Energy balance for large thrust sheets and fault-bend folds

Abstract

Thrust sheet movement over ramps requires energy because of the frictional resistance and deformation within the fault zone, fault-bend folding at the base and top of the ramp, and changes in the gravitational potential energy because of uplift. To model the energy usage, a kinematic model of a foreland thrust sheet is constructed assuming: (1) the ramp is planar and the flats are parallel to bedding; (2) the fault-bend folds are concentric; (3) thickness is preserved for beds that enter the folds parallel to the basal thrust fault and (4) cross-sectional area is preserved for rocks deformed by folding. Equations for the work done within the fault zone, and during uplift and fault-bend folding are derived by combining the kinematic model with stresses that increase in proportion to depth. The relative amounts of energy consumed by friction along the fault, uplift and fault-bend folding are estimated to be 2.7:1.0.25 for a ramp angle of 30°. The energy balance for the movement of large thrust sheets thus depends principally upon friction in the fault zone and changes in the gravitational energy.