As previously shown in Figure F4, high-quality logging data from the upper 600 m of Hole 735B were obtained during Leg 176. In the upper 600 m of Hole 735B, density values range from 1.47 to 3.27 g/cm3, with a mean value for the entire logged section of 2.88 ± 0.14 g/cm3. Low values are related to fractures filled with seawater, especially at the top and bottom of the logged interval. The olivine gabbros of Unit V exhibit a range of values from 2.25 to 2.98 g/cm3, with a mean of 2.88 g/cm3, whereas the oxide gabbros of Unit IV show a range of values from 2.95 to 3.27 g/cm3, with a mean of 3.09 g/cm3. Variations in the density profile mostly correspond to variations in oxide mineralogy (Fig. F2) and increases in porosity (Fig. F4).
The porosity measurements in the same interval show variations between 0.03% and 57.37%, with a mean value for the entire section of 3.2% ± 5.2%. High values generally correspond to fractures and correlate with low peaks in the density log. Several isolated zones corresponding to high porosity and low density occur in the upper 450 m of the hole and were previously documented as high permeability zones from results of Packer experiments (Robinson, Von Herzen, et al., 1989). Below 450 mbsf, several zones showing a decrease in density and an increase in porosity are apparent, especially between 555 and 570 mbsf. This interval has been identified as a zone bounded by two faults.
The sonic logs recorded with the Dipole Sonic Imager (DSI) tool represent the first use of this tool in a lower oceanic crust environment and the first set of good-quality sonic data in Hole 735B. The logs were processed postcruise using a Baker Atlas software package for processing the compressional, shear, and Stoneley waveforms. Compressional wave and shear wave results are presented here. VP has a mean value of 6520 ± 418 m/s, and variations correlate well with changes in both porosity and density measurements. An apparent low-velocity, low-density zone at the top of lithostratigraphic Unit V, which also correlates with high-porosity and caliper readings (Dick, Natland, Miller, et al., 1999), is responsible for the reflector identified during Leg 118 at this depth (Swift et al., 1991). In general, VP gradients increase with depth (Fig. F5B) except for the bottommost 100 m of the logged interval. The sharpest increase is observed between 90 and 260 mbsf as deformation decreases downhole. Between 290 and 480 mbsf, the velocities are high and relatively constant. Below 480 mbsf, there is a decreasing trend in velocities marked by the presence of large fractures and increase deformation. The velocity gradient for the entire logged section is defined by VP = 6373 + 0.43016d with R = 0.15. A more complete analysis will be discussed in the following sections.
The deployment of the DSI tool in Hole 735B also marked the first opportunity to measure high-resolution shear wave velocity and VS anisotropy profiles in a lower oceanic basement environment. The DSI utilizes a directional source and receivers. The dipole source behaves much like a piston, creating a pressure increase on one side of the hole and a decrease on the other. This causes a small flexing of the borehole wall, which directly excites compressional and shear waves in the formation. Propagation of this flexural wave is coaxial with the borehole, whereas displacement is at right angles to the borehole axis and in line with the transducer. The source operates at low frequencies, usually below 4 kHz, where excitation of these waves is optimal.
The DSI tool can record fast and slow shear waves when the x-dipole mode is selected prior to deployment (see "Appendix"). Using the software package from Baker Atlas, data from Hole 735B were analyzed for orientation and degree of anisotropy indicated by the amount of birefringence. After identifying the fast and slow directions, quality curves (S1ISO and S2ISO) were used for the anisotropy estimate (S1ISO/S2ISO) and the anisotropy azimuth (S1S2). The relative difference between the s1/s2 residue error (s1 and s2 are the fast shear wave and slow shear wave polarization angles, respectively) and the wave data-fitting residue error for an isotropic formation is S1ISO/S2ISO. The bigger this value, the higher the confidence in the estimated anisotropy. In determining the anisotropy azimuth, the s1 angle is determined as the one whose wave data residue error is the smallest. If, however, the s1 error minimum is comparable to the s2 minimum, it is difficult to determine which is the fast angle and which is the slow orientation. Therefore, the relative difference between the s1 and s2 residue errors gives a quality indicator for the determined s1 angle and it is stored in the S1S2 curve. The bigger this ratio, the more reliable the s1 angle. Caliper curves (Fig. F4) were also used for determining borehole enlargements and assessing data quality. For more details, refer to "Appendix."
The VS profile shows average velocities slightly increasing with depth from 50 to 260 mbsf (Fig. F5C). The interval from 290 to 480 mbsf also shows a slight increase with depth although average velocities are higher than the previous interval (Fig. F5C). Similar to the VP profile, the VS gradients show a general decrease in the bottommost 100 m of the logged interval. The average velocity gradient for the entire logged section is defined by VS = 3353.5 + 0.43139d with R = 0.17. The velocity analyses based on DSI data also show a mean shear wave velocity of 3518 ± 393 m/s and an average VS anisotropy of 5.8% for the upper 600 m of Hole 735B (Fig. F10). VS anisotropy tends to decrease with depth where the overburden pressure and the age of the crustal section suggests closure of cracks and infilling of fractures by alteration minerals.
The fast-angle azimuth and the average anisotropy determined from DSI analyses also show some interesting results. The mean orientation of the fast-angle azimuth for the entire logged section of Hole 735B is 89° and the maximum and minimum standard deviation values of a 20 point average curve are 138° and 40°, respectively, in what appears to be a bimodal distribution (Fig. F10). The maximum fast-angle azimuth tends to be 138° to an approximate depth of 245 mbsf (Fig. F10). Below this depth, the results show more scatter and an average minimum fast-angle azimuth of 40° from north. Large variations in fast-angle azimuth are also observed in lithostratigraphic Unit IV (from 224 to 272 mbsf). However, the pad 1 azimuth orientation of the Formation MicroScanner (FMS) tool tends to indicate that the magnetization of this Fe-Ti oxide-rich unit may be affecting the orientation of these measurements at this particular interval.
Several factors could be contributing to the observed anisotropy. As discussed earlier, the maximum shear wave splitting in a borehole is controlled by transverse isotropy with a horizontal axis of symmetry or from a nonhorizontal symmetry axis where a maximum anisotropy could not be obtained. VP anisotropy and shear wave splitting has been observed in laboratory velocity measurements because of preferred mineral orientations in isolated intervals. However, laboratory samples do not take into account the effects of large oriented fractures that may be contributing to the observed variations, especially steep (60°-90°) to intermediate (30°-60°) dipping fractures. In addition, stress concentrations can influence the velocity field in the vicinity of the borehole. A recent study of dipole anisotropy measurements (Winkler et al., 1998) has shown that in both experimental and theoretical cases, stress concentrations affect the velocity field around the borehole and consequently, dipole anisotropy measurements. In a borehole, low frequency flexural waves will have a relatively deep penetration; therefore, they will be sensitive to the far-field stresses. In contrast, waves traveling at higher frequencies will be primarily sensitive to near-field stress concentrations because of their shallow penetration depth, and the fast dipole direction will be aligned perpendicular to the far-field maximum stress.
In an attempt to assess the nature of shear wave splitting in Hole 735B, the average, maximum, and minimum fast-wave orientations were compared to the strike of steep and intermediate dipping foliations and fractures that were obtained from the interpretation of FMS images. Most of the fracture dip azimuths have a bimodal distribution in a conjugate set geometry with dips to the south and north (Fig. F11). In addition, focal mechanisms and earthquake magnitudes (Mw) based on the Harvard Centroid Moment Tensor (CMT) solutions (Cornell University GIS Group, 1998) were used to determine the effect of stress concentrations near the borehole.
The results show that the average fast-wave orientation (88°) correlates with the general strike of steep and intermediate dipping structures (Fig. F11). CMT solutions show that three significant strike-slip events have been recorded along the Atlantis II Fracture Zone, with one of the events being in the near vicinity of Hole 735B (Fig. F11). The estimated orientation of the compressional axis for the three events show that the maximum and minimum fast-angle shear wave orientation are approximately perpendicular and parallel to the earthquake P-axis (Fig. F11). The overall results indicate that the average shear wave splitting in Hole 735B might be influenced by preferred structural orientations and the average value of shear wave splitting may not be a maximum because most dips are <90°. Structural features slightly oblique to the maximum fast-wave orientation or near-field stress concentrations could influence the maximum fast-wave orientation values. However, flexural wave dispersion analyses have not been performed to confirm this hypothesis or to indicate to what extent the near-field stresses may be influencing shear wave propagation. At this time, the minimum fast-wave orientation is more difficult to explain because there is no clear indication that preferred structural orientations or stress concentrations are influencing shear wave propagation in this direction. The possibility exists that because anisotropy decreases below 325 mbsf, the DSI velocity analyses have difficulties distinguishing between the fast and slow orientations when the difference between the two is small. However, because the difference between the fast and slow is >90°, further analyses need to be performed to fully explain this orientation.