Central and South Pacific Plate Tectonic Reconstructions

Several aspects of the plate tectonic evolution of the ridge systems and structural development of oceanic crust in the Central and South Pacific basins are investigated. In the South Pacific surveys of magnetic anomaly data and multibeam bathymetry of the Menard and Pitman Fracture Zones, 3̃500 km apart, allowed for significant refinement of the Pacific-Antarctic plate motion history over the last 44 million years. We tested the predictive quality of the high-resolution plate motion model at the southwestern end of the Pacific-Antarctic Ridge close to the Macquarie Triple Junction and showed that reconstructions predict phases of transform fault motion and extension to great detail. In particular, the most recent period of extension since 10 million years ago occurred when a 1̃200 km long transform fault was replaced an array of 15 transform faults and short spreading centers. The detailed plate motion model for the Pacific-Antarctic Ridge was used to test whether abyssal hill tip curvatures correlate to changes in plate motion direction, which lead to periods of transtension and transpression. The tip of the ridge axis can deflect up to 60° into the transform fault in response to changes in the stress field at ridge-transform -intersections. This is observed to have occurred in particular during periods of significant transtension. In addition, abyssal hill curvatures in the opposite sense are observed to have developed during periods of transpression when increased coupling across an oceanic transform fault is partially accommodated by distributed strike-slip deformation rather than solely by discontinuous displacement at the transform fault. In the equatorial Pacific region, where the total field magnetic anomalies are very small for oceanic crust formed at the North-South striking East Pacific Rise, towed and aeromagnetic vector magnetometer data allowed for better constraints of the relative motions of the Pacific-Cocos Ridge near the Galapagos Triple Junction. These data in combination with available magnetic anomaly profiles for most of the East Pacific ridge were used to improve plate reconstructions for the Pacific-Cocos, Pacific-Nazca, Cocos-Nazca, Pacific-Antarctic, Chile Ridge and Mathematician Paleo-Ridge since 9 Ma