Velocities and elastic moduli of basalts from Pacific DSDP sites

Compressional (Vp) and shear (Vs) wave velocities have been measured to 10 kb in 32 cores of basalt from 14 Pacific sites of the Deep Sea Drilling Project. Both Vp and V s show wide ranges (3.70 to 6.38 km/sec for Vp and 1.77 to 3.40 km/sec for V s at 0.5 kb) which are linearly related to density and sea floor age, confirming earlier findings by Christensen and Salisbury of decreasing velocity with progressive submarine weathering based on studies of basalts from five sites in the Atlantic. Combined Pacific and Atlantic data give rates of decreasing velocity of -1.89 and -1.35 km/sec per 100 my for Vp and Vs respectively. New analyses of oceanic seismic refraction data indicate a decrease in layer 2 velocities with age similar to that observed in the laboratory, suggesting that weathering penetrates to several hundred meters in many regions and is largely responsible for the extreme range and variability of layer 2 refraction velocities.

The physical properties of basalt at DSDP Hole 45-395A

A detailed study has been made of the physical properties of core samples from Deep Sea Drilling Project Hole 395A. The properties include: density, porosity, compressional and shear wave velocity, thermal conductivity, thermal diffusivity, and electrical resistivity. Of particular importance are the relations among the parameters. Most of the variations in the basalt properties follow the porosity, with smaller inferred dependence on pore structure, original mineralogy differences, and alteration. The sample measurements give very similar results to (and extend previous data from) Mid-Atlantic Ridge drillholes, the sample data from this site and previous data are used to estimate relations between porosity and other large-scale physical properties of the upper oceanic crust applicable to this area. These relations are important for the analysis and interpretation of downhole logging measurements and marine geophysical data.

Physical properties and elastic constants of ODP Holes 137-504B and 140-504B upper crustal rocks

Seismic velocities have been measured at confining pressures of 100 MPa and 600 MPa for sheeted dike samples recovered during Ocean Drilling Program Legs 137 and 140. The compressional- and shear-wave velocities show an increase with depth at Hole 504B, which is in sharp contrast to the atmospheric pressure velocity measurements performed as part of the shipboard analyses. Rocks exposed to different types of alteration and fracture patterns show distinct changes in their physical properties. The seismic reflectors observed on the vertical seismic profile (VSP) experiment performed during Leg 111 may have been caused by low velocity zones resulting from alteration. The amount of fracturing and hydrothermal alteration in several zones also may have contributed to the acoustic impedance contrast necessary to produce the E5 reflector. Poisson's ratios calculated from laboratory velocity measurements show several low values at depths ranging from 1600 mbsf to 2000 mbsf, which tends to follow similar trends obtained from previous oceanic refraction experiments. A comparison of physical properties between samples recovered from Hole 504B and ophiolite studies in the Bay of Islands and Oman shows a good correlation with the Bay of Islands but significant differences from the measurements performed in the Oman complex.

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) Compressional-wave and shear-wave velocities and elastic constants from DSDP Legs 69 and 70

Shear-wave and compressional-wave velocities of 26 basalt samples collected at Site 504 during Deep Sea Drilling Project Legs 69 and 70 were measured at elevated confining pressures. The young basalts have higher velocities than average DSDP basalts, because of their lack of alteration. Measurements of sample porosity are combined with laboratory and in situ velocity measurements to yield estimates of total crustal porosity: 13% at the top of Layer 2, and very low porosity below a depth of 2.0 km.

Physical properties, compressional-wave velocity and consolidation characteristics of ODP Holes 133-815B and 133-817B

Sites 815 and 817 were drilled near the Townsville Trough during Leg 133 of the Ocean Drilling Program. The physical properties, compressional-wave velocity, and consolidation characteristics indicate that the periplatform carbonate sediments maintain more water content and lower compressional velocity near the Queensland Plateau than the clayey hemipelagic sediments, which have a clay content of up to 60%. Bulk density, void ratio or porosity, water content, and compressional-wave velocity are shown to have a linear relationship with burial depth. Between 3.5 and 5 Ma (about 100-500 mbsf), these physical properties maintained a constant rate vs. the depth in core because of the fast sedimentation-rate effect at Site 815. However, compressionalwave velocity still increases downward in this section. The clay content in this section causes an increase of bulk modulus and compaction effect. At Site 817, scarce terrigenous mud content and abundant carbonate content (88%-97%) cause a straight line relationship between physical properties and burial depth. During the consolidation test, we show that dominant micritic particles may cause faster acoustic velocity than sediments composed mainly of coccoliths. The bulk modulus ratio increasing rate in the clay-rich carbonate sediments is almost 4.5 times higher than in the clay-free periplatform carbonate sediments.

Seismic velocities and elastic properties of gabbroic rocks from ODP Hole 118-735B

The nearly continuous recovery of 0.5 km of generally fresh, layer 3 gabbroic rocks at Hole 735B, especially near the bottom of the section, presents scientists an unusual opportunity to study the detailed elastic properties of the lower oceanic crust. Extending compressional-wave and density shipboard measurements at room pressure, Vp and Vs were measured at pressures from 20 to 200 MPa using the pulse transmission method. All of the rocks exhibit significant increases in velocity with increasing pressure up to about 150 MPa, a feature attributed to the closing of microcrack porosity. Measured velocities reflect the mineralogical makeup and microstructures acquired during the tectonic history of Hole 735B. Most of the undeformed and unaltered gabbros are approximately 65:35 plagioclase/clinopyroxene rocks plus olivine or oxide minerals, and the observed densities and velocities are fully consistent with the Voigt-Reuss-Hill (VRH) averages of the component minerals and their proportions. Depending on their olivine content, the predominant olivine gabbros at 200 MPa have average Vp = 7.1 ± 0.2 km/s, Vs = 3.9 ± 0.1 km/s, and grain densities of 2.95 ± 0.5 g/cm3. The less abundant iron-titanium (Fe-Ti) oxide gabbros average Vp = 6.75 ± 0.15 km/s, Vs = 3.70 ± 0.1 km/s, and grain densities of 3.22 ± 0.05 g/cm3, reflecting the higher densities and lower velocities of oxide minerals compared to olivine. About 30% of the core is plastically deformed, and the densities and directionally averaged velocities of these shear-zone tectonites are generally consistent with those of the gabbros, their protoliths. Three sets of observations indicate that the shear-zone metagabbros are elastically anisotropic: (1) directional variations in Vp, both vertical and horizontal and with respect to foliation and lineation; (2) discrepancies among Vp values for the horizontal cores and the VRH averages of the component minerals and their mineral proportions, suggesting preferred crystallographic orientations of anisotropic minerals; and (3) variations of Vs of up to 7%, with polarization directions parallel and perpendicular to foliation. Optical inspection of thin sections of the same samples indicates that plagioclase feldspar, clinopyroxene, and amphibole typically display crystallographic-preferred orientations, and this, plus the elastic anisotropy of these minerals, suggests that preferred orientations are responsible for much of the observed anisotropy, particularly at high pressure. Alteration tends to be localized to brittle faults and brecciated zones, and typical alteration minerals are amphibole and secondary plagioclase, which do not significantly change the velocity-density relationships.

Seismic velocities at elevated pressures of igneous rocks at DSDP Holes 60-454 and 60-458

The Deep Sea Drilling Project, in addition to providing valuable information on the history and processes of development of the ocean, has significantly contributed to our knowledge of the chemical and physical nature of the upper oceanic crust. Among the important physical properties of the crust are its seismic velocity and structure, the interpretation of which requires laboratory studies of seismic velocities in oceanic rocks.