Axial Zone Research Articles

Channel Migration of the Meandering River Fan: A Case Study of the Okavango Delta

Okavango delta is a typical distributive fluvial system, which is composed of a series of sand island-river-swamp networks. River migration in the Okavango delta is analyzed by using satellite images from Google Earth and Alaska Satellite Facility (ASF). Four configuration characterization parameters are selected to depict and measure the meandering river. These four parameters are sinuosity index (S), curvature (C), the difference of along-current deflection angle (Δθ) and expansion coefficient (Km). In the fan, the channel migration is mainly asymmetric. According to geomorphic elements and associated features, Okavango Delta can be subdivided into three zones: axial zone, median zone and distal zone. Under the influence of slope, climate and vegetation, different migration modes are developed in different zones. As the river moves downstream, the sinuosity index of the river on the Okavango Delta decreases downstream. Based on the characteristics of different zones, the sedimentary facies model of a single source distributive fluvial system of a meandering river is proposed. The models of channel migration and sedimentary facies have wide application. This research will not only provide a basis for the prediction of future river channels but will also provide important theoretical guidance for the study of the sedimentary morphology of underground reservoirs.

C64. New Onset PVC Bigeminy Post Covid-19 Infection: Another Covid-Cardiac Related Problem

Abstract Background Covid-19 is an infectious disease caused by a newly discovered coronavirus that caused acute severe respiratory syndrome and other complication. Many cardiac complication already occurred such as carditis and acute heart failure. PVC bigeminy is a new complication that we found in our post covid-19 patient. Case Summary A 31-years-old female patient admitted to the outpatient department with chief complaint palpitation with chest discomfort in the past 1 month with a history covid-19 infection 3 months ago with prolong covid infection for 2 months. ECG showed sinus rhythm and bigeminy PVC with PVC LBBB morphology left inferior axis and transition zone in V4. The patient planned to do Electrophysiology study and ablation. From the EP study we found 2 type of PVC, both of the PVC have LBBB morphology with left inferior axis and transitional zone in V4. The first PVC had 42ms earliest activation time and the second PVC had 48ms earliest activation. We do ablation with non irrigating catheter on the first PVC but the PVC still remain. Then try with irrigating catheter and do the ablation. After the ablation the PVC already vanished and the ablation was successful. Discussion PVC bigeminy post covid-19 infection is a rare complication in covid-19 patient. PVC bigeminy cause several complain from palpitation, chest discomfort, and shortness of breath. Ablation is one of the best way to treat PVC bigeminy patient with symptoms and in the patient the ablation was successful.

C64. New Onset PVC Bigeminy Post Covid-19 Infection: Another Covid-Cardiac Related Problem

BackgroundCovid-19 is an infectious disease caused by a newly discovered coronavirus that caused acute severe respiratory syndrome and other complication. Many cardiac complication already occurred such as carditis and acute heart failure. PVC bigeminy is a new complication that we found in our post covid-19 patient.Case SummaryA 31-years-old female patient admitted to the outpatient department with chief complaint palpitation with chest discomfort in the past 1 month with a history covid-19 infection 3 months ago with prolong covid infection for 2 months. ECG showed sinus rhythm and bigeminy PVC with PVC LBBB morphology left inferior axis and transition zone in V4. The patient planned to do Electrophysiology study and ablation. From the EP study we found 2 type of PVC, both of the PVC have LBBB morphology with left inferior axis and transitional zone in V4. The first PVC had 42ms earliest activation time and the second PVC had 48ms earliest activation. We do ablation with non irrigating catheter on the first PVC but the PVC still remain. Then try with irrigating catheter and do the ablation. After the ablation the PVC already vanished and the ablation was successful.DiscussionPVC bigeminy post covid-19 infection is a rare complication in covid-19 patient. PVC bigeminy cause several complain from palpitation, chest discomfort, and shortness of breath. Ablation is one of the best way to treat PVC bigeminy patient with symptoms and in the patient the ablation was successful.

Quadruple node of triple junctions of grain boundaries in a Eu2+-doped solid solution of the ions K+, Rb+, Cl- and Br-: an epifluorescence microscopy study using the doping ion as a fluorochrome.

Epifluorescence microscopy was used to unveil the geometry and relative spatial orientation of an arrangement of crystal defects, consisting of a quadruple node of triple junctions of grain boundaries, in a Eu2+-doped solid solution of K+, Rb+, Cl- and Br-. The doping ion was utilized as a fluorochrome. Microscopy images of different optical cross-sections of the arrangement of crystal defects under study were recorded and used to build an electronic three-dimensional reconstruction of this arrangement. The geometry is that of an irregular tristetrahedron, centred at the quadruple node, so that the tristetrahedron legs, deviating from the lattice <111>-zone axis directions, lie along the triple junctions, whereas the tristetrahedron faces collapse onto the grain boundaries. The deviation angles as well as the angles defined by different triple junction pairs and different grain boundary pairs were measured. The orientational deviation is larger than that in a KI single crystal but lower than that in a solid solution of K+, Cl- and Br-, meaning that the structural equilibrium depends on the matrix lattice structural character. Uncompensated local nanostrains, due to the ion substitution, are associated with the observed decrement in structural stability, in relation to the KI case, whereas compensated local nanostrains, provoked by the simultaneous substitution of cations and anions, are argued to be responsible for the observed increment in structural stability, in relation with the K(Cl, Br) case. The optical characterization of the fluorochrome, the recording and processing of the microscopy images, the building of the electronic 3D reconstruction, the angular measure methodology and the geometrical modelling are all carefully described.

Atomic-scale evidence of chemical short-range order in CrCoNi medium-entropy alloy

High (or medium)-entropy alloys (H/MEAs) are complex concentrated solid solutions that may develop chemical short-range order (CSRO). In this regard, CrCoNi, the prototypical face-centered-cubic MEA, has recently kindled a debate in the H/MEA community, as it is uncertain if CSRO can possibly form in such a multi-principal-element solution, where no equilibrium or metastable intermetallic compounds have ever been seen or predicted. To answer this challenging question, here we present firm experimental evidence for the CSRO from electron diffraction as well as atomic-resolution chemical mapping, under an appropriate zone axis. We also develop a methodology to reliably determine the locations of atomic columns from the line scan profiles in the chemical maps, as well as a quantitative covariance-based correlation analysis of the column chemical compositions to reveal the spatial correlations between various atomic pairs. The detailed chemical information affirms the tendency for like-pair avoidance and unlike-pair preference, specifies the preferred atomic packing and plane stacking by the three constituent species, and suggests a proposed atomic configuration that constitutes the CSRO motif. The fraction of CSRO regions is moderately lowered after either plastic deformation or high-temperature heating. A comparison is also made with previous attempts to identify CSROs in H/MEAs.

Atomic scale chemical ordering in franckeite—a natural van der Waals superlattice

The mineral franckeite is a naturally occurring van der Waals superlattice which has recently attracted attention for future applications in optoelectronics, biosensors and beyond. Furthermore, its stacking of incommensurately modulated 2D layers, the pseudo tetragonal Q-layer and the pseudo hexagonal H-layer, is an experimentally accessible prototype for the development of synthetic van der Waals materials and of advanced characterization methods to reveal new insights in their structure and chemistry at the atomic scale that is crucial for deep understanding of its properties. While some experimental studies have been undertaken in the past, much is still unknown on the correlation between local atomic structure and chemical composition within the layers. Here we present an investigation of the atomic structure of franckeite using state-of-the-art high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and atom probe tomography (APT). With atomic-number image contrast in HAADF STEM direct information about both the geometric structure and its chemistry is provided. By imaging samples under different zone axes within the van der Waals plane, we propose refinements to the structure of the Q-layer and H-layer, including several chemical ordering effects that are expected to impact electronic structure calculations. Additionally, we observe and characterize stacking faults which are possible sources of differences between experimentally determined properties and calculations. Furthermore, we demonstrate advantages and discuss current limitations and perspectives of combining TEM and APT for the atomic scale characterization of incommensurately modulated von der Waals materials.

Moho and uppermost mantle structure in the Alpine area from S-to-P converted waves

Abstract. In the frame of the AlpArray project we analyse teleseismic data from permanent and temporary stations of the Alpine region to study seismic discontinuities down to about 140 km depth. We average broadband teleseismic S-waveform data to retrieve S-to-P converted signals from below the seismic stations. In order to avoid processing artefacts, no deconvolution or filtering is applied, and S arrival times are used as reference for stacking. We show a number of north–south and east-west profiles through the Alpine area. The Moho signals are always seen very clearly, and negative velocity gradients below the Moho depth are also visible in a number of profiles. A Moho depression is visible along larger parts of the Alpine chain. It reaches its largest depth of 60 km beneath the Tauern Window. However, the Moho depression ends abruptly near about 13∘ E below the eastern Tauern Window. This Moho depression may represent the crustal trench, where the Eurasian lithosphere is subducted below the Adriatic lithosphere. East of 13∘ E an important along-strike change occurs; the image of the Moho changes completely. No Moho deepening is found in this easterly region; instead the Moho bends up along the contact between the European and the Adriatic lithosphere all the way to the Pannonian Basin. An important along-strike change was also detected in the upper mantle structure at about 14∘ E. There, the lateral disappearance of a zone of negative velocity gradient in the uppermost mantle indicates that the S-dipping European slab laterally terminates east of the Tauern Window in the axial zone of the Alps. The area east of about 13∘ E is known to have been affected by severe late-stage modifications of the structure of crust and uppermost mantle during the Miocene when the ALCAPA (Alpine, Carpathian, Pannonian) block was subject to E-directed lateral extrusion.

Time behavior of an electron beam current pulse in the axial and peripheral zones of an anode in vacuum and gas-filled diodes

The study of the time behavior of a current pulse of an electron beam generated during a high-voltage nanosecond discharge in gas-filled and vacuum diodes has been carried out. As follows from the experimental results, in both cases, the distribution of the beam current density in the plane of a grounded anode is non-uniform. The highest beam current density is recorded in the axial part of the anode. It was established that in the case of a gas-filled diode, ~ 2 ns after the onset of the beam current pulse, its shape in the axial anode zone changes relative to that in the peripheral one. It is assumed that the most probable reason for this is the effect of compensation of the charge of the beam electrons by the positive charge of ions arising in the ionization process in the paraxial zone.

INFLUENCE OF THE POURING DIRECTION ON THE QUALITY OF THE MELT ENTERING THE MOLD AND OBTAINING A GOOD STANDARD CAST IRON SAMPLE

The paper presents the results of computer simulation (on the example of grey cast iron of SCH 20 grade) of the influence of melt pouring direction on the degree of defects of shrinkage origin in the axial zone of the neck of a tensile sample when filling the mold cavity with a standard cast sample for optimal values of melt mass flow rate.

Cross-cutting palaeo-ice streams in NE-Iceland reveal shifting Iceland Ice Sheet dynamics

Ice streams are thought to have regulated the past Iceland Ice Sheet (IIS) during and following the Last Glacial Maximum (LGM) by discharging ice and sediment from the interior of the ice sheet towards the shelf edge. Previous assessments of ice streams within the IIS have produced contradictory reconstructions of ice sheet dynamics and location of ice divides. Here, we reconstruct palaeo-ice streams in NE-Iceland based on detailed mapping of streamlined subglacial bedforms (SSBs) and other glacial lineations. The morphometrics, distribution and orientation of the SSBs indicate the flow-sets of several cross-cutting palaeo-ice streams and the location and migration of ice divides, while high density and packing of SSBs possibly suggest the axial zones of the ice streams. North-trending flow-sets suggest that during maximum glaciation and ice-sheet thickness, ice flow within the study area was largely unaffected by underlying topography allowing ice to flow northwards across valleys and fjords. This ice flow direction correlates with the orientation of previously identified troughs on the north-eastern shelf and indicates that an E-W orientated ice divide was located to the south of the study area. During deglaciation and ice sheet thinning, a N-S orientated ice divide was located to the west of the study area, and palaeo-ice streams became confined to valleys and fjords, so that associated flow-sets crosscut the older ones. The absolute ages of these flow-sets are uncertain, but the simplest interpretation is that the older ones pertain to the LGM and the younger ones to the following deglaciation. This study sheds light on the dynamics and evolution of palaeo-ice streams within the IIS and highlights the importance of further efforts to understand the subglacial processes responsible for the formation of the streamlined subglacial bedforms as well as to constrain the evolution of shelf glaciation and chronology of ice stream retreat.

Eightfold Degenerate Fermions in Two Dimensions.

Multifold degenerate fermions have attracted a lot of research interest in condensed matter physics and materials science, but always lack in two dimensions. In this Letter, from symmetry analysis and lattice model construction, we demonstrate that eightfold degenerate fermions can be realized in two-dimensional systems. In nonmagnetic materials with negligible spin-orbit coupling, the gray magnetic space groups together with SU(2) spin rotation symmetry can protect the two-dimensional eightfold degenerate fermions on a certain high-symmetry axis in the Brillouin zone, no matter whether the system is centrosymmetric or noncentrosymmetric. In antiferromagnetic materials, the eightfold degenerate fermions can also be protected by certain "spin space groups." Furthermore, by first-principles electronic structure calculations, we predict that the paramagnetic phase of the monolayer LaB_{8} on a suitable substrate is a two-dimensional eightfold degenerate as well as Dirac node-line semimetal. Especially, the eightfold degenerate points are close to the Fermi level, which makes monolayer LaB_{8} a good platform to study the exotic physical properties of two-dimensional eightfold degenerate fermions.

Percutaneous hemiepiphysiodesis using transphyseal screws for adolescent tibia vara.

Hemiepiphysiodesis around the knee is becoming the mainstay procedure in adolescents for a wide range of aetiological deformities, when considering adolescent tibia vara (ATV), the published series have variable results. The purpose of this study was to review our experience with the percutaneous transphyseal screw (PETS) in these patients followed until bone maturity. We analysed the charts from 13 patients (20 knees) that underwent lateral tibial hemiepiphysiodesis using PETS. The radiographs were accessed before surgery, at implant removal, when occurred, and at the final follow-up. The clinical evaluation noted if there were complaints regarding pain or range of motion, and the radiographic assessment included: the femorotibial angle, the mechanical axis zone, the anatomic lateral distal femoral angle, and medial mechanical proximal tibial angle. There was one overcorrection, and after the screw removal (14 knees), rebound was observed in two knees modifying the result from excellent to good in all three knees. No bone bars and no implant breakage were observed. At the last appointment, all patients had normal knee range of motion, and two patients had unilateral alignment complaints, one of whom referred to occasional pain. Overall, the surgery was excellent in 12 knees (60%), good in six knees (30%), and poor in two knees (10%). This technique is indicated to be well tolerated and effective for treating ATV. When a complete correction cannot be obtained, in our opinion, it is advantageous to at least stabilise the deformity and postpone osteotomies until after skeletal maturity. Level of Evidence: Level IV - Case Series, Therapeutic Study.

Mapping the Evolution of Surface Strain in PtCo Core-Shell Catalysts By 4D-STEM

With the recent shift in attention from light-duty to heavy-duty vehicles for fuel cell research, the efficiency and durability requirements for catalyst materials have significantly increased [1]. As a result, it is more important than ever to understand the effects of degradation on catalyst nanoparticle surface structure and properties. Surface strain, for example, plays an important role in dictating the activity of catalyst nanoparticles [2], and understanding its evolution over the lifetime of a cell is therefore important. State-of-the-art PtCo core-shell catalyst nanoparticles contain complex strain gradients since, in addition to the surface strain that can occur for even single-metal nanoparticles [3], the lattice constant of the ordered PtCo core is smaller than that of the Pt shell [4]. These strain gradients may evolve over the lifetime of the cell due to cycling-induced metal dissolution, leading to compositional and shell thickness changes. Since these particles are generally <10 nm in diameter and have Pt shells only a few monolayers thick, a technique that can directly reveal their surface strain with high spatial resolution is needed to enable a better understanding of the relationship between surface strain and the performance and durability of heavy-duty fuel cells.High-resolution strain mapping can be performed by conventional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) techniques, for example through geometric phase analysis [5] or direct tracking of atomic column positions [6,7]. These methods require the material to be tilted to a low-order zone axis for data acquisition, a very time-consuming process for randomly oriented nanoparticles that significantly limits the number of particles, and hence the statistically relevant information, that can be included in a given study [6-8]. Four-dimensional (4D)-STEM provides an alternative approach for mapping strain at high resolution by tracking diffraction disk shifts in patterns recorded in a two-dimensional STEM scan [9]. Recently, 4D-STEM was shown to offer advantages over more conventional methods for nanoparticle strain mapping, such as reduced noise and/or artifacts [10]. While initial demonstrations of off-axis 4D-STEM strain measurements have been shown [11], strain analysis based on 4D-STEM typically still requires particles to be oriented on a zone axis, which again limits the number of particles measurable for a given study. A strain mapping technique that incorporates the strengths of 4D-STEM while allowing off-axis measurements to be made would therefore significantly increase the throughput of the technique, enabling statistically relevant information about particle surface strain as a function of size, composition, or cycling history to be obtained.In this talk, we will demonstrate advancements in 4D-STEM strain mapping that allow strain to be mapped in off-axis particles, therefore enabling hundreds of catalyst nanoparticles to be measured with unit-cell spatial resolution in a single instrument session. This allows mean strain profiles to be generated for the particles as a function of distance from the surface and correlate this with particle size, composition, and cycling history. By significantly increasing the throughput of high-resolution strain mapping, this technique paves the way to a better understanding of the role strain plays in catalyst performance and degradation, accelerating development of advanced heavy-duty fuel cell catalyst materials [12].

High-Speed Growth of Epitaxial Rutile GeO2 Thin Film on (001) TiO2 Under Highly Oxygen-Rich Condition and Its Structural Analysis

Recently, rutile germanium dioxide (r-GeO2) has been proposed as a novel ultra-wide bandgap (UWBG) semiconductor with ambipolar dopability, high carrier mobilities, and a higher thermal conductivity than Ga2O3, in addition to its wide bandgap (4.68 eV). These excellent properties indicate that r-GeO2 can be utilized as high-performance bipolar devices with high breakdown voltage. On the other hand, although r-GeO2 is most stable phase at the ambient condition, fabrication of its thin film is reported to be quite difficult, owing to the existence of energetically deep meta-stable phases, namely, amorphous and quartz phase, and high vapor pressure of r-GeO2. In this study, we accomplished fabrication of (001)-oriented r-GeO2 thin films on (001) rutile titanium dioxide (r-TiO2) substrate with high growth rate (≧1 mm/h) by mist chemical vapor deposition (mist CVD). Moreover, we investigated the structural properties of r-GeO2 film by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques.First of all, bis [2-carboxyethylgermanium (IV)] sesquioxide (C6H10Ge2O7) as a precursor. To analyze the decomposition process of C6H10Ge2O7 and optimize the growth conditions, Thermogravimetry-Differential Thermal Analysis (TG-DTA) was conducted in aqueous vapor. By the TG-DTA, GeO2 was found to be synthesized from C6H10Ge2O7 at 553-783 ℃ in aqueous vapor. Therefore, we determined the growth temperature of r-GeO2 thin film to be 550-850 ℃. As shown in XRD 2q/w scan profile (Fig. 1), (001)-oriented r-GeO2 thin film was grown on (001) r-TiO2 substrate at growth temperature (T g) of 700-775 ℃. The growth rates of all the r-GeO2 films were found to be 1.2-1.7 mm/h, which were much higher than those grown by molecular beam epitaxy (MBE) (10 nm/h). Moreover, XRD F scan revealed that r-GeO2 thin film was epitaxially grown without rotational domains.It is considered that the higher growth rate by mist CVD compared to that by MBE is attributed to the lower desorption rate of GeO in mist CVD due to its highly oxygen-rich condition. In Fig. 2, we show the schematics of GeO desorption from the surface in (a) MBE and (b) mist CVD, inspired by Wang et al. In mist CVD, oxygen partial pressure is much higher (~760 Torr) than that in MBE (1×10- 6 Torr), resulting lower vapor pressure of GeO and lower desorption rate in mist CVD system. Higher oxygen partial pressure also suppresses oxygen vacancies in the film. Moreover, in mist CVD, oxygen was continuously supplied by water in the reaction area. Therefore, such highly oxygen-rich condition as seen in mist CVD is considered to contribute to high temperature growth and high growth rate.Cross-sectional TEM image and Selected Area Electron Diffraction (SAED) patterns at r-TiO2 substrate and the r-GeO2/r-TiO2 interface viewed along the <110> zone axis are shown in Fig. 3 (a), (b), and (c), respectively. The r-GeO2 thin film for the TEM observation was grown at lower growth rate (50 nm/h) for higher crystallinity. In Fig. 3 (b), clear diffraction spots of r-TiO2 substrate were visible. On the other hand, in Fig. 3 (c), clear diffraction spots of r-GeO2 thin film were observed as well as those of r-TiO2 substrate at the interface. From the diffraction spots, we calculated lattice parameters of the r-GeO2 thin film to be a=4.5 Å and c=2.7 Å. This indicates that the film suffers from significant tensile stress along the in-plane direction, and in-plane strain value was ~2.3%. Thus, in order to release the strain and achieve higher crystalline quality, using a buffer layer or other substrates with less lattice mismatches to r-GeO2 may be needed.We believe that these results greatly contribute to the future development of r-GeO2 applied for power-electronics devices and exploration of novel materials that are difficult to synthesize under normal growth conditions.This work was supported, in part, by JSPS KAKENHI under Grant No. 21H01811. Figure 1

The Influence of Bilayer Graphene on the Structure of Electrodeposited Pd and CuCl Nanostructures

Pd was deposited on bilayer graphene to study how the interaction between the two affects the structure and strain of the metal. Pd was deposited through surface limited redox replacement using Cu as the sacrificial metal. The use of HCl as the electrolyte in the CuSO4 solution produced a prominent Cu UPD peak, facilitating the deposition. Two structures were found to be produced as a result of the deposition, nanoparticles and films. EELS confirmed that the particles were composed of Pd while the films were composed of Cu, and both contained Cl. Based on the lattices of the two structures imaged using STEM and their behavior under the beam, it was determined that the particles were pure Pd while the films were CuCl. The particles and films seemed to orient along a preferred zone axis, [110] for the Pd and [111] for the CuCl, and the particles and films which showed these preferred orientations also aligned in xy-space and possessed a slight tensile strain, a 1.5% tensile strain in the [001] direction for the Pd, and an isotropic 4.5% tensile strain for the CuCl. A lattice expansion is atypical for metallic nanoparticles, which, along with the consistency of orientation, indicates that the graphene exerts some influence on the two materials, although further study is needed to understand the mechanism behind this influence. The presence of Cl on the nanoparticles indicates that the release of Cl from PdCl2 is the rate limiting step during the reduction reaction.

Evidence of an active rift zone in the northern Okinawa Trough

The character of continental rifting processes in the northern part of the Okinawa Trough remains largely unknown due to the absence of an obvious axial rift zone or recent rift-related volcanism in the region. This paper describes the discovery of a tectonically and magmatically active rift zone in the northern trough, and discusses the nature of the rift. Ship-based bathymetry identified a group of NE–SW-trending surface lineaments on the western margin of the northern trough that form a small rift structure. The newly discovered rift, named the Uji Rift, is ~30 km long, ~10 km wide and ~180 m deep. A seismic reflection profile near the rift confirmed the existence of well-developed normal faults with large displacements, suggesting that the rift was formed by back-arc rifting. A previously unknown volcanic edifice, named West Uji Knoll, which consists of multiple cones with pronounced summits and high acoustic backscatter characteristics, also exists within the rift. High-resolution mapping with an autonomous underwater vehicle at one of the cones confirmed the existence of lava flow and eruptive fissure morphologies. Lava flows cover the pre-existing lineaments that form the Uji Rift. The flows are undissected by any lineaments, suggesting that West Uji Knoll is a young submarine volcano and the most recent eruptions likely occurred after rifting activity in the early or late Pleistocene. Active magmatism is also inferred from evidence of hydrothermal venting, such as the existence of an acoustic plume and sharp temperature increases of up to ~0.06 °C, even at a depth of ~100 m above the seafloor. In addition, three bubble plumes were detected near the cones by the shipboard multibeam sonar, suggesting emission of magmatic gases. Our volcano-tectonic observations show that the Uji Rift developed at an unusual location, ~170 km behind the arc volcanic front. Continental rifting in the northern Okinawa Trough is considered to extend onshore at the Beppu-Shimabara Rift Zone in SW Japan. However, due to the small number of submarine studies in this area, where and how the rifting is developing in the trough and has extended from the ocean to the continent is poorly understood. As an active rift, the Uji Rift provides an important constraint on the continuity of rifting in the Okinawa Trough, as well as new insights into the propagation and migration processes of continental rifting in subduction systems.

The Congo Basin: Subsurface structure interpreted using potential field data and constrained by seismic data

The Congo Basin (CB) occupies a large part of the Congo Craton, which formed from the amalgamation of different cratonic blocks of Archean age. It records the history of deposition of up to 1 Gyr of sediments, one of the longest geological records on Earth above a crystalline basement. It is considered as a typical intracratonic basin, with its slow and long-lived subsidence history and the largely unknown formation mechanisms. In this study, we analyze the gravity field in the central area of the CB, applying two different methodologies to investigate the depth of the crystalline rock basement and crustal structures. The crustal contribution to the gravity observations is determined in one case by a data driven approach, while in the other using an isostatic compensation model that includes the effect of the rigidity of the lithosphere. Both methods are constrained by density measurements from rock samples/boreholes and interpretations of the reflection seismic profiles, crossing the CB. The results obtained are quite consistent with the aeromagnetic anomalies, showing structural highs and lows, NW-SE trending, matching the axial magnetic zone. The maximum basement depth is between 10 and 18 km, depending on the gravity method and assumptions used. Furthermore, we could identify several small basins that from the interpolation of seismic profiles are not clearly resolved, such as Busira (7–9 km deep), Lokoro (10.5–12 km deep), and Salonga basins (10–11.5 km deep), in the northwestern, southeastern, and central sector of the CB, respectively. The analysis of the gravity anomalies mainly related to the crystalline crust supposes the presence of high-density bodies located in the southern part of the CB that we interpreted as related to the extensional phases that initiated the subsidence of the basin.

INVESTIGATING MICROANGIOPATHY USING SWEPT-SOURCE OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHY IN PATIENTS WITH SUSAC SYNDROME.

To determine whether optical coherence tomography angiography is of diagnostic utility for Susac syndrome (SuS) by quantifying microvascular retinal changes. We enrolled 18 eyes of 9 healthy controls and 18 eyes of 9 patients with chronic SuS (12 had previous branch retinal artery occlusions and 6 were clinically unaffected). Images of the fovea were taken using an optical coherence tomography angiography system. Analysis included vessel density, fractal dimension, vessel diameter, and measurements of the foveal avascular zone (area, eccentricity, acircularity index, and axis ratio) in deep and superficial retinal layers. Skeleton density and inner ring vessel density were significantly lower in patients with SuS (skeleton density: Susac 0.11 ± 0.01 vs. controls 0.12 ± 0.01, P = 0.027. VD: SuS 0.39 ± 0.04 vs. controls 0.42 ± 0.02, P = 0.041). Eccentricity and axis ratio were significantly higher in patients with SuS (EC: Susac 0.61 ± 0.11, controls 0.51 ± 0.10, P = 0.003; axis ratio: Susac 1.57 ± 0.28, controls 1.39 ± 0.11, P = 0.005). SuS eyes (affected and unaffected) had poorer outcomes of the remaining vascular parameters compared with controls (P > 0.05). Optical coherence tomography angiography identified chronic microvascular changes in the eyes of patients with chronic SuS. Even clinically unaffected SuS eyes showed poorer vascular parameters. Although further research is needed, this noninvasive imaging modality seems to have the potential to serve as a valuable additive diagnostic tool.

High‐Resolution Isotopic Variability Across EPR Segment 16°N: A Chronological Interpretation of Source Composition and Ridge‐Seamount Interaction

AbstractA high‐resolution sampling profile constituted of 52 basalt samples from across the East Pacific Rise (EPR) was investigated. These samples provide a unique opportunity to study the coeval recording of isotopic signals derived from sub‐marine eruptions at a fast spreading ridge over a time interval of ∼320 kyrs. Additionally, the study area is marked by the presence of a neighboring seamount chain that has recently caused the ridge to jump twice toward it. Combining previous geochemical studies and bathymetry, we established a first‐order chronology between analyzed samples, and have reconstructed the evolution of basalt compositions as the ridge and seamounts advance and finally merge. Our data reveal the existence of two distinct types of isotopic variability within the samples. One that has a low amplitude and frequency and is accounted for by the continuous melting of the ambient mantle, indicating a process with a ∼125 kyr periodicity. The other, of higher amplitude, is discontinuous in time, and likely reflects the seamounts source influence on the ridge. Our results support a two‐step hotspot‐ridge interaction including a first stage (≥600 ka) of regional enrichment of the depleted ridge mantle by hotspot material; and a second, more recent (at least 300 ka) even wherein ambient mantle melts mixed with proximal melts from heterogeneous seamounts sourced nearby. We also propose that the ancient gabbroic component previously identified in this region appeared very recently (&lt;hundreds ya) as erupted lavas are exclusively recorded in a cluster proximal to the axial zone of the ridge.

Variscan U-Th-Pb age for stratabound Pb-Zn mineralization in the Bossòst dome (Pyrenean Axial Zone)

In orogens, pinpointing the timing of mineralization is often complex due to superimposition of magmatic, metamorphic and tectonic events. Unravelling the tectono-metamorphic evolution of deformed terranes is essential for understanding the formation of orogenic mineralization. In the Bossòst dome of the Pyrenean Axial Zone, undated stratabound Pb-Zn mineralization previously considered to be SEDEX-like and devoid of evidence for significant remobilization/deformation, was recently interpreted as orogenic mineralization and structurally-controlled by Variscan tectonics. However, relations with the poly-magmatic and metamorphic events reported in the Pyrenean Axial Zone are still poorly constrained. In the Bossòst dome, laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Th-Pb dating was performed on zircon to constrain the formation ages of two undeformed granitic dykes, and on monazite to identify both the metamorphic imprint recorded by two metapelites and the formation of stratabound Pb-Zn mineralization in the Bentaillou deposit. late-Carboniferous – early-Permian magmatic zircon ages (315–280 Ma) are commonly found in these undeformed dykes (307.4 ± 4.7 Ma and 283 ± 15 Ma), consistent with the main magmatic event, extensively recorded in the Pyrenean Axial Zone. Monazite crystals with co-genetic textural relationships with stratabound sphalerite from Bentaillou were dated at 309 ± 11 Ma. We propose that the stratabound mineralization is formed during one main Variscan remobilization event associated with the first Variscan deformation event (D1) and probable metamorphic fluid circulation in pre-existing metal-rich Ordovician sedimentary levels. Moreover, monazite ages in metapelite rocks (289.1 ± 5.9 Ma and 290.0 ± 3.8 Ma) probably date both the end of high T – low P metamorphism and late fluid circulation in the Variscan tectonothermal event. Visean-Serpukhovian ages (340–325 Ma) are found in one inherited zircon and in two monazites included in garnet and staurolite, which probably record the end of the first Variscan magmatic-metamorphic event, mainly recorded in the core of the Pyrenean Axial Zone. Based on our ages and a regional synthesis, we propose a new tectono-metamorphic model for the Bossòst dome and the related formation of Pb-Zn mineralization.