Fracture initiation, development, and reactivation in folded sedimentary rocks at Raplee Ridge, UT

Fractures at Raplee Ridge, a 14 km long monoclinal fold of Pennsylvanian sedimentary rock in southeast Utah, are examined to determine their sequential development and to explain the mechanical relationships among the three primary fracture sets and the fold. Folding is attributed to slip on an unexposed thrust fault of Laramide age. Fracture orientations were measured at 98 sites and fracture patterns were mapped at 8 sites on two sandstone and two limestone units of the Rico Formation, revealing at least four stages of fracture formation. Stages 1 and 2 occurred pre-folding and are defined, respectively, by E–W trending fracture Set I and N–S trending fracture Set II. Set I fractures are characterized by a unimodal distribution of attitudes, an intermediately-developed outcrop pattern, and continuous, linear traces on bedding surfaces. Set II fractures have a greater dispersion of orientations and are poorly-to intermediately-developed. Abutting relationships confirm that Set I fractures are older than Set II. Stage 3 occurred during folding when NW–SE trending Set III fractures initiated both from isolated flaws and as wing-cracks due to right-lateral shearing along reactivated Set I fractures. Set III fractures increase in density as the fore-limb dip increases from 10° to 30°. Later stages of deformation are characterized by the formation of fracture Subsets IV and V, structurally associated with fore-limb dips greater than 30°. Fracture Sets I and II are found throughout the surrounding Monument Upwarp and are attributed to that structure, whereas Set III is attributed to local stresses induced by monoclinal folding and/or thrust faulting.