High-energy bow tie multi-pass cells for nonlinear spectral broadening applications

Multi-pass cells have emerged as very attractive tools for spectral broadening and post-compression applications. We discuss pulse energy limitations of standard multi-pass cells considering basic geometrical scaling principles and introduce a novel energy scaling method using a multi-pass cell arranged in a bow tie geometry. Employing nonlinear pulse propagation simulations, we numerically demonstrate the compression of 125mJ, 1 ps pulses to 50 fs using a compact 2 m long setup and outline routes to extend our approach into the Joule-regime.

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.

Analogue modelling of strike-slip tectonics from basin to structural-scale comparing silica sand and new rock-analogue materials

<p>Strike-slip fault zones commonly display complex 3D geometries, with high structural variability along strike and with depth and their architecture and evolution are difficult to analyse. In this regard, analogue modelling represents a powerful tool to investigate the structural, kinematic and mechanical processes in strike-slip fault systems with variable scales. In detail, dynamically scaled experiments allow the direct comparison between model and nature. The geometrical scale factor defines the model resolution, in terms of model/prototype length equivalence, and depends on the physical properties of prototype and model material. Therefore, the choice of the analogue material is critical in scaled analogue experiments.<br>Granular materials like dry silica sand are ideal for the simulation of upper crustal deformation processes due to similar non-linear strain-dependent deformation behaviour of granular flow and brittle rock deformation. Comparing the geometrical scaling factor of the common analogue materials applied in tectonic models, we identified a model resolution gap for the simulation of fault-fracture processes corresponding to the structural scale (1 m – 100 m) observed in fault zones and damage zones in outcrops, field studies or subsurface well data. We developed a new Granular Rock-Analogue Material (GRAM) for the simulation of fault-fracture processes at the structural scale. GRAM is an ultra-weak sand aggregate composed of silica sand and hemihydrate powder capable to deform by tensile and shear failure under variable stress conditions. Based on dynamical shear tests, the new GRAM is characterised by a similar stress-strain curve as dry silica sand and has a geometrical scaling factor L<sup>*</sup>= L<sub>model</sub>/L<sub>nature</sub> = 10<sup>-3</sup> (1 cm in model = 20 m in nature).<br>We performed strike-slip experiments at two different length scales, applying as model material dry silica sand and the new GRAM. Digital Image Correlation (DIC) time-series stereo images of the experiments surface allowed the comparison of the developed structures at different stages of dextral displacement above a single planar basement fault. The analysis of fractures localisation and growth in the strike-slip zone with displacement and strain components enabled the comparison of the different structural styles characterising dry silica sand and GRAM models. The application of the developed GRAM in scaled experiments can provide new insights to the multi-scale investigation of complex deformation processes with analogue models. </p>

Box canyon erosion along the Canterbury coast (New Zealand): A rapid and episodic process controlled by rainfall intensity and substrate variability

Box canyon formation has been associated to groundwater seepage in unconsolidated sand to gravel sized sediments. Our understanding of box canyon evolution mostly relies on experiments and numerical simulations, and these rarely take into consideration contrasts in lithology and permeability. In addition, process-based observations and detailed instrumental analyses are rare. As a result, we have a poor understanding of the temporal scale of box canyon formation and the influence of geological heterogeneity on their formation. We address these issues along the Canterbury coast of the South Island (New Zealand) by integrating field observations, optically stimulated luminescence dating, multi-temporal Unmanned Aerial Vehicle and satellite data, time-domain electromagnetic data, and slope stability and landscape evolution modelling. We show that box canyon formation is a key process shaping the sandy gravel cliffs of the Canterbury coastline. It is an episodic process associated to groundwater flow that occurs once every 227 days on average, when rainfall intensities exceed 40 mm per day. The majority of the box canyons in a study area SE of Ashburton has undergone erosion, predominantly by elongation, during the last 11 years, with the most recent episode occurring 3 years ago. The two largest box canyons have not been eroded in the last 2 ka, however. Canyons can form at rates of up to 30 m per day via two processes: the formation of alcoves and tunnels by groundwater seepage, followed by retrogressive slope failure due to undermining and a decrease in shear strength driven by excess pore pressure development. The location of box canyons is determined by the occurrence of hydraulically-conductive zones, such as relict braided river channels and possibly tunnels, and of sand lenses exposed across sandy gravel cliff. We also show that box canyon formation is best represented by a linear diffusive model and geometrical scaling.

Geometrical Scaling Relations of Drainage Basins During Basin Evolution

<p>Fluvial drainage systems are organized in drainage basins, whose boundaries are defined by water divides. The network of divides determines the geometry of the basins and the distribution of drainage area along flow. Drainage basins obey global geometric-geomorphic scaling relationships. These include Hack’s law that predicts the relation between channel length (L) and drainage area (A): L = c∗A<sup>h</sup>  where c and h are referred to as Hack’s coefficient and exponent, respectively. These parameters have a relatively narrow range of 1.1 ≤ c ≤ 2.7 and 0.45 ≤ h ≤ 0.6. Additionally, the distance between basin outlets (S) has been shown to scale linearly with the distance between the main divide and the mountain front (W) and is expressed by the ratio: R = W ⁄ S , where R is within the range of 1.91 ≤ R ≤ 2.23. When the tectonic and climatic conditions change through time, drainage basins can change their geometry. It is not clear, however, if and how the global scaling relations evolve when basins change their shape and size. This gap in our understanding specifically relates to the links between geomorphic processes and surface forms. A promising approach to study fluvial landscape evolution is by using physical laboratory-scale models. These models provide a unique opportunity to study the details of drainage network evolution and geometrical changes by constraining climate and uplift and by maintaining the lithological parameters constant and uniform. In the current study, we utilize DULAB (Differential Uplift LAndscape-evolution Box), an experimental apparatus that simulates mountainous landscape evolution, to study the evolution of basin geometrical scaling relations. Our experimental scheme consists of two distinct settings: (1) uniform uplift, with basins that grow by incising backward towards an uplifting and shrinking plateau, and (2) differential uplift, where the main drainage divide migrates towards the higher uplift rate side, and the drainage basins adjust accordingly. During the experiments, precipitation is held constant, and we document the landscape geometry in predefined time intervals by applying a “Structure from Motion” algorithm on a series of photos. Experimental results show that while basins drastically change their size and shape, they tend to maintain the globally observed geometrical scaling relations. Hack’s parameters are computed to be c = 2.29 ± 0.08  and h = 0.51 ± 0.02 and the spacing ratio, R is R = 2.95 ± 0.4. This is achieved as only a subset of basins grow towards the migrating divide, while other basins maintain their former geometry or shrink. Additionally, processes of reorganization, such as basins merging close to their outlets and inter-basin divide migration, assist in maintaining the geometrical scaling relations.</p>