Reassessing the Thermal Structure of Oceanic Lithosphere With Revised Global Inventories of Basement Depths and Heat Flow Measurements

Abstract

Half‐space cooling and plate models of varying complexity have been proposed to account for changes in basement depth and heat flow as a function of lithospheric age in the oceanic realm. Here, we revisit this well‐known problem by exploiting a revised and augmented database of 2,028 measurements of depth to oceanic basement, corrected for sedimentary loading and variable crustal thickness, and 3,597 corrected heat flow measurements. Joint inverse modeling of both databases shows that the half‐space cooling model yields a mid‐oceanic axial temperature that is >100°C hotter than permitted by petrologic constraints. It also fails to produce the observed flattening at old ages. Then, we investigate a suite of increasingly complex plate models and conclude that the optimal model requires incorporation of experimentally determined temperature‐ and pressure‐dependent conductivity, expansivity, and specific heat capacity, as well as a low‐conductivity crustal layer. This revised model has a mantle potential temperature of 1300 ± 50°C, which honors independent geochemical constraints and has an initial ridge depth of 2.6 ± 0.3 km with a plate thickness of 135 ± 30 km. It predicts that the maximum depth of intraplate earthquakes is bounded by the 700°C isothermal contour, consistent with laboratory creep experiments on olivine aggregates. Estimates of the lithosphere‐asthenosphere boundary derived from studies of azimuthal anisotropy coincide with the 1175 ± 50°C isotherm. The model can be used to isolate residual depth and gravity anomalies generated by flexural and sub‐plate convective processes.

Analysis and Mapping of an Updated Terrestrial Heat Flow Data Set

“…The resulting seafloor subsidence is significantly different from that predicted by GDH1 for age >65 Ma but in good agreement with Sclater () Model (PS; Figure ) or the more recent analysis of Richards et al (), which selected 2,028 reliable sites where sediment thickness and crustal thickness are well known. In order to check if these differences were related to methodological problems, the same analysis was performed using just the global topography (GTOPO30).…”

Global distribution of sediment-hosted metals controlled by craton edge stability

“…Mapping the lithosphere-asthenosphere boundary (LAB). A recent study on the thermal structure of oceanic lithosphere found that the 1175 ± 50 • C isotherm provides a good match to seismological observations of the lithosphere-asthenosphere boundary (Richards et al, 2018). In this study, we therefore adopt this isotherm as a proxy for lithospheric thickness beneath the continents.…”

“…For each rift scenario, we select an initial lithospheric template. For regular continental lithosphere, the crustal thickness is set to 30 km and the total lithospheric thickness to 140 km, which matches results from plate cooling models of oceanic lithosphere (Richards et al, 2018) and places the 1175 • C isotherm at ∼ 120 km. Radiogenic heat production in the mantle is set to zero, whilst the crustal value is tuned to 1.0 µW m −3 such that the steady state geotherm yields a surface heat flow of ∼63 mW m −2 , which is the average for Phanerozoic continental lithosphere (Lucazeau, 2019) For cratonic lithosphere, we assume an initial crustal thickness of 50 km, lithospheric thickness of 280 km (1175 • C isotherm at ∼ 240 km), and crustal radiogenic heat production of 0.57 µW m −3 , which yields an initial surface heat flux consistent with the average of ∼48 mW m −2 for Archean and cratonic areas (Lucazeau, 2019) Based on the typically low paleowater depth of sediments found in proximal portions of these basins and the high supply of clastic material from adjacent cratons, we assume the basin is constantly filled by sediments.…”

“…A high resolution regional LAB map over Australia is obtained from the FR12 model and is calibrated using nine local paleogeotherms derived from thermobarometry of mantle peridotite xenoliths and xenocrysts. To expand our analysis to other continents, a global LAB is also produced using the SL2013sv model (Schaeffer & Lebedev, 2013) and calibrated using multiple constraints, including the latest thermal structure of cooling oceanic lithosphere (Richards et al, 2018). This global LAB exhibits a bi-modal thickness distribution, with peaks at 90 km and 190 km, separated by a minimum at 150 km (Supplementary Information).…”

“…An alternative approach to constraining these material properties is to invert real-Earth observations of the relationship between temperature, shear-wave velocity, attenuation and viscosity in the upper mantle (Priestley & McKenzie, 2006;Afonso et al, 2013a,b;. The general philosophy is that there are certain properties that are 'known' about Earth, including the typical thermal structure of oceanic lithosphere (Richards et al, 2018), the average adiabatic gradient within the convecting mantle (Connolly, 2009), the attenuation structure of the upper mantle beneath old oceanic lithosphere , and the bulk diffusion creep viscosity of the upper mantle from studies of glacial isostatic adjustment (Lau et al, 2016). Thus any thermal model inferred from shear-wave velocities should be compatible with these observations.…”