Plate-like behavior in mantle convection models is commonly obtained using a strongly temperature-dependent viscosity together with a low yield stress that caps lithospheric strength. Yet, alternative mechanisms for limiting strength, including low-temperature plasticity, have been proposed. Here, we investigate how different rheological formulations promote lithospheric-scale strain localization, using 2-D thermo-mechanical simulations of upper mantle extension. We test a suite of olivine flow laws (diffusion creep and dislocation creep at low- and/or high-temperature), with and without a yield-stress cap, and introduce diagnostics that attribute plate weakening to either increasing strain rate (mechanical weakening) or increasing temperature (thermal weakening). Without a yield-stress cap, deformation remains distributed in all cases, indicating that weakening of the whole lithosphere is required for strain localization. In such simulations, a new extensional plate boundary develops in two stages: progressive narrowing of the deforming zone followed by rapid thinning of the weakened lithosphere. Across plate ages of 10–100 Myr, localization depends weakly on age but strongly on divergence rate, primarily through Stage-1 narrowing, and is controlled more by the stiffest lithospheric region than by the presence of a shallow weak layer. Including dislocation creep allows both mechanical and thermal weakening to operate within the deforming plate, enhancing feedbacks and accelerating localization relative to yield-stress plus diffusion creep. This implies that yield-stress/diffusion-only rheologies may overestimate break-up timescales in whole-mantle convection models. Low-temperature dislocation creep weakens the plate at 800–1000 K, providing a physically grounded mechanism for limiting strength in this temperature range. Finally, we propose that thermal weakening may in some cases promote rapid strength loss and accelerated extension during the late stages of natural rift evolution.
Brittle damage around tunnels in the form of spalling is common in massive crystalline rocks subjected to high in-situ stresses. Understanding the short-term development and long-term evolution of the Excavation Damage Zone (EDZ) is particularly important for deep geological repositories (DGRs) for heat-generating nuclear waste, as the EDZ may provide preferential pathways for radionuclide migration. This paper presents an overview of the PRECODE mine-by experiment at BedrettoLab, a newly developed underground research laboratory (URL) in southern Switzerland designed to investigate brittle fracturing and EDZ evolution in crystalline rocks under repository-relevant conditions. The PRECODE tunnel was excavated in Rotondo Granite using both controlled drill-and-blast (D&B) and line-drilling and rock-breaking (LDRB) methods to evaluate the influence of excavation technique on EDZ characteristics. The experiment combines multidisciplinary in-situ monitoring including acoustic emission, distributed acoustic and strain sensing, hydraulic monitoring, electrical resistivity tomography, and terrestrial laser scanning with laboratory-based fracture mechanics and time-dependent testing under varying environmental conditions. The main objectives are to characterize short-term EDZ formation, investigate long-term crack growth and stress corrosion processes, quantify permeability evolution, and assess the interaction between excavation-induced damage and nearby fault zones. Preliminary results indicate clear differences in AE activity and EDZ development between excavation methods, while observations from the mature EDZ of the more than 40-year-old Bedretto Tunnel suggest ongoing time-dependent brittle fracturing. The coupled in-situ and laboratory datasets provide a unique basis for improving predictive models and long-term safety assessments for nuclear waste repositories in crystalline rocks.
In the present work, the elastic properties of both 1 M and 2 M 1 phlogopite polytypes, KMg 3 Si 3 AlO 10 (OH) 2 (monoclinic crystal system) were investigated from PV equation of state fitting and by analysis of the fourth-rank elastic tensor. The analysis was performed within the Density Functional Theory framework, using all-electron Gaussian-type orbitals basis sets and the B3LYP functional corrected a posteriori to include long-range interactions (B3LYP-D*). In general, the elastic properties of the two polytypes were strongly anisotropic, with the axial moduli ratio M ( a ) : M ( b ) : M ( c ) being close to 4:4:1. The volume-integrated third-order Birch-Murnaghan equation of state fitting parameters at 0 K were K 0 =57.9(2) GPa, K′=8.29(7) and V 0 =489.82(3) Å 3 for phlogopite-1 M , which were very close to those of the 2 M 1 polytype, i.e., K 0 =58.3(1) GPa, K′=8.71(8) and V 0 =978.96(9) Å 3 . The monoclinic elastic tensors obtained for the two polytypes of phlogopite, which have never been experimentally reported for both minerals so far, were in line with the PV behaviour of the mineral, providing further data related to the directional dependence of the elastic properties and seismic wave propagation. The elastic properties from both PV hydrostatic compression and from the elastic moduli tensor were discussed against the available experimental and theoretical data in the scientific literature, extending the knowledge on this important trioctahedral phyllosilicate.
Bathymetry (ocean-bottom topography) beneath ice shelves is challenging to observe yet is vitally important for modelling how ice sheets will evolve into the future. An alternative to direct observation of bathymetry is to invert airborne gravity data for the bathymetric signal. Appropriate gravity data can be collected via remote sensing above the ice shelf and be used to provide an initial estimate of sub-ice shelf bathymetry, typically at wavelengths of ∼5 km and above. However, lateral variations in density associated with the underlying geology can distort the gravity field biassing the results. We show that techniques which tie inversion results to known bathymetry and topography, although solving some of these issues, may be insufficient in the case of large and deep basins lacking centrally located tie points. Using new direct bathymetry observations of the Dotson and Crosson ice shelves as a case study, we show that gravity inversion for bathymetry can be improved by considering and removing a model of the gravity field due to crustal isostatic compensation prior to inversion. We finally present our updated and improved bathymetric model for the Dotson-Crosson and Thwaites Glacier Ice Shelf system and discuss where our method can be best applied in future.
Gas hydrates and deltaic deep-sea fans are main features in continental margin systems, influencing slope stability, fluid migration, and carbon cycling. In the northwestern Black Sea, the Danube Fan remains poorly constrained with respect to subsurface structure, sediment strength, and hydrate dynamics. Here, we present high-resolution multichannel seismic (MCS) and ocean-bottom seismometer (OBS) data to characterise sedimentary structure and fluid-related features. Two integrated OBS–MCS profiles reveal under-consolidated, clay-rich levee deposits interspersed with mass-transport units, chaotic facies, and gas-related anomalies. Derived P- and S-wave velocity models indicate low shear strength and high Vp/Vs-ratios in shallow units, consistent with soft, water-saturated sediments. Deeper layers display compaction-driven velocity increases but remain mechanically weak, rendering the slope prone to failure. Our findings suggest that vertical gas migration is widespread, expressed by seismic chimneys, polarity reversals, and velocity pull-downs, with free gas confined below bottom simulating reflectors and in stratigraphic traps. Hydrates likely occur as sparse, patchy pore-filling accumulations, and the lack of S-wave velocity anomalies suggests they do not act as cementing phases, implying little direct influence on sediment strength or slope stability. The hydrate system appears hydrate-poor, possibly reflecting post-glacial re-equilibration. Overall, lithology, gas migration pathways, and high sedimentation rates emerge as primary controls on hydrate formation and slope instability in the Danube Fan.
