Abstract
One of the most difficult tasks in the interpretation of oceanic crustal seismic data is determining the nature of the reflectivity or the components responsible for the acoustic impedance contrast producing observed reflections. Therefore, the scientific objectives of this project were to study the seismic properties and parameters controlling the reflective nature of oceanic crustal and upper mantle rocks and correlate laboratory and log measurements to seismic data. Results from Hole 504B, the deepest continuous penetration into oceanic crustal basement located in the equatorial Pacific Ocean, show that crack porosity is a very important factor in controlling seismic velocities at shallow crustal depths. The pillow lava, transition zone and sheeted dike sequence observed in Hole 504B produce significant acoustic impedance contrasts that can be imaged seismically. Also, in the upper crust, shear zones serve as pathways for hydrothermal circulation causing rock alteration, lowering bulk densities and producing significant reflectors. Results obtained from Vp/Vs and Poisson's ratios show that these parameters may be useful for determining brittle-ductile deformation boundaries within the oceanic crust. Sites 735 and 894 located in the Indian and equatorial Pacific Oceans, respectively, show that ductile deformation and preferred mineral orientation are the principal contributors to the reflective nature of the gabbroic layer of the lower oceanic crust. Compositional variations within Layer 3 may also contribute to the reflectivity of the lower oceanic crust. Finally, results from Site 895 located in the equatorial Pacific Ocean, show that the reflective nature of the lower crustal-upper mantle boundary may vary significantly due to hydration processes. The seismic velocities are highly dependant on the serpentine content and the pyroxene to olivine ratios found in these upper mantle rocks.