Page 92 - Global Tectonics

SEA FLOOR SPREADING AND TRANSFORM FAULTS 79

the field is horizontal, negative anomalies coincide with which is equivalent to geographic latitudes greater than
normally magnetized blocks and positive anomalies 45°), north-south trending ridges at all latitudes and
with reversely magnetized blocks, precisely the reverse east-west trending ridges at the magnetic equator.
situation to that at high latitudes. In addition, the ampli-
tude of the anomaly decreases from the poles to the

equator as the geomagnetic field strength, and hence 4.1.6 Magnetostratigraphy
the magnitude of the remanence, decreases in this
direction. Figure 4.6 illustrates how the shape and
Once the geomagnetic reversal timescale has been cali-
amplitude of the magnetic anomalies over an ocean
brated, oceanic magnetic anomalies may be used to
ridge striking east–west vary with latitude.
date oceanic lithosphere. The method has been progres-
The orientation of the ridge also affects anomaly
sively refined so that it is now possible to deduce ages

shape and amplitude, because only that component of
back to mid-Jurassic times with an accuracy of a few
the magnetization vector lying in the vertical plane
million years.

through the magnetic profile affects the magnetic
The Vine–Matthews hypothesis explains the
anomaly. This component is at a maximum when the
sequence of magnetic anomalies away from ocean

ridge is east-west and the profile north-south, and at a
ridges in terms of normal and reversed magnetizations
minimum for ridges oriented north-south. The varia-
of the oceanic crust acquired during polarity reversals
tion in amplitude and shape of the magnetic anomalies
of the geomagnetic fi eld. Verification of the hypothesis

with orientation for a ridge of fixed latitude is shown in
was provided by the consistency of the implied reversal
Fig. 4.7. In general, the amplitude of magnetic anoma-
sequence with that observed independently on land.
lies decreases as the latitude decreases and as the strike
Cox et al. (1967) had measured the remanent magneti-
of the ridge progresses from east-west to north-south.
zation of lavas from a series of land sites. The lavas
The symmetry of the anomalies is most apparent for
were dated by a newly refi ned potassium-argon
ridges at high magnetic latitudes (e.g. greater than 64°,
method, which allowed the construction of a reversal
timescale back to 4.5 Ma. The timescale could not be
extended to earlier ages, as the errors involved in K-Ar
dating become too large. Similarly, polarity events of
less than 50,000 years duration could not be resolved.
The timescale to 5 Ma before present, as later refi ned
by Cande & Kent (1992), is given in Fig. 4.8. In mag-
netostratigraphic terminology, polarity chrons are

Figure 4.6 Variation of the magnetic anomaly pattern Figure 4.7 Variation of the magnetic anomaly pattern
with geomagnetic latitude. All profiles are north-south. with the direction of the profile at a fixed latitude.

Angles refer to magnetic inclination. No vertical Magnetic inclination is 45° in all cases. No vertical
exaggeration. exaggeration.