The relationship between depth and age in the North Pacific Ocean

The North Pacific contains active mid-oceanic ridges and the oldest, Jurassic (166.8 ± 4 Ma), drilled oceanic crust. Its bathymetry is therefore critical to studies of the applicability of thermal contraction models (e.g., the infinite half-space and cooling plate) to the subsidence of seafloor with crustal age. The bathymetry, however, contains seamounts and oceanic islands (e.g., Mid-Pacific Mountains), oceanic plateaus (e.g., Hess, Magellan, and Shatsky), and midplate topographic swells (e.g., Hawaii), which are unrelated to the current plate-scale thermal state of the oceanic lithosphere. We use here a regional-residual separation algorithm called MiMIC to remove these features and to isolate the depths associated with the subsidence of North Pacific oceanic crust. These depths, z (m), increase with time, t (Ma), as z = 3010 + 307 until 85 Ma. For greater ages the depths “flatten” and asymptotically approach ∼6.1 km and are well described by z = 6120 − 3010 exp(−0.026t). The flattening is not “abrupt” as recently described in z-t curves produced using the mean, median, and mode. As a result, the depths of both young and old seafloor are fit well (mean difference between and observed and calculated depths of 75 ± 54 m 1σ) by a single cooling plate model. Using a thermal conductivity, k, of 3.138 mW m−2 as previous studies, however, gives a plate of similar thickness (i.e., thermal thickness, L, of ∼115 km) but one which is unreasonably hot (i.e., temperature at the base of the plate, Tb, of 1522 °C) and inexpansive (i.e., coefficient of thermal expansion, α, of 2.57×10−5 °C−1). More reasonable values (i.e., Tb = 1363°C, k = 3.371 W m−1 °C−1, α = 2.77×10−5 °C−1, and L = 120 km) are obtained if the crustal thickness is used to constrain Tb and a certain amount of the surface heat flow is allowed to be radiogenically generated within the oceanic lithosphere.