35. AN EXPERIMENT OF IN-SITU STRESS ESTIMATION ON BASALTIC ROCK CORE SAMPLES FROM HOLE 758A, NINETYEAST RIDGE, INDIAN OCEAN1

We carried out an experiment to estimate in-situ stresses at ODP Hole 758A (water depth: 2924 m) from the basaltic core samples by a new method, deformation rate analysis (DRA). Stress previously applied to a rock specimen is identified in inelastic strain behavior of the specimen under uniaxial compression by this method. Although the identification of the previous stresses by the DRA was difficult because of small fracture strengths of the samples, we could find the previous stresses which are considered to correspond to the in-situ stresses. The vertical, the maximum, and the minimum horizontal in-situ stresses estimated are (37.8, 36.6, 26.8 MPa), (40.6,40.1,28.6 MPa), and (41.2,40.6,30.4 MPa), respectively, for Cores 121-758A-55R (502 mbsf), -64R (583 mbsf), and -69R (639 mbsf). The estimated in-situ vertical stress is equal to the calculated overburden pressure with an error less than 3% at each depth. The maximum horizontal stresses are nearly equal to the vertical ones. This suggests that the site is in the stress field of the normal fault regime at the depths. We measured the stable magnetization directions of our samples in order to estimate their azimuthal orientations. The site was affected by a maximum of 10° of counterclockw ise rotation since the basaltic rocks were magnetized. Referring to the paleomagnetic orientations, the estimated in-situ directions of the maximum horizontal stress range from N16°W to N63°W. The directions are not greatly different from the regional stress field at the site. The DRA needs more experiments in deep-sea cores, in conjunction with another in-situ stress measurement such as a borehole televiewer and the packer/hydrofracturing method, to become a useful method.