Page 80 - Global Tectonics

CONTINENTAL DRIFT 67

magnetization, that is, the ancient north direction, the 3.6.5 Apparent polar
apparent location of the paleopole can be computed.
Such computations, combined with age determina- wander curves
tions of the samples by radiometric or biostratigraphic
methods, make possible the calculation of the appar- Paleomagnetic data can be displayed in two ways. One
ent location of the north magnetic pole at a particular way is to image what is believed to be the true situa-
time for the continent from which the samples were tion, that is, plot the continent in a succession of posi-
collected. Paleomagnetic analyses of samples of a tions according to the ages of the sampling sites (Fig.
wide age range can then be used to trace how the 3.13a). This form of display requires the assumption of
apparent pole position has moved over the Earth’s the paleolongitudes of the sites. The other way is to
surface. regard the continent as remaining at a fi xed position
It is important to recognize that remanent magne- and plot the apparent positions of the poles for various
tization directions cannot provide an estimate of times to provide an apparent polar wander (APW) path
paleolongitude, as the assumed dipole field is axisym- (Fig. 3.13b). As discussed above, this representation

metric. There is a consequent uncertainty in the ancient does not reflect real events, but it overcomes the lack

location of any sampling site, which could have been of control of paleolongitude and facilitates the display

situated anywhere along a small circle, defined by the of information from different regions on the same
paleolatitude, centered on the pole position. diagram.
If a paleomagnetic study provides a magnetic pole The observation that the apparent position of the
position different from the present pole, it implies pole differed for rocks of different ages from the same
either that the magnetic pole has moved throughout continent demonstrated that continents had moved
geologic time, that is, the magnetic pole has wandered over the surface of the Earth. Moreover, the fact that
relative to the rotational pole, or if the poles have APW paths were different for different continents
remained stationary that the sampling site has moved, demonstrated unequivocally that relative movements
that is, continental drift has occurred. It appears that of the continents had taken place, that is, continental
wandering of the magnetic pole away from the geo- drift had occurred. Paleomagnetic studies thus con-
graphic pole is unlikely because all theoretical models firmed and provided the first quantitative measure-

for the generation of the field predict a dominant dipole ments of continental drift. Figure 3.14a illustrates the
component paralleling the Earth’s rotational axis APW paths for North America and Europe from the
(Section 4.1.3). Consequently, paleomagnetic studies Ordovician to the Jurassic. Figure 3.14b shows the
can be used to provide a quantitative measure of con- result of rotating Europe and its APW path, according
tinental drift. to the rotation parameters of Bullard et al. (1965), to
An early discovery of paleomagnetic work was that close up the Atlantic Ocean. The APW paths for
in any one study about half of the samples analysed Europe and North America then correspond very
provided a primary remanent magnetization direction closely from the time the continents were brought
in a sense 180° different from the remainder. Although together at the end of the Caledonian orogeny,
the possibility of self-reversal of rock magnetism approximately 400 Ma ago, until the opening of the
remains, it is believed to be a rare phenomenon, and Atlantic.
so these data are taken to reflect changes in the polar- APW paths can be used to interpret motions,

ity of the geomagnetic field. The field can remain collisions, and disruptions of continents (Piper, 1987),
normal for perhaps a million years and then, over an and are especially useful for pre-Mesozoic continents
interval of a few thousand years, the north magnetic whose movements cannot be traced by the pattern
pole becomes the south magnetic pole and a period of of magnetic lineations in their surrounding ocean
reversed polarity obtains. Polarity reversals are random, basins (Section 4.1.6). Figure 3.15 represents the full
but obviously affect all regions of the Earth synchro- Wilson cycle (Section 7.9) of the opening and closure
nously so that, coupled with radiometric or paleonto- of an ocean basin between two continents. Before
logic dating, it is possible to construct a polarity rifting, the two segments A and B of the initial
timescale. This subject will be considered further in continent have similar APW paths. They are unlikely
Chapter 4. to be identical as it is improbable that the initial rift