Crack Distribution in the Upper Oceanic Crust and Its Effects Upon Seismic Velocity, Seismic Structure, Formation Permeability, and Fluid Circulation

Abstract

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 tectonic activity. The total volume of free interstitial water has been calculated 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 permeability of 3.9 × 1O 4 cm 2 . 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 convection cells in the upper oceanic crust. Cross-correlation of the permeability 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.

Ridge-Flank Alteration of Upper Ocean Crust in the Eastern Pacific: Synthesis of Results for Volcanic Rocks of Holes 504B and 896A

“…Core recovery in Hole 417A is similar in both pillows and breccias (both -66%), but lower in massive units (45%). In contrast, recovery in Hole 418A is greater in massive units (81%) than in pillows or breccias (64%-68%), and in Hole 417D, breccias had better recovery than pillows or massive units (92% vs. 72%, respectively; Donnelly, Francheteau, Bryan, Robinson, Flower, Salisbury, et al, 1979). The data for these holes support the conclusion from Holes 504B and 896A, that is, that there is no simple relation- ship between recovery and the abundance of veins, breccias, and massive units.…”

“…In nearby Hole 504B, a similar variation in magnetic inclination occurs beneath the 56-m-thick Unit 1, where inclinations fit those predicted for the site, and this likewise may reflect tilting of the deeper section before extrusion of the uppermost units (Furuta and Levi, 1983;Allerton et al, this volume). This record of tilting of the uppermost crust during accretion at the axis is commonly observed in other DSDP drill cores (e.g., Dmitriev, Heirtzler, et al, 1978;Melson et al, 1978;Natland, 1979;Donnelly, Francheteau, Bryan, Robinson, Flower, Salisbury, et al, 1979). General decreases of natural remanent magnetization intensity and magnetic susceptibility below -350 mbsf in Hole 896A are similar to trends observed in the volcanic section of Hole 504B, and reflect a general increase in the abundance of massive units having larger grain sizes downward in both holes (Al- Adamson (1985) and Alt, Kinoshita, Stokking, et al (1993).…”

“…Overall core recoveries in DSDP Holes 417A, 417D, and 418A (62%, 72%, and 72%, respectively) are much greater than in Holes 504B and 896A (27%-30%), presumably because the former holes are located in much older crust (110 Ma in the Atlantic) and are more tightly cemented by secondary minerals (Donnelly, Francheteau, Bryan, Robinson, Flower, Salisbury, et al, 1979). Core recovery in Hole 417A is similar in both pillows and breccias (both -66%), but lower in massive units (45%).…”

“…This general lack of correlation between closely spaced drillholes in upper oceanic crust is typical, even for holes as close as 100 m, and must reflect the geometry and size of lava flows (Melson et al, 1978;Aumento, Melson, et al, 1977;Natland, 1978;Donnelly, Francheteau, Bryan, Robinson, Flower, Salisbury, et al, 1979).…”

“…Calculations of permeability similar to those made here were made by Johnson (1979) for Hole 418A in 110 Ma old crust in the Atlantic. The abundance of filled veins in that core is only about onehalf that measured for the volcanic sections of Holes 504B and 896A (-14 vs. -30 veins/m, respectively), but the average width of veins in Hole 418A is an order of magnitude larger than that for the other two holes (2 vs. 0.3 mm; Johnson, 1979;J.C.…”