Inconsistent correlation of seismic layer 2a and lava layer thickness in oceanic crust

At mid-ocean ridges with fast to intermediate spreading rates, the upper section of oceanic crust is composed of lavas overlying a sheeted dyke complex.

Seismic reflection data acquired over young oceanic crust commonly image a reflector known as 'layer 2A', which is typically interpreted as defining the geologic boundary between lavas and dykes

Although the layer 2A reflector is imaged near the top of the sheeted dyke complex at fast-spreading crust located adjacent to the Hess Deep rift, it is imaged significantly above the sheeted dykes section at intermediate-spreading crust located near the Blanco transform fault

Although the lavas and underlying transition zone thicknesses differ by about a factor of two, the shallow seismic structure is remarkably similar at the two locations.

Anomaly :

Seismic data

The two-way travel time (TWTT) between the sea floor and the layer 2A event varies from 0.1 to 0.8 s, with lateral variability in TWTT to the layer 2A event occurring over a shorter wavelength in the scarp-parallel direction 

We convert the TWTT between the sea floor and the layer 2A event to layer 2A thickness using interval velocities of 2,600 m s-1 at the HDR and 2,700 m s-1 at the BTF; details of our velocity analyses can be. Our estimated mean layer 2A thicknesses of 0.48 km at the HDR and 0.49 km at the BTF are remarkably similar

Seismic results

The first hypothesis is that the layer 2A/2B boundary corresponds to the geologic boundary between lavas and dykes. Although both lavas and dykes are composed of basaltic material, the lavas will have a higher porosity, and hence lower seismic velocity, than the dykes, owing to a greater fracture density and volume of void spaces

The alternative hypothesis for the nature of the layer 2A/2B boundary is that it corresponds to an alteration boundary within the upper crust, probably in the lava unit. Results at Deep Sea Drilling Program and Ocean Drilling Program (ODP) show a general increase in temperature and intensity of alteration with depth; hydrothermal mineralization associated with alteration fills cracks, decreases porosities, and increases seismic velocities.

Nature of 2A/2B layer

In summary, this study indicates that the seismic layer 2A/2B boundary does not universally correlate with the structural boundary between lavas and dykes. It is more likely that the primary control on the depth of the layer 2A/2B boundary is crack closure enhanced by hydrothermal alteration and sealing.

Conclusion