GEOLOGICAL AND GEOPHYSICAL OBSERVATIONS OF NORMAL OCEANIC CRUST

Observations of exposed oceanic crust in tectonic windows and analogies to ophiolites have helped advance our understanding of the geologic nature of oceanic crust. In recent years, the Integrated Ocean Drilling Program (IODP) has succeeded in reaching through lavas and dikes to gabbros for the first time in a section of normal oceanic crust. IODP Hole 1256D provides a unique opportunity to integrate geologic and geophysical data at several scales. Samples and geophysical data logged in the open hole allow us to ground-truth the geophysical layers identified by regional seismic experiments. To determine the influence of cracks on seismic velocity at several scales, we first need an accurate ground-truth, in the form of laboratory velocity of crack-free, or nearly crack-free samples. Hand samples include few cracks since drilling recovery generally excludes cracks except those that have been filled or are small enough to be preserved within the 6 cm diameter core. The influence of cracks on seismic velocity is then determined as the difference between seismic velocities of hand samples and seismic velocities logged in the open hole or from regional experiments. Crack-free velocities calculated from simple models of logging and laboratory porosity data for different lithologies and facies suggest that crustal velocities in the lavas and upper dikes are strongly influenced by porosity. In particular, our models demonstrate significant large-scale porosity in the lavas, especially in the units previously identified as fractured flows and breccias. In the lower dikes and gabbros porosity drops to less than 1% and crustal velocities are controlled by other factors. At this location, seismic velocity and porosity both change noticeably at the transition between lavas and dikes and the seismic layer 2/3 boundary is estimated to be within about 100 m of the bottom of the hole and likely near or coincident with the transition between dikes and gabbros.