Triangle zones as mechanical gages - results from numerical models and the Alpine Carpathian Belt

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. We present numerical models of brittle-ductile wedges, varying dip and strength of the basal décollement systematically. We show that a new parameter, cumulative vorticity, is well-suited to characterize wedges based on dominant fault vergence. 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 décollement strength and basal dip may control thrust vergence, comparison with natural examples indicates this cannot be the only explanation for the formation of backthrusts. We probe into this using a compilation of structures along strike the fold-thrust belt of the entire Alpine-Carpathian Belt. Our results show that indeed triangle zones are associated with weak décollements, while additionally syn-tectonic sedimentation, rheological changes across strike, or structural inheritance may play a role. In some cases, interpretations of backthrusts at depth is challenging due to geometric uncertainty. We show uncertainty modeling of triangle zones using a well-known example from the boundary between Eastern and Central Alps. These uncertainty estimates may be combined with thermochronological data with the goal to distinguish the presence or absence of triangle zones from exhumation estimates. When present, triangle zones and backthrusts may serve as mechanical gages, providing tight constraints on fold-thrust belt mechanics.