3. IN-SITU STRUCTURE AND PROPERTIES OF 110-MA CRUST FROM GEOPHYSICAL LOGS IN DSDP

DSDP Hole 418A, drilled through 324 m of sediments and 544 m of basalts during DSDP Legs 51-53 at the southern end of the Bermuda Rise, was reoccupied during ODP Leg 102 for the purpose of obtaining a comprehensive suite of geophysical logs. Although a hole restriction at 469 m below seafloor (145 m into basement) prevented the recording of the complete log suite throughout the open-hole interval, two passes of the first Schlumberger tool combination, consisting of resistivity-sonic-gamma-ray-caliper, were recorded from just below the sediment/basalt interface at 324 to 788 mbsf. The remaining 80 m was not logged owing to the suspected presence of a logging tool and cable lost during DSDP Leg 53. The second Schlumberger lowering, consisting of compensated neutron porosity, natural gamma spectroscopy, and lithodensity, and the third lowering, dual resistivity laterolog and gamma ray, were both recorded in open hole from 464 to 788 mbsf. The second combination was also recorded through the pipe to the mud line. Careful calibration of these measurements in fresh flow basalts, corrections for known borehole effects, and comparison of gamma-ray results to core mineralogy result in an excellent set of physical-property measurements throughout the logged interval. Differences in gamma-ray activity, mostly due to the potassium-40 decay series, separate the basement interval into three zones on the basis of smectite content. The uppermost zone 1 consists of a series of relatively unaltered, low-porosity (<15%), high \p (>5 km/s) massive basalts and more porous lower velocity pillows capped by a 9-m-thick, more highly altered pillow unit, the total extending from 324 to 388 mbsf (log lithologic Units 1 to 4). In zone 2, from 388 to 514 mbsf (log lithologic Unit 5 and Subunit 6A), the pillow basalts contain large amounts of smectite within fractures and voids; porosities generally range from 15% to 25%, and velocities vary inversely with smectite content and porosity from 3.5 to 5 km/s. The bottom of the Subunit 6A breccia at 514 mbsf marks a geophysical boundary separating these altered pillows from the much fresher pillows and flows of Subunit 6B to Subunit 13C in the remainder of the logged interval, which we refer to as zone 3. In this zone velocities are generally above 4 km/s and reach 6 km/s in the massive flows. Porosities are generally below 20%; smectite is generally less than 5%. The density log is more sensitive to well-bore rugosity than the other logs, but aside from this effect, density ranges from 2.4 to 2.95 g/cm. After correction for the presence of smectite, density is generally inversely proportional to porosity, as expected in a two-phase (seawater-basalt) system. Although velocities within the altered section (zone 2) are somewhat lower than in the remainder of the hole, nowhere are they as low as in seismic Layer 2A. In the uppermost 190 m of the basement where smectite is replaced by void space, the average velocity can be reduced to less than 3.5 km/s. Thus, alteration and smectite infilling over the 110 Ma since the formation of these rocks can raise in-situ velocities and eliminate Layer 2A as part of the crustal aging process.