Relationship between depth and age in the North Pacific Ocean

[1] 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 ffiffi t p 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 1s) 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, a, of 2.57 10 5 C ). More reasonable values (i.e., Tb = 1363 C, k = 3.371 W m 1 C , a = 2.77 10 5 C , 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.