THE CONSTANCY OF STRESS FIELDS DURING FRACTURING INDICATED BY JOINT SET NORMAL VECTOR DISTRIBUTIONS

Joints open parallel to the least principal stress and contain in their planes the maximum and intermediate principal stresses. As such, the tightness, shape and orientation of the concentration of joint normal vectors in a set reflect stress field complexity and host rock strength contrast at the time of jointing. Joint normal vectors for a set formed in a homogeneous stress field, hosted by an isotropic material and measured with a perfectly precise compass would plot as a point on a stereographic projection. However, in real sets, whose joints vary in strike and dip, the normal vectors populate an area on the plot, with concentration decreasing away from a mean orientation. Joint vector distributions from fold belt and basin settings are quantified with an eigenvalue method, and used to determine the relative constancy of the horizontal tectonic and vertical gravitational stresses in the rock volume over the jointing interval.

The variations in strike and dip within joint sets are random at neither the outcrop nor the regional scale. Joint vector distributions in horizontal beds, and especially in rock sequences with low strength anisotropy, demonstrate that the vertical dimension of joints, while not constant, is better constrained in the joint set than the strike dimension in both passive and active tectonic settings. Thus, the stress trajectory due to overburden is more constant than the horizontal tectonic stresses during jointing. This result holds in rock volumes at the outcrop and geological province scales, and over time, when stress rotations are observed in the form of joints reactivated with slip. Rocks with strength anisotropy, such as interbedded stiff siltstone and compliant shale, contain joints that demonstrate increased constancy of the vertical stress relative to the horizontal stresses. Bed rotation with respect to the vertical stress during folding, fractures that propagate across bedding interfaces or irregular mechanical boundaries are necessary to effect joint sets with dip dispersion comparable to strike dispersion.