A Magnetic Survey of the Southwest Pacific Ocean
The design and construction of a free precession proton magnetometer which give a reading of the field directly in gamma is described. This instrument has been used to obtain magnetic profiles across the Southwest Pacific Ocean during the 1963-65 summer Antarctic supply cruises of H.M.N.Z.S. Endeavour. The magnetic and bathymetric profiler obtained on these cruises have been analysed to determine the nature and structure of the oceanic crust in this region. The region is divided into four divisions. (l) The New Zealand Plateau, with an almost continental crustal thickness. (2) The Southwest Pacific Basin, at a depth of 3,000 fathoms. (3) The Pacific-Antarctic Ridge, pert of the world encircling mind-ocean ridge system. (4) The Ross Sea, an epicontinental sea across the Antarctic continental shelf. Subtraction of the regional field form the magnetic results has enabled a regional field map of the area to be drawn. Comparison with earlier results indicates a westward drift of the earth's field of approximately 0.25 degrees /yr. Some discussion of regional anomalies (~ 100 miles period) has been given. Because of the excellent correlation of magnetic anomalies from track to track across the basin it has been possible to draw an anomaly contour map of this part. This map illustrates the predominantly east-west trend of features over the basin. To the north the features parallel the edge of the New Zealand Plateau. To the south the features swing more towards the Pacific-Antarctic Ridge. A major discontinuity is indicated along a direction 9 degrees E of S meeting the Plateau just southwest of Antipodes Islands. If this discontinuity is extrapolated south to the ridge it meets it at approximately 180 degrees E, where the ridge turns N-E towards Easter Island. Across the ridge the magnetic pattern shows three distinct regions. Over the northern flanks large anomalies are evident but the correlation of anomalies from track to track is very poor. Further south, across the upper flanks, the magnetic records are very much subdued. The extent of this region varies appreciably from track to track. Over the axis of the ridge large, steep-sided anomalies are obtained. These correlate well over part of the region studied. The southern flanks of the ridge are hidden by the Balleny Plateau which seems to form a link between Antarctica and the ridge in this region. The bathymetry records obtained indicate a step-type formation over the ridge. A narrow median valley appears to exist along the axis of the ridge. Some preliminary experimentation has been carried out with continual seismic profiling techniques over the region. The equipment that has been developed and the preliminary results obtained with it are discussed.