Triple junction reorganization

Abstract

For at least the past 20 m.y. the Bouvet triple junction appears to have existed predominantly in a ridge-fault-fault rather than a ridge-ridge-ridge configuration when it has an isosceles velocity vector triangle that allows either configuration to be stable. In this paper two conceptual models originally developed to explain the general orthogonality of ridges and transform faults in spreading center systems are examined to see if they can explain the observed predominance of the ridge-fault-fault geometry for this triple junction. The first hypothesis is that a ridge axis will form and evolve to remain oriented perpendicular to the direction of the maximum local tens fie stress at the ridge axis; at a triple junction this can be strongly modified by the local plate boundary geometry. The second hypothesis is that plate boundary geometries adjust to minimize the power dissipation associated with plate boundary motions. Both hypotheses successfully predict the observed predominance of the ridge-fault-fault geometry. We suspect that the minimum dissipation hypothesis is successful for spreading center problems, despite its questionable theoretical underpinning, because for these problems it is equivalent to plate geometries assuming a minimum resistive force configuration. Since a minimum resistive force configuration is a partial corollary of a local stress-controlled ridge axis geometry, the resulting minimum dissipation hypothesis plate geometry predictions will be similar to stress-controlled plate boundary geometries. Because of its simplicity, the minimum dissipation hypothesis may remain a useful tool for predicting plate boundary configurations provided it is realized that this is more an approximate rule of thumb than a hard physical theory. Observations of the occurrence and orientation of episodic overshooting of ridge segments past the stable triple junction location should provide a better test that both fine-and large-scale triple junction tectonics are stress controlled.

What built Shatsky Rise, a mantle plume or ridge tectonics?

“…This simple-minded calculation ignores mantle convection or basal lithosphere topography that might steer the plume head to the ridge (e.g., Courtillot et al, 1999;Braun and Sohn, 2003;Jellinek et al, 2003). Although it is true that a ridge or triple junction in the vicinity of a plume will jump or reorganize to stay near the plume (e.g., Kleinrock and Morgan, 1988), this assumes that the ridges are already near the plume. Having a plume head find a triple junction as it rises from the deep mantle is a lowprobability event unless there is some connection that steers the plume head impact point toward the triple junction.…”

Minimum‐work mountain building

“…Sleep et al [1979] went on to show that, for the particular case of ridge-transform systems, the problems identified by Bird and Yuen [1979] could be reasonably neglected and reiterated the basic conclusion that models based on minimizing dissipation yielded results comparable to those based on force-balance analysis. This conclusion was verified by Kleinrock and Morgan [1988], who suggested that the success of the minimum-dissipation technique derived from the fact that plate boundaries minimizing dissipation also minimized resistive forces.…”

Mesozoic magnetic anomaly lineations and seafloor spreading history of the northwestern Pacific

“…Our results as well as the velocity triangle analyses do not support the existence of any microplates after chron M18. Morgan [1988] indicated that spreading ridges and triple junctions with one or more ridges are adjusted to remain near a hotspot. This indicates that the Pacific-Izanagi-Farallon triple junction continued to be situated on or very near to the Shatsky hotspot after the jump.…”