The complexity of 3D stress-state changes during compressional tectonic inversion at the onset of orogeny

Compressional tectonic inversions are classically represented in 2D brittle failure mode(BFM) plots that illustrate the change in differential stress (s1 2s3) versus the pore-fluid pressureduring orogenic shortening. In these BFM plots, the tectonic switch between extension and compressionoccurs at a differential stress state of zero. However, mostly anisotropic conditions arepresent in the Earth’s crust, making isotropic stress conditions highly questionable. In thisstudy, theoretical 3D stress-state reconstructions are proposed to illustrate the complexity of triaxialstress transitions during compressional inversion of Andersonian stress regimes. These reconstructionsare based on successive late burial and early tectonic quartz veins which reflect earlyVariscan tectonic inversion in the Rhenohercynian foreland fold-and-thrust belt (High-ArdenneSlate Belt, Belgium, Germany). This theoretical exercise predicts that, no matter the geometryof the basin or the orientation of shortening, a transitional ‘wrench’ tectonic regime shouldalways occur between extension and compression. To date, this intermediate regime has neverbeen observed in structures in a shortened basin affected by tectonic inversion. Our studyimplies 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 switchbetween the extensional and compressional regimes.status: publishe