Tectonic History and New Isochron Chart of the South Pacific (Paleoceanographic Mapping Project Progress Report No. 43-0888)

We have developed an internally consistent isochron chart and a tectonic history of the South Pacific using a combination of new satellite altimeter data and shipboard magnetic and bathymetric data. Highly accurate, vertical deflection profiles (1-2 µrad), derived from 22 repeat cycles of Geosat altimetry, reveal subtle lineations in the gravity field associated with the South Pacific fracture zones. These fracture zone lineations are correlated with sparse shipboard bathymetric identifications of fracture zones and thus can be used to determine paleo-spreading directions in uncharted areas. The high density of Geosat altimeter profiles reveals previously unknown details in paleo-spreading directions on all of the major plates. Magnetic anomaly identifications and magnetic lineation interpretations from published sources were combined with these fracture zone lineations to produce a tectonic fabric map. The tectonic fabric was then used to derive new plate reconstructions for twelve selected times in the Late Cretaceous and Cenozoic. This is the first time that the tectonic history of the entire South Pacific has been studied as a whole. From our reconstructions, we estimated the former location of the spreading centers in order to derive a new set of isochrons (interpreted lines of equal age on the ocean floor). We believe that the use of new Geosat altimeter data in combination with a multi-plate reconstruction has led to a major improvement in our understanding of South Pacific tectonics. There are three times of important changes in the tectonic history of the South Pacific. Just prior to Chron 34 (84.0 Ma) spreading initiated between Marie Byrd Land and the New Zealand block (the Campbell Plateau and the Chatham Rise). Spreading in the southwest Pacific was occurring along two different spreading centers between Chron 34 (84.0 Ma) and Chron 25 (58.9 Ma): the Pacific-Antarctic Spreading Center to the west and the Pacific-Bellingshausen Spreading Center· to the east at around the time of Chron 21 (49.4 Ma), the eastern and western spreading centers began spreading about a common pole of rotation. In addition, the Pacific-Antarctic Spreading Center broke through old crust to the north, transferring a piece of crust created at the Pacific-Aluk spreading center to the Antarctic plate. The next major change in the South Pacific occurred between Chron 7 (25.8 Ma) and Chron 5 (10.6 Ma) when spreading initiated at the Galapagos Spreading Center and the East Pacific Rise was reoriented from spreading in a northwesterly direction to spreading in a northeasterly direction.