Hole 735B provides a unique opportunity to compare the in situ properties of a thick sequence of oceanic gabbros with the properties of oceanic Layer 3 observed seismically worldwide. For example, the correlation of laboratory tests, sonic log, and vertical seismic profile (VSP) compressional wave velocities from Hole 735B with typical refraction velocities of oceanic Layer 3 (6.5-6.8 km/s) was well established after the early work during Ocean Drilling Program (ODP) Leg 118 (Shipboard Scientific Party, 1989; Iturrino et al., 1991; Swift et al., 1991; Von Herzen et al., 1991). It was also observed, however, that there was anomalously high attenuation in the Site 735 gabbros (Goldberg et al., 1991, 1992; Swift and Stephen, 1992). If oceanic Layer 3 consisted largely of the types of gabbros observed at Site 735, then so much seismic energy would be absorbed on propagation through the crust that we would not observe mantle and Moho arrivals. Because we clearly do observe lower crustal and upper mantle arrivals on refraction seismic experiments, it was hypothesized that oceanic gabbros could compose only a small percentage of seismic Layer 3 material. A major objective of the physical properties program during Leg 176 was to test this hypothesis with measurements on additional cores from deeper in the section. Other objectives were to identify the nature of the reflections observed below the borehole during Leg 118 at depths of 560 and 760-825 meters below seafloor (mbsf) and to check for seismic anisotropy in the gabbros.
The "Physical Properties" section of the "Site 735" chapter in the Leg 176 Initial Reports volume (Shipboard Scientific Party, 1999) presents the results of shipboard measurements of compressional wave velocity. The average value of measurements on 217 minicores was 6.777 ± 0.292 km/s, which is in agreement with the Leg 118 results of 6.713 ± 0.383 km/s based on 228 minicores (Shipboard Scientific Party, 1989). These measurements were made at room temperature and pressure. We have carried out shore-based laboratory measurements on a small subset of the Leg 176 cores to (1) confirm the compressional wave velocities under in situ conditions of pressure, (2) measure shear wave velocities, (3) measure compressional and shear wave attenuation, and (4) check for anisotropy. For the last objective, we specifically acquired three sets of three mutually orthogonal whole-core samples.
1Stephen, R.A., 2001. Data report: Physical properties measurements in ODP Hole 735B. In Natland, J.H., Dick, H.J.B., Miller, D.J., and Von Herzen, R.P. (Eds.), Proc. ODP, Sci. Results, 176 [Online]. Available from World Wide Web: <http://www-odp. tamu.edu/publications/176_SR/chap_02/chap_02.htm>. [Cited YYYY-MM-DD]
2Woods Hole Oceanographic Institution, Woods Hole MA 02543, USA. rstephen@whoi.edu
Initial
receipt: 7 April 2000
Acceptance: 24 July 2001
Web publication: 18 December 2001
Ms 176SR-017