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Volcanic calderas are delimited by a ‘caldera wall’ which can be several hundred meters in height. This represents the degraded scarp of a fault that accommodates roof subsidence. Here, we assess the roles of friction and cohesion on caldera wall morphology by: (i) analysing the slope properties of several young natural calderas in the ALOS-3D global digital surface model (DSM), and (ii) comparing those observations to the results of a text-book analytical solution and of new Distinct Element Method (DEM) modelling.Our analysis of the DSM suggest that caldera wall heights are not as closely linked to slope angle as previously suggested. Slope angles range from 20 – 65° and slope heights range from 99 m - 1085 m. We find that the smaller slope heights are not robustly tied to greater slope angle. When compared to analytical predictions, these slope-height data yield expected rock mass cohesion values of less than 0.25 MPa for all calderas, which is 2-3 orders of magnitude less than typical laboratory-scale values.The DEM models explicitly simulated the process of progressive caldera collapse, wall formation and destabilisation, enabling exploration of the emergence of slope morphology as a function of increasing subsidence and of mechanical properties. Results confirm that low bulk cohesion values

The reactivation of faults can occur when the effective stresses acting on them are perturbed. In some cases man-made changes in effective stress can result in fault reactivation that can have enormous impacts including loss of integrity of underground storage facilities. Current practical methods for making this assessment are generally based on shear stresses calculated over fault surfaces. Depending on the absolute or relative magnitudes of these resolved shear stresses, which depend primarily on the local orientation of the fault surface relative to the regional stress field, different faults or parts of faults are said to be closer or further from the Coulomb failure envelope and are therefore more likely to slip due to changes in effective stress. This proximity to failure/slip is referred to as the slip tendency or reactivation tendency.Although the slip/reactivation tendency approach is firmly grounded in Coulomb theory and laboratory experiments, there may be issues applying it to the reactivation of irregular fault surfaces. A key assumption of the approach is that an area of a fault surface can be treated in isolation so that the slip tendency can be evaluated once its orientation and frictional properties are known. However, it is well established that fault surfaces are not planar but often have highly irregular geometries and fault rock distributions so that the likelihood that a particular part of a fault will reactivate must also depend, not only on the properties at that point but also on adjacent areas of the fault surface.To investigate the significance of fault surface irregularity for the evaluation of fault slip/reactivation tendency, we conduct numerical modelling of fault reactivation resulting from an increase in pore pressure within a normal faulting stress regime. The modelling employs a form of the Discrete Element method that uses rigid blocks. This approach provides for both accurate representation of the geometry and frictional properties of the irregular slip surfaces and also failure in the surrounding wall-rock and is capable of modelling the variety of ways in which slip may initiate on, or adjacent to an irregular fault.

Although the origin of outer-arc extension fractures in folded sequences is well-understood and documented in many natural examples, geometric and geomechanical factors controlling their spacing are hitherto unexplored. This study investigates the formation of bending-induced tensile fractures during constant-curvature forced folding using two-dimensional Distinct Element Method (DEM) numerical modelling. The DEM model comprises a central brittle layer embedded within elastic layers; the layer interfaces are cohesionless. Folding of this three-layer system is enforced by a velocity boundary condition at the model base, while a constant overburden pressure is maintained at the model top.The models illustrate several key stages of fracture array development: (i) Prior to the onset of fracture, the neutral surface is located midway between the layer boundaries, consistent with pure bending; (ii) Once the outer-fibre stress equals the tensile strength of the layer, fractures nucleate and propagate through the brittle layer; (iii) The rate of fracture formation as a function of curvature decreases nonlinearly, with new fractures developing approximately midway between two existing fractures; (iv) Eventually no new fractures form, irrespective of any further increase in fold curvature, a state referred to as fracture saturation.On the basis of these numerical model results, an approximate analytical solution for fracture spacing based on classic beam theory is developed. The predicted range of fracture spacing as a function of fold curvature is in good agreement with the numerical model results. Importantly, the analytical solution reveals which geometric and geomechanical factors control fracture spacing, namely layer thickness, radius of curvature, Young’s modulus, tensile strength and confining pressure. The fracture spacing to layer thickness ratio at saturation however depends only (nonlinearly) on the ratio of tensile strength to overburden pressure.The numerical model results are qualitatively compared with field observations at outcrops located in the Montpellier Fold region. The folded lithologies are of Jurassic age and comprise (brittle) limestones, with (ductile) marl intercalations. Fracture-bound limestone blocks located within the fold hinges and observed on the fold-profile plane are laterally bound by V-shaped veins that thin towards the fold core. In the inner-arc of each vein-bound limestone block, the marl interbed thickens towards the veins, whereas in the outer-arc it thins towards the veins. Clearly, the thickness distribution of the marl interbeds reflects non-uniform loading, which is consistent with the loading conditions hypothesised on the basis of the theoretical models.

Stylolites are common microstructures in rocks, where mineral dissolution in a fluid localises in more or less discrete seams. Stylolite formation strongly affects rock porosity, pore connectivity and thus fluid flows. However, details of porosity evolution associated with stylolite nucleation, propagation and growth remain unclear, leading to the debate whether stylolites are conduits or barriers for fluids.In this contribution, we use an exceptionally large high-resolution SEM-BSE mosaic (102,600 × 18,239 pixels, 0.17µm/pixel) to investigate the detailed microstructures of stylolites from the Eilean Dubh limestone of the NW Highlands in Scotland. Advanced image analyses indicate that porosity self-organises systematically around stylolites suggesting a four-stage growth process for stylolites: In stage 1, primary pyrites in the rock matrix concentrate stress and dissolution, leading to the formation of porosity. In stage 2, the dissolution porosity self-organises, triggering a chemical-hydraulic-mechanical feedback loop that facilitates further dissolution. In stage 3, the porous zones enlarge and grow along the direction of the smallest principal stress (process zone), concentrating the synkinematic precipitation of pyrites from an external fluid in the core (core zone). In stage 4, the isolated domains connect forming a stylolite with a core zone in the center (highest porosity) surrounded on both sides by a process zones (higher-than-matrix porosity), suggesting a conduit phase during stylolite formation. Since the stylolite-hosting Eilean-Dubh limestones were episodically imbricated below the Ullapool thrust and the stylolites formed sub-parallel to the internal thrust planes, we speculate that synkinematic fluids during the stylolite formation were pumped episodically into the rock during activity of the Ullapool thrust. The detailed stylolite microstructures and the proposed process for stylolite growth suggest that stylolite in carbonate may act as a conduit for a fluid flow.

Time-lapse photogrammetry in conjunction with high-resolution digital elevation models is used to quantify the surficial velocity field and the ablation of the Pasterze, a rapidly retreating alpine valley glacier in Austria. Three automatic time-lapse cameras were installed along the orographic left valley flank, c. 150 m above the glacier’s surface, to monitor retreat and ablation from July to September 2020. Although time-lapse photogrammetry offers spatial and temporal high-resolution data, the various processing steps to calculate the glacier’s velocity field are challenging. Digital image correlation of the time-lapse series photos is achieved using a published Python library (How et al. (2020) PyTrx: a Python-based monoscopic terrestrial photogrammetry toolset for glaciology. Frontiers in Earth Science 8:21, doi:10.3389/feart.2020.00021), which was adapted for the Pasterze data set. Typical time intervals for the digital image correlation were two to ten days. Factors that hamper computing flow velocities from time-lapse series photos alone are low contrast of the glacier’s surface and ablation rates exceeding the horizontal flow velocity. The latter problem is solved with the aid of two high-resolution digital elevation models (DEMs), which were calculated using 790 drone images from flight missions on July 13 and September 15 2020. The photogrammetric (Structure from Motion) software Agisoft Metashape (v. 1.8.4) is used to calculate the two DEMs from dense point clouds with a resolution of 5 cm/pixel. Linear interpolation of the glacier’s elevation between the July and September DEM was used to provide an approximate surface for a given date within the monitoring period. With this additional processing step, we can project tracked pixels from the digital image correlation process on an adapted DEM, which reflects the absolute height on a certain date best. Our workflow illustrates that time-lapse series photos taken obliquely to the glacier’s surface can be used to compute the surficial velocity field with the aid of digital elevation models that yield the glacier’s surface at the beginning and at the end of the monitoring period. In fact, the glacier’s velocity field computed in that fashion is consistent with direct measurements from a sparse network of stakes and mapped structures.

The Gepatschferner in the Öztal Alps is Austria’s second largest glacier and is the subject of a monitoring campaign from 2019 onwards which was initially focussed on sedimentology and geomorphology of the forefield. Emphasis was placed on the styles and rates of sediment cannibalisation, with implications for transcription of the evidence into the deep time sedimentary record. This included the mapping of flutes, crag and tail structures, roches moutonées, fluvial sediments, till and rockfall deposits in the proglacial area. Their evolution over time is documented by repeated fieldwork and drone surveys. However, cognizant of the complexity of the subglacial environment (deforming bed areas, rigid bed areas and shifting meltwater systems) our work has expanded to ground-penetrating radar (GPR) surveys, enabling us to map subglacial conduits, englacial channels, and glacier structure. This structure involves the mapping of foliation, folds and fractures in the glacier, supported by field measurements. Repeated survey of both GPR and drones allows the 4D evolution of surficial glacier drainage, elevation, and forefield to be characterised. This work thus encompasses sedimentology, geomorphology, structural glaciology and bedrock geology. We argue that investigating the temporal and spatial landsystem-scale interactions between cryosphere (glacier and its structure), hydrosphere (meltwater pathways), and lithosphere (geomorphology, bedrock geology, sedimentology) will lead to breakthrough interpretations. These will include (i) controls on the evolution of the meltwater system, (ii) controls on the genesis of subglacial bedforms, (iii) the relationship between geology, geomorphology and glacier structure. Repeated, iterative surveys allow us to explore the teleconnections between cryosphere, hydrosphere and lithosphere, and their predictive capacity.