In this study, magnetic and bathymetric data from an R/V Endeavor survey are combined with earthquake mechanism studies to produce a tectonic model for the Juan Fernandez microplate at the Pacific-Nazca-Antarctic triple junction. Magnetic lineations along the East ridge of the microplate fan to the south, indicating that the Nazca-Juan Fernandez pole is near the northern end of this feature. Relative motions of the Juan Fernandez-Pacific-Nazca-Antarctic four-plate system are determined using relative plate motion inversions. The resulting tectonic model predicts that the northern boundary of the Juan Fernandez microplate is a zone of compression and that the West Ridge and southwestern boundary are spreading obliquely. The southeastern boundary is predicted to be obliquely divergent, but present data fail to constrain its geometry. We also present a schematic evolutionary model which relates the formation of the Juan Fernandez microplate to differential spreading rates at the triple junction, and discuss related complexities. the future evolution of the four-plate system depends on whether one or both ridges remain active.