Geoscience Reference
In-Depth Information
Figure 2.16 gives the conditions for stability of the various types of triple
junction and also gives examples of some of the triple junctions occurring around
the Earth at present. Many research papers discuss the stability or instability of
the Mendocino and Queen Charlotte triple junctions that lie off western North
America. These are the junctions (at the south and north ends, respectively) of
the Juan de Fuca plate with the Pacific and North American plates and so are
subjects of particular interest to North Americans because they involve the San
Andreas Fault in California and the Queen Charlotte Fault in British Columbia.
Another triple junction in that part of the Pacific is the Galapagos triple junction,
where the Pacific, Cocos and Nazca plates meet; it is an RRR junction and thus
is stable.
2.6.2 The significance of triple junctions
Work on the Mendocino triple junction, at which the Juan de Fuca, Pacific and
North American plates meet at the northern end of the San Andreas Fault, shows
why the stability of triple junctions is important for continental geology. The
Mendocino triple junction is an FFT junction involving the San Andreas Fault,
the Mendocino transform fault and the Cascade subduction zone. It is stable, as
seen in Fig. 2.16, provided that the San Andreas Fault and the Cascade subduction
zone are collinear. It has, however, been suggested that the Cascade subduction
zone is after all not exactly collinear with the San Andreas Fault and, thus, that
the Mendocino triple junction is unstable. This instability would result in the
northwards migration of the triple junction and the internal deformation of the
continental crust of the western U.S.A. along pre-existing zones of weakness. It
would also explain many features such as the clockwise rotation of major blocks,
such as the Sierra Nevada, and the regional extension and eastward stepping of
the San Andreas transform. The details of the geometry of this triple junction
are obviously of great importance to the regional evolution of the entire west-
ern U.S.A. Much of the geological history of the area over approximately the
past thirty million years may be related to the migration of the triple junction,
so a detailed knowledge of the plate motions is essential background for any
explanation of the origin of Tertiary structures in this region. This subject is dis-
cussed further in Section 3.3.3. The motions of offshore plates can produce major
structural changes even in the continents.
The Dead Sea Fault is similar to the San Andreas Fault system in that it is
an intra-continental plate boundary. It is the boundary between the Arabian and
African plates and extends northwards from the Red Sea to the East Anatolian
Fault (Fig. 10.18). It is a left-lateral strike-slip fault with a slip rate of
5mmyr
−
1
.
That such a major strike-slip boundary is located close to the continental edge, but
still within the continent, is because that is where the plate is weakest - a thinned
continental margin is weaker than both oceanic and continental lithosphere. The
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