LEADING EDGE RAMP FORMATION BY ROTATION OF EARLY FORMED FLANK THRUSTS ALONG THE KANARRA FOLD, SEVIER THRUST BELT, SW UTAH

The role of early formed flank thrusts in the termination of a fault propagation fold, development of a frontal ramp, and advancement of the leading edge of fold and thrust belts is examined using a combination of detailed field mapping, structural interpretation, and forward modeling of the Kanarra Fold. The Kanarra Fold is an anticline exposed between Zion National Park and Cedar City, UT along the leading edge of the Sevier Thrust Belt. To the south, in Kolob Canyons, the Kanarra Fold is upright and open - here the well exposed Taylor Creek Thrusts are east dipping, west directed flank thrusts along the east limb of the fold. Northwards along strike near Kanarraville, the east verging Kanarra Fold is tight and overturned. In this area, outcrop patterns, minor fold vergence, and slickensides reveal the presence of west-dipping reverse faults that show normal separation. These faults occur along strike in Spring Creek and offset the same units (e.g., Springdale) as the Taylor Creek Thrusts. Immediately to the east and just north of Spring Creek, a laterally continuous, ~0.25 km wide cataclasite zone in the Jurassic Navajo Sandstone marks the beginning of a significant east directed thrust fault along the edge of the Kanarra Fold that can be traced north of Cedar City. We propose the reverse faults in Spring Creek are “Taylor Creek” flank thrusts that were rotated during shortening into an optimal orientation to promote ramp formation and advancement of the leading edge of the Sevier Fold and Thrust Belt. We use trishear forward modeling in Move™ to investigate this process. Preliminary results confirm that as shortening progresses, the flank thrusts rotate into favorable orientations for east-directed movement, the spacing between them decreases, they link together, and reactivate as east-directed thrusts. These reactivated faults then form a ramp in the Navajo Sandstone, initiating fault breakthrough on the eastern limb of the Kanarra Fold. We suggest that this fault is correlative with the east-directed thrusts that cut through the Jurassic Navajo along the western edge of Parowan Gap. Rotation of early formed flank thrusts promoted ramp formation and advancement of the leading edge of the Sevier Thrust Belt as demonstrated by the Iron Springs Thrust within the Cretaceous sequence in Parowan Gap.