MICROFRACTURING, PALEOSTRESS AND THE GROWTH OF FAULTS

We have analyzed the microfractures in samples taken near one thrust fault and five normal faults of known displacement to test the dependence of microfracture density and distribution on fault slip. We have also recorded the orientation of microfractures as a function of distance from the associated fault surface. In order to control as many parameters as possible, shallow faults with similar lithologies were chosen. They showed the same kind of deformation, namely, microfracturing without block rotation, recrystallization, extensive granulation or crystal plasticity. The results of over 8000 microfracture measurements suggest that maximum microfracture density is independent of net slip. We interpret this relationship as indicating that the preponderance of microfracturing occurs in proximity to the propagating fault tip. The angular relationship between the shear plane and microfractures was commonly found to be between 5-degrees and 20-degrees, significantly less than the approximately 30-degrees expected between the far-field maximum principal stress and the shear surface. This low angle between microfracture orientation and shear plane orientation is predicted for a local stress field associated with a propagating mode II fault tip. Other possible explanations for the small angle for tensional microfractures and the shear surface are: (1) the effective confining pressure was very low; or (2) shear strength at failure was much larger than expected.