Constraints on the Geometry and Fracturing of Hole 894G, Hess Deep, from Formation MicroScanner Logging Data

Abstract

Ocean Drilling Program Hole 894G, sited on a small horst within Hess Deep, was drilled in lower oceanic crust that was generated at the East Pacific Rise and then rifted by the propagating Cocos/Nazca divergent plate boundary. Difficult drilling conditions in fractured gabbros resulted in a washed-out hole. Despite these unfavorable circumstances, open-hole logs from the Formation MicroScanner (FMS) tool string provide strong constraints on the fracture orientations and suggest a few directions of preferential enlargement. These results were obtained because five logging passes were recorded to compensate for the expected low data quality by redundancy and because the now classical analysis of FMS data has been adapted to this situation of multiple passes and washed out borehole. An east enlargement direction is thought to be related to the east-west strike of the dense network of fractures observed both in cores and on the FMS resistivity images, and an east-northeast enlargement direction may be the result of drill pipe wear along the borehole plunge direction. Additional east-southeast to south-southeast and north-northeast enlargements may reflect the influence of either the Nazca-Galapagos or the Cocos-Nazca plate boundary state of stress, respectively. However, the interpretation of these poorly defined directions remains uncertain. The analysis of the FMS resistivity images shows mainly east-west striking and southward dipping fractures and very few north-south trending features. This suggests a strong Cocos-Nazca control on brittle deformation in the Hess Deep. The distribution of orientation of the fractures identified on the FMS images is very similar to that obtained from core veins reoriented relative to paleomagnetic north. Matching the two sets requires that the remanent magnetization direction points north-northwest. The preferred interpretation of this result is a vertical-axis component of tectonic rotation of 30° to 40° counterclockwise.