Geological Sciences Dissertation Defense- Solomon Seyum

Deformation at Raplee Ridge, Utah, inferred from Joints, Veins, and Pressure Solution Seams

Joints, and arrays of echelon veins with intersecting pressure solution seams are two common structures in sedimentary rocks. Their systematic geometries across large areas makes them useful indicators of tectonic stress state. I describe the physical, geologic conditions at Raplee Anticline and Comb Monocline in Utah that led to the formation of systematic echelon vein and pressure solution seam arrays in limestone, and contrast their deformation mechanism with jointing in adjacent sedimentary strata.

Building on published mechanical model results of echelon veins, vein-intersecting pressure solution seams are introduced, an elastic-plastic constitutive relationship is assigned to the limestone, and model results are compared to measurements made in the field. Four physical quantities are necessary to describe the shape of model veins in a 2D, homogeneous, isotropic material: vein spacing, vein-array angle, limestone elastic stiffness, and closing of intersecting pressure solution seams.

Pressure solution seam closing influences the mechanical interaction between adjacent veins, and for a range of conditions, causes triangular vein shapes with large apertures and encourages straight vein propagation, which approximates field measurements. For limestones with stiffness values typical of laboratory measurements, and for stresses inferred from the burial history of the strata at Raplee Ridge, model results show that small spacing of veins with seams and large vein-array angles produce the observed straight vein traces.

Echelon veins and pressure solution seams at Raplee Anticline and Comb Monocline formed in response to a horizontal, remote greatest compressive stress oriented 103° to 119° from north during the Laramide Orogeny, and during the early stages of folding. Supposing a 3 km depth and 30 MPa hydrostatic fluid pressure, the greatest compressive stress was 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. Limestones in the Rico and Honaker Trail Formations ranged in stiffness from 10 GPa to 25 GPa, and could have been as stiff as 40 GPa to 50 GPa.