Hidden Faults: The Late Pleistocene Transpression of the Königssee–Lammertal–Traunsee Fault Inferred from Caves Deformation (Eastern Alps)

Abstract The Eastern Alps have undergone lateral extrusion since the Late Oligocene, with major crustal-scale strike-slip faults still active, as evidenced by earthquakes up to M6, despite scant geological record. Research has focused more on the Salzach-Ennstal-Mariazell-Puchberg (SEMP) and Mur-Mürz faults, leaving the central part of the Northern Calcareous Alps cut by the 110 km long Königssee–Lammertal–Traunsee (KLT) fault system under-studied. We took advantage of a cave environment isolated from erosion, providing unparalleled structural indicators exposure to fill the Pleistocene deformation history gap of the KLT. We reconstruct paleostress for twenty-six reverse, strike-slip, and oblique reactivated faults that offset passages in seven caves close to sinistral KLT and dextral Lammertal faults. 230Th/U dating of faulted and broken speleothems revealed three reactivation events since the Middle Pleistocene. The oldest event could be pinpointed to 560 ka (+100/−60). The middle event dates can be constrained to 331 (+89/−54) to 287 ± 6 ka, or 297–281 ka if the KLT and Lammertal faults were reactivated simultaneously. The youngest event occurred between 130 and 90 ka, aligning with fault reactivations in the Eastern Alps. Cave observations allowed us to analyze reverse faults in the positive flower structure of the KLT and mode I fracture with minor antithetic dextral slip, suggesting a sinistral component of regional rejuvenation. Our findings indicate that the KLT was reactivated due to simple shear with NNE compression. Along the Lammertal fault, we recorded a strike-slip regime driven by NNW compression. The KLT and Lammertal faults form a system of conjugated shears that efficiently accommodate N-S shortening compared to the SEMP fault, which is perpendicular to the compression. Combining our neotectonic data with current seismicity shows that the KLT plays a key role in the Quaternary extrusion process in the studied Alpine sector, surpassing the more prominent SEMP fault.