Micromechanics of shear rupture and the control of normal stress

Conjugate microfault zones and distributed tensile microcracks were observed in low porosity sandstone samples deformed by shear box rupture at different normal stresses. Statistical data collected from scanning electron microscopic mapping show that tensile microcracks and microfaults are more frequent in the higher normal stress cases. The average thickness of the cataclastic microfault zones increases proportionally to rupture normal stress. The density of tensile microcracks increases locally in microfault relay and intersection zones where their formation is an important mode of microfault zone cataclasis. Concentration of longer en echelon tensile microcracks occurs in process zones ahead of the microfault tips, connected by cross-cracks in an incipient breakdown zone to further propagate the fault. Fracture densities are much lower in calcite cement grains whose compliancy serves to arrest microfault growth temporarily, yet also aid intragranular microcracking in adjacent stiff grains and at grain boundaries. Tensile microcrack and microfault average dips from the sample rupture surface increase with rupture normal stress, with conjugate microfault orientation variability at higher normal stresses aiding their connectivity. Results of this investigation demonstrate how the fracture porosity and connectivity around rupture zones increase dramatically at increased rupture normal stress, impacting on hydrodynamic properties of rupture zones. (C) 2000 Elsevier Science Ltd. All rights reserved.