The Implications of Fracture and Void Distribution from Bore-Hole Televiewer Imagery for the Seismic Velocity of the Upper Oceanic Crust at Deep Sea Drilling Project Holes 501 and 504B

Abstract

To examine closely in situ lithostratigraphy and the distribution of fracture and void zones in oceanic Layer 2A, a comprehensive survey of Deep Sea Drilling Project Holes 501 and 504B on the south flank of the Costa Rica Rift was conducted with an ultrasonic bore-hole televiewer. The televiewer records clearly outline the stratigraphic interrelationship between pillow basalts and massive flows, as well as the natural fracture distribution and voidand clay-zeolite-rich zones. A direct correlation is found between low-velocity intervals on the ultrasonic velocity logs and fracture and void zones. The ultrasonic velocity appears to be a good indicator of the composition of materials filling cracks and voids in the upper oceanic crustal Layer 2A. At the top of the basement, massive flow units and large pillow zones are interrupted with fractures and voids filled at least by a 2:1 ratio of sea water to clay, whereas at a sub-basement depth of 100 to 200 meters small pillow flow units are interfilled with at least a 2:1 ratio of clay and zeolite to sea water. The increase with depth in the degree to which clay and zeolite fill fractures and voids likely explains the increase in seismic velocity as Layer 2A merges into 2B with depth in the oceanic crust, as well as the order-of-magnitude drop in permeability observed in Hole 504B. INTRODUCTION Heat-flow measurements on the flanks of mid-ocean ridges record a transition from convective heat transfer near ridge axes to predominantly conductive heat flow in old ocean basins (Anderson et al., 1977). This transition in the heat-transfer mechanism is mirrored in an increase in seismic velocity of the upper 500 meters of the oceanic crust, to the point that Layer 2A (with characteristic velocities ranging from 2 to 5 km/s) essentially disappears on the flanks of mid-ocean ridges (Houtz and Ewing, 1976). In order to investigate possible interrelationships between these major changes in the oceanic crust, we participated in an integrated suite of in situ down-hole experiments at DSDP Holes 501 and 504B, on the south flank of the Costa Rica Rift of the Galapagos Spreading Center (Fig. 1). The experiments included permeability and pore-pressure measurements (Zoback and Anderson, this volume), sonic velocity logging (Cann and Von Herzen, this volume), and formation-fluid sampling (Mottl et al., this volume). In this paper, we report ultrasonic bore-hole televiewer imagery that was acquired to examine the lithostratigraphy and fracture variation with depth in Layer 2A. The studies in these drill holes are only a starting point for the examination of the evolution in physical properties away from mid-ocean ridges, but they yield fascinating insights into the structure and permeability of the shallow oceanic crust and the factors controlling seismic velocity and its increase with depth and age. Cann, J. R., Langseth, M. G., Honnorez, J., Von Herzen, R. P. , White, S. M., et al., Init. Repts. DSDP, 69: Washington (U.S. Govt. Printing Office). One of the primary difficulties with interpretation of the lithostratigraphy of the oceanic crust using deep-sea drilling results from the D/V Glomar Challenger has been the low percentage of core recovery. The drilling operations crush and wash away an average of 70% of all drilled-oceanic crust. Previously, the only measurements over entire drill holes were geophysical logs, but it has been extremely difficult to interpret geophysical observations, because of lack of comparable examples on land for correlations. That is, most geophysical logging techniques have been used very rarely in volcanic terranes on land (much less in alternating layers of altered pillow and flow basalts). Thus, geophysical logs from deep-sea drill holes have been difficult to correlate with specific structural and stratigraphic relationships in the oceanic crust. THE BORE-HOLE TELEVIEWER A seldom-used logging tool developed by Mobil Oil Corp. has been used in this study to provide the data needed to interpret more fully the structure and stratigraphy of the ocean crust as well as results of geophysical logging in deep-sea drill holes. With the support of Amoco Oil Field Research, we deployed a bore-hole televiewer (BHTV) in Holes 501 and 504B on DSDP Legs 68 and 69 on the south flank of the Costa Rica Rift of the Galapagos Spreading Center (Fig. 1). The BHTV contains a transducer which rotates three times per second and emits a 1-mHz pulse 180 times per second as the tool is moving upor down-hole at 0.6 meters per minute. By displaying the amplitude of the acoustic pulse reflected off the wall of the hole (referenced to magnetic north), the image produced by the 255 R. N. ANDERSON, M. D. ZOBACK 86°W 84°W 82°W 80°W 78°W