Geometric, kinematic and mechanical analyses of faulting, folding and fracturing at Raplee Ridge, UT

The relationship between folding and fracturing is one long studied by structural geologists. Folds are common fluid (e.g. hydrocarbons) traps and fractures act as fluid conduits, therefore understanding the relationship between the two is critical not only to understanding regional tectonics, but also to societal concerns such as oil exploration and extraction. In this research, we examine as a case study the relationship between fold geometry and fracture characteristics in order to determine the predictive capabilities of fold shape on fracturing, as suggested by existing models. Our field area is Raplee Ridge, Utah, a Laramide aged, 14 km long monocline in the Monument Upwarp of the Colorado Plateau. Fracture characteristics were determined by detailed field studies which measured over 3700 individual fracture attitudes at 100 sites across and near the fold. From these data, along with detailed fracture maps at 8 sites, three fracture sets and two subsets were identified, as well as a developmental history of the fractures. This history, based on the geometry and kinematics of the fractures observed in the field, indicated increased fracture complexity and density as the fold increased in amplitude. Models of the fold geometry were created by using extracted x,y,z coordinates of the tops of individual stratigraphic units from acquired Airborne Laser Swath Mapping data as constraints. These data were interpolated using Inequality Constrained Minimum Curvature Splines and by fitting elastic models using the Boundary Element Model Poly3D. Curvature analyses of the resulting models when compared to the fracture data indicated no relationship between curvature and fracturing. Stress analyses from the Poly3D models, however, indicate very strong correlations between both the least compressive and Critical Coulomb stresses and folding related fractures. These stresses are the result of near tip stress from slip along the underlying fault. Therefore, our findings are that stress from fault slip, and not deformation of the stratigraphic layers from folding, created the folding related fracturing seen at Raplee Ridge.