FINDING THE RIGHT PLACE IN MOHR CIRCLE SPACE

The formation of natural brittle fracture systems in rocks and, especially the state of stress under which they form, have long been a subject of interest to the scientific and engineering communities. Because fractures are important conduits for subsurface fluid flow, there is immense importance attached to fracture analysis in many applications in hydrology, petroleum systems, carbon capture and storage, induced seismicity, and ore forming systems. Both linear and nonlinear relationships between crustal stress and brittle rock failure are characterized by empirically derived failure criteria. The utilization of a linear vs nonlinear failure criterion is inconsistent among laboratory, field, and modeling-based analyses. Unified parabolic failure envelopes have been shown to apply at compressive and/or trans-tensile conditions of low differential stress, but commonly they are ignored in favor of the simpler linear failure criteria. In a study area of Jurassic Moab Tongue (Entrada Formation) bordering Arches National Park (Utah), we have documented field evidence, augmented by primary data gathered through experimental testing, for a conjugate fracture system whose character and kinematics can be explained by nonlinear failure within a kilometer-scale gradient of declining differential stress. The gradient of declining differential stress was likely produced by mechanical buffering of the Moab Tongue sandstone by a major subjacent (Salt Valley) salt wall and accompanying structures. Utilizing Mohr circle space, we integrate these findings with previous work to describe a characterization for the mechanical conditions of faulting and fracturing. Our results concur with previous experimental insights and provide the first recognition of nonlinear relationship between stress conditions and failure angle at a subregional scale and natural geologic environment.