Page 134 - Global Tectonics

120 CHAPTER 5

single point (the circumcenter of the triangle). The and only the Mendocino and Murray fracture zones are
FFF triple junction is never stable, as the velocity lines shown. The Farallon Plate was being underthrust
coincide with the vector triangle, and, of course, the beneath the North American Plate, and, since the rate
sides of a triangle never meet in a single point. The of consumption exceeded the rate of spreading at the

other possible triple junctions are only stable for East Pacific Rise, the ridge system moved towards the

certain particular orientations of the juxtaposed plate trench. The first point of the ridge to meet the trench
margins. was the eastern extremity of the Mendocino Fracture
Zone. A quadruple junction existed momentarily at
about 28 Ma, but this devolved immediately into two
5.11 PRESENT DAY triple junctions (Fig. 5.28b). The more northerly was of
FFT type, the more southerly of RTF type, and both
TRIPLE JUNCTIONS were stable (insets on Fig. 5.28b). Because of the geom-
etry of the system the northern triple junction moved
north along the trench and the southern triple junction
moved south. Thus the dextral San Andreas Fault
Only six types of triple junction are present during the formed in response to the migration of these triple
current phase of plate tectonics. These are RRR (e.g. junctions. The southerly migration of the southern

the junction of East Pacific Rise and Galapagos Rift triple junction ceased as the eastern extremity of the
Zone), TTT (central Japan), TTF (junction of Peru– Murray Fracture Zone reached the trench (Fig. 5.28c).
Chile Trench and West Chile Rise), FFR (possibly at The triple junction changed to FFT type and began to
the junction of Owen Fracture Zone and Carlsberg move northwards. The Farallon Plate continued to be
ridge), FFT (junction of San Andreas Fault and Men- subducted to the north and south of the San Andreas
docino Fracture Zone), and RTF (mouth of Gulf of Fault, until the geometry changed back to that shown

California). in Fig. 5.28b when the East Pacific Rise to the south of
The evolution of the San Andreas Fault illustrates the Murray Fracture Zone reached the trench. The
the importance of the role of triple junctions. In Oli- triple junction then reverted to RTF type and changed
gocene times (Fig. 5.28a), the East Pacific Rise sepa- to a southerly motion along the trench. This represents

rated the Pacific and Farallon plates. The transform the situation at the present day at the mouth of the
faults associated with this ridge have been simplifi ed, Gulf of California.