Heterogeneous cooling subsidence of oceanic lithosphere controlled by spreading rate

Abstract

Ocean age-dependent cooling subsidence with seafloor deepening is traditionally described by models of thermo-chemical buoyancy of oceanic plates with globally constant parameters, that specify a linear correlation between square-root of seafloor age, sqrt(age), and bathymetry. Here I present a worldwide analysis of the ocean floor split into 94 segments, delineated by wide-offset transform faults and mid-ocean ridges, to demonstrate a strong heterogeneity of sediment-corrected isostatic cooling subsidence both between and within normal oceans. Anomalous oceans are identified from bathymetry deviation from age-dependent predictions during data processing. Subsidence parameters for individual ocean segments significantly deviate from global constants in conventional models and show a large variability of subsidence rate (270–535 m/Ma^1/2) and zero-age depth (−1.30 to −3.03 km) with plate thickness estimated between 50 and 160 km for cooling models with constant mantle properties.

Statistically strong correlations (R²=0.80–0.94) between major characteristics of cooling subsidence and spreading rate indicate that ocean evolution is essentially controlled by spreading rate, despite this factor is not included in conventional models of ocean subsidence. Normal oceans with slower spreading rate have, statistically, higher subsidence rate which implies faster gravitational collapse caused by faster plate cooling with moderate-to-low mantle temperatures at mid-ocean ridges. Fast-spreading oceans have the opposite characteristics. The ultraslow SW Indian and the fast-spreading Central Pacific Oceans are the end-members in ocean cooling subsidence trends, with the Atlantic/NW Indian Oceans tending towards the ultraslow end, and the Pacific/SE Indian Oceans being closer to the fast-spreading end. The Arctic Ocean and the Atlantics north of the Charlie-Gibbs Fracture Zone with an atypical subsidence behavior often deviate from the global trends. Strong correlation between spreading rate, ocean half-width and the type of ocean margins implies that ridge-push dominates tectonic forces in slower-spreading, narrower oceans with passive margins, while slab-pull at active margins is a dominant tectonic force in faster-spreading oceans with half-width exceeding 4250 km.

The age of bathymetry departure from cooling subsidence, controlled by distribution of hotspots on ocean floor, correlates (R²=0.76) with spreading rate, and thus is not fully random. Slower-spreading oceans follow normal cooling subsidence to older ages (7.5–9.5 Ma^1/2) than faster-spreading oceans (5–7 Ma^1/2). Recognition that spreading rate controls ocean evolution with formation of active or passive ocean margins dominated by slab-pull or ridge-push contributes to advances in understanding driving forces in geodynamics.