Compressional tectonic inversions are classically represented in 2D brittle failure mode (BFM) plots that illustrate the change in differential stress (σ1−σ3) versus the pore-fluid pressure during orogenic shortening. In these BFM plots, the tectonic switch between extension and compression occurs at a differential stress state of zero. However, mostly anisotropic conditions are present in the Earth's crust, making isotropic stress conditions highly questionable. In this study, theoretical 3D stress-state reconstructions are proposed to illustrate the complexity of triaxial stress transitions during compressional inversion of Andersonian stress regimes. These reconstructions are based on successive late burial and early tectonic quartz veins which reflect early Variscan tectonic inversion in the Rhenohercynian foreland fold-and-thrust belt (High-Ardenne Slate Belt, Belgium, Germany). This theoretical exercise predicts that, no matter the geometry of the basin or the orientation of shortening, a transitional ‘wrench’ tectonic regime should always occur between extension and compression. To date, this intermediate regime has never been observed in structures in a shortened basin affected by tectonic inversion. Our study implies that stress transitions are therefore more complex than classically represented in 2D. Ideally, a transitional ‘wrench’ regime should be implemented in BFM plots at the switch between the extensional and compressional regimes.
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