Page 139 - Global Tectonics

OCEAN RIDGES 125

6.2 BROAD is slightly thinner than encountered in the main ocean
basins, and that the upper mantle velocity beneath the
STRUCTURE OF THE crestal region is anomalously low (Fig. 6.4). Oceanic
layer 1 rocks (Section 2.4.5) are only present within
topographic depressions, but layers 2 and 3 appear to
UPPER MANTLE be continuous across the ridge except for a narrow
region at the crest. A similar structure has been
BELOW RIDGES determined for the Mid-Atlantic Ridge (Fig. 6.5). The
suggestion of this latter work that layer 3 is not con-
tinuous across the ridge was subsequently disproved
(Whitmarsh, 1975; Fowler, 1976).
Gravity measurements have shown that free air anoma- As the crust does not thicken beneath ridges, iso-
lies are broadly zero over ridges (Figs 6.4, 6.5), indicat- static compensation must occur within the upper
ing that they are in a state of isostatic equilibrium mantle by a Pratt-type mechanism (Section 2.11.3).
(Section 2.11.6), although small-scale topographic fea- Talwani et al. (1965) proposed that the anomalously low
tures are uncompensated and cause positive and nega- upper mantle velocities detected beneath ridges corre-
tive free air anomalies. The small, long wavelength, spond to the tops of regions of low density. The densi-
positive and negative free air anomalies over the crests ties were determined by making use of the Nafe–Drake
and fl anks, respectively, of ridges are a consequence of relationship between P wave velocity and density (Nafe
the compensation, with the positives being caused by & Drake, 1963), and a series of models produced that
the greater elevation of the ridge and the negatives satisfied both the seismic and gravity data. One of these

from the compensating mass deficiency. The gravita- is shown in Fig. 6.6, and indicates the presence beneath
tional effects of the compensation dominate the gravity the ridge of a body with a density contrast of
field away from the ridge crest, and indicate that the −0.25 Mg m extending to a depth of some 30 km. This

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compensation is deep. large density contrast is difficult to explain geologically.
Seismic refraction experiments by Talwani et al. An alternative interpretation, constructed by Keen &

(1965) over the East Pacific Rise showed that the crust Tramontini (1970), is shown in Fig. 6.7. A much lower,

Fig. 6.4 Heat flow, free air gravity anomaly and crustal structure defined by seismic refraction across the East Pacific

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Rise at 15–17°S. P wave velocities in km s (redrawn from Talwani et al., 1965, by permission of the American
Geophysical Union. Copyright © 1965 American Geophysical Union).