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Using sonic full waveforms recorded by a borehole compensated logging sonde during Leg 144, we determined Pand 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, Vp ranges from 4.8 to 5.1 and V̂ ranges from 2.4 to 2.6 km/s, yielding Vp/Vs between 1.9 and 2.0. In the tholeiites, Vp ranges from 5.2 to 6.2 and V̂ ranges from 2.8 to 3.4 km/s, yielding Vp/Vs 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.

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