A curious case of agreement between conventional thermobarometry and phase equilibria modelling in granulites: New constraints on P–T estimates in the Antarctica segment of the Musgrave–Albany–Fraser–Wilkes Orogen

The Windmill Islands region in Wilkes Land, east Antarctica, preserves granulite facies metamorphic mineral assemblages that yield seemingly comparable P–T estimates from conventional thermobarometry and mineral equilibria modelling. This is uncommon in granulite facies terranes, where conventional thermobarometry and phase equilibria modelling generally produce conflicting P–T estimates because peak mineral compositions tend to be modified by retrograde diffusion processes. In situ U–Pb monazite geochronology and calculated metamorphic phase diagrams show that the Windmill Islands experienced two phases of high thermal gradient metamorphism during the Mesoproterozoic. The first phase of metamorphism is recorded by monazite ages in two widely separated samples and occurred at c. 1,305 Ma. This event was regional in extent, involved crustally derived magmatism and reached conditions of ~3.2–5 kbar and 690–770°C corresponding to very high thermal gradients of >150°C/kbar. The elevated thermal regime is interpreted to reflect a period of extension or increased extension in a back‐arc setting that existed prior to c. 1,330 Ma. The first metamorphic event was overprinted by granulite facies metamorphism at c. 1,180 Ma that was coeval with the intrusion of charnockite. This event involved peak temperatures of ~840–850°C and pressures of ~4–5 kbar. A phase of granitic magmatism at c. 1,250–1,210 Ma, prior to the intrusion of the charnockite, is interpreted to reflect a phase of compression within an overall back‐arc setting. Existing conventional thermobarometry suggests conditions of ~4 kbar and 750°C for M₁ and 4–7 kbar and 750–900°C for M₂. The apparent similarities between the phase equilibria modelling and existing conventional thermobarometry may suggest either that the terrane cooled relatively quickly, or that the P–T ranges obtained from conventional thermobarometry are sufficiently imprecise that they cover the range of P–T conditions obtained in this study. However, without phase equilibria modelling, the veracity of existing conventional P–T estimates cannot be evaluated. The calculated phase diagrams from this study allow the direct comparison of P–T conditions in the Windmill Islands with phase equilibria models from other regions in the Musgrave–Albany–Fraser–Wilkes Orogen. This shows that the metamorphic evolution of the Wilkes Land region is very similar to that of the eastern Albany–Fraser Orogen and Musgrave Province in Australia, and further demonstrates the remarkable consistency in the timing of metamorphism and the thermal gradients along the ~5,000 km strike length of this system.     

Field theoretical techniques in statistical fluid dynamics: With application to nonlinear wave dynamics

Abstract

A derivation of two-point Markovian closure is presented in classical statistical field theory formalism. It is emphasized that the procedures used in this derivation are equivalent to those employed in the quantum statistical field theory derivation of the Boltzmann equation. Application of these techniques to the study of two-dimensional flow on a β-plane yields a quasi-homogeneous, quasi-stationary transport equation and a renormalized dispersion relation for Rossby waves     

Reducing vertical acceleration in ballast layers of railways to mitigate geometrical disorders in high‐speed lines

According to field feedbacks from high‐speed lines (HSL), the increase of train operating speeds is responsible for unusual fast evolving geometrical disorders in ballasted tracks. This paper deals with the search of solutions applicable at the design stage to mitigate these disorders. The starting point of the present work relies on the assumption, comforted by the literature, of a strong correlation between disorders and vertical accelerations in the ballast layer induced by the train passages. This led us focus herein on the calculation and the analysis of accelerations in the railway structure. The vertical accelerations (γ_z) are computed using the in‐house developed numerical program ViscoRail and on the basis of a reference HSL. These are shown to increase strongly with the train speed attesting to the link between the train speed and the geometrical disorders in ballast. Then, other simulations are run varying some structural parameters to evaluate their impact on the acceleration field γ_z. In that way, we show that decreasing the stiffness of the mechanical connection between the rails and the ballast, increasing the moment of inertia of the rails or the Young modulus of the sub‐ballast layer, leads to a decrease of γ_z and could provide solutions for the design of future HSL. The solution consisting in the incorporation of an asphalt sub‐ballast layer, as already experimented on sites, is finally examined in more details. Copyright © 2017 John Wiley & Sons, Ltd.     

River–aquifer interactions in a semi‐arid environment stressed by groundwater abstraction

Rivers and aquifers are, in many cases, a connected resource and as such the interactions between them need to be understood and quantified for the resource to be managed appropriately. The objective of this paper is to advance the understanding of river–aquifer interactions processes in semi‐arid environments stressed by groundwater abstraction. This is performed using data from a specific catchment where records of precipitation, evapotranspiration, river flow, groundwater levels and groundwater abstraction are analysed using basic statistics, hydrograph analysis and a simple mathematical model to determine the processes causing the spatial and temporal changes in river–aquifer interactions. This combined approach provides a novel but simple methodology to analyse river–aquifer interactions, which can be applied to catchments worldwide. The analysis revealed that the groundwater levels have declined (~ 3 m) since the onset of groundwater abstraction. The decline is predominantly due to the abstraction rather than climatic changes (r = 0.84 for the relationship between groundwater abstraction and groundwater levels; r = 0.92 for the relationship between decline in groundwater levels and magnitude of seasonal drawdown). It is then demonstrated that, since the onset of abstraction, the river has changed from being gaining to losing during low‐flow periods, defined as periods with flow less than 0.5, 1.0 or 1.5 GL/day (1 GL/day = 1 × 10⁶ m³/day). If defined as < 1.0 GL/day, low‐flow periods constitute approximately 65% of the river flows; the periods where the river is losing at low‐flow conditions are thus significant. Importantly, there was a significant delay (> 10 years) between the onset of groundwater abstraction and the changeover from gaining to losing conditions. Finally, a relationship between the groundwater gradient towards the river and the river flow at low‐flow is demonstrated. The results have important implications for water management as well as water ecology and quality. Copyright © 2012 John Wiley & Sons, Ltd.     

A Computationally Efficient,Time-Dependent Model of the Solar Wind for Use as a Surrogate to Three-Dimensional Numerical Magnetohydrodynamic Simulations

Solar radiation variability spans a wide range in time, ranging from seconds to decadal and longer. The nearly 40 years of measurements of solar irradiance from space established that the total solar irradiance varies by \(\approx 0.1\%\) in phase with the Sun’s magnetic cycle. Specific intervals of the solar spectrum, e.g., ultraviolet (UV), vary by orders of magnitude more. These variations can affect the Earth’s climate in a complex non-linear way. Specifically, some of the processes of interaction between solar UV radiation and the Earth’s atmosphere involve threshold processes and do not require a detailed reconstruction of the solar spectrum. For this reason a spectral UV index based on the (FUV-MUV) color has been recently introduced. This color is calculated using SORCE SOLSTICE integrated fluxes in the FUV and MUV bands. We present in this work the reconstructions of the solar (FUV-MUV) color and Ca ii K and Mg ii indices, from 1749–2015, using a semi-empirical approach based on the reconstruction of the area coverage of different solar magnetic features, i.e., sunspot, faculae and network. We remark that our results are in noteworthy agreement with latest solar UV proxy reconstructions that exploit more sophisticated techniques requiring historical full-disk observations. This makes us confident that our technique can represent an alternative approach which can complement classical solar reconstruction efforts. Moreover, this technique, based on broad-band observations, can be utilized to estimate the activity on Sun-like stars, that cannot be resolved spatially, hosting extra-solar planetary systems.

     

Modified Structured Cam Clay: A generalised critical state model for destructured,naturally structured and artificially structured clays

This paper presents a generalised constitutive model for destructured, naturally structured and artificially structured clays that extends the Structured Cam Clay (SCC) model. This model is designated as “Modified Structured Cam Clay (MSCC) model”. The influence of structure and destructuring on the mechanical behaviour of clay can be explained by the change in the modified effective stress, which is the sum of the current mean effective stress and the additional mean effective stress due to structure (structure strength). The presence of structure increases the modified mean effective stress and yield surface, enhancing the cohesion, peak strength and stiffness. The destructuring begins when the stress state is on the virgin yield surface. After the failure (peak strength) state, the abrupt destructuring occurs as the soil–cementation structure is crushed; hence the strain softening. The soil structure is completely removed at the critical state when the yield surface becomes identical to the destructured surface. The destructuring law is proposed based on this premise. In the MSCC model, the yield function is the same shape as that of the Modified Cam Clay (MCC) model. A plastic potential is introduced so as to account for the influence of structure on the plastic strain direction for both hardening and softening behaviours. The required model parameters are divided into those describing destructured properties and those describing structured properties. All the parameters have physical meaning and can be simply determined from the conventional triaxial tests. Thus, the MSCC model is a useful tool for geotechnical practitioners. The capability of the model is verified by the test results of destructured, natural structured and artificially structured clays.     

Behaviour of cemented clay simulated via the theoretical framework of the Structured Cam Clay model

In this paper, the mechanical properties of cemented clay are studied. The theoretical framework of the Structured Cam Clay (SCC) model is extended to describe the behaviour of cemented clay. The SCC model is modified to take into account special features of the behaviour of cemented clay. The widely used mean effective stress parameter is modified to include the influence of cementation on the strength and the plastic deformation of cemented clay. A new destructuring function is also suggested to represent the removal of the cementation structure, especially in the formation of the final failure state for artificially strongly cemented clay. The revised model is then employed to simulate and predict the behaviour of cemented clay with various degrees of cementation and confining stresses. The main features of the complicated behaviour of cemented clay can be reasonably represented well by the theoretical framework of the SCC model. Finally, the influence of cementation on the model parameters is discussed, and empirical equations are proposed to determine some of the key model parameters for engineering practice.     

The trace of the Pacific‐Cocos‐Nazca triple junction in the Central Pacific and the formation of an overlapping spreading centre

The `plate tectonic mirror image' to the region of the Cocos and Nazca plates, which are currently being subducted beneath Central America, is preserved in the Central Pacific around 120°W just south of the equator. Cruise SO‐180 investigated this remote area during project CENTRAL and acquired new magnetic and bathymetric data. A plate tectonic model for the ‘mirror image’ is presented based on the newly acquired as well as reprocessed existing data. Discordant magnetic anomaly patterns and bathymetric structures indicate at least two major reorganization events (19.5 and 14.7 Myr), which can be detected both in the Cocos‐Nazca spreading system and in the East Pacific Rise. Irregularities in the anomaly pattern and curvilinear structures on the sea floor of the survey area are interpreted in terms of a fossil overlapping spreading centre at the location where the Farallon break‐up originated.