Tectonics of the Nazca-Antarctic plate boundary
- S. Stein
- Earth Planet. Sci. Lett.,
A new Chile ridge tectonic framework isdeveloped based on satellite altimetry data, shipboard geophysical d ta and, primarily, 38,500 km of magnetic data gathered on a joint U.S.Chilean aeromagnetic survey. Eighteen active transforms with fossil fracture zones (FZs), including two complex systems (the Chile FZ and Valdivia FZ systems), have been mapped between the northern end of the Antarctic-Nazca pl te boundary (Chile ridge) at 35øS and the Chile margin triple junction at 47øS. Chile ridge spreading rates from 23 Ma to Present have been determined and show slowdowns in spreading rates that correspond to times of ridge-trench collisions. The Valdivia FZ system, previously mapped as two FZs with an uncharted seismically active region between them, is now recognized tobe a multiple-offset FZ system composed of six FZs separated by short ridge segments 22 to 27 km long. At chron 5A (-12 Ma), the Chile ridge propagated from the Valdivia FZ system northward into the Nazca plate through crust formed 5 Myr earlier at the Pacific-Nazca ridge. Evidence for this propagation event includes the Friday and Crusoe troughs, located at discontinuities n the magnetic anomaly sequence and interpreted as pseudofaults. This propagation event led to the formation of the Friday microplate, which resulted in the transferal of crust from the Nazca plate to the Antarctic plate, and in a 500-km northward stepwise migration of the Pacific-Antarctic-Nazca triple junction. Rift propagation, microplate formation, microplate xtinction, and stepwise triple junction migration are found to occur during large-scale plate motion changes and plate boundary changes in the southeast Pacific.