Downhole electrical images obtained in the bottom 200 m of Hole 504B during ODP Leg 148 are analyzed here in terms of geometry, with a precise description of fracturing given. Because core recovery was poor, these images are essential to the understanding of the structural context within which this deep hole is drilled. Nearly 4500 traces of fractures were mapped over 167 m of images (26.9 per m on average). The steep to nearly vertical structures that dominate throughout this interval are oriented mainly in a 015×N strike direction, which is identical to the maximum horizontal stress (SH_max) determined higher up in Hole 504B from BHTV images.

Two zones with intense horizontal fracturing are described (1920–1950 and 1975–2005 mbsf) from FMS images. Although the top one is also detected at a meter scale by deep-reaching electrical resistivity measurements, the bottom one is not. The top interval is also the site of a change in hole deviation (at 1950 mbsf), possibly in relation to a change in dike orientation or in structure of the stress field near the bottom of the hole.

The large number of nearly vertical fractures reported from FMS images in this bottom section were probably induced in response to drilling. Particularly concentrated over a 70-m-long interval (1975–2045 mbsf), these steep features coincide in azimuthal direction with those of borehole enlargements. This might arise from tension-generated failures under a strike-slip and strongly deviatoric environment, as suggested also by modes of failure obtained in cores. In such a case, borehole enlargements developed in the direction of tensile failure and pointing 015×N would not correspond to the traditional "breakout" terminology, but would confirm the previous determination of SH_max orientation in this hole.

From 1980 to 2003 mbsf, steep features are also detected at depth into the rock from electrical measurements. Located at the bottom of the section drilled during Leg 140, this interval is that in which cooling from fluid circulation was applied at the beginning of Leg 148 in order to protect the core bit from high temperatures. Therefore, it is likely that borehole enlargements and nearly vertical fracturing detected from 1975 to 2045 mbsf originate in drilling, as the propagation of failures deep into the diabase is particularly efficient at depths where cooling was specifically performed.

Date of initial receipt: 23 August 1994
Date of acceptance: 22 February 1995

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