Analysis of propagators along the Pacific–Antarctic Ridge: evidence for triggering by kinematic changes

Abstract

We analyze the structure and evolution of two propagators along the Pacific–Antarctic Ridge (PAR) that we surveyed during the Pacantarctic cruise of the N/O L’Atalante. A large propagator at 63°30′S, 167°W shows a N50°E-trending segment of the PAR propagating southwestward, while the adjacent, N45°E-trending segment retreats. The propagating and doomed ridges are offset by about 43 km. They both curve into the offset to define an overlap zone about 25 km long. The inner pseudofault is juxtaposed to a series of E–W-trending ridges inferred to represent the failed axes. Their direction and arrangement suggest an evolution as an overlapping propagator with cyclic rift failure. The pseudofaults are 35±5° oblique to the propagating ridge, which implies a rate of propagation of 44±8 mm/yr, using a 62 mm/yr full spreading rate, comparable to that of other propagators with similar morphology. The age of the initiation of the propagation from the Heirtzler fracture zone is estimated to be 5–6 Ma, which coincides with the age of a clockwise change in spreading direction. A second, smaller, southwestward propagator is observed northeast of the major one, at 63°15′S, 165°10′W, with a morphology very similar to that of the larger one. It is inferred to have started near 1 Ma, again at the time of a clockwise change in spreading direction along the PAR. These two propagators are likely to have evolved from extensional relay zones which developed within the Heirtzler transform fault (TF) valley following clockwise changes in spreading direction. The present-day axial discontinuity is less than 40 km in offset and may not be a TF anymore. The development of propagators in this area of the PAR appears to be triggered by kinematic changes rather than by thermal gradients along the ridge. Other propagators that have left similar signatures on the flanks of the PAR, appear to have developed at similar spreading rates near 50–60 mm/yr full rate, as a result of kinematic changes.