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.