An experiment of in-situ stress estimation on basaltic rock core samples from Hole 758A, Ninetyeast Ridge, Indian Ocean

Abstract

Baslatic core samples were analysed 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. 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. Stable magnetization directions of the sample were measured in order to estimate their azimuthal orientations. The site was affected by a maximum of 10° of counterclockwise 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. -from Authors