Movement history of faults under variable stress fields – insights derived from 3D seismics in the central northern Upper Rhine Graben, Germany

Abstract. The tectonic history of the Upper Rhine Graben (URG) is still unclear, despite decades of research. This is because of the lack of good quality 3-D seismic data to investigate the subsurface. We reconstruct the tectonic evolution of the central northern Upper Rhine Graben (NURG) by analyzing a 3D seismic survey that covers approximately 300 km² between Worms in Rhineland-Palatinate and Darmstadt in Hesse, Germany.

We aim to understand the interactions between the faults, the sedimentation pattern near the fault areas, their evolution in relation to the major structures of the URG, and to deduce the associated stress regime. Seven of the largest normal faults were selected (five strike NNW–SSE and two strike WNW–ESE), and the sediment thickness maps in each stratigraphic zone were generated using interpreted seismic horizons. Based on the thickness maps and the syn-sedimentary fault activity, the normalized Expansion Index (EI) was calculated. The analysis indicates that NNW–SSE fault activity was dominant pre-24 Ma, while the fault activity of WNW–ESE faults became more prominent post-24 Ma. Independent structural evidence, including the geometrical refraction of fault intersection traces (attributed to the reactivation of inherited Variscan basement trends) and the unilateral southward migration of displacement maxima, physically documents the rotation from E-W extension to NW-SE transtension. We postulate that all seven faults are active to the present day, as they can be traced seismically to the near surface. Recently acquired geophysical data (including shear-wave seismics) by our research group confirms recent activity on two of these faults.

We conclude, based on the seismic interpretation of our study area and the derived results, that URG underwent through two distinct tectonic phases: 1) a pre-24 Ma rifting stage and 2) a post-24 Ma transtensional stage that has lasted to the present-day. This finding aligns with other recent studies on the tectonic evolution of the URG. Rift systems evolving under variable stress regimes can develop complex fault architectures as changing stress orientations reactivate inherited structures and redistribute strain through time. Our results show that such stress-field changes can transform initially simple extensional fault systems into more segmented and interacting networks, where older rift faults and newly activated transfer structures jointly control fault propagation, basin subsidence, and sedimentation patterns.