Deformation at Raplee Ridge, Utah, inferred from joints, veins, and pressure solution seams [electronic resource]
Keywords: Geological and Environmental Sciences.

The mechanisms for fracturing sedimentary strata at Raplee Ridge and Comb Ridge during folding are described in this study using field and numerical methods, shedding light on how residual and tectonic stresses acting within the Earth's crust can give way to inelastic deformation. Systematic fractures, veins, and pressure solution seams are pervasive within the Pennsylvanian to Permian, marine sedimentary strata at Raplee Ridge, Utah. Raplee Ridge is a 14-km-long anticlinal fold adjacent to, and west of the 130-km-long monoclinal fold at Comb Ridge. Fractures, veins, and pressure solution seams are examined here to determine their sequential development and to explain the mechanical relationships for each deformation event relative to folding. Four major fracturing events are classified at Raplee Ridge. The earliest two fracture sets, Sets I and II, are opening-mode fractures, or joints, that formed prior to folding. Set I joints are perpendicular to bedding and trend E-W. They occur on and off the folds, and single joints within the set are often continuous through multiple strata. Set II joints are orthogonal (N S) to Set I joints, often terminating against Set I joints and, thus, are inferred to be younger. Set III joints trend NW-SE, and their occurrence is restricted to Raplee Anticline. Therefore, Set III fractures are inferred to have formed during folding. Systematic fracture Subsets IV and V occur exclusively at the fore-limb of Raplee Anticline, and are inferred to have formed during later stages of folding. A N-S least compressive stress was active during Set I fracture formation, and an E-W least compressive stress is associated with Set II fracture formation. A regional stress rotation, or a gradual change in magnitudes in those directions may explain the development of orthogonal fracture sets. During folding, Set III, IV, and V fractures formed in response to local stresses induced by monoclinal folding and/or reverse faulting. Two sets of systematic arrays of echelon veins and orthogonal seams are observed to exist exclusively within limestone strata at Raplee Anticline and Comb Monocline, adjacent to clastic sedimentary units that are predominantly deformed by the aforementioned systematic fracture sets. The two array sets are inferred to have developed contemporaneously. The arrays occur exclusively on the folds and are suggested to have formed during the Laramide orogenic event between 2 km and 4 km depth while the strata was approximately horizontal. Therefore, they are inferred to have formed after Set II joints, and prior to Set III joints. Existing mechanical models of echelon veins show that the strain field surrounding the veins in the limestone is heterogeneous, and that the interaction between closely spaced veins causes curved vein propagation paths. A finite element mechanical model is developed to describe how orthogonal, intersecting pressure solution seams and an elastic-plastic constitutive relationship for limestone affect echelon vein shapes. Thirteen physical quantities are defined for the mechanical system of echelon veins and seams in two-dimensions. Four of those quantities are determined to be significant: vein spacing, vein-array angle, limestone elastic stiffness, and closing of orthogonal pressure solution seams. Plasticity at the vein tips causes a negligible change in vein opening amounts when closing occurs at orthogonal seams. Seam closing greatly contributes to the mechanical interaction effects between adjacent veins, influencing the propagation path of veins, causing straight vein traces. Seam closing also causes an approximately linear distribution of vein opening, resulting in triangular vein shapes. In the presence of closing seams, small vein spacing and large vein-array angles produce straight vein traces in limestone with stiffness typical of laboratory measurements. Triangular vein shapes and straight vein traces are common in the field. A remote stress state in which the principal stresses bisect the array sets, and are at an angle to the vein traces, can form the observed echelon vein shapes for specific ranges of remote stress magnitudes, limestone stiffness, and pressure solution seam closing. To reproduce the range of measured echelon vein geometries, model results suggest that the greatest compressive stress was oriented between 103° and 119° from north in the horizontal plane with a magnitude of approximately 65 MPa and the least compressive stress ranged between 10 MPa and 30 MPa for a 30° to 50° range of vein-array angles and a fluid pressure of 30 MPa. Model results suggest that limestones in the Rico and Honaker Trail Formations ranged in stiffness between 10 GPa to 25 GPa, and may have been as stiff as 40 GPa to 50 GPa. For closely spaced echelon veins, lesser seam closing will form straight vein traces, and is thus, associated with stiffer limestone. A conceptual model is introduced to show how a typical array of echelon veins and pressure solution seams at Raplee Ridge may have developed, giving consideration to the mechanical interaction between closely-spaced veins and seams in limestone under geologic stress conditions. Perhaps a similar physical description can be used to explain the formation of echelon veins and pressure solution seams in other geologic settings.