The formation of conjugate normal fault systems in folded sandstone by sequential jointing and shearing, Waterpocket monocline, Utah

[1] We report conjugate normal fault development by the formation and subsequent shearing of joints during flexure and associated extension of the Jurassic Wingate and Navajo Sandstones at the core of the Waterpocket monocline, Utah. Geometry and structural components across fault zones were quantified from an incipient stage to a relatively well-developed stage where the zone is 120 m wide and accommodates more than 3.5 m of slip. Shearing of joints leads to the formation of splay fractures near the joints' tip. Subsequently, these splay fractures are sheared and a new generation of splay fractures forms. Multiple generations of splay fractures cause the width of the fault damage zone to expand into the surrounding host rock, producing unusually wide fault damage zones with respect to fault offset. These extensive fault damage zones are stratigraphically confined and develop with low strains at small bed dips during folding. The faults formed by this sequential process develop a characteristic fault architecture distinguished by the type of component structures, their geometric relationships, and their distribution. We conclude that cyclic joint formation, shearing accompanied by splay fracture formation, and repeated shearing produce a geometric pattern similar to a classical conjugate fault system. However, the mechanism identified here has different implications for fault intersection angles, stress inversion, and fault architecture.