Morphology of extrusive basalts and its relationship to seismic velocities in the shallow oceanic crust

Abstract

Recent seismic experiments have revealed that a shallow, low velocity zone is a common feature within the uppermost few hundred meters of very young oceanic crust at "fast spreading' ridges. The shape of this zone suggests that the low velocities are related to the constructional morphology of the extrusive (layer 2A) portion of the crust. With this in mind, simple analytical techniques are used to study the effects of characteristic morphologic elements of the extrusive basalt on their elastic properties. High compliance of the contacts between pillows and pillow fragments can account for all of the observed velocity anomaly. Voids between pillows are, by comparison with their effects on porosity, considerably less important. Radial (cooling) cracks and voids within the pillows themselves will affect the velocities in direct proportion to their effect on the individual pillows. Because of the importance of such subtle variations in morphology, there can be no one-to-one relationship betwen seismic velocities and porosities within the extrusive layer of the oceanic crust. Therefore independent measures of crustal porosity must be found (for example, using sea floor gravity surveys). -from Authors