Page 96 - Global Tectonics

SEA FLOOR SPREADING AND TRANSFORM FAULTS 83

over oceanic crust dating back to the Jurassic. Although rate of spreading in the South Atlantic (Maxwell et al.,
there is no oceanic crust older than this, paleomagnetic 1970). A series of holes was drilled in the South Atlantic
investigations on land have shown that geomagnetic along a traverse at right angles to the Mid-Atlantic Ridge
reversals have occurred at least back to 2.1 Ga. (Fig. 4.12a). The age of the oceanic crust would ideally
That spreading rates have varied with time is apparent have been determined by radiometric dating of the layer

from an examination of magnetic profiles from different 2 basalts that were penetrated in each hole. However the
oceans. Examples are given in Fig. 4.11 in which the basalts were too weathered for this to be possible, and
spreading rate in the South Atlantic is assumed to be so their ages were determined, albeit slightly underesti-
constant and the distances to various magnetic anomalies mated, by paleontologic dating of the basal sediments
from ridge crests in other oceans are plotted against the of layer 1. In Fig. 4.12b oldest sediment age is plotted
distance to the same anomaly in the South Atlantic. against distance from the ridge axis, and it is readily

Inflection points in the curves for the other oceans indi- apparent that there is a remarkable linear relationship,
cate when the spreading rates changed there if the implicit with crustal age increasing with distance from the ridge.
assumption that the spreading rate has remained con- The predicted ages imply a half spreading rate in this
−1
stant in the South Atlantic is correct. However, spreading region of 20 mm a , as predicted, and hence agree well

rates may have changed with time in all oceans. with the age of the ocean floor and the reversal times-
The first long-term geomagnetic timescale was con- cale proposed by Heirtzler et al. (1968) (Fig. 4.10).

structed by Heirtzler et al. (1968). Again they made the A thorough review of the calibration of this polar-
assumption that spreading in the South Atlantic had ity timescale was carried out by Cande & Kent (1992,
remained constant at the same rate as had been 1995). It drew on oceanic magnetic anomaly data,
deduced for the last 4 Ma. A model of normal and magnetostratigraphic studies of sedimentary sequences
reversely magnetized blocks was constructed which on land and at sea, and radiometric dating of nine
simulated the observed anomaly pattern, and the dis- specific stratigraphic horizons. From this they con-

tance axis transformed into a geomagnetic timescale of cluded that sea floor spreading in the South Atlantic
reversals extending back in time nearly 80 Ma. Promi- had been continuous, with some variation about an
nent anomalies corresponding to periods of normal essentially constant rate, and that it was still appropri-
polarity were numbered from 1 to 32 with increasing ate to use the South Atlantic as a standard against
time (Fig. 4.10). which the spreading history in the other ocean basins
Leg 3 of the Deep Sea Drilling Program (DSDP), in could be compared. The revised timescale for the past

1968, was specifically designed to test the hypothesis of 80 Ma suggested by Cande & Kent (1995) is illustrated
sea floor spreading and the assumption of a constant in Fig. 4.13.

Figure 4.11 Relationship between the distance to a given anomaly in the South Atlantic and the distance to the same
anomaly in the South Indian, North Pacific and South Pacific Oceans. Numbers on the right refer to magnetic anomaly

numbers (redrawn from Heirtzler et al., 1968, by permission of the American Geophysical Union. Copyright © 1968
American Geophysical Union).