GEOMECHANICAL MODELING OF FOLDING ADJACENT TO SALT DIAPIRS: AN EVALUATION OF SPREADING, GLIDING AND STEM PUSH AT THE ONION CREEK DIAPIR, UTAH

The movement and storage of fluids in the vicinity of salt structures is controlled by the hydrological properties of near-salt strata, salt dissolution, the thermal properties of the evaporites, the geometry of the salt-sediment interface, and the structural and stratigraphic architecture of rocks flanking the diapir. The evolution of these variables during diapirism makes it challenging to exploit salt structures as hydrocarbon reservoirs, storage sites for CO₂ sequestration, and sources of geothermal energy and critical minerals. In this study we use a series of simple, 2-D numerical models to examine the origin of sub-kilometer scale folds that are developed in a 300-500 m wide burial wedge of Permian Cutler Group strata above a salt shoulder along the northern margin of the Onion Creek diapir in the Paradox Basin of Utah. These folds increase in wavelength away from the shallow diapir, often have kink-style geometries, extend along strike for hundreds of meters, and have amplitudes of tens to over a hundred meters. We model the salt as a viscoelastic material and the sedimentary rocks as an elastic-plastic material and evaluate the relative significance of stem push, gravity spreading and gravity gliding as mechanisms to create the folds. We also examine the influence of burial wedge geometry and the presence of strong caprock over the top of the diapir. Our models demonstrate that stem push is incapable of folding burial wedge strata unless the yield strength of those strata is < 1 MPa. Gravity spreading of the salt is positively correlated to the vertical flux rate and relief on the top of the diapir, but primarily causes uplift of strata in the burial wedge, rather than horizontal shortening. This uplift is reduced by the presence of caprock and by rapid sedimentation, both of which preserve structural relief on the top of the diapir and inhibit gravity spreading. Gravity gliding produces significant horizontal shortening and folding of burial wedge strata only if there is a lower viscosity, weakened layer of salt near the top of the diapir. In this circumstance, folds initiate at local thicks or thins in the burial wedge strata, or simply because of the wedge taper. Lateral salt flow during gravity gliding causes uplift of salt at the diapir margin, and may cause overlying strata to dip into the diapir, a geometry that has previously been attributed primarily to salt dissolution.