THE CHILE RIDGE - A TECTONIC FRAMEWORK

A new Chile ridge tectonic framework is developed based on satellite a ltimetry data, shipboard geophysical data and, primarily, 38,500 km of magnetic data gathered on a joint U.S.-Chilean aeromagnetic survey. E ighteen 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 plate boundary (Chile ridge) at 35 degrees S and the Chile margin triple junction at 47 degrees S. Chile ridge spreading rates from 23 Ma to Present have b een determined and show slowdowns in spreading rates that correspond t o 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 to be a multiple-offset FZ system composed of six FZs separated by short ridge segments 22 to 27 km long. At chron 5A (similar to 12 Ma), the Chile ridge propagated from the Valdivia FZ system northward into the Nazca plate through crust formed 5 Myr earl ier at the Pacific-Nazca ridge. Evidence for this propagation event in cludes the Friday and Crusoe troughs, located at discontinuities in th e magnetic anomaly sequence and interpreted as pseudofaults. This prop agation event led to the formation of the Friday microplate, which res ulted in the transferal of crust from the Nazca plate to the Antarctic plate, and in a 500-km northward stepwise migration of the Pacific-An tarctic-Nazca triple junction. Rift propagation, microplate formation, microplate extinction, and stepwise triple junction migration are fou nd to occur during large-scale plate motion changes and plate boundary changes in the southeast Pacific.