Dike Formation, Cataclastic Flow, and Rock Fluidization During Impact Cratering: an Example from the Upheaval Dome Structure, Utah

Patterns of deformation within the Upheaval Dome structure, Utah, provide important clues for assessing its possible impact origin. The complex structure of the innermost part of the dome, in particular of the White Rim Sandstone (WRS), indicated almost complete loss of internal coherence during deformation. The WRS displays extreme thickness variations, blind terminations and frequent embranchments at nodular-like points. This, together with discordant contacts to the country rock, shows that WRS builds up a dike network that was emplaced and injected during formation of the central dome. Microstructural analysis reveals that the macroscopically ductile appearance is achieved by distributed cataclastic flow. Beside intergranular fracturing, dislocation pile-ups, dislocation arrays and tangles indicate additional dislocation glide activity in quartz of WRS during deformation. Disseminated brittle fault zone networks postdate the cataclastic flow and represent the final deformation increments during formation of the central uplift. The distributed cataclastic flow in the sandstones was initiated by grain crushing, collapse of pore space, and subsequent intergranular shear. In accordance with experimental data for a similar sandstone (Berea sandstone), it is suggested that a high effective confining pressure, most likely in excess of 250 MPa, was necessary to cause this flow. At shallow crustal levels (maximum possible depth of burial of WRS is 3 km) such a high confining pressure can only be build up dynamically by impact processes. From deformation mechanisms, a shock wave attenuation to magnitudes below the Hugoniot elastic limit of quartz can be deduced and correlates with a burial of the studied sediments of 2-3 km at the time of the impact