Insights into the growth of basement uplifts deduced from a study of fracture systems in the San Rafael monocline, east central Utah

Abstract

We examined networks of joints, small faults, and minor cleavage exposed in the lower parts of the Jurassic Carmel Formation along the San Rafael monocline in east central Utah. By collecting data from the same 3 m stratigraphic interval we effectively guaranteed that variations in the architecture of fracture systems that we examined were not caused by influences that could be attributed to stratigraphic variables like lithology or environmental variables such as burial depth. We consequently interpret that the spatial variability of fracture system architecture we observe was due to local variations in fold kinematics or mechanics. Steeply dipping portions of the monocline contain suborthogonal sets of dominant strike‐normal and strike‐parallel joints. Shallowly dipping portions of the monocline contain multiple joint sets with no single dominant set and evidence for shear reactivation of joints. Sets of small offset conjugate faults are present throughout the monocline and are used to determine the orientation of paleostresses that caused them. This stress inversion yields a thrust faulting configuration with maximum compression bearing between 285° and 312°. There is a systematic 27° clockwise rotation of these stress directions from north to south along the monocline. We interpret the spatial variability in fracture systems in the monocline as a consequence of local cylindrical or noncylindrical folding associated with three‐dimensional flexure of the sedimentary cover during dominantly dip‐slip motion on the underlying basement fault. This interpretation suggests that the monocline did not grow laterally but evolved with fixed tips in response to a northwest‐southeast compression that is kinematically incompatible with accepted Laramide convergence directions.