Australia: The Land Where Time Began
Triple Junction Stability
The relative velocity vectors determine the stability of plate boundaries, unstable boundaries existing transiently, transforming instantly into a stable configuration.
As trenches can consume crust in a single direction, in a situation such as occurs at the Alpine Fault that passes through New Zealand, a dextral transform fault develops to accommodate the movements where a dextral transform fault links the Tonga-Kermadec Trench, where Pacific Plate lithosphere is underthrusting in a southwesterly direction, to a trench south of New Zealand, where the Tasman Sea is being consumed towards the northeast (McKenzie & Morgan, 1969).
At a triple junction, where 3 plates come into contact, the situation is more complex and maybe unstable. Quadruple junctions are always unstable, developing immediately into 2 stable triple junctions.
Triple junctions must exist because there are more than 2 plates covering the surface of the Earth, the stability of these triple junctions being dependent on the relative velocity vector directions of the contacting plates. Where a triple junction involves a ridge, a trench and a transform fault, the triple junction must be capable of moving up or down between the 3 boundaries between the plate pairs for the junction to be stable.
At the present there are 6 types of triple junction, at the junction of the East Pacific Rise and the Galapagos Rift Zone, Central Japan, the junction of the Peru-Chile Trench and the West Chile Rise, possibly at the junction of the Owen Fracture Zone and the Carlsberg Ridge, the San Andreas Fault and the Mendocino Fracture Zone, and at the mouth of the Gulf of California.
The importance of the role of triple junctions is illustrated by the evolution of the San Andreas Fault. The Farallon Plate and the Pacific Plate were separated by the East Pacific Rise during the Oligocene. The consumption rate of the Farallon Plate that was being thrust beneath the North American Plate exceeded the spreading rate at the East Pacific Rise, the result being that the ridge system moved towards the subduction trench. The eastern extremity of the Mendocino Fracture Zone was the first point of the Rise to reach the trench. At about 28 Ma there was a quadruple junction that existed momentarily, immediately devolving into 2 stable triple junctions, the northernmost being of the FFT type, while the southernmost one was of the RTF type. The northern triple junction moved north along the trench and the southern triple junction moved south, the movement of both as a result of the geometry of the system.
As a response to the migration of these triple junctions, the dextral San Andreas Fault formed. When the eastern extremity of the Murray Fracture Zone reached the trench, the southern triple junction stopped migrating south, the triple junction changing to an FFT and started migrating north. To the north and south of the San Andreas Fault the consumption of the Farallon Plate continued until the East Pacific Rise to the south of the Murray Fracture Zone reached the trench, when the geometry changed back (See Source 1, p. 119). After changing to the FFT type the triple junction began moving to the north, the triple junction reverted to RTF type, and changed to a southerly movement along the trench, the situation at the mouth of the Gulf of California at the present
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading