Age, Depth, and Residual Depth Anomalies in the North Pacific: Implications for Thermal Models of the Lithosphere and Upper Mantle

We present an empirical basement depth versus age relation for the North Pacific Ocean, based on the statistical treatment of an ocean-wide gridded data set. The SYNBAPS bathymetry was averaged into half-degree intervals and corrected for the effects of sediment loading. The resulting basement depths were plotted against ages determined from a revised isochron chart based on a recent compilation of magnetic lineations and various published plate reconstructions. On crust older than 80 Ma, the depths are skewed to the shallow side of the depth versus age distribution by large numbers of seamounts. Therefore the mean and standard deviations are not useful representations of the data. A more appropriate representation is the mode (or greatest concentration of points) and contours around the mode. The contours around the mode show that most ocean floor increases in depth with the square root of age out to crust of 80 Ma. Beyond this the majority of the data oscillates about a line that remains essentially constant as the age in-creases. Approximately 56% of all the data points lie within a + 300m band about the mode. If the sediment thickness data in the older basins of the western North Pacific is correct then the flattening of the depths favor a model in which extra heat is supplied to the base of the lithosphere on older ocean floor. Residual depth anomalies were calculated by removing the depths predicted by such a model. These anomalies correlate with bathymetric features and occur predominantly on crust of 120 and 160 Ma. They account for the rises in the mode at these two ages. The overall subsidence of the ocean floor can be accounted for by the cooling of a thermo-mechanical boundary layer. Correlations between geoid height and depth are evidence that many of the residual depth anomalies result from convective plumes which reset the thermal structure of the lithosphere. It is possible that this process observed at different times after the initial resetting of the isotherms may account for many of the depth anomalies in the western North Pacific.