The architecture of the crust in intracontinental Western and Central Europe is well constrained by multidisciplinary geoscientific data. This low-strain intraplate setting is known for its widely distributed seismicity with earthquake localization, however, being difficult to explain by the observed crustal configuration. Also, observed variations in crustal thickness do not provide clear evidence to explain lateral shifts in depositional and erosional centers over geological time. This raises questions regarding the underlying forces controlling crustal tectonics in this region, located far from active plate boundaries. Shear-wave velocity models obtained from seismic full waveform inversion methods show that the upper mantle is strongly heterogeneous pointing to thermomechanical contrasts that potentially could impact crustal tectonics. Therefore, we convert mantle shear-wave velocities to thermodynamically consistent temperature and density configurations by following a Gibbs's free energy minimization approach. We find spatial correlations between lithospheric thickness, respectively shallow lithospheric temperature and density variations, and crustal deformation patterns (including seismicity). This indicates that thermomechanical instabilities in the mantle could be the origin of relative vertical movements which would (i) cause laterally variable surface uplift and/or subsidence and (ii) facilitate strain localization in the mantle (ductile shear movements) above which the overlying crust would locally respond by brittle deformation.

The Asse salt structure is a salt-cored anticline with steep, locally overthrusted flanks located in the Subhercynian Basin. It is an excellent example of salt structures in the North German Basin containing a wedge-shaped intrusion of Upper Permian into Upper Triassic salt (‘salt wedge’). The Asse is also known as a location for the disposal of low- and intermediate-level radioactive waste emplaced in the former salt mine Asse II during the 1970s. Detailed knowledge of the tectonic structures is indispensable for planning new retrieval infrastructure and for long-term safety analysis. The Asse salt structure has been thoroughly explored for over 100 years by surface geological mapping, 2D seismic and drilling. Ongoing exploration provides further insights into the salt structures and strengthens our understanding of its evolution.

A new 3D seismic data set (2019-2020) provides new insights into the details of this salt structure. Here we present first results of the seismic interpretation revealing substantial changes in the structural style opposing previous geological models. It can be shown that the sub-horizontal northern flank terminates at the base of the southern flank implying a north-ward instead of a south-ward directed overthrusting. Small-scale faults crossing the crest were previously interpreted as transpressional faults developed during the Late Cretaceous inversion. Ongoing kinematic modeling suggests that these faults originated as steeply dipping pre-Cretaceous normal faults that were overprinted during flank-rotation. These implications as well as observations of thickness variations and unconformities in Mesozoic layers will help to reconstruct the evolution of the Asse salt structure.

Salt rocks are mechanically weak and behave like viscous fluids when deforming at geological time scales and strain rates. The weight of an ice sheet advancing into a salt-bearing basin may cause sufficient differential load to induce salt flow. Ice loading has been postulated as a trigger for Pleistocene deformation at a number of salt structures in the Central European Basin System. We conducted 2D-finite-element models (ABAQUS) with a setup representing a simplified salt diapir to test existing conceptual models and evaluate the controlling factors. Different parameter sets for the rheology of salt and overburden rocks, including linear versus non-linear viscosity of the salt, were tested. Model results show lateral salt flow into the diapir and diapiric rise during the ice advance, while a transgression of the diapir by the ice sheet leads to overall downwards displacement. During unloading, displacements are largely restored due to the dominance of the elastic response. Displacements never exceed few metres and are always larger in models with linear viscosity than in those with non-linear viscosity. Linear viscous salt behaviour seems reasonable, considering the low differential stresses caused by the load of a few hundred-metres-thick ice sheet and the time-scale of several thousand years. The elastic parameters also have a strong impact, with lower Young's moduli leading to larger displacements. Our findings demonstrate that both the viscosity and the elasticity exert a fundamental control on ice-load driven salt movement during glacial-interglacial cycles and highlight the importance of a careful parameter choice in numerical modelling.

The lateral collapse of oceanic volcanoes poses a high risk for the population living in coastal areas since the sudden displacement of large amount of material can trigger tsunami waves impacting the surrounding coastlines. One recent example is the lateral collapse of the SW-flank of Anak-Krakatau (Sunda Strait, Indonesia) in December 2018 that generated a tsunami wave impacting the Sunda Strait coastlines and causing several hundred fatalities. Even though, the lateral collapse of oceanic volcanoes are hazardous events, the precursors of such events are poorly understood. It is suggested that external triggers such as the movement of a décollement, the rise of magma during enhanced activity, or earthquakes can cause a lateral collapse. Yet, the internal state of stability of a volcano needs to be known to evaluate the impact of external triggers. We carry out direct shear tests on samples from Anak-Krakatau and implement the results into finite-element models to evaluate the influence of the volcano’s geometry and the rock mechanical properties on the stability of Anak-Krakatau’s flank before the collapse in 2018. The preliminary results suggest that the volcanic edifice of Anak-Krakatau was unstable before the collapse in 2018 solely due to the geometry of the volcano and the rock mechanical properties. Whether the instability of the volcanic edifice is enough to cause the lateral collapse of Anak-Krakatau in 2018 or whether an external trigger is needed, needs further testing.

There is ample evidence of transient high stresses of several hundred MPa at the base of the seismogenic zone in the continental crust, i.e. at greenschist-facies conditions. The microstructural evidence from these depths includes twinning and kinking of jadeite and amphibole, as well as quasi-instantaneous cataclastic deformation of garnet and quartz. At interseismic strain rates, known flow laws for dislocation creep of the rheological dominant mineral, quartz, and/or dissolution precipitation creep of crustal rocks predict lower stresses at the given pressure-temperature conditions. Thus, fast stress-loading rates are required to explain the inferred high-stress crystal-plasticity at greenschist-facies conditions, i.e. loading rates from few tens of MPa to several hundred of MPa within minutes, corresponding to the rupture times for major earthquakes in the seismogenic zone. Although high-stress crystal-plasticity is not allowing to accumulate a high amount of strain, as high stresses prevail only transiently, it provides a driving force for accelerated but rapidly decaying creep, where higher amounts of strain can be accumulated. The strength of both, fault rocks and their host rocks, is strongly depending on the stress conditions that control whether they behave by high-stress crystal-plasticity or creep at given pressure-temperature conditions. Thus, the rheology of crustal rocks is dependent on the stress-loading rates during the seismic cycle controlled by the distance to the tip of the seismic active fault.