How large are tectonic deviations from lithostatic pressure in a continent-derived, lithologically heterogeneous Alpine UHP nappe (Koralpe-Saualpe-Pohorje Complex, Austria and Slovenia)?

• Applicants: Professor Dr. Timm John; Dr. Jan Pleuger
• Subject Area: Palaeontology ; Mineralogy, Petrology and Geochemistry
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 442578564

The rheological properties control how rocks deform in a non-uniform stress field. Large differences in the strengths of neighbouring rocks inevitably lead to a deviation of the mean normal stress (i.e. the pressure) from lithostatic pressure. Several theoretical studies have shown that such deviations may reach hundreds of MPa in which case they may influence the petrological pressure record. Since pressures recorded by rocks are commonly used for estimating their burial depth according to a lithostatic pressure-to-depth conversion, such estimates may be biased. On the other hand, unambiguous evidence for tectonic deviations from lithostatic pressure is very scarce and their magnitude and scale at which they occur are poorly known. We propose a novel approach of evaluating the magnitudes by systematically comparing peak pressures of different rock types (mostly eclogites and metapelites) along an orogen-perpendicular transect through an Alpine ultrahigh-pressure nappe. For each rock type, we will determine peak pressure profiles as a function of distance along the transect. If these profiles coincide, there will be no evidence for significant tectonic pressures. If they show a systematic shift, this can be interpreted as an effect of the rheological properties of the different lithologies and can easiest be explained by tectonic pressures. Previous literature data suggest that this may indeed be the case in the Austroalpine Koralpe-Saualpe-Pohorje Complex. A second approach is to investigate if there are outcrop-scale structurally controlled tectonic pressure variations in the near field of rigid lenses and layers surrounded by a weaker matrix. We will investigate samples of the Koralpe-Saualpe-Pohorje Complex with a straightforward thermobarometric methodology that relies as little as possible on complex mineral reactions. If the rock record does not provide evidence for tectonic pressures, this would comply with geodynamic models applying lithostatic pressure-to-depths conversion. If they do, the possibility of tectonic deviation from lithostatic pressure should also be considered for other cases of lithologically diverse (ultra)high-pressure units, both in kinematic and geodynamic models. We will account for potential tectonic overpressure when constraining the maximum burial depth of the Koralpe-Saualpe-Pohorje Complex in a kinematic restoration of an orogen-parallel cross section. By this restoration we aim specifically at estimating the rates of motion of the Koralpe-Saualpe-Pohorje Complex in the Cretaceous Alpine subduction zone and the amount of continental crust that may have been subducted without returning to the surface. These estimates for the Eo-Alpine subduction zone may also give us an idea of the proportion of continental crust in slabs that were imaged below the Alps and are to be resolved in greater detail by seismological methods in SPP 2017.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2017: Mountain Building Process in Four Dimensions (4D-MB)
• Co-Investigators: Dr. Johannes C. Vrijmoed; Dr. Xin Zhong