Seismic properties of volcanic rocks from DSDP Hole 62-465A

Hess Rise, a major structural feature in the northern Pacific, is one of several oceanic plateaus which apparently possess anomalous seismic structures (Sutton et al., 1971). Hence, Laboratory measurements of compressional- and shear-wave velocities in rocks from oceanic plateau regions are of considerable interest. Several questions come to mind: (1) Are compressional - wave velocities of volcanic rocks from oceanic plateaus similar to basalts of equivalent age from normal oceanic crust? (2) Do velocity-density relations for plateau rocks fit the well-established trends for Layer 2 basalts? (3) How do Poisson's ratios, determined from compressional- and shear-wave velocities, of oceanic plateau rocks compare with those of normal sea-floor basalts? To answer these questions, we have selected for velocity measurements five volcanic samples from different depths from Hole 465A, on southern Hess Rise. It is particularly significant that all the rocks are vesicular and have been highly altered, which significantly affects their elastic properties.

Physical properties and minerals of Leg 65 Holes basalts

Velocities and densities of submarine basalts obtained during Leg 65 are distinguished by their relatively high values (about 6.3 km/s and 2.9 g/cm/**3, respectively). This is consistent with their low degree of alteration. The range of velocities covered by these samples correlates well with porosity, and a comparison with in situ velocities from refraction data suggests maximum porosities ih the upper 100 meters of the crust of about 10%.

Mineralogy and diagenesis: Their effect on acoustic and electrical properties of pelagic clays at DSDP Leg 86 Holes

Analysis of pelagic clay samples from Sites 576, 578, and 581 shows that physical, acoustic, and electrical trends with increasing burial depth are related to mineralogical and diagenetic changes. The properties of interest are bulk density (roo), porosity (phi), compressional-wave velocity (Vp) and velocity anisotropy (Ap), and electrical resistivity (Ro) and resistivity anisotropy (Ar). In general, as demonstrated in particular for the brown pelagic clay, the increase in roo, Vp, Ro, and to a lesser extent Ap and Ar with increasing depth is primarily caused by decreasing phi (and water content) as a result of compaction. The mineralogy and chemistry of the pelagic clays vary as a function of burial depth at all three sites. These variations are interpreted to reflect changes in the relative importance of detrital and diagenetic components. Mineralogical and chemical variations, however, play minor roles in determining variations in acoustic and electrical properties of the clays with increasing burial depth.

Anisotropy and acoustic properties at DSDP Holes 62-463 and 62-465A

Variations of acoustic properties within the sediment column may significantly affect the propagation of acoustic energy in the upper portion of the oceanic crust. Moreover, the acoustic properties of sediments reflect their mineral compositions, fabrics, and degrees of compaction and cementation. Hence, the physical properties of indurated deep-sea sediments are of considerable geophysical and geological interest. Chalks and limestones are particularly important because substantial accumulations of biogenic carbonates are generally present at the base of the deep-sea sediment column, and high-standing features such as Hess Rise are capped by calcareous deposits. This paper constitutes a preliminary report of the compressional-wave velocities and densities of 31 indurated calcareous sediment samples recovered at DSDP Sites 463 and 465, in the Mid-Pacific Mountains and on Hess Rise, respectively. The sample set includes nine pairs of samples in which velocities were measured parallel and perpendicular to bedding to determine the velocity anisotropy of the sediment. This research is part of an ongoing study of the seismic properties of indurated deep-sea carbonates.

Compressional and shear wave velocities and elastic constants at DSDP Leg 59 Holes

Prior to the Deep Sea Drilling Project the composition of the oceanic crust could only be inferred from seismic-refraction and gravity data and the recovery of a wide variety of dredged rocks. Through the success of the Deep Sea Drilling Project, it is now clear that the top of oceanic Layer 2 usually consists of basalt. Several laboratory studies (e.g., Fox et al., 1972; Christensen and Shaw, 1970; Hyndman and Drury, 1976) have demonstrated that the seismic velocities of oceanic basalt are similar to velocities reported from refraction studies of Layer 2 and that the variability in Layer 2 velocities has many causes, the most important being fracturing and sea-floor alteration produced by the interaction of basalt and sea water (Christensen and Salisbury, 1973). To date, most reported measurements of velocities in oceanic basalts are from samples obtained from the main ocean basins. With the exception of an earlier study of velocities and related elastic properties of a suite of rocks from DSDP Sites 292, 293, 294, and 296 located in the Philippine Sea (Christensen et al., 1975; Fountain et al., 1975), elastic properties have not been determined for oceanic rocks from marginal basins. In this chapter compressional- and shear-wave velocities and elastic constants are reported at elevated confining pressures for basalt and volcanic breccias from Holes 447A, 448, and 448A.

(Table 1) Summary of acoustic properties at DSDP Leg 59 Holes

Pyroclastic and other sediments derived from volcanic terranes are prominent constituents of the sediment column in the central and eastern parts of the Philippine Sea. On the Palau-Kyushu Ridge (Site 448), basement is overlain by over 100 meters of vitric-tuff deposits, which are overlain in turn by about 170 meters of nannofossil chalk and ooze. In contrast, thick accumulations of vitric tuff are overlain by minor accumulations of pelagic clay in the east-central Parece Vela Basin (Sites 53, 54, and 450), (Fischer, Heezen, et al., 1971), and almost 900 meters of vitric tuff, ash, and breccia overlie igneous basement at Site 451 on the adjacent West Mariana Ridge. The seismic velocities of these vitric tuffs at in situ pressures can be usefully applied in the interpretation of seismic-reflection data collected in this region.