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The depth to the temperature at which rocks lose their magnetization (Curie point depth) can be calculated by estimating the average depth to the bottom of the magnetized bodies which make up the crust. Using the method of spectral analysis on magnetic data, as suggested by Bhattacharyya and Leu, we have calculated the depth to the Curie point temperature in Bligh Water, Fiji.
Measurements of the magnetic susceptibility of rocks versus temperature at field strengths approaching that of the earth's main field suggest that the effective Curie point temperature of the magnetic crust is about 500°C. Using this temperature with the calculated Curie point depth and reasonable ocean-bottom temperatures, we have prepared a crustal-thermal-gradient map of the Bligh Water basin. The thermal gradients thus calculated are in reasonable agreement with gradients determined from conventional oceanographic heat-flow measurements in surrounding deep-water regions. The results for Bligh Water, Fiji, suggest that hydrocarbon maturation temperatures are probably reached in the relatively shallow lower to middle Miocene reefs.
We are presently analyzing marine magnetic data over the Fiji Plateau west of Fiji, where an active back-arc spreading center is postulated. The results of this investigation will provide a direct comparison between thermal gradients calculated from Curie point data and marine heat-flow measurements, as well as provide insight into the regional geophysics of the area.
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