Tectonometamorphic evolution of the Atbashi high‐P units (Kyrgyz CAOB,Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

The South Tien Shan (STS) belt results from the last collision event in the western Central Asian Orogenic Belt (CAOB). Understanding its formation is of prime importance in the general framework of the CAOB. The Atbashi Range preserves high‐P (HP) rocks along the STS suture, but still, its global metamorphic evolution remains poorly constrained. Several HP units have been identified: (a) a HP tectonic mélange including boudins of mafic eclogites in a sedimentary matrix, (b) a large (>100 km long) high‐P metasedimentary unit (HPMU) and (c) a lower blueschist facies accretionary prism. Raman Spectroscopy on carbonaceous material combined with phengite and chlorite multiequilibria and isochemical phase diagram modelling indicates that the HPMU recorded homogeneous P–T conditions of 23–25 kbar and 560–570°C along the whole unit. ⁴⁰Ar/³⁹Ar dating on phengite from the HPMU ranges between 328 and 319 Ma at regional scale. These ages are interpreted as (re‐) crystallization ages of phengite during T_max conditions at a pressure range of 20–25 kbar. Thermobarometry on samples from the HP tectonic mélange provides similar metamorphic peak conditions. Thermobarometry on the blueschist to lower greenschist facies accretionary prism indicates that it underwent P–T conditions of 5–6 kbar and 290–340°C, highlighting a 17–20 kbar pressure gap between the HPMU‐tectonic mélange units and the accretionary prism. Comparison with available geochronological data suggests a very short time span between the prograde path (340 Ma), HP metamorphic peak (330 Ma), the T_max (328–319 Ma) and the final exhumation of the HPMU (303–295 Ma). Extrusion of the HPMU, accommodated by a basal thrust and an upper detachment, was driven by buoyant forces from 70–75 km up to 60 km depth, which directly followed continental subduction and detachment of the HPMU. At crustal depths, extrusion was controlled by collisional tectonics up to shallow levels. Lithological homogeneity of the HPMU and its continental‐derived character from the North Tien Shan suggest this unit corresponds to the hyper‐extended continental margin of the Kazakh continent, subducted southward below the north continental active margin of the Tarim craton. Integration of the available geological data allows us to propose a general geodynamic scenario for Tien Shan during the Carboniferous with a combination of (a) N‐dipping subduction below the Kazakh margin of Middle Tien Shan until 390–340 Ma and (b) S‐dipping subduction of remaining Turkestan marginal basins between 340 and 320 Ma.     

Evaluation of a high-resolution regional climate simulation over Greenland

A simulation of the 1991 summer has been performed over south Greenland with a coupled atmosphere–snow regional climate model (RCM) forced by the ECMWF re-analysis. The simulation is evaluated with in-situ coastal and ice-sheet atmospheric and glaciological observations. Modelled air temperature, specific humidity, wind speed and radiative fluxes are in good agreement with the available observations, although uncertainties in the radiative transfer scheme need further investigation to improve the model’s performance. In the sub-surface snow-ice model, surface albedo is calculated from the simulated snow grain shape and size, snow depth, meltwater accumulation, cloudiness and ice albedo. The use of snow metamorphism processes allows a realistic modelling of the temporal variations in the surface albedo during both melting periods and accumulation events. Concerning the surface albedo, the main finding is that an accurate albedo simulation during the melting season strongly depends on a proper initialization of the surface conditions which mainly result from winter accumulation processes. Furthermore, in a sensitivity experiment with a constant 0.8 albedo over the whole ice sheet, the average amount of melt decreased by more than 60%, which highlights the importance of a correctly simulated surface albedo. The use of this coupled atmosphere–snow RCM offers new perspectives in the study of the Greenland surface mass balance due to the represented feedback between the surface climate and the surface albedo, which is the most sensitive parameter in energy-balance-based ablation calculations.     

Environmental Radioxenon Levels in Europe: a Comprehensive Overview

Activity concentration data from ambient radioxenon measurements in ground level air, which were carried out in Europe in the framework of the International Noble Gas Experiment (INGE) in support of the development and build-up of a radioxenon monitoring network for the Comprehensive Nuclear-Test-Ban Treaty verification regime are presented and discussed. Six measurement stations provided data from 5 years of measurements performed between 2003 and 2008: Longyearbyen (Spitsbergen, Norway), Stockholm (Sweden), Dubna (Russian Federation), Schauinsland Mountain (Germany), Bruyères-le-Châtel and Marseille (both France). The noble gas systems used within the INGE are designed to continuously measure low concentrations of the four radioxenon isotopes which are most relevant for detection of nuclear explosions: ^131mXe, ^133mXe, ¹³³Xe and ¹³⁵Xe with a time resolution less than or equal to 24 h and a minimum detectable concentration of ¹³³Xe less than 1 mBq/m³. This European cluster of six stations is particularly interesting because it is highly influenced by a high density of nuclear power reactors and some radiopharmaceutical production facilities. The activity concentrations at the European INGE stations are studied to characterise the influence of civilian releases, to be able to distinguish them from possible nuclear explosions. It was found that the mean activity concentration of the most frequently detected isotope, ¹³³Xe, was 5–20 mBq/m³ within Central Europe where most nuclear installations are situated (Bruyères-le-Châtel and Schauinsland), 1.4–2.4 mBq/m³ just outside that region (Stockholm, Dubna and Marseille) and 0.2 mBq/m³ in the remote polar station of Spitsbergen. No seasonal trends could be observed from the data. Two interesting events have been examined and their source regions have been identified using atmospheric backtracking methods that deploy Lagrangian particle dispersion modelling and inversion techniques. The results are consistent with known releases of a radiopharmaceutical facility.     

An improved spectral turning-bands algorithm for simulating stationary vector Gaussian random fields

We propose a spectral turning-bands approach for the simulation of second-order stationary vector Gaussian random fields. The approach improves existing spectral methods through coupling with importance sampling techniques. A notable insight is that one can simulate any vector random field whose direct and cross-covariance functions are continuous and absolutely integrable, provided that one knows the analytical expression of their spectral densities, without the need for these spectral densities to have a bounded support. The simulation algorithm is computationally faster than circulant-embedding techniques, lends itself to parallel computing and has a low memory storage requirement. Numerical examples with varied spatial correlation structures are presented to demonstrate the accuracy and versatility of the proposal.     

A Top-Cut Model for Deposits with Heavy-Tailed Grade Distribution

In some ore deposits, the grade distribution is heavy-tailed and high values make the inference of first- and second-order statistics nonrobust. In the case of gold data, high values are usually cut and the block estimation is performed using truncated grades. With this method, a part of the deposit is omitted, resulting in a potential bias on resources figures. Ad-hoc corrections can be applied on the final figures to take into account the tail of the grade distribution, but no theoretical guidelines are available. A geostatistical model has been developed to handle high values based on the assumption that for high grade zones, the only tangible information is the geometry. The grade variable z can be split into three parts: the truncated grade ( $\operatorname{Min} (z, z_{\mathrm{e}})$ where z _e is the top-cut grade), a weighted indicator above top-cut grade (1{z≥z _e}), and a residual. Within this framework, the residual is poorly structured, and in most cases is a pure nugget-effect. Moreover, it has no spatial correlation with the truncated grade and the indicator above top-cut grade. This decomposition makes the variographic study more robust because variables (indicator and truncated grade) do not keep high grade values. The underlying hypotheses can be tested on experimental indicator variograms and the top-cut grade can be justified. Finally, the estimation is based on a truncated grade and indicator cokriging. The model is applied to blast holes from a gold deposit and on a simulated example. Both cases illustrate the benefits of keeping the high values in the estimation process via an appropriate mathematical model.     

Spatiotemporal patterns of burned areas,fire drivers,and fire probability across the equatorial Andes

Field-based fire studies in the equatorial Andes indicate that fires are strongly associated with biophysical and anthropogenic variables. However, fire controls and fire regimes at the regional scale remain undocumented. Therefore, this paper describes spatial and temporal burned-area patterns, identifies biophysical and anthropogenic fire drivers, and quantifies fire probability across 6° of latitude and 3° of longitude in the equatorial Andes. The spatial and temporal burned-area analysis was carried out based on 18 years(2001-2018) of the MCD64 A1 MODIS burned-area product. Climate, topography, vegetation, and anthropogenic variables were integrated in a logistic regression model to identify the significance of explanatory variables and determine fire occurrence probability. A total of 5779 fire events were registered during the 18 years of this study, located primarily along the western cordillera of the Andes and spreading from North to South. Eighty-eight percent of these fires took place within two fire hotspots located in the northwestern and southwestern corners of the study area. Ninety-nine percent occurred during the second part of the year, between June and December. The largest density of fires was primarily located on herbaceous vegetation and shrublands. Results show that mean monthly temperature, precipitation and NDVI during the prefire season, the location of land cover classes such as forest and agriculture, distance to roads and urban areas, slope, and aspect were the most important determinants of spatial and temporal fire distribution. The logistic regression model achieved a good accuracy in predicting fire probability(80%). Probability was higher in the southwestern and northern corners of the study area, and lower towards the north in the western and eastern piedmonts of the Andes. This analysis contributes to the understanding of fires in mountains within the tropics. The results here presented have the potential to contribute to fire management and control in the region.     

Geostatistical simulation of random fields with bivariate isofactorial distributions by adding mosaic models

This work deals with the geostatistical simulation of a family of stationary random field models with bivariate isofactorial distributions. Such models are defined as the sum of independent random fields with mosaic-type bivariate distributions and infinitely divisible univariate distributions. For practical applications, dead leaf tessellations are used since they provide a wide range of models and allow conditioning the realizations to a set of data via an iterative procedure (simulated annealing). The model parameters can be determined by comparing the data variogram and madogram, and enable to control the spatial connectivity of the extreme values in the realizations. An illustration to a forest dataset is presented, for which a negative binomial model is used to characterize the distribution of coniferous trees over a wooded area.     

Analysis of seafloor backscatter strength dependence on the survey azimuth using multibeam echosounder data

The sediment backscatter strength measured by multibeam echosounders is a key feature for seafloor mapping either qualitative (image mosaics) or quantitative (extraction of classifying features). An important phenomenon, often underestimated, is the dependence of the backscatter level on the azimuth angle imposed by the survey line directions: strong level differences at varying azimuth can be observed in case of organized roughness of the seabed, usually caused by tide currents over sandy sediments. This paper presents a number of experimental results obtained from shallow-water cruises using a 300-kHz multibeam echosounder and specially dedicated to the study of this azimuthal effect, with a specific configuration of the survey strategy involving a systematic coverage of reference areas following “compass rose” patterns. The results show for some areas a very strong dependence of the backscatter level, up to about 10-dB differences at intermediate oblique angles, although the presence of these ripples cannot be observed directly—neither from the bathymetry data nor from the sonar image, due to the insufficient resolution capability of the sonar. An elementary modeling of backscattering from rippled interfaces explains and comforts these observations. The consequences of this backscatter dependence upon survey azimuth on the current strategies of backscatter data acquisition and exploitation are discussed.     

Tagging-the-triangle algorithm for partitioning features with inconsistent boundaries

In spatial data sets, gaps or overlaps among features are frequently found in spatial tessellations due to the non-abutting edges with adjacent features. These non-abutting edges in loose tessellations are also called inconsistent boundaries or slivers; polygons containing at least one inconsistent boundary are called inconsistent polygons or sliver polygons. The existing algorithms to solve topological inconsistencies in sliver polygons suffer from one or more of three major issues, namely determination of tolerances, excessive CPU processing time for large data sets and loss of vertex history. In this article, we introduce a new algorithm that mitigates these three issues. Our algorithm efficiently searches the features with inconsistent polygons in a given spatial data set and logically partitions them among adjacent features. The proposed algorithm employs the constrained Delaunay triangulation technique to generate labelled triangles from which inconsistent polygons with gaps and overlaps are identified using label counts. These inconsistent polygons are then partitioned using the straight skeleton method. Moreover, each of these partitioned gaps or overlaps is distributed among the adjacent features to improve the topological consistency of the spatial data sets. We experimentally verified our algorithm using the real land cadastre data set. The comparison results show that the proposed algorithm is four times faster than the existing algorithm for data sets with 200,000 edges.