Thermal conductivity measurements were made at 219 intervals through the borehole section. Over the section the thermal conductivity is 2.276 ± 0.214 W/(m K). The thermal conductivity varied considerably in the region of felsic veins. These values lie within the range measured for the upper 500 m of Hole 735B and for gabbroic rocks in general (Clark, 1966).
The MST track system measured in excess of 22,000 magnetic susceptibility points. These are shown in Figure 21 and exhibit two characteristic features. The first is an overall decrease in susceptibility downhole defined largely by a gradual decrease in the baseline value with depth. The second is the occurrence of more than 600 extreme spikes of high susceptibility that can be individually correlated to the location of intervals of oxide-rich gabbros and gabbronorites enclosed within oxide-poor olivine gabbro and crosscutting felsic veins in the Hole 735B cores. The MST spikes indicate that the typical interval is no more than 10 to 15 cm thick, and at times significantly smaller. These intervals should be most abundant in the upper 500 m of the core, where susceptibility measurements on individual samples define the highest overall unit susceptibility. The proportion and frequency of these oxide-rich intervals decrease systematically downhole from 274 m at the base of the Unit 4 massive oxide olivine gabbro, but were not measured by MST during Leg 118. The occurrence of oxide gabbros correlates remarkably well with both the intensity of deformation and average oxide percent in the gabbros.
Mass and volumetric measurements were made on 218 minicores with a mean porosity of 0.649 ± 2.884% (the population being heavily skewed by the large number of minimal porosities and the small number of porosities significantly greater than 1%). The mean bulk density was 2.979 ± 0.10 g/cm3, and a mean grain density was 2.991 ± 0.107 g/cm3, close to the density of the typical olivine gabbro (2.96 g/cm3). Density varied with mineral content and was highest in oxide gabbros (3.21 g/cm3) due to the presence of substantial ilmenite and magnetite (4.7 5.2 g/cm3), and lowest in troctolitic gabbros and olivine gabbros, where the proportion of plagioclase (2.7 g/cm3) was greatest. Density shows variations consistent with the lithologic variations downhole, with the greatest scatter in the upper 500 m where oxide gabbros are most abundant (between 2.8 and 3.3 g/cm3). Density also shows increasing scatter below 925 mbsf (2.8 and 2.9 g/cm3). A slight decrease in mean density in samples from the bottom of the hole reflects an increasing proportion of plagioclase rich olivine gabbro and troctolite.
Vertical Incidence Seismic Profile Reflectors
The physical properties measurements provide considerable insight into the origin of the vertical incidence seismic profile reflectors identified from the Leg 118 VSP (Swift et al., 1991). The compressional velocities of 217 minicores were measured; they averaged 6777 ± 292 m/s, and are show together with the data from Leg 118 in Figure 22. No significant variation in minicore compressional velocities occurs for the Leg 176 cores downhole, which could correspond to either the VSP reflector at 560 m, or that between 760 and 825 m. Higher in the hole, however, there is a clear dip in velocities for the massive oxide gabbro Unit IV that corresponds to a striking increase in the density of minicores and the vertical-incidence seismic profile reflector at 225-250 mbsf (Iturrino et al., 1991; Dick et al., 1991a). A sharp increase in seismic velocity of the Leg 118 cores near the top of the hole occurs in the vicinity of another reflector at 50 m. The latter corresponds to a zone of amphibolites with intense shear and crystal-plastic deformation that has produced a strong crystal fabric orientation. Iturrino et al. (1991) demonstrated that these are elastically anisotropic, with strong directional variations in Vp related to foliation. The reflector is located at a break in the deformed interval where highly deformed amphibolites are juxtaposed against relatively undeformed ones.
The absence of either a break in density or compressional P-wave velocity corresponding to the two lower VSP reflectors at 560 m and 760 825 m, then, demonstrates that these are not due to the intrinsic physical properties of the gabbros themselves, either the development of a strong preferred mineral fabric, or variations in density. Rather, they must correspond to another variable such as the presence of faults and fractures. This is confirmed by examination of the recovery record from Hole 735B (Fig. 22). Recovery in both massive fine- and coarse-grained, foliated and unfoliated gabbros was high, generally close to 100% in Hole 735B. Drilling rates, however, varied considerably in these lithologies, dropping dramatically for fine-grained intervals. Where the rocks were believed to be highly fractured, drilling rates increased dramatically, and recovery dropped to as low as 31%. The precise coincidence of the two lower VSP reflectors with intervals of dramatically reduced recovery, therefore, apparently confirms the hypothesis that these are highly fractured zones corresponding to some form of faulting associated with the late uplift of the platform.
The downhole measurements performed at the end of the leg confirmed that the fault centered at 560 mbsf is about 4 m thick, that it has reduced formation velocities, densities, and resistivities, and elevated porosities. Additional information from the logs, including an attempted, but regrettably degraded, formation microscanner (FMS) log, and a reasonably successful VSP experiment, await shore-based processing.
In summary, the sequence of rocks observed in Hole 735B is unlike that found in well-studied ophiolites. A full on-land counterpart to these rocks has yet to be found. Nor does this sequence of rocks resemble a layered igneous intrusion. Hole 735B provides a first example of synkinematic igneous differentiation in which the upper levels of the gabbroic crust were enriched in the late differentiated melts through tectonic processes, rather than simple gravitationally driven crystallization differentiation.
To 176 Conclusions
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