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 (less than 100 kyr) 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, we use simple analytical techniques to study the effects of characteristic morphologic elements of the extrusive basalts on their elastic properties. We find that 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 to 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. The relatively rapid increase in velocity with age in these very young extrusives can be explained by welding of pillow contacts and stiffening of the individual pillows by infilling of radial fractures, both of which may take place without a large reduction in porosity. If voids between pillows are filled initially by sediments, the cementation of these materials can also cause large increases in velocity without a large addition of mass. Because of the importance of such subtle variations in morphology, there can be no one‐to‐one relationship between 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).

Distribution of porosity in a section of upper oceanic crust exposed in the Troodos Ophiolite

“…Our data also indicate, however, that the evolution of upper crustal porosity cannot be uniquely described by seismic data alone, as predicted by Moos and Marion [ 1994]. Summary 1.…”

“…For example, Wilkens et al [1991] predict that only a small decrease in porosity is required to produce the observed increase in seismic velocity with depth and age if low aspect ratio (thin) cracks are preferentially filled. Moos and Marion [1994] show that velocities may increase without a large reduction in porosity if void-filling material or the contacts between pillows morphology and alteration produce a spatially heterogeneous porosity structure that reflects the local environment and, to a lesser extent, crustal age. The geometry of the volcanic morphologies and porosity types were also determined in order to units that are tens to 200 m thick .…”

“…Because the effects of large-scale porosity on the seismic signal cannot be investigated in the laboratory, estimates for the absolute abundance of bulk porosity remain poorly constrained. By exploring the relationship between the geometry and distribution of porosity and the seismic signal, however, several studies have made important predictions about how and why porosity evolves [Spudich and Orcutt, 1980;Purdy, 1987;Wilkens et al, 1991;Moos and Marion, 1994;Shaw, 1994]. For example, Wilkens et al [1991] predict that only a small decrease in porosity is required to produce the observed increase in seismic velocity with depth and age if low aspect ratio (thin) cracks are preferentially filled.…”

Physical properties of upper oceanic crust: Ocean Drilling Program Hole 801C and the waning of hydrothermal circulation

“…Subsequent refraction experiments using improved techniques demonstrate that seismic velocities of the upper oceanic crust increase rapidly for the first 5–8 Myr, from an average of 2.3 km/s at the spreading center to 4.3–4.4 km/s at 5–10 Ma [ Grevemeyer and Weigel , 1996; Carlson , 1998]. Velocity modeling suggests that the large near‐ridge velocity increases may be accomplished by changing the shape of pore spaces via secondary mineralization with relatively small overall porosity reduction [ Wilkens et al , 1991; Shaw , 1994; Moos and Marion , 1994]. Beyond 10 Ma, no significant change in upper crustal velocity is evident.…”

Microstructure and Physical Properties of Samples from Hole 896A

“…It has been realized that much of the variation in the velocity structure of the ocean crust lies within the upper 500 m or so. Critical factors such as eruptive record, pore structure, and alteration history influence seismic velocity in the seafloor (Wilkens et al, 1991;Moos and Marion, 1994). In this study we examine laboratory velocity measurements of a set of basaltic samples from Ocean Drilling Program (ODP) Hole 896A.…”