REACTIVATION OF CONJUGATE FAULTS IN THE FOOTWALL OF BRYCE CANYON’S RUBY’S INN THRUST FAULT

Conjugate faults exposed in the Eocene Claron Formation’s hoodoos in and around Bryce Canyon National Park likely were formed under a north-south principal axis of compression, based on the conjugate faults’ acute angle bisector. Researchers have related these conjugate faults to the south-directed Ruby’s Inn thrust fault (Oligocene), the fault trace of which stretches from the Paunsaugunt normal fault on the eastern side of Bryce Canyon to the Sevier normal fault approximately 20 km to the west (both late Miocene to recent). The Paunsaugunt and Sevier normal faults are the easternmost faults related to Basin and Range extension. Southern Utah University undergraduate researchers working near Tropic Reservoir discovered Claron Formation outcrops in the footwall of the Ruby’s Inn thrust fault that display multiple orientations of slickensides with chatter marks. A detailed structural analysis of the slickensides revealed evidence for primarily normal dip-slip faults, indicating a change in the orientation of the principal axis of compression from north-south to vertical. One logical sequence of events involves the production of reverse conjugate faults during Ruby’s Inn thrust contraction, followed by normal fault slickenline production along pre-existing weaknesses during Basin and Range extension. Recent and potentially future movement along these pre-existing weaknesses could contribute to hoodoo development and rock fall hazards in and around Bryce Canyon National Park.