Geoscience Reference
In-Depth Information
Figure 3.27.
A present-
day tectonic map of the
western part of the North
American plate. Shaded
regions mark the extent of
expected arc magmatism.
(After Dickinson (1979).)
volcanoes
NORTH
JUAN DE FUCA
PLATE
Mendocino
Triple
Junction
AMERICAN
PACIFIC
PLATE
PLATE
Rivera
Triple
Junction
COCOS PLATE
been translated northwards since its initial rift from North America at about 12-
14 Ma. Details of the past plate motions and their effect on the interior of the
North American plate will no doubt continue to be revised for many years, but
the main features of the model as presented here are probably nearly correct.
Farther south, to the west of the Middle Americas Trench, magnetic lineations
striking east-west are sandwiched between north-south anomalies (Fig. 3.28).
This region has undergone a series of plate reorganizations since 30 Ma when the
Pacific-Farallon Ridge first intersected the North American Subduction Zone.
In general terms these reorganizations can best be described as the breaking
up of large plates into smaller plates as the ridge was progressively subducted.
At 30 Ma one plate (Farallon) lay to the east of the ridge. At 25 Ma the east-
west anomalies indicate that the Cocos-Nazca Ridge (otherwise known as the
Galapagos spreading centre) started spreading; so two plates, the Nazca plate and
the Cocos or Guadalupe plate, lay to the east of the ridge. At about 12 Ma, the
Cocos plate subdivided, spawning the tiny Rivera plate; and now three plates lie
to the east of the ridge we call the East Pacific Rise (Fig. 2.2). A schematic flat
model of the plates in this region was presented in Chapter 2, Problem 10.
The magnetic lineations in the northwestern Pacific region are much older
and more complex than those of the northeastern Pacific (Fig. 3.28). As plates
