Control of décollement strength and dip on fault vergence in fold-thrust belts and accretionary prisms

Abstract

Fault vergence in fold-and-thrust belts and accretionary prisms is characterized by mainly forward verging thrusts and pop-up structures, and only few examples exist where backthrusting dominates. However, backthrusting and triangle zones are known from most if not all fold-thrust belts in the world. The circumstances under which backthrusts instead of forethrusts form are still incompletely understood. Previous studies suggest that the strength and dip of the basal décollement of Coulomb wedges plays a key role. However, a systematic study of these parameters is still missing. In this study we model numerically the formation of brittle-ductile wedges, systematically, varying dip and strength of the basal décollement. To characterize wedges, we use cumulative vorticity as a measure of dominant fault vergence within the entire wedge. We find that backthrust-dominated wedges form in setups of very low basal dip (≤ 0.5°) and a weak décollement. Increasing décollement strength or basal dip results in the formation of pop-up-dominated wedges, before forethrust dominated wedges form. While our models corroborate the idea that thrust vergence and basal dip may control thrust vergence, comparison with natural examples indicates this cannot be the only explanation for the formation of backthrusts. Consequently, additional factors such as syntectonic sedimentation or changes of décollement rheology across strike may be required to explain backthrusting in fold-thrust belts.