Variations in thickness of layer 3 dominate oceanic crustal structure

Abstract

Seismic refraction studies have determined that, while much of the crust underlying the world's oceans is about 7 km thick, it also occurs in lesser and considerably greater thicknesses. Here we show that vastly different thicknesses of oceanic crust (formed at ridges by seafloor spreading) are generally characterized by the two-gradient velocity structure recognized as typical of 6–7 km thick oceanic crust. Adopting a standard, we study ninety published seismic velocity structures of oceanic crust ranging in thickness from 2 to 37 km. Several structures formed in midplate settings (ocean islands and seamounts) are also considered for comparison. Regardless of origin (ridge or midplate) we find that the percentage of the whole crust formed by Layer 2 systematically decreases with increasing total crustal thickness. While the average velocity of Layer 2 varies widely for all crust, the average velocity of Layer 3 increases systematically with increases in the average whole crustal thickness and velocity. Crust formed in midplate settings differs in that the thickness of Layer 2 velocity crust is generally two to three times that of crust formed at ridges. Residual depths were calculated for oceanic crust by removing the effects of sediment loading and subsidence due to lithospheric cooling. Most of the data fall close to a model whereby different crustal thicknesses result from different extents of partial melting of oceanic upper mantle. The crustal sections were restored to ‘zero-age’, assuming the average depth of emplacement for 7 km thick oceanic crust is 2.5 km below sea level. From this restoration it appears that the depth to the top of Layer 3 at the time it was formed is 4.25 (± 1.25) km below sea level regardless of whole crustal thickness or spreading rate. These results suggest that the mechanism of construction of oceanic crust is, in many ways, remarkably uniform despite very large changes in the total thickness of the crust produced. Very basic questions remain. It is, for instance not at all clear why Layer 2 and Layer 3 exist in the proportions they do. While neutral buoyancy or melt trapping at the base of a brittle lid may account for local properties of the Layer 2/3 boundary, they do not seem adequate to account for the global phenomenon.