69. CRACK DISTRIBUTION IN THE UPPER OCEANIC CRUST AND ITS EFFECTS UPON SEISMIC VELOCITY, SEISMIC STRUCTURE, FORMATION PERMEABILITY, AND FLUID CIRCULATION

The open cracks and filled veins visible in the basalts drilled at DSDP Hole 418A in the North Atlantic have been analyzed and counted for the entire cored section of 546 meters. A discrimination of crack types was developed by counting separately the carbonate-filled, clay-filled, and open (unfilled) cracks, and recounting for crack orientation (horizontal, oblique, and vertical). The crack-depth profiles show several regions of intense fracture, some peaks correlating with lithology change and tec-tonic activity. The total volume of free interstitial water has been calcu-lated from the primary porosity and open cracks, demonstrating that more water is available from Layer 2 than from the overlying sediments, thus eliminating the need to subduct sediments at the trench to provide fluid for serpentinization and the formation of andesites. From the crack data, bulk fluid permeability of the rock was computed, with an average per-meability of 3.9 × 1O4 cm2. The permeability profiles reveal a pattern of thick, highly permeable sequences bounded above and below by less fractured, thin, low-permeable zones, suggesting a model for fluid con-vection cells in the upper oceanic crust. Cross-correlation of the permea-bility functions with the downhole geochemistry analyses of the fresh basalts resulted in zero correlation; but, future cross-correlation of the permeability profiles with the geochemical variations in altered materials in the hole may yield important details of the alteration phenomena. The crack data were utilized in conjunction with shipboard physical property measurements to calculate the true bulk density, compressional wave velocity, and total porosity for the entire hole. This revealed a series of layers of high and low velocities, and demonstrated that the laboratory physical property measurements, in concert with the crack-distribution information, can accurately simulate the seismic structure of the oceanic crust. Using the combined physical property data, synthetic seismograms have been computed for Hole 418A, showing that the zones of high crack density are at least one of the causes of sub-basement reflectors in the upper crust