Past plate motions - The Solid Earth - page 76

The fine details of the motions of the plates in this region and the exact
positioning of the continents as they were prior to anomaly M25 when they
formed the mega-continent Gondwanaland remain to be firmly established, but
on a broad scale the motions are fairly well understood.
3.3.4 The Pacific Ocean
The magnetic-anomaly pattern off the west coast of North America was the first
magnetic-anomaly pattern to be studied in detail (Fig. 3.7). An interpretation
of these anomalies based on the plate motions and earthquakes in the area is
shown in Fig. 3.22. The details of the plate boundaries and their relative motions
and ages are much clearer. Although the ridges in this region are spreading
relatively slowly, the plates are small, so the lithosphere was only about 10 Ma
old when it was subducted beneath the North American plate. Another feature of
the magnetic anomalies in this region is the difference of some 20 ◦ between the
present-day trend of the Juan de Fuca Ridge and the strike of the older anomalies
on the Pacific plate (the southwest part of Fig. 3.22(a)). This difference is caused
by changes in the rotation pole and subsequent reorientation of the ridge. The
diagonal pseudofaults which offset the magnetic anomalies in this region are
also due to the adjustment of the ridge system to changes in the rotation poles
(Fig. 3.23). Thus, when an area is studied in detail, the original questions may
well be answered and theories validated, but usually new questions are also raised
(in this instance, the new problem is the exact method by which ridges adjust to
changes in rotation poles, or vice versa). The past plate motions in other parts of
the Pacific are more difficult to interpret than those in the region flanking North
America, where the presence of an active ridge system means that both sides of
the anomaly pattern are preserved.
Further to the south and west on the Pacific plate, the oceanic anomaly pattern
is, on a broad scale, fairly simple (Figs. 3.24 and 3.28). Anomalies strike almost
north-south and are offset by fracture zones (see Section 9.5.1). The central part
of the ocean was formed during a period which included the Magnetic Quiet Zone.
Thus, there are not many anomalies to be observed. However, much farther north
towards the Aleutian islands the pattern changes. The anomalies change direction
so that they are striking approximately east-west. This feature is called the Great
Magnetic Bight. The other main feature of the northern Pacific is that the north-
south anomalies represent only the western half of the pattern and, except for the
short ridge segments such as the Juan de Fuca Ridge, the mid-ocean ridge that
created the oceanic plate no longer exists. This vanished ridge has been subducted
under the North American plate. With it went much of the Farallon plate, the
name given to the plate which once was to the east of the ridge and had the
matching half of the symmetrical anomaly pattern (Fig. 3.24). For the Farallon
plate and the Pacific-Farallon Ridge to have been subducted in this manner,
the rate of subduction must have been greater than the rate at which the ridge