SOUTHEAST PACIFIC TECTONIC EVOLUTION FROM EARLY OLIGOCENE TO PRESENT

Plate tectonic reconstructions of the Nazca, Antarctic, and Pacific pl ates are presented from late Oligocene to Present. These reconstructio ns document major plate boundary reorganizations in the southeast Paci fic at chrons 6C (24 Ma), 6(o) (20 Ma), and 5A (12 Ma) and a smaller r eorganization at chron 3(o) (5 Ma). During the chron 6(o) reorganizati on it appears that a ridge propagated into crust north of the northern most Pacific-Antarctic Ridge, between the Chiloe fracture zone (FZ) of the Chile ridge and Agassiz FZ of the Pacific-Nazca ridge, which resu lted in a northward jump of the Pacific-Antarctic-Nazca (PAC-ANT-NAZ) mid-ocean triple junction. During the chron 5A reorganization the Chil e ridge propagated northward from the Valdivia FZ system to the Challe nger FZ, through lithosphere formed roughly 5 Myr earlier at the Pacif ic-Nazca ridge. During this reorganization a shea-lived microplate (th e Friday microplate) existed at the PAC-ANT-NAZ triple junction. The P AC-ANT-NAZ triple junction jumped northward 500 km as a result of this reorganization, from a location along the Valdivia FZ to a location a long the Challenger FZ. The chron 5A reorganization also included a ch ange in spreading direction of the Chile and Pacific-Antarctic ridges. The reorganization dt chron 3(o) initiated the formation of the Juan Fernandez and Easter microplates along the East Pacific rise. The mann er of plate boundary reorganization at chron 6(o) and chron 5A (and po ssibly today at the Juan Fernandez microplate) included a sequence of rift propagation, transfer of lithosphere from one plate to another, m icroplate formation, and microplate abandonment and resulted in northw ard migration of the PAC-ANT-NAZ triple junction. The associated micro plate differs from previously studied microplates in that there is no failed ridge.