Unravelling Fault Reactivation History: Geochronological Insights from a Major Intraplate Fault in the Bavarian Forest, Germany

The tectonic history of Central Europe, located within the interior of the Eurasian plate, is characterised by episodic fault reactivations extending into the Cenozoic. Determining the exact timing of repeated activity along continental intraplate faults is key to understanding the underlying forces driving lithospheric deformation, mantle convection, and geodynamic processes. In particular, lithospheric flow has been proposed as a mechanism capable of reactivating pre-existing fault zones, but its contribution to deformation in Central Europe is not yet well-constrained.

The Variscan Bohemian Massif provides a key setting for this study, with granitic plutons featuring a complex structural and lithological architecture that reflects a prolonged history of deformation. The area is predominantly composed of 312–325-Ma-old granitic rocks intruded into the metamorphic basement during the Variscan orogeny. These rocks are crosscut by numerous fault zones, including the prominent NW–SE-striking Danube fault zone, which has been periodically reactivated under varying stress regimes. Despite its young morphology, the post-Variscan deformation history of the Danube fault zone remains poorly constrained.

By integrating ⁴⁰Ar/³⁹Ar thermochronology with U-Pb dating of calcite slickenfibres and K-Ar dating of illite from fault gouges in nine different quarries in bedrock northeast of the Danube fault, we reconstruct the temporal and kinematic evolution of these faults. Our results reveal a multi-phase reactivation history, with significant tectonic activity persisting into the Cenozoic. ⁴⁰Ar/³⁹Ar analysis of K-bearing minerals from deformed host rock yielded the oldest dates, ranging from 232 to 331 Ma, with K-feldspars showing the largest intra-outcrop variations of up to 10 Myr, likely indicating localised resetting of the ⁴⁰Ar/³⁹Ar clock. K-Ar dates of illite, spanning from 173.2 ± 4.0 Ma to 204 ± 5.3 Ma, reveal evidence of brittle deformation resulting in clay gouge formation. Complementary U-Pb dating of synkinematic calcite slickenfibres on subsidiary fault planes up to 10 km from the main fault, with ages ranging from 45.7 Ma to 0.82 Ma, provides precise temporal constraints and preliminary insights into the timing of deformation. The complementary analysis of mineral parageneses within the dated faults reveals multiple phases of mineral formation and distinct fluid compositions, indicating varying low temperature and pressure conditions (50 – 200 °C; <1.2 GPa). We observed a transition of the Danube Fault from higher-temperature deformation (200–300°C) in the Triassic to near-surface faulting and fluid activity (<150°C) during the Cenozoic. The thermal evolution inferred from our detected mineral assemblages aligns with previously obtained Apatite Fission Track (AFT) ages, indicating low-temperature thermal events (<120°C) related to near-surface exhumation processes.

Our results underscore the importance of detailed analysis of deformation inventory in intraplate setting over geological timescales. The temporal and kinematic data from this study provide a critical contribution to refining the timing and constraining the duration over which currently available stress field models are applicable. Additionally, these data offer a framework for understanding the evolution of intraplate fault systems through integrated radiometric, petrological, and geochemical analyses.