New granular rock-analogue materials for simulation of multi-scale fault and fracture processes

In this study, we present a new granular rock-analogue material (GRAM) with a dynamic scaling suitable for the simulation of fault and fracture processes in analogue experiments. Dynamically scaled experiments allow the direct comparison of geometrical, kinematical and mechanical processes between model and nature. The geometrical scaling factor defines the model resolution, which depends on the density and cohesive strength ratios of model material and natural rocks. Granular materials such as quartz sands are ideal for the simulation of upper crustal deformation processes as a result of similar nonlinear deformation behaviour of granular flow and brittle rock deformation. We compared the geometrical scaling factor of common analogue materials applied in tectonic models, and identified a gap in model resolution corresponding to the outcrop and structural scale (1–100 m). The proposed GRAM is composed of quartz sand and hemihydrate powder and is suitable to form cohesive aggregates capable of deforming by tensile and shear failure under variable stress conditions. Based on dynamical shear tests, GRAM is characterized by a similar stress–strain curve as dry quartz sand, has a cohesive strength of 7.88 kPa and an average density of 1.36 g cm−3. The derived geometrical scaling factor is 1 cm in model = 10.65 m in nature. For a large-scale test, GRAM material was applied in strike-slip analogue experiments. Early results demonstrate the potential of GRAM to simulate fault and fracture processes, and their interaction in fault zones and damage zones during different stages of fault evolution in dynamically scaled analogue experiments.

Orogen-scale transpression accounts for GPS velocities and kinematic partitioning in the Southern Andes

The Southern Andes are regarded as a typical subduction orogen formed by oblique plate convergence. However, there is considerable uncertainty as to how deformation is kinematically partitioned in the upper plate. Here we use analogue experiments conducted in the MultiBox (Multifunctional analogue Box) apparatus to investigate dextral transpression in the Southern Andes between 34 °S and 42 °S. We find that transpression in our models is caused mainly by two prominent fault sets; transpression zone-parallel dextral oblique-slip thrust faults and sinistral oblique-slip reverse faults. The latter of these sets may be equivalent to northwest-striking faults which were believed to be pre-Andean in origin. We also model variable crustal strength in our experiments and find that stronger crust north of 37 °S and weaker crust to the south best reproduces the observed GPS velocity field. We propose that transpression in the Southern Andes is accommodated by distributed deformation rather than localized displacements on few margin-parallel faults.

Controlling friction and wear with anisotropic microstructures in MoN coated surfaces

Steel surfaces have been patterned by laser surface texturing (LST) to create anisotropic microstructures with typical dimensions of a few 10 micrometers. The subsequent deposition of thin molybdenum nitride coatings by high power impulse magnetron sputtering (HiPIMS) leads to surfaces that conserve the surface microstructures and exhibit an extraordinarily large resistance against mechanical wear. Tribological experiments with steel counter bodies show a substantial influence of the relative orientation of structures and wear track on friction and wear. It is pointed out that the main effect is the removal of abrasion particles from the mechanical contact. Analogue experiments with alumina counter bodies that hardly provide wear particles show that the orientation effect is absent.

Internal deformation of the Dolomites Indenter, eastern Southern Alps: An integrated field, thermochronology and physical analogue modelling approach

<p>The Dolomites Indenter (DI) represents the front of the Neogene to ongoing N(W)-directed continental indentation of Adria into Europe. Deformation of the DI is well studied along its rim, documented by important fault zones as, e.g., the Periadriatic fault system (PFS), the Giudicarie belt, and the Valsugana and Montello fault systems. With this study, we aim to investigate the internal deformation of the DI and its eastern continuation towards the Dinarides including the interference of Dinaric SW-directed and Alpine SE-directed folds and thrusts. What also remains unsolved at present is the relationship between deep-seated mantle dynamics and their control on the geometry and internal deformation of the DI. Our approach to unravel this tectonic history is a combination of (i) compilation and acquisition of detailed structural field data within the DI, (ii) collection of a new and comprehensive low-temperature thermochronological dataset covering the entire DI, and (iii) crustal- to lithospheric scale physical analogue experiments.</p><p>The existing but limited thermochronological dataset already indicates the presence of relative vertical motions within the DI after the onset of indentation, including mostly Miocene Apatite fission track (AFT) ages along the PFS and the Valsugana fault and two age clusters of Triassic to Jurassic AFT data. One cluster represents the Monti Lessini east of Riva del Garda, the second is located SE of Bozen, in the footwall of the Truden line. Are these Mesozoic AFT age clusters resulting from tectonic vertical movements and/or are they linked to inhomogeneities within the DI, like the Mesozoic platform-basin geometries or the Permian Athesian Volcanic Complex? Ongoing thermochronological investigations aim to clarify these issues.</p><p>By using crustal-scale (as a first step) physical analogue models, we aim to study (i) the impact of Jurassic E-W extension and (ii) the effect of crustal strengthening on the NW-SE directed deformation of the DI since Neogene times. Jurassic NNE-SSW trending normal faults led to a platform-basin-topography resulting, from west to east, in the Lombardian basin, Trento platform, Belluno basin, and Friuli platform (Winterer & Bosellini, 1981) but were inverted during Alpine orogeny. Moreover, the Trento platform approximately coincides with the extent of the up to ~2 km thick (Avanzini et al., 2013) Permian Athesian Volcanic Group. We simulate rigid Permian magmatic rocks, which could have led to a critical strengthening of the crust, in our analogue experiments by incorporating an additional strong domain to the lower upper crust. This, together with studying the influence of structural inheritance on the geometry and kinematics of Dinaric and Alpine deformation in the Southern Alps, allows us to model various deformational styles and -wavelengths of the DI during Neogene indentation.</p><p>This study will contribute substantially to the understanding of internal deformation and thus enable conclusions to be drawn about the processes at lithospheric scale also addressed by AlpArray.</p><p>References:</p><p>Avanzini, M. et al. (2013): Note illustrative della carta geologica d'Italia, foglio 026 Appiano. Roma, Servizio Geologico d'Italia, 324 pp.<br>Winterer, E. L., & Bosellini, A. (1981): Subsidence and Sedimentation on Jurassic Passive Continental Margin, Southern Alps, Italy. AAPG Bulletin, 65(3), 394-421.</p>

Insights into lava dome and spine extrusion using analogue sandbox experiments

<p>Lava dome formation is a common process at stratovolcanoes involving the shallow intrusion or extrusion of viscous lava and may lead to the rise of spines. Spines are protrusions observed to extrude episodically during lava dome growth, yet the structural and mechanical factors controlling their formation are only partially understood. Here, we provide new, detailed insight into lava dome growth and the production of spines using a novel set of analogue experiments extruding sand-plaster mixtures from a fixed-diameter conduit under isothermal conditions. We trace displacement and strain with photogrammetric methods for precise and detailed monitoring of the extrusion process. Results show initial dome growth forming a steep-sided and flat-topped shape through extrusion of new material, leading to slumping of oversteepening slopes, forming a talus. Spines are found to protrude at a later stage through the dome surface along discrete circular faults that originate from the conduit walls, starting a cycle of spine growth and collapse. As our spines only appear after prolonged extrusion, we relate their appearance to the compaction and strengthening of material within the conduit. We find that spine diameter, height and volume are positively correlated with increasing cohesion and therefore material strength. The spine diameter was also observed to be smaller or equal to the diameter of the underlying conduit, as shear extrusion occurs along vertical to outward-dipping fault planes. For natural domes, our findings imply that spine growth may be the consequence of compaction and densification via porosity loss, shearing and/or outgassing of conduit magma during ascent. More efficient compaction will yield wider and taller spines as a result of increasing rock strength. Our study further highlights the relevance of analogue experiments in the study of lava domes and spines, which remain one of the most hazardous and unpredictable features at dome-forming volcanoes worldwide.</p>

Clustering of Experimental Seismo-Acoustic Events Using Self-Organizing Map (SOM)

The analogue experiments that produce seismo-acoustic events are relevant for understanding the degassing processes of a volcanic system. The aim of this work is to design an unsupervised neural network for clustering experimental seismo-acoustic events in order to investigate the possible cause-effect relationships between the obtained signals and the processes. We focused on two tasks: 1) identify an appropriate strategy for parameterizing experimental seismo-acoustic events recorded during analogue experiments devoted to the study of degassing behavior at basaltic volcanoes; 2) define the set up of the selected neural network, the Self-Organizing Map (SOM), suitable for clustering the features extracted from the experimental events. The seismo-acoustic events were generated using an ad hoc experimental setup under different physical conditions of the analogue magma (variable viscosity), injected gas flux (variable flux velocity) and conduit surface (variable surface roughness). We tested the SOMs ability to group the experimental seismo-acoustic events generated under controlled conditions and conduit geometry of the analogue volcanic system. We used 616 seismo-acoustic events characterized by different analogue magma viscosity (10, 100, 1000 Pa s), gas flux (5, 10, 30, 60, 90, 120, 150, 180 × 10−3 l/s) and conduit roughness (i.e. different fractal dimension corresponding to 2, 2.18, 2.99). We parameterized the seismo-acoustic events in the frequency domain by applying the Linear Predictive Coding to both accelerometric and acoustic signals generated by the dynamics of various degassing regimes, and in the time domain, applying a waveform function. Then we applied the SOM algorithm to cluster the feature vectors extracted from the seismo-acoustic data through the parameterization phase, and identified four main clusters. The results were consistent with the experimental findings on the role of viscosity, flux velocity and conduit roughness on the degassing regime. The neural network is capable to separate events generated under different experimental conditions. This suggests that the SOM is appropriate for clustering natural events such as the seismo-acoustic transients accompanying Strombolian explosions and that the adopted parameterization strategy may be suitable to extract the significant features of the seismo-acoustic (and/or infrasound) signals linked to the physical conditions of the volcanic system.

Constraining the preservation of organic compounds in Mars analog nontronites after exposure to acid and alkaline fluids

The presence of organic matter in lacustrine mudstone sediments at Gale crater was revealed by the Mars Science Laboratory Curiosity rover, which also identified smectite clay minerals. Analogue experiments on phyllosilicates formed under low temperature aqueous conditons have illustrated that these are excellent reservoirs to host organic compounds against the harsh surface conditions of Mars. Here, we evaluate whether the capacity of smectites to preserve organic compounds can be influenced by a short exposure to different diagenetic fluids. We analyzed the stability of glycine embedded within nontronite samples previously exposed to either acidic or alkaline fluids (hereafter referred to as “treated nontronites”) under Mars-like surface conditions. Analyses performed using multiple techniques showed higher photodegradation of glycine in the acid-treated nontronite, triggered by decarboxylation and deamination processes. In constrast, our experiments showed that glycine molecules were preferably incorporated by ion exchange in the interlayer region of the alkali-treated nontronite, conferring them a better protection against the external conditions. Our results demonstrate that smectite previously exposed to fluids with different pH values influences how glycine is adsorbed into their interlayer regions, affecting their potential for preservation of organic compounds under contemporary Mars surface conditions.