MESOSCOPIC STRAIN AND STRESS INDICATORS AT UPHEAVAL DOME, UTAH, THAT SUPPORT AN IMPACT ORIGIN

Upheaval dome is a well preserved, small, fairly deeply eroded ring structure in the Phanerozoic rocks of the Colorado Plateau. The degree of erosion and quality of outcrops make it the finest exposed complex crater on the Earth, and probably in the solar system. As expected, Mesoscopic structures dating from the crater collapse and gravity modification stage are the best preserved within it.

As the transient crater was excavated, material moved away from the hypocenter in flow paths that arched downward from the point of impact and curved upward to the surface. Thrust sheets riding on thrust faults oriented parallel to these flow paths carried Mesozoic and Paleozoic strata outward from the point of impact, allowing the excavated material to pile up along the rim of the crater.

These thrust faults were reactivated in listric normal form, allowing material on the crater rim to ride back into the crater during the collapse and modification stage. As the material converged on the rising central uplift, the faults cut up section at their leading edges which allowed the strata to pile up against the core in radial convergent imbricated thrust sheets. Simultaneously, as the circumferences of the converging ring-like sheets contracted, they deformed into crenelation folds having outward and downward oriented axis.

The Permian White Rim Sandstone now stands vertically as a crown of thinned thrust sheets surrounding an exposed core of the underlying Organ Rock Shale. The sheets are bounded by bedding parallel thrust faults now rotated to the vertical. Cross cutting relations between the fault surfaces and enclosing strata reveal that dikes injected downward from the White Rim Sandstone were carried en mass within the converging sheets toward the crater center, revealing that the injection of the dikes dates from the crater excavation stage or before.