Ribbed bedforms in palaeo-ice streams reveal shear margin positions, lobe shutdown and the interaction of meltwater drainage and ice velocity patterns

Conceptual ice stream landsystems derived from geomorphological and sedimentological observations provide constraints on ice-meltwater-till-bedrock interactions on palaeo-ice stream beds. Within these landsystems, the spatial distribution and formation processes of ribbed bedforms remain unclear. We explore the conditions under which these bedforms develop and their spatial organisation with (i) an experimental model that reproduces the dynamics of ice streams and subglacial landsystems and (ii) an analysis of the distribution of ribbed bedforms on selected examples of paleo-ice stream beds of the Laurentide Ice Sheet. We find that a specific kind of ribbed bedforms can develop subglacially from a flat bed beneath shear margins (i.e., lateral ribbed bedforms) and lobes (i.e., submarginal ribbed bedforms) of ice streams. These bedforms initiate where the ice flow undergoes high velocity gradients and the ice-bed interface is unlubricated. We suggest that (i) their orientation reflects the local stress state along the ice-bed interface and (ii) their development reveals distinctive types of subglacial drainage patterns below these two kinds of margins: linked-cavities and efficient meltwater channels respectively. These ribbed bedforms are thus convenient geomorphic markers to reconstruct palaeo-ice stream margins and constrain palaeo-ice flow dynamics and palaeo-meltwater drainage characteristics.

APPLICATION OF LOCAL SPECTRUM OF NATURAL OSCILLATIONS FOR MONITORING OF STRESS STATE IN NON-METALLIC ELEMENTS OF STRUCTURES

This work offers a method based on extraction of specially selected localized forms of oscillations from the general spectrum of frequencies of natural oscillations of a part by means of which non-destructive monitoring of stress state in various points of non-metallic parts is performed. Frequency spectrum is localized by use of special oscillation transducers with adjustable mechanic filter at the inlet before piezoelectric plate. Such filter allows both to extract localized frequency spectrum and to ensure adjustment of frequencies of natural oscillations of transducer rod wrt frequencies of natural oscillations of the part being monitored. The transducers operated as a part of acoustic impedance flaw detector «ДАМИ-С09». Stress variations at the points of the part were captured on the screen of the flaw detector by energy redistribution between the forms of natural oscillations of transducer rod. Captured energy redistribution between the forms of oscillations took place due to shift of frequencies of natural oscillations of the part during stress variation wrt to frequencies of natural oscillations of transducer rod and summing up of their amplitudes. Here, the amplitudes of some oscillations intensified and some subsided. The results which were experimentally gained during monitoring demonstrate variations of local stress state at the point on the perimeter of the square plate made of sphere plastic under effect of compressive stresses on the perimeter of the plate and in the centre. In the first case at the point of measurement “conditional compression” took place and in the second case it was “conditional tension”.

Exploring the effects of a time- and space-dependent eruption efficiency on planetary evolution.

<p>It has been shown that melting and crust production strongly influences the convection regime of terrestrial planets, potentially even more than the vigor of convection. A planet producing and erupting a lot of crust can hardly remain in the stagnant lid regime and produces resurfacings or even reaches some mobile-lid regime. On the other hand, a planet that intrudes its melt in the lithosphere tends to have a larger conductive heat flux and cools efficiently without much lid mobility. Thus, the question of the amount of melts being erupted or intruded might dominate the cooling of terrestrial planets. So far, an "eruption efficiency", which gives the ratio of melt that erupts over the remaining melt fraction, has been imposed in numerical simulations. The eruption efficiency in the convection code StagYY has thus far been treated as a constant in time and space. Here, we explore the effects of a time- and space-dependent eruption efficiency on planetary evolution in the planetary convection code StagYY. An equation was devised that describes how eruptive a system is, based on the main characteristics of lithospheric melt transport: the amount of melt and the local stress state. In a range of systematic simulations, we explore the consequences of this parameter. </p><p>In a first set of simulations this parameter is explored while keeping the eruption efficiency constant. Results show that the most important parameters are the amount of melt, where the stress has smaller local effects. Additionally, changing the yield stress, viscosity or constant eruption efficiency has a large effect on what the eruptivity should be based on this equation. Parameters that govern the global mantle temperature are less important for the eruptivity. </p><p>A second set of simulations was performed with the eruption efficiency behaving in a fully self-consistent manner. These models tend to behave like intrusive systems, except during resurfacing episodes when the models become very extrusive. Models that show mobile behaviour at almost all times in the planetary evolution will have an almost constant spatially averaged eruption efficiency. In these models the eruption efficiency does vary locally however.</p>

Multiscale analysis of interlaminar stresses near a free-edge in a [±45/0/90]s laminate

A multiscale model for a [±45/0/90]s tape laminate under uniaxial extension was used to investigate the effect of modeling the heterogeneous microstructure near a free-edge. A random fiber arrangement was used for the 0° and 90° plies and homogenized properties for the 45° and −45° plies. The predicted interlaminar normal stress was compared to the prediction using a classical homogeneous model. When fibers and matrix were modeled discretely, the local stress state was shown to be sensitive to the proximity of fibers, which caused a complex stress distribution along the 0–90 ply interface. Next, the effect of reducing the size of region modeled at the microscale was investigated, since this would significantly reduce the computational effort. Reducing the region modeled at the microscale in the direction normal to the 0–90 ply interface to a size that was 25% and 10% of the ply thickness only changed the peak stresses by 3% and 8%, respectively. Reducing the microscale region in the direction normal to the free-edge to be one and two ply thicknesses in size did not have a significant effect on the predicted interlaminar normal stress at points within 75% of a ply thickness of the free-edge.

Poor performance of a common crevasse model at marine-terminating glaciers

Crevasses are both affected by and effect stresses and surface mass balance of glaciers, potentially exerting important controls on meltwater routing, glacier viscosity, and iceberg calving, yet there are few direct observations of crevasse depth. Here we assess one of the most common models for crevasse formation, in which crevasse depths depend on the local stress state, through analysis of 52644 crevasse depth observations from 19 Greenland glaciers. We find that modeled depths are uncorrelated with observed depths and are generally too deep. Model performance can be improved with glacier-by-glacier tuning of viscosity and water depth parameters, but spatial variations in tuning parameters are unlikely to have a physical basis, and the model still fails to capture smaller-scale variations in crevassing that may control calving. Thus, numerical ice flow models drawing on this parameterization are likely to yield inaccurate projections of glacier mass change or crevasse depth-driven terminus position changes.