To protect electronic systems against high-power microwave (HPM), a plasma waveguide limiter is presented. In response to the above phenomenon, a self-consistent 3-D multiphysics ...electromagnetic-plasma fluid model coupling full-wave Maxwell's equations with plasma fluid equations is established to describe the operating mechanism of a 3-D simplified sandwich structure plasma limiter filled with four easily ionized inert gases. The plasma limiter system is analyzed with the spectral-element time-domain (SETD) method. The choice of filling easily ionizable inert gas, influence of the number of layers, and pressure on the plasma limiter are discussed. Numerical results demonstrate that the gas with low critical breakdown field strength is more suitable for protecting HPMs. In the case of 20 torr, the order of microwave breakdown is xenon, argon, neon, and helium, and the two-layer plasma limiter (plasma-slab-plasma) has better protective characteristics than a one-layer plasma limiter (slab-plasma-slab) with the same length of gas chamber. Our research can provide theoretical guidance for designers and give the complete physical physics process.
Slab orphaning is a newly discovered phenomenological behavior, where the slab tip breaks off at the top of the lower mantle (~660 km depth) and is abandoned by its parent slab. Upon orphaning, ...subduction continues uninterrupted through the lateral motion of the parent slab above 660 km depth. In this work, we present a regime diagram for the range of conditions under which slabs can orphan at the top of the lower mantle. Our models show that a viscosity jump at 1,000 km depth not coincident with the endothermic phase change responsible for the 660 km seismic discontinuity, is necessary for orphaning as is the presence of a low viscosity channel between 660 and 1,000 km depth. We show that orphan slabs, similar to other deep slab morphologies, can be the end result for a wide range of physical parameters governing slab dynamics: slab orphaning persists across wide variations in slab dip, slab yield stress/strength, Clapeyron slope values, and overriding plate nature. The diversity in orphan slab sizes and orphaning periods is tied to the orphaning regime space, which describes a hitherto unexplored region between deflected and penetrating deep-subduction modes. Orphaning provides a simple dynamic link between the well-known deflection and penetration, and provides one possible way for slabs to switch from direct penetration to deflection, littering the mantle with abandoned fragments. Orphan slabs are therefore the intermediary between these two extensively studied slab morphologies.
The study of slab‐slab interactions has come to the front of geodynamics researches to explain geological and geophysical observations from tectonically complex areas. Here we aim to better ...understand the geodynamics of the Central Mediterranean, where the Adria plate subducts on its two opposite sides. Additionally, the slab below the Central South Apennines has been progressively breaking off during the last 3 Myr. The role of a slab window in a single slab or in an outward dipping double‐sided subduction system is addressed by analog models at the scale of the upper mantle, realized using glucose syrup and silicone putty, to model the interaction between the Earth's mantle and the lithosphere. Our results show that the presence of a slab window modifies the pattern of mantle circulation, as well as the trench geometry and kinematics. In particular, the opening of the slab window induces the formation of two arcs flanking the window, while the mantle flows through it and turns toward the arcs, creating a small‐scale toroidal flow. The effect of a slab window is more pronounced on double subduction systems, as the outflow through the window is amplified, while internal deformation is induced in the plate by the opposite slab pull force. These experimental results suggest that the origin of the Apenninic and the Calabrian arcs is the result of the formation of a slab window, providing a new interpretation of the surface deformation and the SKS shear wave splitting pattern of the Adria microplate.
Plain Language Summary
The Adria microplate in the Central Mediterranean is an important part of the tectonic puzzle between the converging Africa and Eurasia plates. The heavy lithosphere sinks into the underlying mantle on two sides of the Adria plate, under the Apennines in the west and under the Dinarides in the east. During the most recent few millions of years, the lithosphere started to detach from the surface in the Central South Apennines while sinking into the mantle. This opened a slab window, an additional escape route for the mantle, which is squeezed by the two subducting lithospheres. We scaled down and simplified this setting in order to model and investigate the role of the dual subduction and the opening slab window in the evolution of the Central Mediterranean by means of analog models. Our results show that the two opposite subduction zones have a regional tectonics effect via the complex subduction‐induced mantle circulation. Furthermore, the detaching lithosphere modifies the geometry of the subduction zone, which results in a double‐arc geometry resembling the Northern Apennines and Calabria.
Key Points
The combination of slab‐slab interactions and an opening slab window defines the recent tectonic evolution of the Central Mediterranean
Outward dipping double‐sided subduction drives the mantle to flow from below the Adria plate toward the Alps and the Pannonian Basin
The opening of a slab window under the Central Apennines changes the trench geometry and the subduction‐induced mantle flow pattern
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Reconfigurations of magmatic arcs through time have been recognized since pioneering works, describing inland and trenchward arc migrations or magmatic shut-off lasting for several millions of years. ...These modifications present variable magnitudes and rates of arc migration and magmatic broadening, and different arc quiescence time spans. The time-space behavior of magmatic arcs has been attributed to a diversity of geodynamic processes acting at convergent margins largely associated with modifications in the upper-plate or changes in plate kinematics. Identifying a geodynamic process responsible for a particular spatiotemporal arc history from the geological record is not straightforward. This task is further complicated where more than one process influencing arc position acted in concert. To assess these issues, it is essential to have a deep understanding of how each process influences the space-time arc behavior and modifies the geological context. To date, a joint comparison highlighting similarities and differences in how these phenomena impact arc dynamics and the associated geological framework is still missing from the literature. In this study, we provide a state-of-the-art synthesis of processes controlling arc migration and quiescence. Then, we extract diagnostic elements from the literature to build a synthetic table to aid in the task of discerning a dominant geodynamic process from the geological record. In this task, we considered the first-order characteristics of the space-time arc evolution and diagnostic features of the geological context associated with each geodynamic process. Finally, this synthesis stresses that the combination of both perspectives, understanding the space-time arc pattern and the associated geological framework, provides the best approach to unravel a dominant process controlling arc migration and shut-off.
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Seismic tomography shows that subducting slabs can either sink straight into the lower mantle, or lie down in the mantle transition zone. Moreover, some slabs seem to have changed mode from ...stagnation to penetration or vice versa. We investigate the dynamic controls on these modes and particularly the transition between them using 2D self-consistent thermo-mechanical subduction models. Our models confirm that the ability of the trench to move is key for slab flattening in the transition zone. Over a wide range of plausible Clapeyron slopes and viscosity jumps at the base of the transition zone, hot young slabs (25 Myr in our models) are most likely to penetrate, while cold old slabs (150 Myr) drive more trench motion and tend to stagnate. Several mechanisms are able to induce penetrating slabs to stagnate: ageing of the subducting plate, decreasing upper plate forcing, and increasing Clapeyron slope (e.g. due to the arrival of a more hydrated slab). Getting stagnating slabs to penetrate is more difficult. It can be accomplished by an instantaneous change in the forcing of the upper plate from free to motionless, or a sudden decrease in the Clapeyron slope. A rapid change in plate age at the trench from old to young cannot easily induce penetration. On Earth, ageing of the subducting plate (with accompanying upper plate rifting) may be the most common mechanism for causing slab stagnation, while strong changes in upper plate forcing appear required for triggering slab penetration.
•Hot young slabs penetrate the lower mantle more easily than cold old slabs.•Sinking mode can switch by altering upper-plate mobility, Clapeyron slope or slab age.•It is easier and quicker to switch from penetration to stagnation than vice versa.
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From among the several types of slabs, there is uncertainty about the most cost-effective one for structures with short, medium, and long spans. A study comparing the cost of the solid, flat, hollow ...block (HB), and post-tensioned (PT) slabs for 4, 6, and 8m spans was conducted using SAFE software. For each span length, the total quantities of concrete and steel bars were found and compared for 4 slabs. Then, the total cost of steel bars, concrete, blocks, tendons, and formwork was tabulated and compared. The findings revealed that the quantity of concrete in slabs with 4 and 6m spans is the least in the HB slabs, while for slabs with 8m spans, the quantity of concrete is the least in the PT slab. Besides, the quantity of steel bars in slabs with 4, 6, and 8 spans is the least in PT slabs. In terms of economy, the most cost-effective option is the flat slab for 4m spans, with savings of about 14, 9, and 17% compared to solid, HB, and PT slabs, respectively. On the other hand, for 6 and 8m spans, the most cost-effective option is the PT slab, with savings of about 10, 6, and 6% for the 6m span, and 10, 20, and 22% for 8m span compared to solid, HB, and flat slabs, respectively. Therefore, for 6m spans and more, PT slabs offer significant cost savings.
Slab break-offs in the Alpine subduction zone Kästle, Emanuel D.; Rosenberg, Claudio; Boschi, Lapo ...
International journal of earth sciences : Geologische Rundschau,
03/2020, Volume:
109, Issue:
2
Journal Article
Peer reviewed
Open access
After the onset of plate collision in the Alps, at 32–34 Ma, the deep structure of the orogen is inferred to have changed dramatically: European plate break-offs in various places of the Alpine arc, ...as well as a possible reversal of subduction polarity in the eastern Alps have been proposed. We review different high-resolution tomographic studies of the upper mantle and combine shear- and body-wave models to assess the most reliable geometries of the slabs. Several hypotheses for the tectonic evolution are presented and tested against the tomographic model interpretations and constraints from geologic and geodetic observations. We favor the interpretation of a recent European slab break-off under the western Alps. In the eastern Alps, we review three published scenarios for the subduction structure and propose a fourth one to reconcile the results from tomography and geology. We suggest that the fast slab anomalies are mainly due to European subduction; Adriatic subduction plays no or only a minor role along the Tauern window sections, possibly increasing towards the Dinarides. The apparent northward dip of the slab under the eastern Alps may be caused by imaging a combination of Adriatic slab, from the Dinaric subduction system, and a deeper lying European one, as well as by an overturned, retreating European slab.
•SFRC flat slab of the 7c strength class (considering mean values) was tested.•SFRC slab demonstrated no loss of structural integrity under the load of 16 kN/m2.•Constructed SFRC element revealed a ...suitable cracking and deflection control.•The average fibre contribution in the horizontal plane resulted to be 76.8%.•An increment of fibre amount from the top to the bottom of the slab was depicted.
The total substitution of traditional reinforcement in the form of steel bars by fibres can be mainly found in elements with favourable boundary conditions and subjected to low-moderate load levels. However, the rigorous study of fibre reinforced concrete (FRC) and its potential fields of application over the last decades permitted this material to face structural application with greater responsibility in terms of structural integrity and mechanical capacity – construction of FRC flat slabs. This promising technology was used in a dozen buildings with recognition of positive outcomes with respect to the optimization of resources, reduction of execution time, and environmental impacts. Despite these achievements, the application of FRC in flat slabs is still limited in the building sector because of certain concerns of the material capacities, and existence of some aspects related both to service and ultimate limit states, which are still unclear. With this in mind, an extensive experimental programme was carried out and focused on the construction of a full-scale FRC flat slab and its loading protocol in order to analyse both crack and deflection patterns. Likewise, the structure was led to failure, which also allowed assessing both the bearing and deformability capacities as well as the fibre distribution and orientation. The results derived from this experimental program are expected to increase the confidence of designers and practitioners on the use of FRC as structural material for column-supported flat slabs.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Kinematic punching theory for SFRC slabs considering flexural and translational deformations.•Application of Variable Engagement Model to estimate the fiber contribution to punching ...strength.•Validation of the kinematic theory by means of systematic test series and a databank evaluations.•Investigation of the influence of fiber properties on punching strength.
There are several possibilities to enhance the punching shear behavior of reinforced concrete slabs. Among them, various shear reinforcement systems have been found to efficiently increase both the punching strength and the deformation capacity compared to similar slabs without shear reinforcement. Nevertheless, the biggest disadvantage of shear-reinforced concrete slabs is the large effort associated with the installation of the reinforcement elements. Another efficient solution to improve the punching shear behavior of slab-column connections is the application of steel fibers in the region of the column. Depending on the fiber volume and fiber properties, steel fibers may significantly enhance the tension-softening behavior of concrete leading to higher punching strengths combined with a more ductile failure mode.
In this paper, the existing two-parameter kinematic theory for punching shear in reinforced concrete slabs is extended considering the beneficial effects of steel fibers on punching shear capacity. The contribution to the punching strength provided by fibers is determined based on a theoretical model described in literature. The model allows for the description of the tension-softening behavior of reinforced composites containing randomly orientated discontinuous fibers as a function of the normal opening of the failure crack. The proposed theory is validated by means of systematic punching test series with varying fiber volume and databank evaluations. Further parametric studies were conducted to analyze the influence of fiber properties on punching strength more in detail.
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We study the age of the subducting Pacific slab beneath East Asia using a high-resolution model of P-wave tomography and paleo-age data of ancient seafloor. Our results show that the lithosphere age ...of the subducting slab becomes younger from the Japan Trench (∼130 Ma) to the slab's western edge (∼90 Ma) beneath East China, and the flat (stagnant) slab in the mantle transition zone (MTZ) is the subducted Pacific plate rather than the proposed Izanagi plate which should have already collapsed into the lower mantle. The flat Pacific slab has been in the MTZ for no more than ∼10–20 million years, considerably less than the age of the big mantle wedge beneath East Asia (>110 million years). Hence, the present flat Pacific slab in the MTZ has contributed to the Cenozoic destruction of the East Asian continental lithosphere with extensive intraplate volcanism and back-arc spreading, whereas the destruction of the North China Craton during the Early Cretaceous (∼140–110 Ma) was caused by the subduction of the Izanagi (or the Paleo-Pacific) plate.
•Age distribution of the subducting Pacific slab beneath East Asia is estimated.•The current flat slab in the MTZ beneath East Asia is the subducting Pacific plate.•The big mantle wedge has existed much longer than any slabs in the MTZ.
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