402 TECTONICS/Propagating Rifts and Microplates At Mid-Ocean Ridges propagates through to the opposite spreading boundary, eliminating coupling to one of the bounding plates Dual spreading would no longer occur, spreading would continue on only one bounding ridge, and 106–107 km3 of microplate lithosphere would accrete to one of the neighbouring major plates Active microplates are thus modern analogues for how largescale (hundreds of kilometres) spreading centre jumps occur There is evidence in the older seafloor record that this happened many times along the ancestral East Pacific Rise All large right-stepping offsets along the Pacific– Nazca spreading centre are transform faults, whereas all large left-stepping offsets are microplates or the giant duelling propagators (possible protomicroplate) between the Easter and Juan Fernandez microplates (Figure 6) The Galapagos microplate at the Pacific– Cocos–Nazca triple junction also fits this pattern This suggests that a recent clockwise change in Pacific–Nazca plate motion could have been an important factor triggering the formation of these microplates Earth’s fastest active seafloor spreading occurs in this area, and all parts of the plate boundary presently spreading faster than 142 km My are reorganizing by duelling propagators or microplates (Figure 6) The combination of thin lithosphere produced at these ‘superfast’ seafloor spreading rates, and the unusually hot asthenosphere produced by an Easter mantle plume, would reduce the forces resisting propagation and thus make these plate boundary reorganizations easier, perhaps explaining their common occurrence in this area Microplates also occur in convergent settings, where small pieces of lithosphere are caught between large plates A well-studied continental convergence example is the Mediterranean, where small plates the size of Turkey and the Aegean adapt to Africa–Eurasia convergence Oceanic microplates can also form along convergent margins, e.g., in the West Pacific, to accommodate Pacific, Australia, and Asia convergence Figure Propagating rift model for continental breakup (map view) (A) Original continent under tension; (B) initial rifting, with the amount of extension represented between parallel lines; (C) mid rifting (seafloor spreading is occurring in the lower half of the continent while crustal thinning and extension occur in the upper half); (D) rifting complete (continental edges have undergone extension that increases in the direction of rifting); (E) My after rifting is complete (oldest seafloor is found in the part of the ocean where rifting began; oldest isochrons converge with the ocean continent boundary; continental edge is not an isochron); (F) reconstruction of the pre rift configuration (extension due to rifting results in apparent overlap when the continents are returned to their pre rift geometry) Reproduced with permission from Vink GE (1982) Continental rifting and the implications for plate tectonic reconstructions Journal of Geophysical Research 87: 10 677 10 688