Eocene Propagating Rifts in the Southwest Pacific and Their Conjugate Features on the Nazca Plate

We have mapped a 1200 km long NW-SE trending lineament in Seasat and Geosat radar altimeter data crossing a remote portion of the Southwest Pacific Basin. This lineament runs obliquely both to the fracture zones (FZs) and magnetic lineations between the Austral and Agassiz FZs. By examining shipboard magnetics, gravity, and bathymetric profiles in this region and in the conjugate regions of the Nazca plate, we identify this feature as the site of a series of small ridge jumps, highly asymmetrical spreading and small propagating rifts, during the mid-to-late Eocene (chron C21 to chron C13). Between the Austral and Agassiz FZs there is roughly 250 to 350 km of missing Pacific plate crust, which must have been transferred to the Nazca plate by the propagating rifts and small ridge jumps. On the Nazca plate, the conjugate area to the gravity lineament is now being subducted beneath the Chile Trench. North of the Challenger FZ this area corresponds to a region of undecipherable magnetic anomalies. South of the Challenger FZ, there is evidence for a small 50 km ridge jump offshore of the Chile Trench. This small ridge jump by itself is not sufficient to account for the missing crust on the Pacific plate, and we conclude that there must have been repeated small ridge jumps at the same site. One consequence of these ridge jumps is that crust on the Nazca plate entering the trench is not as old as would be predicted by simply mirror imaging the anomaly pattern on the Pacific plate.