“ROLLOVER-BREAK THRUSTS” AND ADVANCEMENT OF THE LEADING EDGE OF THE SEVIER FOLD AND THRUST BELT, SOUTHWEST UTAH

Detailed mapping of the Taylor Creek Fault Zone (TCFZ), an initially west verging fault on the forelimb of the Kanarra Fold in southwest Utah, indicates rotation of early formed flank thrusts during progressive shortening, significantly enhanced development of a frontal ramp, and advancement of the leading edge of the Sevier fold and thrust belt along a “rollover-break thrust”. The spatial and temporal variation of the architecture of the leading edge is imaged in along-strike cross-sections of the TCFZ, constructed using Move™, from regions of low strain in the south (e.g., Kolob Canyons) to high strain to the north (e.g., Kanarraville). The role of flank-thrust rotation is further evaluated using forward kinematic modeling in Move™ and analysis of slickenside data from the TCFZ with FaultKin to determine relative timing of faulting with fold development.

Results from mapping, kinematic modeling, and fault data analysis show the TCFZ rotates and reactivates to form a rollover-break thrust. Northeastward from Zion National Park, mapping reveals a substantial increase in shortening, from 10% in the south to approximately 42% in the north. Strata abruptly become overturned near Kanarraville. North of this critical transition, significant thinning due to forced folding of Kayenta mudstones (90 m) and Navajo Sandstone (400 m) is located proximal to a 250 m wide cataclastic zone in the Navajo Sandstone. This zone marks the beginnings of a thrust breaking through the fold that can be traced past Cedar City. Trishear forward models of cross sections in Move™ along the fold limb reflect observations from mapping. As folding progresses, flank thrusts rotate, and forelimb strata are thinned. When displacement on the main fault exceeds 3 km, the flank thrusts reactivate, linking together to form a ramp. Unfolding algorithms in FaultKin also indicate rotation and reactivation of the TCFZ. Slickenside sets in overturned Springdale Sandstone at the Kanarraville transition show thrust faulting before folding, while TCFZ slickensides 4.5 km northeast of the transition show thrust faulting after folding.

Rotation of flank thrusts into favorable orientations for reactivation circumvents development of triangle zones and advances the leading edge of the fold and thrust belt to higher structural levels along rollover-break thrusts.