EXTENT OF FOOTWALL SHEAR ADJACENT TO THE RUBY'S INN THRUST FAULT, SOUTHERN UTAH

In the late 1980s and early 1990s several researchers attributed the south-directed Ruby’s Inn Thrust Fault in the Bryce Canyon National Park region to gravitational spreading and collapse of the Miocene Marysvale Volcanic Complex directly to the north. In George Davis’s 1997 Field Guide to the Geologic Structures in the Bryce Canyon Region, Utah, he reported conjugate shear structures in the Paleocene to Eocene Claron Formation within the footwall of the Ruby’s Inn Thrust Fault that are relatable to the thrusting. In our study we determined the north-south extent of a broad shear zone along a traverse immediately west of Bryce Canyon National Park. We found that shearing intensifies slowly from the main thrust at the northern end of our traverse to a maximum intensity at 13 kilometers south of the thrust, where it then slowly diminishes until an abrupt end approximately 29 kilometers south of the thrust. We found no evidence of conjugate shear structures in the hanging wall of the thrust. The footwall outcrops adjacent to the thrust and at the southern portion of the traverse contained the structures, but they were more difficult to visually recognize, whereas the structures within the outcrops of the central region were obvious. The conjugate shear structures crosscut bedding and vary from small scale (a few centimeters) to large scale (tens of meters) throughout each outcrop. They are best observed parallel to their east-west strike, and they contain distinct structural planar surfaces that include very well developed slickensides and slickenlines. The orientations of conjugate shear structures, slickensides, and slickenlines are generally consistent throughout the traverse and corroborate Davis’s proposal that these structures are related to the Ruby’s Inn Thrust. We have recognized similar structures in Cedar Canyon adjacent to Cedar City, Utah. The extent and orientation of the Ruby’s Inn shear zone suggest that it could extend westward and connect with south-directed structures found in Cedar Canyon. This potential connection could have implications for the relationship between the Ruby’s Inn Thrust Fault and the Marysvale Volcanic Complex.