Page 94 - Global Tectonics

SEA FLOOR SPREADING AND TRANSFORM FAULTS 81

6

defined with durations of the order of 10 years. quent work on other cores extended the reversal history
Chrons may be dominantly of reversed or normal back to 20 Ma (Opdyke et al., 1974).
polarity, or contain mixed events. Pitman & Heirtzler (1966) and Vine (1966) used the
Further verification of the geomagnetic reversal tim- radiometrically dated reversal timescale to compute

escale was provided by paleomagnetic investigations of the magnetic profiles that would be expected close

deep sea cores (Opdyke et al., 1966). Unlike lava fl ows, to the crestal regions of mid-ocean ridges. By varying
these provide a continuous record, and permit accurate the spreading rate it was possible to obtain very close
stratigraphic dating from their microfauna. This method simulations of all observed anomaly sequences (Fig.
is most conveniently applied to cores obtained in high 4.9), and consequently to determine the spreading rates.
magnetic latitudes where the geomagnetic inclination A compilation of such rates is shown in Table 4.1. Exten-
is high, because the cores are taken vertically and are sions of this work show that the same sequence of mag-
not oriented azimuthally. Excellent correlation was netic anomalies, resulting from spreading and reversals
found between these results and those from the lava of the Earth’s magnetic field, can be observed over

sequences, and confirmed that at least 11 geomagnetic many ridge flanks (e.g. Fig. 4.10). Later work has shown

field reversals had occurred over the last 3.5 Ma. Subse- that similar linear magnetic anomalies are developed

51° S

Figure 4.9 Magnetic anomaly profiles and models of several spreading centers in terms of the reversal timescale
(redrawn from Vine, 1966, Science 154, 1405–15, with permission from the AAAS).