ROLLOVER BREAK-THRUST FORMATION AND ADVANCEMENT OF THE LEADING EDGES OF FOLD AND THRUST BELTS THROUGH FAULT PIRACY

Detailed mapping of the Taylor Creek Fault Zone (TCFZ), an initially west verging flank thrust on the forelimb of the Kanarra Fold in southwest Utah, indicates that flank thrust rotation enhances frontal ramp development and advancement of the leading edges of fold and thrust belts along “rollover break-thrusts”. The spatiotemporal variation of the architecture of the leading edge Kanarra Fold is imaged in along-strike cross sections of the TCFZ 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 in fold development is evaluated with trishear modeling in Move™ and analysis of fault data from the TCFZ with FaultKin to determine relative timing of faulting.

Results from mapping, fault data solutions, and trishear modeling show the TCFZ rotates and forms 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 overturn near Kanarraville. North of this critical transition, significant thinning due to forced folding of the Navajo Sandstone (66%) occurs near a 250 m wide cataclastic zone in the unit. This zone marks the beginnings of a thrust breaking through the fold that can be traced past Cedar City. Trishear modelling of cross sections across the transition indicates a mechanism for rollover-break thrust development via “fault piracy”. The leading edge thrust steepens as it branches towards the flank thrusts and induces thinning in the overlying units. The change in leading edge thrust geometry as flank thrusts overturn suggests rotated flank thrusts introduce a zone of weakness that transfers strain from the leading edge thrust and attracts the thrust tip. Strain transfer zones adjacent rotated flank thrusts are expressed in map and cross section views through duplexing and ductile thinning. Fault piracy links the leading edge thrust to the flank thrusts, enabling development of a rollover-break thrust.

Fault piracy of leading edge thrusts by rotated flank thrusts alters leading edge thrust geometry and trajectory, circumventing development of triangle zones and advancing the leading edge of the fold and thrust belt to higher structural levels along rollover break-thrusts.