Geoscience Reference
In-Depth Information
Figure 3.25.
Reconstructions of the
positions of the Pacific
plate and the Farallon
plate with respect to the
North American plate
(shaded). P, Pacific plate;
F, Farallon plate; V,
Vancouver plate; J, Juan
de Fuca plate; G,
Guadalupe plate; and C,
Cocos plate. (From
Atwater (1989), after Stock
and Molnar (1988).)
subduction zone. This meant that subduction ceased on that part of the North
American plate boundary where the North American plate was adjacent to the
Pacific plate as the relative motion between the Pacific and North American plates
was parallel to the boundary. Thus, the San Andreas Fault and the Mendocino
Triple Junction were born. By 10 Ma, the Farallon-Guadalupe-Cocos plate was
very small, and the San Andreas Fault system had lengthened. At about 9 Ma and
5 Ma, as discussed earlier, the strike of the Juan de Fuca Ridge changed by some
20
◦
in total, resulting in the present configuration of the plates. The geological
evolution of western North America was controlled by the motions of these
oceanic plates. If the relative motion between the Pacific and North American
plates is assumed to have been parallel to the subduction zone between the Farallon
and North American plates (the present-day Cascadia Subduction Zone), the
Mendocino Triple Junction, where the Farallon, North American and Pacific
plates meet, must have been stable (Figs. 2.16, 3.26 and 3.27). However, since
the Cascadia Subduction Zone is not, at present, collinear with the San Andreas
Fault, the triple junction is unstable and may well always have been unstable
(Fig. 3.27). The evolution of this unstable triple junction is shown in Fig. 3.26(e).
If the three plates are assumed to be rigid, a hole must develop. Such a hole would
presumably fill with rising mantle material from below and sediments from above.
It would become in effect a microplate. Alternatively, if the continental North
American plate is allowed to deform, then the triple junction evolves as shown in
Fig. 3.26(f), with internal deformation involving both extension and rotation over
