A review of deformation pattern templates in foreland basin systems and fold-and-thrust belts: Implications for the state of stress in the frontal regions of thrust wedges

Aesthetically appealing thrust systems and related large-scale anticlines, in both active and fossil foreland fold-and-thrust belts, and the economic potential associated with them, have captured the interest of structural geologists for many decades. As a consequence, a large amount of data on sub-seismic deformation patterns from thrust-related anticlines is available in the literature. We provide a review of deformation pattern templates from field data in foreland fold-and-thrust-belts and show that the most frequent trends of sub-seismic syn-orogenic deformation structures hosted in km-scale thrust-related folds frequently and paradoxically indicate a syn-thrusting strike-slip stress field configuration, with a near-vertical σ2 and a sub-horizontal σ3, rather than a contractional one where the latter is expected to be the vertical principal axis of the stress ellipsoid. This apparent inconsistency between sub-seismic syn-orogenic deformation structures and stress field orientation is here named “the σ2 paradox”. Field data support a possible explanation of the paradox, provided by the major role played by inherited early-orogenic extensional deformation structures on thrust fault nucleation. Nucleation of major thrusts and their propagation is facilitated and driven by the positive inversion and linkage of the early-orogenic sub-seismic extensional inheritance developed in the foreland basin. This process eventually leads to the development of large reverse fault zones and can occur both in contractional and strike-slip stress field configurations.