Formation of curved seafloor fabric by changes in rift propagation velocity and spreading rate: application to the 95.5° W Galapagos propagator

The geometry of oceanic spreading centers can change by rift propagation, in which a new ridge segment propagates and preempts a preexisting one. As the locus of relative motion between the two plates shifts, the dying and growing ridge segments are joined by a boundary, usually a transform, which migrates with time, causing seafloor to be transferred from one plate to the other. As a result, originally ridge-parallel isochrons in a zone between the growing and failed rifts can be reoriented to trends oblique to both the ridge and spreading direction. An attractive feature of this model is that the reorientation occurs by strictly rigid plate tectonics, since relative motion occurs only between points on different plates. Recent surveys of the Galapagos propagating rift system at 95°W show seafloor fabric curving away from the growing rift and toward the failed rift. These observations have been interpreted as requiring a zone of distributed simple shear between the two rifts. Such behavior would represent a local deviation from rigid plate tectonics, since relative motion would be distributed over a finite region not part of either of the two plates. Here we explore possible rigid plate models for the formation of curved seafloor lineaments by rift propagation. -from Authors