Microplate and shear zone models for oceanic spreading center reorganizations

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Abstract
The reorganization of oceanic spreading centers is often accomplished through the process of rift propagation. We first examine general rift propagation geometries for the simple two‐plate case in which the propagating rift instantaneously reaches full spreading rate while the other rift dies instantaneously, such that the propagating and dying rifts never overlap. A velocity space representation provides a framework for describing the geometry of rift propagation and its various tectonic elements, and shows how both the failed rift and isochrons between the two rifts are reoriented as a consequence of purely rigid plate tectonics. We then discuss the more complex geometries that result when both growing and dying rifts spread simultaneously during propagation, such that a technically independent overlap region, like the Easter or Juan Fernandez plates along the East Pacific Rise, is produced. We examine the effect of the propagation rate and rise time of spreading on the evolution of overlap regions between dual oceanic spreading centers. The tectonics and evolution of such regions can be modeled by assuming that they act either as rigid microplates or as zones of distributed shear. Rigid plate models are suggested by the reasonable success of Euler vectors derived from inversions of plate motion data (spreading rates, transform azimuths, and slip vectors) both in describing available data and in predicting subsequently acquired data. On the other hand, recent structural data showing fabric oblique to the surrounding spreading centers might suggest shearing, rather than rigid behavior, in the interior of the Easter microplate. We use schematic evolutionary simulations for an Easter‐style geometry to examine whether plate motion data and orientations of preexisting and present structures can be used to discriminate between rigid and shear models. In a microplate model, rigid plate tectonics requires that relative motion occur only along the boundaries of the overlap region, while structures within the overlap zone rotate rigidly. In contrast, in a shear model, relative motions at the boundaries differ from those predicted by rigid plate tectonics, and the angular relationships of structures within the overlap region are altered by internal deformation. Thus inversion of relative motion data yields different results for rigid and shear behavior. Similarly, under favorable circumstances, structural trends could be diagnostic of the different cases. We find that much of the Easter structural data suggestive of shearing are also consistent with rigid plate tectonics. We conclude that microplate models offer useful testable hypotheses for describing overlap systems and that shearing need be invoked only to explain features not describable by rigid plate tectonics. Furthermore, the approaches used here to study current overlap regions should be useful in the analysis of similar regions formed during past oceanic plate boundary reorganizations.