies to the rate of motion of crust on one side of the ridge only. In most cases spreading has been symmetric on each side of the ridge (i.e., the opposite sides are moving away from the ridge at equal speeds), so the full rate of separation at a ridge axis is double the calculated half-rate of spreading (Fig. 1.11).

Fig. 1.15 Computation of half-rates of sea-floor spreading at different spreading centers by measuring the distances to anomalies with known radiometric ages (after Vine, 1966).

The rates of current plate motion determined from axial anomaly patterns (Fig. 1.11) are average values over several million years. Modern geodetic methods allow these rates to be tested directly (see Section 2.4.6). Satellite laser-ranging (SLR) and very long baseline interferometry (VLBI) allow exceptionally accurate measurement of changes in the distance between two stations on Earth. Controlled over several years, the distances between pairs of stations on opposite sides of the Atlantic ocean are increasing slowly at a mean rate of 17 mm yr^-1 (Fig. 1.16). This figure is close to the long-term value of about 20 mm yr"¹ interpreted from model NUVEL-1 of current plate motions (Fig. 1.11).

Knowing the spreading rates at ocean ridges makes it possible to date the ocean floor. The direct correlation between polarity sequences measured in continental lavas and derived from oceanic anomalies is only possible for the last 4 Ma or so. Close to the axial zone, where linear spreading rates are observed (Fig. 1.15), simple extrapola­tion gives the ages of older anomalies, converting the striped pattern into an age map (Fig. 1.13). Detailed mag­netic surveying of much of the world's oceans has revealed a continuous sequence of anomalies since the Late Cretaceous, preceded by an interval in which no reversals occurred; this Quiet Interval was itself preceded by a Mesozoic reversal sequence. Magnetostratigraphy in sedimentary rocks (Section 5.7.4) has enabled the identifi­cation, correlation and dating of key anomalies. The polarity sequence of the oceanic anomalies has been con­verted to a magnetic polarity timescale in which each polarity reversal is accorded an age (e.g., as in Fig. 5.78). In turn, this allows the pattern of magnetic anomalies in the ocean basins to be converted to a map of the age of the ocean basins (Fig. 5.82). The oldest areas of the oceans lie close to northwest Africa and eastern North America, as well as in the northwest Pacific. These areas formed during the early stages of the breakup of Pangaea. They are of Early Jurassic age. The ages of the ocean basins have been confirmed by drilling through the sediment layers that cover the ocean floor and into the underlying basalt layer. Beginning in the late 1960s and extending until the present, this immensely expensive undertaking has been carried out in the Deep Sea Drilling Project (DSDP) and its successor the Ocean Drilling Project (ODP). These multinational projects, under the leader­ship of the United States, are prime examples of open sci­entific cooperation on an international scale.

Fig. 1.16 Changes in separation between Westcott (Massachusetts, USA) and (a) Onsala (Sweden) and (b) Wettzell (Germany), as determined by very long baseline interferometry (after Ryan et al., 1993).

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