Neotectonics of the Eastern Alps investigated in caves

Numerous field studies, seismic data, and GPS observations suggest ongoing activity along major tectonic fault systems in the Eastern Alps. However, discrete geological proofs characterizing the recent activity of individual fault systems are rather scarce. As tectonically damaged speleothems and micro-displacement measurements in karst caves have proven to provide unique quantitative information on neotectonic activity, this study aims on providing the first comprehensive and detailed assessment of Quaternary tectonic activity and recent dynamics of micro-displacements along major fault systems of the Eastern Alps registered in caves. In the first step, traces of active faults in caves will be documented on the basis of field observations, literature research and by checking hints from the speleological community. Cave passages that have been dislocated by active faulting and which are coated by flowstone that grew before as well as after the tectonic event will be dated by the U- series disequilibrium method for achieving the age of respective neotectonic events. In order to reconstruct the local stress conditions during the (re)activation of the faults, the paleostress field will be, if possible, calculated from fault-slip data of the recent dislocations. In order to discriminate between fault displacements caused by seismic slip or aseismic creep, which is an essential task for identifying paleo-earthquakes, micro-tectonic studies of faulted flowstones using cathodoluminescence images with electron backscattered diffraction will be the basic methods. At selected prominent test sites, recent dynamics of micro-displacements along single faults that are related to major fault systems will be monitored by means of the high-accuracy 3D crack-gauge TM71. The potentially recorded micro-displacements will be correlated with regional seismic activity in the study area of the Eastern Alps in Austria and with other influencing factors. Finally the obtained data are a solid base for updating the paleoseismic record of the Eastern Alps for regional earthquake hazard assessment.

Earthquakes occur when rock masses move abruptly relative to each other along faults. Field studies, seismic data, and GPS observations revealed an ongoing and recurrent activity of the major fault systems and related earthquake hazard in the Eastern Alps. So far, however, geologic field data on active faults are rare, as surface erosion has obliterated traces of young tectonics, and temperature variations inhibit exact measurements of movements along faults. In contrast, caves containing tectonically deformed dripstones and flowstones provide good opportunities to study active tectonics, and their constant climate allows measurements of fault movements at sub-millimetre level. Therefore, in the frame of the SPELEOTECT-Project, the first comprehensive investigation of active tectonics of East Alpine fault systems was conducted by means of cave findings. Beginning of the project included documenting the caves showing indicators of active tectonics, through their field investigations, literature search, and hints from cavers. Flowstones along the cave walls, offset by faults and sealed by new flowstone again, were used to date the age of the tectonic event by means of the Thorium-Uranium-method. An outstanding example of fault exhibit 40 cm of offset, formed 19 to 42 thousand years ago, and was found in the Obir Caves. This fault-related offset of the cave wall is unique evidence that the plate tectonic border between the Eastern- and Southern Alps (i.e. the Periadriatic fault system) is still active. Furthermore, the question whether these offsets occurred abruptly, thus causing an earthquake, or slowly, meaning a-seismically, was aimed to be answered by means of electron microscopy of the deformed flowstone calcite crystals. This turned out to be more challenging than expected, considering the lack of a clear-cut distinction between different faulting architecture in calcite, reported both in former literature and laboratory experiments conducted within this project. Nevertheless, using other geologic observations, it can be concluded that the fault movements, such as the one in the Obir Caves, triggered strong earthquakes in recent past. Additionally, caves near the epi centre of the strong earthquake near Kos (Greece, June 2017) were studied with the purpose of comparison with Alpine caves and faulting mechanisms. To better understand the current tectonic activity of the Eastern Alps, a monitoring network was set up in seven caves to record micro-displacements along faults and compare them to local earthquakes. It turned out, that movements along faults coincide with phases of increased earthquake activity but do not occur at the same time as earthquakes. Understanding these processes based on the analysis of long time-series could significantly contribute to understanding local tectonic processes and seismicity. In summary, the SPELEOTECT-Project provides an important step in better understanding the fault activity in the Eastern Alps. Based on direct geological observations the data can help to directly constrain earthquake hazards, which are otherwise only indirectly constrained by geophysical data.