Elastic-Wave Velocities in Jurassic-Age Oceanic Crust from Analysis of Sonic Full Waveform Logs in Hole 801C

Abstract

Using sonic full waveforms recorded by a borehole compensated logging sonde during Leg 144, we determined P-and S-wave velocities in the uppermost basement interval (from 480 to-560 mbsf) of Jurassic crust penetrated by Hole 801C. The basement consists of three principal units: an uppermost ~157-Ma alkali basalt, erupted off-axis; a zone of hydrothermal deposits; and a section of ~167-Ma mid-ocean ridge tholeiites. In the alkali basalts, V p ranges from 4.8 to 5.1 and V^ ranges from 2.4 to 2.6 km/s, yielding V p /V s between 1.9 and 2.0. In the tholeiites, V p ranges from 5.2 to 6.2 and V^ ranges from 2.8 to 3.4 km/s, yielding V p /V s between 1.75 and 1.85. We were unable to confirm the extremely low f-wave velocities reported previously within sections of the hydrothermal interval. Within the basalts, low-velocity zones mark the boundaries between individual lithostratigraphic units. A comparison of sonic log velocities within crusts of a wide variety of ages with laboratory-determined ultrasonic velocities reveals that (1) lower average in situ velocities in young crust are a consequence of the presence of macroscopic voids and fractures and of systematic sampling bias; (2) these effects are minimized in old crust by the infilling of fractures and voids and by a decrease in sampling bias in older, more altered basalts; (3) velocities within pillows and interflow zones never reach values equivalent to those in massive units; and (4) alteration of the basalt matrix can significantly reduce its velocity. Velocities, therefore, are controlled by the relative percentages of pillows and flows, the progressive increase in velocity within the pillows resulting from infilling, and an alteration-dependent decrease in velocities within the basalt matrix with increasing crustal age.

Age dependent porosity of young upper oceanic crust: Insights from seafloor gravity studies of recent volcanic eruptions

“…Specifically, the four seafloor gravity studies of recent volcanic eruptions (less than 14 years after eruption) show that pillow flows can have high porosities of 29–36%, while similar studies appear to show a rapid decrease of this initial porosity to 10–15% in the first million years. Although the number of ocean bottom gravity measurements decreases strongly with distance from the ridge axis, drillhole logging experiments confirm that the density of the upper crust remains between 2500–2800 kg/m 3 in older crustal sections [ Kirkpatrick , 1979; Mathews et al , 1983; Cann and von Herzen , 1983; Carlson et al , 1988; Moos et al , 1995]. This observation agrees with geological observations, since young eruptions may be either massive (with correspondingly low porosity) or, at lower effusion rates, exhibit the large voids and drain‐back features (high porosity) such as is common to pillow flows [ Gregg and Chadwick , 1996].…”

“…[6] The outcrop density of the new lava flow was estimated using the density-profiling method [Nettleton, 1954;Parasnis, 1986]. An iterative (re-)weighted least squares technique was used to determine the density contrast at the water/seafloor interface, with errors estimated from bootstrap error analysis [Pruis and Johnson, 1998;Johnson et al, 2000].…”