Horizontal residual stresses in the vicinity of a Breccia Pipe

Abstract

This study determined the relative magnitudes and orientations of the horizontal residual stresses in the vicinity of the 755 Brecci Pipe at Cananea, Sonora, Mexico. These stresses were determined from eleven surface and underground orientated rock samples by the stress-relief overcoring technique. An elastic stress analysis for cylindrical and spherical inclusions was examined and compared to the calculated residual stresses. A petrographic examination for preferred mineral orientation was performed on orientated thin sections made from each of the overcores. Indications are that more confidence can be placed in residual stress orientations than in magnitudes. The secondary maximum principal horizontal residual stress in the host rocks has an orientation of S83°E and is normal to the secondary maximum principal horizontal residual stress in the 755 Breccia Pipe which acts in a N6°E direction. Comparison with the elastic analyses suggests the existence of a significant current tectonic N-S stress field in the 755 Area such that the in situ maximum principal horizontal stress is N-S. At the time of breccia pipe intrusion, it is estimated the magnitude of this N-S maximum principal horizontal stress was approximately 1·4 times the magnitude of the associated minimum principal horizontal stress. Large dispersions are noted in the calculated magnitudes of residual stresses. In general, little confidence can be placed in individual residual stress magnitudes and relative magnitudes are more meaningful. This study indicates that the stress-relief overcoring technique is acceptable for determining residual stress orientations; however, less confidence can be placed in residual stress magnitudes.