Evolution of passive margin escarpments: what can we learn from low-temperature thermochronology?

Recent studies integrating geomorphology, thermochronology, cosmogenic erosion rate estimates, and numerical modeling suggest that escarpment evolution may take place following two dramatically different modes: (1) parallel retreat from the escarpment's original position at the continent-ocean boundary to its present-day inland position and (2) formation-in-place by progressive downwearing of a plateau initially located between the coast and a preexisting inland drainage divide. Using a three-dimensional finite element model to solve the heat transfer equation, we show that the mode of migration of a passive margin escarpment can be constrained by low-temperature (apatite (U-Th)/He) thermochronology. We first couple the heat equation solver to a surface processes model that predicts the two different escarpment evolution modes from only slightly different initial conditions. We predict (U-Th)/He age distributions that are markedly different for the two scenarios. We perform a thorough investigation of the model behavior to determine under which circumstances thermochronological data can be used to constrain passive margin escarpment dynamics. These conditions include (a) a tall escarpment, (b) a high geothermal gradient, and/or (c) a low flexural rigidity of the lithosphere. We demonstrate that to determine the rate and mode of escarpment migration from low-temperature thermochronology, one needs to collect samples along transects perpendicular as well as parallel to the escarpment. Tightest constraints on escarpment development are provided by (in ascending order) the minimum (U-Th)/He age encountered seaward of the escarpment, the location of where the minimum age is found, the slope of the age-distance relationship (in a direction perpendicular to the coast), and the slope of the age-elevation relationship (from a transect parallel to the escarpment). We finally demonstrate that there are situations where thermochronological data sets do not provide constraints on the mode of escarpment migration, such as along the escarpment of southeastern Australia, where migration has possibly been very rapid. Using the Neighborhood Algorithm method, we are, however, able to extract from an existing apatite (U-Th)/He data set very useful constraints on the evolution of the southeastern Australian escarpment, including the duration of the migration event (<15 Myr), the local geothermal gradient (32°–40°C km−1), and the effective elastic thickness of the underlying lithosphere (6–8 km)

A quantification of the glacial imprint on relief development in the French western Alps

International audienceThe morphology of the western Alps has been strongly influenced by Quaternary glaciations. On the basis of observations of glacial morphology in the Belledonne, Grandes Rousses, Taillefer and Pelvoux-Ecrins Massifs (south-eastern France), we reconstitute the glacial trimline and Equilibrium Line Altitude (ELA) during the most extensive glaciation (MEG). Our best estimate of the MEG ELA is 1800 ± 100m. Using digital elevation models, we compare our glacial reconstruction with the relief structure of nine major catchments draining the massifs. Modal elevations of the largest catchments occur at 2000–2500m and coincide with minima in plots of mean slope angles as a function of elevation. Modal elevations and slope minima occur between the modern and MEG ELAs, confirming a strong glacial imprint on relief. In order to quantify glacial valley carving in the massifs, we isolated high-elevation, low-relief surfaces that form rock shoulders adjacent to the glacial valleys from a Digital Elevation Model and constructed an interpolated surface passing through these. Subtracting the present-day topography from this surface allows us to quantify the maximum glacial valley depths. Maximum valley depths determined in this manner are typically > 1000m, with spatial maxima occurring around the location of the MEG ELA in most valleys. These numbers do not take into account glacial valley widening and local glacial overdeepenings. The also neglect, however, potential pre-glacial fluvial valley incision, which could account for 20–50% of the measured valley depths. In spite of these problems, inferred valley depths are reasonably well correlated with the mean reconstructed ice thickness, and constitute about half of the sub-ridgeline relief of the studied catchments. These results lead us to propose a significant Quaternary increase in the relief of the French western Alps, controlled by climate and associated with the initiation of alpine glaciations. For reasonable values of the effective elastic thickness of the lithosphere, the isostatic response to glacial valley carving reaches values of not, vert, similar 300m across the massifs. This number is insufficient to substantially offset topographic lowering due to regional denudation, and we conclude that the isostatic response to glacial valley carving has not increased peak elevations significantly

Fluvial incision into bedrock: Insights from morphometric analysis and numerical modeling of gorges incising glacial hanging valleys (Western Alps, France)

International audienceBedrock gorges incising glacial hanging valleys potentially allow measurements of fluvial bedrock incision in mountainous relief. Using digital elevation models, topographic maps, and field reconnaissance, we identified and characterized 30 tributary hanging valleys incised by gorges near their confluence with trunk streams in the Romanche watershed, French Western Alps. Longitudinal profiles of these tributaries are all convex and have abrupt knickpoints at the upper limit of oversteepened gorge reaches. We reconstructed initial glacial profiles from glacially polished bedrock knobs surrounding the gorges in order to quantify the amount of fluvial incision and knickpoint retreat. From morphometric analyses, we find that mean channel gradients and widths, as well as knickpoint retreat rates, display a drainage area dependence modulated by bedrock lithology. However, there appears to be no relation between horizontal retreat and vertical downwearing of knickpoints. Assuming a postglacial origin of these gorges, our results imply high postglacial fluvial incision (0.5-15 mm yr−1) and knickpoint retreat (1-200 mm yr−1) rates that are, however, consistent with previous estimates. Numerical modeling was used to test the capacity of different fluvial incision models to predict the inferred evolution of the gorges. Results from simple end‐member models suggest transport‐limited behavior of the bedrock gorges. A more sophisticated model including dynamic width adjustment and sediment‐dependent incision rates predicts present‐day channel geometry only if a significant supply of sediment from the gorge sidewalls (∼10 mm yr−1) is triggered by gorge deepening, combined with pronounced inhibition of bedrock incision by sediment transport and deposition

In-plane field-induced vortex liquid correlations in underdoped Bi_2Sr_2CaCu_2O_8+\delta

The effect of a magnetic field component parallel to the superconducting layers on longitudinal Josephson plasma oscillations in the layered high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ is shown to depend on the thermodynamic state of the underlying vortex lattice. Whereas the parallel magnetic field component depresses the Josephson Plasma Resonance (JPR) frequency in the vortex solid phase, it may enhance it in the vortex liquid. There is a close correlation between the behavior of microwave absorption near the JPR frequency and the effectiveness of pancake vortex pinning, with the enhancement of the plasma resonance frequency occurring in the absence of pinning, at high temperature close to the vortex melting line. An interpretation is proposed in terms of the attraction between pancake vortices and Josephson vortices, apparently also present in the vortex liquid state.Comment: 8 pages, 7 Figures, submitted to Phys. Rev.

How intrinsically motivating are swimming instructors/lessons in the Netherlands? An observational study through the lens of self-determination theory

BackgroundIt is desirable that (more) children continue swimming after having completed their swimming lessons to preserve their swimming skills and water safety, and as part of an active, healthy lifestyle. This may be encouraged by stirring children's intrinsic motivation for swimming during swimming lessons. However, it is currently unknown how intrinsically motivating swimming lessons are in Western countries.PurposeThis study examined to what extent swimming instructors in the Netherlands cater to the basic needs of autonomy, competence, and relatedness, which, according to Self-Determination Theory (SDT), promote intrinsic motivation. Additionally, it examined whether an SDT-based teaching program prompts instructors to better meet these needs, and to what extent the teaching program, the education and experience of the instructor, and the group size predict the employment of SDT in swimming lessons.MethodsA total of 128 swimming lessons given by equally many instructors were observed in the Netherlands and rated on a modified version of the SDT teaching style scale to assess autonomy, competence, and relatedness support. The swimming lessons referred to four teaching programs, one of which was explicitly based on SDT.ResultsInstructors exhibited autonomy-thwarting, weakly competence-supportive, and relatedness-supportive behaviors. The SDT-based teaching program scored higher on the provision of autonomy, competence, and relatedness in lessons. This finding was significant for autonomy. Teaching program was the only significant predictor of SDT employment by instructors.ConclusionFurther improvement is desirable in catering to the basic needs, particularly autonomy, which can be achieved by deliberately implementing the principles of SDT into teaching programs for swimming

Українська культура як чинник української державності: історичний аспект

У статті подано особливості формування української національної культури кінця XIX - XX століть. Охарактеризовано розвиток культури за умови становлення української державності. Автор намагався простежити вплив національного суспільства на розвиток культури в конкретний історичний період.The article introduces the features of formation of the Ukrainian national culture of the 19th-20th centuries. Characterized by the development of culture during becoming of Ukrainian statehood. The author tried to trace the influence of the national society for the development of culture in a specific historical period

Stepping strategies used by post-stroke individuals to maintain margins of stability during walking

AbstractBackgroundPeople recovering from a stroke are less stable during walking compared to able-bodied controls. The purpose of this study was to examine whether and how post-stroke individuals adapt their steady-state gait pattern to maintain or increase their margins of stability during walking, and to examine how these strategies differ from strategies employed by able-bodied people.MethodsTen post-stroke individuals and 9 age-matched able-bodied individuals walked on the Computer Assisted Rehabilitation Environment. Medio-lateral translations of the walking surface were imposed to manipulate gait stability. To provoke gait adaptations, a gait adaptability task was used, in which subjects occasionally had to hit a virtual target with their knees. We measured medio-lateral and backward margins of stability, and the associated gait parameters walking speed, step length, step frequency, and step width.FindingsPost-stroke participants showed similar medio-lateral margins of stability as able-bodied people in all conditions. This was accomplished by a larger step width and a relatively high step frequency. Post-stroke participants walked overall slower and decreased walking speed and step length even further in response to both manipulations compared to able-bodied participants, resulting in a tendency towards an overall smaller backward margins of stability, and a significantly smaller backward margin of stability during the gait adaptability task.InterpretationPost-stroke individuals have more difficulties regulating their walking speed, and the underlying parameters step frequency and step length, compared to able-bodied controls. These quantities are important in regulating the size of the backward margin of stability when walking in complex environments

Postural threat during walking:Effects on energy cost and accompanying gait changes

Background: Balance control during walking has been shown to involve a metabolic cost in healthy subjects, but it is unclear how this cost changes as a function of postural threat. The aim of the present study was to determine the influence of postural threat on the energy cost of walking, as well as on concomitant changes in spatiotemporal gait parameters, muscle activity and perturbation responses. In addition, we examined if and how these effects are dependent on walking speed. Methods: Healthy subjects walked on a treadmill under four conditions of varying postural threat. Each condition was performed at 7 walking speeds ranging from 60-140% of preferred speed. Postural threat was induced by applying unexpected sideward pulls to the pelvis and varied experimentally by manipulating the width of the path subjects had to walk on. Results: Results showed that the energy cost of walking increased by 6-13% in the two conditions with the largest postural threat. This increase in metabolic demand was accompanied by adaptations in spatiotemporal gait parameters and increases in muscle activity, which likely served to arm the participants against a potential loss of balance in the face of the postural threat. Perturbation responses exhibited a slower rate of recovery in high threat conditions, probably reflecting a change in strategy to cope with the imposed constraints. The observed changes occurred independent of changes in walking speed, suggesting that walking speed is not a major determinant influencing gait stability in healthy young adults. Conclusions: The current study shows that in healthy adults, increasing postural threat leads to a decrease in gait economy, independent of walking speed. This could be an important factor in the elevated energy costs of pathological gait