“ROLLOVER BREAK-THRUST” OR TRIANGLE ZONE - THE ROLE OF FLANK THRUSTS ON DEFINING THE ARCHITECTURE OF THE LEADING EDGE OF FOLD AND THRUST BELTS

Detailed mapping, analysis, and kinematic modelling of the Kanarra Fold at the leading edge of the Sevier fold and thrust belt, SW Utah, indicate early-formed flank thrusts control the architecture of leading edge structures. Understanding processes leading to rollover break-thrusts or triangle zones will benefit hydrocarbon exploration in fold and thrust belts and is important in predicting hydrocarbon trap location, geometry, and fault seal behavior. We suggest the following process: 1) Rotation of early-formed flank thrusts within the forelimb of a fault propagation fold leads to formation of a rollover break-thrust. 2) The advancing thrust merges with the rollover break-thrust to form a ramp. 3) The leading edge of the fold and thrust belt advances up section. 4) Development of a triangle zone is terminated. We investigate the role of mechanical stratigraphy (layer thickness, lithology, and competence) and location of flank thrusts within the stratigraphy as important factors influencing competing processes – formation of a rollover break-thrust or a triangle zone. We integrate rock strength and elastic properties, derived from Schmidt hammer data collected in the field, into our geomechanical models. We evaluate the structural position and geometry of the advancing thrust and the presence/absence of a mechanically weak layer on flank thrust evolution. The absence of a mechanically weak layer and development of a fault propagation fold above the advancing thrust promotes flank thrust rotation to form a rollover break-thrust. The presence of a mechanically weak layer, development of a thrust flat, and formation of a fault bend fold above the advancing thrust favors flank thrusts forming a roof thrust and development of a triangle zone. Steep to overturned strata in leading edge structures are commonly difficult to resolve, and in these areas distinction between the two structures will benefit from mechanical stratigraphy-based geomechanical model predictions.