Heat flow maintained in ocean basins of great age: investigations in the north‐equatorial West Pacific

Multipenetration heat flow measurements have been made at four sites in deep basins of the west‐central Pacific Ocean: the West Mariana Basin, Central Mariana Basin, Nauru Basin and Central Pacific Basin. The final heat flows are, respectively, 46.6 /pm 0.5, 49.4 /pm 0.2, 44.2 /pm 0.9 and 49.5 /pm 1.1 mW m^‐2. Each site was surveyed by single‐channel seismic reflection profiling, and provided a gravity core. The instrument measured thermal conductivity in situ over the entire depth intervals used for determination of the gradients, and the reduction scheme iterated conductivity and heat‐capacity changes into the fitting procedure, both of entry frictional decays and of conductivity heat pulse decays. The absolute accuracy of the instrument should approach 2 per cent and the first site would make a good intercalibration standard for heat flow measurement. The heat flow variation between the sites is real, and there is also a significant variation in the isostatically reduced depths of the sites. There is no age progression of either depth or heat flow, and, when five other good multidata points are included, the relationship between depth and heat flow conforms to that expected from simple cooling models only in an average sense for the whole group. The most plausible explanation for the variations is that heat flow and thermal elevation are dependent on different levels of deep lithosphere reheating at different times between 70 and 120 Myr ago. It is suggested that additional topographic variation is caused by the different accumulations of sediment and lava flows at each site, and to errors in the isostatically reduced depths due to incomplete knowledge of the stratigraphy down to the crust‐mantle interface. These explanations of the topographic variation could be tested by seismic refraction measurements.