18. Constraints on the Geometry and Fracturing of Hole 894g, Hess Deep, from Formation Microscanner Logging Data

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. INTRODUCTION Logging for Tectonics in Hess Deep Hess Deep is an intraoceanic rift located between the East Pacific Rise and the western tip of the Cocos Nazca spreading propagator. The rift exhumed deep levels of oceanic lithosphere generated at the East Pacific Rise and make it one of the rare places where the lower oceanic crust and uppermost mantle of a fast spreading ridge are exposed (Fig. 1) (Searle and Francheteau, 1986; Lonsdale, 1988; Searle, 1989; Francheteau et al, 1990). ODP Leg 147 drilled such rocks for the first time (ODP Leg 147 Shipboard Scientific Party, 1993; Mével et al., 1993). The drilled formations are expected to display structures relating both to the early north-south-trending East Pacific Rise accretion and the later east-west-trending rifting. An extensive study of fractures within the core was undertaken on board (Shipboard Scientific Party, 1993c) but the orientation of these data is critical to a better understanding of the tectonics of Hess Deep, its relationship to hydrothermal processes, and, in particular, which of the north-south-trending 'Mével, C, Gillis, K.M., Allan, J.F., and Meyer, P.S. (Eds.), 1996. Proc. ODP, Sci. Results, 147: College Station, TX (Ocean Drilling Program). Université de Montpellier II CNRS, URA 1767, Case courrier 57, 34095 Montpellier Cedex 5, France, [email protected] 'Department of Earth Sciences, University of Wales College of Cardiff, P.O. Box 914, Cardiff CF1 3YE, United Kingdom. Formerly at: Institute of Oceanographic Sciences, Brook Road, Wormley, Surrey GU8 5UB, United Kingdom; and Borehole Research, Department of Geology, University of Leicester, Leicester LEI 7RH, United Kingdom, [email protected] Borehole Research, Department of Geology, University of Leicester, Leicester LEI 7RH, United Kingdom, [email protected] East Pacific Rise or east-west-trending Cocos-Nazca rift exerted the greater influence on brittle tectonics and hence fracture permeability. There are four ways to gather oriented data from an ODP borehole: the first one is to reorient the core pieces by using the direction of the remanent magnetization (e.g., Cannat and Pariso, 1991; Shipboard Scientific Party, 1992a, 1992b), the second one involves reorienting the core piece by matching it with the corresponding oriented borehole images provided by the Formation MicroScanner (FMS; see Table 1 for full description of acronyms and abbreviations) or the Borehole Televiewer (BHTV) (Shipboard Scientific Party, 1985; Newmark et al., 1985a; Kessels and Kuck, 1992; MacLeod et al, 1992, 1994), the third one is to directly obtain an oriented core using a core scriber device, but this is unreliable and expensive thus rarely done, and the fourth one is to directly pick the feature on the oriented FMS or BHTV images (Ekstrom et al., 1987; Pezard and Luthi, 1988; Héliot et al., 1990; Pezard et al., 1990; 1992). This fourth method is the most direct in that it yields a continuous record of the borehole and thus avoids the commonly poor recovery of cores in fractured intervals. The FMS and BHTV also provide borehole diameter measurements that can be used to identify breakouts (Newmark et al., 1984; Newmark et al., 1985b; Zoback et al., 1985; Hickman et al., 1985; Plumb and Hickman, 1985); these breakouts, when stress induced, indicate the directions of the horizontal principal stress (Hottman et al., 1979, Bell and Gough, 1979) and hence constrain the present tectonic regime. For all these reasons, the use of FMS and BHTV logging tool strings was planned during Leg 147. However, only Hole 894G could be logged and only the FMS operated properly and provided usable data. This paper therefore presents an analysis of the FMS data in order to characterize both the borehole geometry and the fractures at Site 894. Because the low core recovery and poor FMS data quality did not