400 TECTONICS/Propagating Rifts and Microplates At Mid-Ocean Ridges Figure Tectonic boundaries, magnetic isochrons, and structures of the Easter microplate (EMP) and Juan Fernandez microplate (JFMP) EPR, East Pacific Rise; FZ, fracture zone; NAZ, Nazca; PAC, Pacific; SA, South America; ANT, Antarctic; WOPF, WIPF, EOPF, and EIPF are western outer and inner and eastern outer and inner pseudofaults, respectively Numbers (e.g 2, 2A) identify magnetic chrons from magnetic reversal time scale, J is Jaramillo reversal $1 Ma, PT is paleotransform Reproduced with permission from Bird RT and Naar DF (1994) Intratransform origins of mid ocean ridge microplates Geology 22: 987 990 rift of the Juan Fernandez microplate, and the dominant (West) rift of the duelling propagators between the microplates, are all propagating away from the Easter mantle plume (or the intersection of this plume with the ridge axis), suggesting that microplate formation as well as rift propagation can be driven by plume-related forces Both microplates have large ($100 Â 200 km), complex, pervasively deformed cores, which may have formed by bookshelf faulting in overlap zones during an initial large-scale propagating rift stage of evolution Both show more recent stages of growth as independent microplates, with deformation concentrated along the plate boundaries At present, both microplates are rotating clockwise very rapidly about poles near the microplate centres, spinning like roller-bearings caught between the major bounding plates The Easter microplate rotation velocity is about 15 My and the Juan Fernandez velocity is about 9 My The roller-bearing analogy has been quantified in an idealized edge-driven model of microplate kinematics If microplate rotation is