Observations from Vancouver Island, British Columbia, that may indicate compression-driven mode I fracturing occurs on the geologic scale

This paper applies fracture mechanics models of rock failure in compression to the formation of concretion fractures and joint sets in a sandstone unit on eastern Vancouver Island, British Columbia. Geologists have traditionally hypothesized that crack driving stresses are tensile in nature; however, new observations obtained from the Cretaceous Nanaimo sedimentary rocks can be better explained using the theoretical models of compression-driven fracturing of Horii and Nemat-Nasser (1985 ) and Kemeny and Cook (1987). Fractures in the study area occur in a thick bedded, massive, arkosic sandstone unit with randomly distributed spherical concretions of the same lithology, but of higher elastic moduli. These inclusions have diameters ranging in size from 0.1 meters to three meters and have concentrated remote compressive stresses, consequently fracturing with precisely the same orientations as one or more of the neighboring joint sets. The fractures in the concretions are sometimes separated from one another by only centimeters, making tension or fluid pressure unlikely causes. Many of the inclusions have additionally produced indentation fracturing in the sandstone rock mass along their contact with the matrix, also in the direction of the joint sets. We suggest that closely-spaced extensile fractures in the concretions and joint sets have a common origin and are caused by a maximum compressive stress acting parallel to the fracture direction. Tensile stresses would have more likely debonded the concretions than have been concentrated in them. Fracturing in a tensile remote stress field would also create stress shadow zones that would prevent joints from forming closer than the depth of fracturing. Consequently, compression-induced extensile fracturing better explains the observations of concretion fractures and the associated regional joint sets.