Global variations in gravity-derived oceanic crustal thickness and implications on oceanic crustal accretion processes

Abstract:

We present a new global model of oceanic crustal thickness based on inversion of global oceanic gravity anomaly and constrained by seismic crustal thickness profiles. Models of residual mantle Bouguer gravity anomaly (RMBA) were first calculated by removing from free-air gravity anomaly all gravitational effects that can be estimated using independent constraints, including seafloor topography, sediment thickness, and age-dependent thermal structure of the oceanic lithosphere. Models of gravity-derived crustal thickness were then calculated through inversion of the RMBA using best-fitting gravity-modeling parameters obtained from comparison with seismically determined crustal thickness profiles. Model results reveal that bout 5% of the global model crustal volume (or 9% of the global oceanic surface area) is associated with model crustal thickness <5.2 km (designated as “thin” crust). About 56% of the global crustal volume (or 65% of the surface area) is associated with model crustal thickness of 5.2-8.6 km thick (designated as “normal” crust). The remaining 39% of the global crustal volume (or 26% of the surface area) is associated with model crustal thickness >8.6 km (designated as “thick” crust) and is interpreted to have been affected by excess magmatism. Distribution of thickened model crust (>8.6 km) seems to depend on spreading rate and lithospheric age: (1) On ocean basins younger than 5 Ma, regions of thickened model crust are predominantly associated with slow and ultraslow spreading ridges. The relatively strong lithospheric plate at slow and ultraslow ridges might facilitate to support the loading of large magmatic emplacements. (2) In contrast, model crustal thickness near fast and intermediately fast spreading ridges typically does not exceed 7-8 km. As the crustal age increases and the lithospheric plate thickens, however, regions of thickened crust start to develop. (3) The integrated crustal volume depends on both spreading rate and age: For fast and intermediately fast ocean crust, peak values in the integrated crustal volume were reached at about 40-50 Ma and 70-80 Ma. The newly constructed global models of gravity-derived crustal thickness, combining with geochemical and other constraints, can be used to investigate the processes of oceanic crustal accretion and evolution.