Oceanic axial depth and age-depth distribution of oceanic lithosphere: Comparison of magnetic anomaly picks versus age-grid models

Abstract

The oceanic age-depth relationship is a fundamental consequence of Earth’s convective system. The new analysis presented here integrates four major data sets to assess the global age-depth distribution of the oceanic lithosphere. First, the distribution of depths along the modern mid-oceanic-ridge axis was obtained using an updated digitized divergent plate-boundary data set for the main ocean basins. Mid-ocean ridges vary in depth by ∼8 km, from −6453 m to +1998 m, with a median depth of −3000 m, modal depth of −2670 m (with a second mode at −2770 m), and area-weighted mean depth of −3044 m. Ridge depth has some correlation with spreading rate up to about 50 mm/yr if the hotspot-impacted North Atlantic and Sheba Ridges are excluded. Second, zero-age crustal thickness, based on a compilation of ∼1000 estimates, was used to test the relationship of oceanic crustal thickness with axial depth. There is some correlation, but it is not a compact correlation, implying that other processes are contributing to both axial depth and crustal thickness variations. Third, a revised sediment-load correction is presented based on assessment of 9,260,185 records from 1263 deep-sea drilling sites with wet bulk density measurements. This can be extrapolated to thicknesses in excess of 8000 m, with an exponential porosity-depth relationship with an initial porosity of ϕ0 = 0.61. Dispersion of integrated wet bulk density measurement-derived sediment-load corrections for the 1263 sites implies ∼±100 m uncertainty in the sediment-load correction at 1300 m sediment thickness. Fourth, the age-depth relationship of oceanic lithosphere was derived based on sediment-load corrected depths of ∼90,000 explicitly dated, globally distributed magnetic reversal picks restricted to the main ocean basins and filtered to exclude large igneous provinces and trench-impacted locations. A best-fit global age-median sediment-corrected depth relationship was derived using a plate model with zero-age ridge depth of −2309 m, temperature difference of 1304 °C, and asymptotic plate thickness of 122 km. The dispersion of sediment-load corrected depths as a function of age is comparable to that observed along the mid-oceanic-ridge system, suggesting that zero-age depths contribute to dispersion at all ages. This is borne out for oceanic lithosphere at relatively young ages (ca. <70 Ma), but the dispersion at older ages appears to be dominated by superposition of hotspots and other large-scale perturbations. In addition, a comparison was made with various vintages of age-grid–derived age-depth distributions, as well as previous age-depth models. The implied age-depth relationship using older versions of the age grid deviates significantly from the age-depth relationship obtained using the magnetic reversal picks alone. There is considerable disparity between pick ages and corresponding grid cell ages for these data sets. In contrast, the most recent age grid (2016) more closely matches the age-depth relationship derived here and shows significantly less disparity with magnetic reversal pick ages. A qualitative suggestion is made that the deviation of the age-depth relationship from a half-space cooling to exponential model reflects the advective contribution of asthenospheric flow associated with asthenospheric tractions contributing to plate-driving forces.