The seismic characterisation of layer 2 in oceanic crust around ODP borehole 504B

Abstract

The seismic velocity structure of the oceanic crust is commonly used to ascertain its lithological structure and, as such, understand the dynamic processes of its formation at spreading ridges, its evolution as it ages off-axis, and its composition when it enters subduction zones. However, the interpretation of velocity models is complicated and hindered by a number of factors, namely the differences between various seismic modelling approaches; discrepancies between how the different velocity layers within the crust are defined; and a scarcity of velocity models with direct groundtruth. The complex nature of the controls on the measured velocity, such as the effects of hydrothermal alteration and the general increase in velocity as the crust ages, has lead to disparities in terminology used to describe the structure (i.e. seismic layers 2A, 2B, 2C and 3) and debates about whether the seismic layers represent different crustal lithologies, or other changes such as alteration fronts.

In this study, 3D seismic velocity models are produced of a 40 x 40 km-squared area of ~7 Ma oceanic crust at the location of DSDP/ODP borehole 504B - the deepest borehole currently drilled into in-situ oceanic crust - which are then interpreted alongside results from other seismic modelling approaches to produce a consistent definition of the seismic structure. At borehole 504B, the P-wave velocity structure comprises a 0.8 km-thick layer 2A, consisting of an upper, 0.5 km-thick low gradient region at ~4.5 km/s overlying a 0.3 km-thick high gradient region where velocity increases from 4.7 to 5.5 km/s, termed the layer 2A/2B transition zone. Layer 2B increases in velocity gradually to reach 6.5 km/s at 1.7 km below top basement, which is interpreted as the layer 2/3 boundary.

Using the velocity-depth gradient profile to define the seismic structure, rather than absolute velocity alone, results in greater consistency between different modelling approaches and different ages of crust. At borehole 504B, the layer 2A/2B transition zone is characterised by Vp gradients of >2 /s in smooth tomographic inversion models, consistent with results from analysis of the 2A caustic arrival from long-offset MCS gathers. Ground-truthing to the borehole shows that this high velocity gradient region corresponds to a significant decrease in porosity, primarily caused by the change in lithology from extrusive lavas to sheeted dykes but also likely exacerbated by an alteration front to greenschist facies mineralisation. Similar correlations observed at another deep crustal borehole, ODP Hole 1256D, suggest that the gradient proxy could help to locate the extrusive-intrusive transition from seismic studies of magmatically-accreted crust which do not have a groundtruth.

The velocity structure at borehole 504B is typical of the surrounding oceanic crust, with little lateral variation within the study area. The relatively high layer 2A velocity shows that hydrothermal alteration has resulted in the decrease in open porosity in the crust since formation. Anisotropy and Poisson's ratio analyses, however, indicate that open, wider fractures remain in the crust, with a strong ridge-axis alignment below ~0.5 km into the basement.