RAMP FORMATION AND FAULT BREAKTHROUGH OF THE KANARRA FOLD SYSTEM, SEVIER THRUST BELT, SW UTAH

Processes contributing to the termination of a fault propagation fold, development of a frontal ramp, and eastward advancement of the leading edge of the Sevier fold and thrust belt in southwest Utah are being investigated along the east limb of the Kanarra Fold, an anticline exposed between Zion National Park and Cedar City. Detailed mapping in Spring Creek Canyon, UT documents faulting along the edge of the fold limb, and as shortening progressed, folding was terminated by ramp formation and fault breakthrough. These processes are evaluated using a combination of geologic mapping, cross section analysis, and forward modeling using Move™. Field data from geologic mapping, including sedimentary and fault contacts, bedding, and slickensides, are used in the construction of cross sections across strike. Results from cross sections define A) the presence of a newly identified west dipping propagating thrust fault, the “Kanarra Thrust”, which drove fold formation, and B) early formed east dipping flank-thrusts along the fold limb which rotated to dip west and now show normal fault separation. This interpretation is supported by field data of fault attitudes. Observations of outcrop patterns and slickensides in the Jurassic Navajo Sandstone (up-section from the “rotated” faults) also indicate the presence of a laterally continuous, nearly 250 m wide fault zone.

We propose the following mechanism for fault breakthrough: with increased shortening, the flank-thrusts rotated into favorable orientations for east-directed thrust ramp formation, linked together, and reactivated as east-directed thrusts. These reactivated faults formed a ramp in the Navajo Sandstone breaking through the eastern limb of the Kanarra Fold. This model is consistent with structures found north along strike near Cedar City, where east-directed thrust faulting cut through the Jurassic sequence, and in Parowan Gap, where east-directed thrust faulting cut through the Navajo Sandstone and continued within the Cretaceous sequence. Based on this breakthrough model, forward modeling in Move™ is used to test hypotheses whether the flank-thrusts are linked with the main propagating fault or led to abandonment of this fault. The results of this study will give better insights into how deformation may evolve along the leading edge of fold and thrust belts.