Southeast Pacific tectonic evolution from early Oligocene to present

Abstract

Plate tectonic reconstructions of the Nazca, Antarctic, and Pacific plates are presented from late Oligocene to Present. These reconstructions document major plate boundary reorganizations in the southeast Pacific at dirons 6C (24 Ma), 6(o) (20 Ma), and 5A (12 Ma) and a smaller reorganization at chron 3(o) (5 Ma). During the chron 6(o) reorganization it appears that a ridge propagated into crust north of the northernmost Pacific-Antarctic Ridge, between the Chiloe fracture zone (FZ) of the Chile ridge and Agassiz FZ of the Pacific-Nazca ridge, which resulted in a northward jump of the Pacific-Antarctic-Nazca (PAC-ANT-NAZ) mid-ocean triple junction. During the chron 5A reorganization the Chile ridge propagated northward from the Valdivia FZ system to the Challenger FZ, through lithosphere formed roughly 5 Myr earlier at the Pacific-Nazca ridge. During this reorganization a short-lived microplate (the Friday microplate) existed at the PAC-ANT-NAZ triple junction. The PAC-ANT-NAZ triple junction jumped northward 500 km as a result of this reorganization, from a location along the Valdivia FZ to a location along the Challenger FZ. The chron 5A reorganization also included a change in spreading direction of the Chile and Pacific-Antarctic ridges. The reorganization at chron 3(o) initiated the formation of the Juan Fernandez and Easter microplates along the East Pacific rise. The manner of plate boundary reorganization at chron 6(o) and chron 5A (and possibly today at the Juan Fernandez microplate) included a sequence of rift propagation, transfer of lithosphere from one plate to another, microplate formation, and microplate abandonment and resulted in northward migration of the PAC-ANT-NAZ triple junction. The associated microplate differs from previously studied microplates in that there is no failed ridge.