ICE 2022


Extensional reactivation of former thrust planes in thrust fold belts, designated as ""negative inversion"", received much less attention by both the petroleum industry and the academia than the opposite process. This obvious imbalance in the understanding level of positive versus negative inversion is clearly reflected by the preference in the petroleum industry to focus on what has been simplified to just ""inversion"" instead of positive inversion. This choice is obviously driven by the fact that many hydrocarbon fields are associated with positive inversion. Negative inversion, occurring by the extensional reactivation of pre-existing compressional structures, has been first observed in the exposed interior segments of thrust fold belts. In general, it is more difficult to make a convincing case for negative rather than positive structural inversion, especially when only subsurface data sets are available. As negative inversion has an extension event or period as the last deformational phase, the resulting basin fill typically covers up all the surface evidence for the pre-existing contractional fabric. Therefore negative inversion manifests itself dominantly in subsurface data sets. Based on the structural review of many case studies of positive and negative inversion they display contrasting kinematic patterns. One of the obvious structural differences is the development of shortcut structures during the advanced stage of inversion. In the case of positive inversion a shortcut thrust develops within the footwall of the major inverted fault to better accommodate the ongoing shortening. In contrast, a shortcut normal fault develops within the hangingwall of the partially inverted master fault during negative inversion. While the shortcut thrust associated with positive inversion typically does not produce traps, the shortcut extensional listric fault during negative inversion is trap-forming, both within its footwall and hangingwall. In the broader Alpine-Carpathian-Pannonian region, there are many examples of hydrocarbon fields where some of the trap elements are due to negative structural inversion. Therefore it is suggested here that even though negative inversion may not be as important for petroleum exploration as its positive counterpart, yet, it may produce more traps in the internal parts of thrust fold belt than currently perceived.