Present Day Triple Junctions

Only six types of triple junction are present during the current phase of plate tectonics. These are RRR (e.g. the junction of East Pacific Rise and Galapagos Rift Zone), TTT (central Japan), TTF (junction of PeruChile Trench and West Chile Rise), FFR (possibly at the junction of Owen Fracture Zone and Carlsberg ridge), FFT (junction of San Andreas Fault and Mendocino Fracture Zone), and RTF (mouth of Gulf of California).

The evolution of the San Andreas Fault illustrates the importance of the role of triple junctions. In Oligocene times (Fig. 5.28a), the East Pacific Rise separated the Pacific and Farallon plates. The transform faults associated with this ridge have been simplified, and only the Mendocino and Murray fracture zones are shown. The Farallon Plate was being underthrust beneath the North American Plate, and, since the rate of consumption exceeded the rate of spreading at the East Pacific Rise, the ridge system moved towards the trench. The first point of the ridge to meet the trench was the eastern extremity of the Mendocino Fracture Zone. A quadruple junction existed momentarily at about 28 Ma, but this devolved immediately into two triple junctions (Fig. 5.28b). The more northerly was of FFT type, the more southerly of RTF type, and both were stable (insets on Fig. 5.28b). Because of the geometry 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 formed in response to the migration of these triple junctions. The southerly migration of the southern triple junction ceased as the eastern extremity of the Murray Fracture Zone reached the trench (Fig. 5.28c). The triple junction changed to FFT type and began to move northwards. The Farallon Plate continued to be subducted to the north and south of the San Andreas Fault, until the geometry changed back to that shown in Fig. 5.28b when the East Pacific Rise to the south of the Murray Fracture Zone reached the trench. The triple junction then reverted to RTF type and changed to a southerly motion along the trench. This represents the situation at the present day at the mouth of the Gulf of California.

Ocean ridges

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