Page 108 - Global Tectonics

THE FRAMEWORK OF PLATE TECTONICS 95

are frequently offset laterally by transform faults
(Section 4.2.1). Because of inaccuracies involved in
mapping oceanic fracture zones, the great circles rarely
intersect at a single point. Consequently, statistical
methods are applied which are able to predict a circle
within which it is most probable that the relative rota-
tion pole lies.
A second method is based on the variation of
spreading rate with angular distance from the pole of
rotation. Spreading rates are determined from mag-
netic lineations (Section 4.1.6) by identifying anomalies
of the same age (usually number 3 or less so that the
movement represents a geologically instantaneous rota-
tion) on either side of an ocean ridge and measuring
the distance between them. The velocity of spreading
is at a maximum at the equator corresponding to the
Euler pole and thence decreases according to the cosine
of the Euler pole’s latitude (Fig. 5.4). The determina-
tion of the spreading rate at a number of points along
the ridge then allows the pole of relative rotation to be
found. Figure 5.4 Variation of spreading rate with latitudinal
distance from the Euler pole of rotation.
The fi nal, and least reliable, method of determining
the directions of relative motion between two plates
makes use of focal mechanism solutions of earthquakes
(Section 2.1.6) on their common margins. If the inclina- plate margins to be able to compute relative velocities
tion and direction of slip along the fault plane are at convergent margins.

known, then the horizontal component of the slip The first study of this type was undertaken by Le
vector is the direction of relative motion. The data are Pichon (1968). He made use of globally distributed esti-
less accurate than the other two methods described mates of relative plate velocities derived from transform
above because, except in very well determined cases, faults and spreading rates, but not of information
the nodal planes could be drawn in a range of possible obtained from focal mechanism solutions. Le Pichon
orientations and the detailed geometry of fault systems used a subdivision of the Earth’s surface based on only
at plate boundaries is often more complex than implied six large plates: the Eurasian, African, Indo-Australian,

here (Section 8.2 and below). American, Pacific, and Antarctic plates. In spite of this
Divergent plate boundaries can be studied using simplification his model provided estimates of spread-

spreading rates and transform faults. Convergent ing rates that agreed well with those derived from mag-
boundaries, however, present more of a problem, and netic anomalies (Section 4.1.6).
it is often necessary to use indirect means to determine Subsequently, more detailed analyses of global plate
relative velocities. This is possible by making use of motions were performed by Chase (1978), Minster &
information from adjoining plates and treating the rota- Jordan (1978), and DeMets et al. (1990). These studies
tions between plate pairs as vectors (Morgan, 1968). recognized a number of additional plate boundaries and
Thus, if the relative movements between plates A and hence additional plates. The latter included the Carib-
B and between plates B and C are known, the relative bean and Philippine Sea plates, the Arabian plate, the
movement between plates A and C can be found by Cocos and Nazca plates of the east Central Pacifi c, and
vector algebra. the small Juan de Fuca plate, east of the Juan de Fuca
This approach can be extended so that relative ridge, off western North America (Fig. 5.5). The Amer-
motions can be determined for any number of inter- ican plate was divided into two, the North American
locking plates. Indeed, the method can be applied to the and South American plates, and the Indo-Australian
complete mosaic of plates that make up the Earth’s plate similarly, into the Indian and Australian plates.

surface, provided that there are suffi cient divergent The new boundaries identified within the American and