During the last few decades, drilling results and geophysical studies have demonstrated that the structural and compositional complexity of the oceanic crust is far greater than that of a simplistic three-layer seismic model. Porosity is a significant contributor to the velocity profile of the upper oceanic crust. Results from Hole 504B show velocities increasing with depth as a direct result of a decrease in porosity (Dick, Erzinger, Stokking, et al., 1992). Consequently, the reflectivity of the top of the sheeted dikes is mainly associated with changes in porosity. For the most part, the overall porosity trend in Hole 735B tends to decrease with depth except for fault zones located at 555 and 570 mbsf. However, high-porosity intervals at the top of the hole and these faulted intervals below show that porosity also plays an important role in the reflectivity of this lower crustal section. In addition, laboratory studies have shown that cracks tend to reduce laboratory seismic velocities at low pressures even though the porosities for most samples are on the order of tenths of a percent. The same effect has been observed in gabbros from the Hess Deep area (Iturrino et al., 1996), thus emphasizing the need for high-pressure laboratory measurements for comparison with in situ log measurements and seismic records. At higher pressures, laboratory velocity measurements are primarily related to mineralogy. For the most part, low velocities in Hole 735B are attributed to either the presence of Fe-Ti oxides, variations in SiO2 content, or fractures. The presence of alteration products (i.e., phyllosilicates) in the upper part of Hole 735B also lowers seismic velocities (Iturrino et al., 1991); however, this does not seem to be the case in the bottom 1000 m, where compressional wave velocities remain relatively high, with an average of 7036 ± 161 m/s (Table T3).
The compressional wave velocity gradient based on log data for the upper 600 m of Hole 735B shows that the depth to the upper mantle would be 3.32 km. This calculation is based on a upper mantle velocity of 7800 m/s that was obtained from P-wave seismic velocity models based on ocean-bottom seismometer (OBS) data (Fig. F3). The same depth can be obtained using the shear wave velocity gradient for the entire logged section and an upper mantle velocity (VS = 4785 ms) consistent with ophiolite results (Christensen, 1978; Salisbury and Christensen, 1978). These results are consistent with observations from dredged samples (Dick et al., 1991) and inversion of rare element concentrations in basalts dredged from the conjugate site to the north of the Atlantis Bank (Muller et al., 1997) that suggest a crustal thickness of 3 ± 1 km. Using the VP gradient from in situ laboratory measurements, a depth to the upper mantle of 5.02 km/s is obtained. These depth estimates are closer to the seismic velocity models from Muller et al. (1997) than the results from the logging data. However, because laboratory measurements do not take into account the effects of large fractures on velocity and the seismic models are based on projections that are 1 km west of this area, the estimates from the logging data seem to be more typical of in situ conditions in the Atlantis Bank.
Five prominent reflectors in the synthetic seismograms compare favorably with a VSP section and lithologic variations found in the drill core recovered from Hole 735B. The first reflection, located at a depth of 55 mbsf, appears to correlate with the first reflection on the corederived synthetic seismogram and the boundary between the foliated metagabbros of Unit I and the olivine-bearing gabbros of Unit II. The second and third reflection sequences appear to mark the top and bottom of the Fe-Ti oxide gabbros of Unit IV and correlate with reflections at these depths in the synthetic seismogram caused by marked changes in density and velocity. The contact between Units IV and V, which may be responsible for the third reflection sequence, is characterized by the presence of mylonitic Fe-Ti oxide gabbros and brecciated gabbros overlying a thick sequence of olivine gabbros. The fourth reflection package is not as clear as the previous sequences but may correspond to the interlayered olivine gabbros, troctolites, altered gabbros, and mylonites found in Unit VI. A seismic reflection between 400 and 475 mbsf in the synthetic seismogram may explain this event. Finally, the last event is attributed to fault zones characterized by high porosity values.
Isolated shear zones also seem to play a very important role in the hydrological and seismological properties as well as in the mineralogical composition of the oceanic crust. As shown in the synthetic seismograms (Fig. F13), high-porosity, low-velocity features and fairly strong seismic reflectors characterize these shear zones. They also serve as pathways for hydrothermal circulation causing rock alteration that generally produces lower seismic velocities than those in the surrounding unaltered crust. These changes in physical properties allow such shear zones to be imaged seismically and they are strong candidates for producing lower crustal dipping reflectors as those imaged in the MidAtlantic Ridge (McCarthy et al., 1988). A fairly good correlation between compositional variations, reflection synthetic seismogram models, and the VSP section for Hole 735B suggests that the reflectivity of the lower oceanic crust may be caused by a combination of deformation and intrusive events. Plastic deformation and mineral orientation due to regional stresses and intrusive events, such as the Fe-Ti oxide sill found in Unit IV, seem to have had a significant effect in the reflectivity of Hole 735B and may very well represent important events along the tectonic evolution of the lower oceanic crust found throughout the ocean basins. Thus, the layering and mineral orientation associated with ductile deformation at the top of magma chambers may also be responsible for the lower crustal reflections observed in seismic sections of the oceanic crust (McCarthy et al., 1988; Becker et al., 1989).
Shear waves have proved to be a powerful tool for studying the properties of the oceanic crust. Several highly deformed samples have shown a large degree of shear wave splitting due to preferred mineral orientations of plagioclase, amphiboles, and pyroxenes along highly deformed intervals. These findings, which are consistent with results of gabbros from the Hess Deep area (Iturrino et al., 1996), suggest that plastic deformation may play an important role on the seismic properties of the lower oceanic crust. The fast-angle azimuth and the average anisotropy determined from DSI analyses also show that, at least on the borehole scale, shear wave splitting may be influenced by preferred structural orientations and that the average value of shear wave anisotropy may not be a maximum because the structures are dipping <90°. In addition, shear wave analyses may have the potential for determining the orientation of predominant near-field stresses.
Finally, most of the Q measurements from Hole 735B are consistent with previous VSP results and measurements in samples from the upper 500 m of the hole. Seismic estimates of Q from refraction studies in oceanic crust indicate high attenuation values (QP <100) in the topmost 500 m of the crust, whereas Layer 3 has yielded Q values of ~300 (Vera et al., 1990), which is 1-2 orders of magnitude higher than the majority of the values reported here. Several samples from the bottom 1000 m of Hole 735B have values in this range (Table T1). This may indicate a less altered, more homogeneous section that may be representative of large portions of the lower oceanic environments. However, experimental limitations may require more intensive laboratory work to determine the effects of scattering and sample heterogeneities on Q.