Downhole measurements were recorded within the complete basement section of Hole 504B during ODP Leg 148. Because of poor core recovery, these data are essential for understanding the structural context of this deep borehole. A much better understanding of the tectonic, thermal, hydrological, and alteration regimes of the sheeted dike complex is required to evaluate whether further penetration is within reach of future drilling legs.

The set of geophysical measurements from Hole 504B has been analyzed, and 4500 traces of fractures from FMS images were mapped over the bottom 167 m of the hole. Steep to near-vertical structures dominate and are oriented mainly in a 15×N strike direction at the base of the hole, that of maximum horizontal stress direction. From electrical resistivity, three porosity estimates were computed (1) on the basis of the Archie formula, (2) taking into account the presence of clays and the associated contribution of counter-ions to total rock conductivity, and (3) from the difference between two electrical resistivity measurements with different depths of investigations. Whereas the different estimates are in reasonable agreement and compare favorably with measurements made on core, the differences are indicative of the origin and structure of the inferred high porosity zones. Although subvertical conductive structures dominate throughout the sheeted dikes, many might be related to drilling. Subhorizontal structures are more numerous in the upper part. A correlation is obtained between changes in thermal gradient recorded at the end of Leg 140 and relatively high porosity sections, probably associated with fractures and relatively high permeability.

In all, this integrated description of the fractured sheeted dike complex of Hole 504B provides a more dynamic understanding of the penetrated structure. Many of the steep features coincide in azimuthal direction with that of borehole enlargements, which might arise for failures generated in tension. These borehole enlargements developing in the direction of tensile failure confirm earlier determinations of SH_max orientation. It is therefore likely that borehole enlargements and near-vertical fracturing originate from specific cooling of the hole.

Date of initial receipt: 23 August 1994
Date of acceptance: 28 May 1995

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