ECHELON VEINS AND PRESSURE SOLUTION SEAMS RELATED TO FOLDING AT RAPLEE RIDGE, UTAH

Two conjugate sets of systematic echelon vein arrays with intersecting pressure solution seams are observed within limestone strata across Raplee Anticline and Comb Monocline in southeastern Utah. The anticline and monocline are kilometer-scale folds with less than 0.1 amplitude-to-wavelength ratios. Echelon veins and seams are not observed in the surrounding, unfolded limestone. Therefore, they are inferred to have occurred during folding, in the late Cretaceous at depths of 2 – 3.5 km based on the published burial history of the strata.

We introduce a mechanical model to show how physical, geologic quantities affect the shape and propagation direction of echelon veins, and use the results to quantify the tectonic setting during folding. From this, we infer the limestone stiffness and regional stress directions and magnitudes at the time of echelon vein and seam formation. These physical attributes are indeterminable from commonly used simple-shear kinematic models that lack an explicit relationship between stresses and strain through a constitutive equation. Additionally, a mechanical description gives reason for contrasting the material properties of limestone with adjacent, folded sedimentary strata predominantly deformed by jointing.

Limestone stiffness, E, echelon vein spacing, s, the angle of echelon veins to the array trace, α, and pressure solution seam closing, Δu_n, are the most critical quantities of eleven necessary to describe echelon vein shapes. Veins have larger apertures when E and s are decreased and when α and Δu_n are increased. Vein surfaces are straighter with decreasing s and increasing Δu_n, and can explain observations of triangular vein shapes. Additionally, closing of vein-orthogonal pressure solution seams can cause veins to propagate in their own plane to form straight veins despite an inclination of the veins to the remote stresses. This discovery offers an explicit reason for the orientation of the principal remote stresses to be angled to echelon vein and seam arrays, and to bisect conjugate arrays. The range of measured echelon vein geometries at Raplee Anticline and Comb Monocline formed when the limestone stiffness was 10 to 50 GPa, the fluid pressures were 20 - 35 MPa, and the regional, maximum, horizontal compressive stress was about 65 MPa oriented 103° to 119° from north.