We have measured the unconfined strength and the tangent Young's modulus under uniaxial loading conditions of 16 minicores sampled from material recovered over the interval 1500–2100 mbsf from Ocean Drilling Program Hole 504B during Legs 137, 140, and 148. Relationships between strength and physical properties such as porosity and density are similar to those of rocks sampled at shallower depth in the same hole. However, trends in these data are different from those predicted from analysis of cores from other boreholes drilled into crust significantly older than that at Site 504. This agrees with several studies which demonstrate that changes in crustal properties with age continue to occur even in much older crust. Relationships between rock strength and electrical properties such as formation factor are tenuous, in part because of the fairly narrow range of properties of these rocks. Observation of tensile failure in 4-arm caliper logs in the deepest section of Hole 504B, and a relative lack of compressional breakouts, may be an indication of a strike-slip stress regime, in contrast to a shallower strike-slip/reverse faulting regime inferred previously. However, stress magnitudes in the deeper sections of Hole 504B should not be quantified solely on the basis of these core measurements and on elongations detected in 4-arm caliper logs, because of the poor sample coverage, the lack of sensitivity of the caliper log to breakouts when compared to a borehole televiewer, and the possible influence of drilling-induced microcracks on the core properties.

Date of initial receipt: 15 August 1994
Date of acceptance: 24 February 1995

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