Internal geometry of the central Sesia Zone (Aosta Valley, Italy): HP tectonic assembly of continental slices

Detailed field mapping reveals that the Sesia Zone is subdivided into two complexes with the Barmet Shear Zone (BSZ) outlining the tectonic contact between them. This greenschist-facies contact reflects a metamorphic gap between the Internal Complex (eclogite facies, eclogitic micaschists dominant) and the External Complex (epidote blueschist facies). The BSZ comprises a wedge shape area in which fragments and slices of orthogneiss and paragneiss are wrapped by siliceous dolomite marbles displaying a mylonitic foliation. Conspicuous cornieules and high pressure breccias occur along this contact. We propose that the eclogite facies Internal Complex is subdivided into three basement units, called sheets, delimited by discontinuous metasedimentary trails of probable Mesozoic age. Thin monocyclic bands thus separate kilometre scale polycyclic sheets. The External Complex comprises three epidote blueschist facies sheets of comparable size, which are separated by lenses retaining a pre-Alpine high temperature imprint. These weakly overprinted fragments (parts of the classically termed 2DK zone) are aligned along greenschist facies shear zones that separate the gneissic sheets. The BSZ, with a wedge rich in meta-sediments, chiefly siliceous dolomite marbles, is a key element in which fragmentation and reworking of materials from the internal and external complexes are evident. A carbonate breccia occurs in this shear zone, with clasts displaying a HP foliation randomly oriented in a ductile carbonate matrix. Siliceous dolomite marbles appear to have acted as lubricants to accommodate deformation related to the juxtaposition of the two basement complexes during exhumation. We propose a model of the Sesia Zone, with the BSZ as the thrust responsible for the juxtaposition of eclogite facies rocks of the Internal Complex on top of epidote blueschist facies rocks of the External Complex. The two complexes were already assembled when this shear zone became active. The entire stack was finally rotated (40\u201360\ub0) during the Vanzone Phase

The Effects of Retrograde Reactions and of Diffusion on 40Ar-39Ar Ages of Micas

The effects of metamorphic reactions occurring during decompression were explored to understand their influence on the 40Ar-39Ar ages of micas. Monometamorphic metasediments from the Lepontine Alps (Switzerland) reached lower amphibolite facies during the Barrovian metamorphism related to the collision between European and African (Adria) continental plates. Mineral assemblages typically composed of garnet, plagioclase, biotite, muscovite and paragonite (or margarite) were screened for petrological equilibrium, to focus on samples that record a minimum degree of retrogression. X-ray diffraction data indicate that some mineral separates prepared for 40Ar-39Ar stepwise heating analysis are monomineralic, whereas others are composed of two white micas (muscovite with paragonite or margarite), or biotite and chlorite. In monomineralic samples 37Ar/39Ar and 38Ar/39Ar (proportional to Ca/K and Cl/K ratios) did not change and the resulting ages can be interpreted unambiguously. In mineral separates containing two white micas, Ca/K and Cl/K ratios were variable, reflecting non-simultaneous laboratory degassing of the two heterochemical Ar reservoirs. These ratios were used to identify each Ar reservoir and to unravel the age. In a chlorite-margarite-biotite calcschist equilibrated near 560°C and 0·65 GPa, biotite, margarite, and muscovite all yield ages around 18 Ma. At slightly higher grade (560-580°C, 0·8-0·9 GPa), the assemblage muscovite-paragonite-plagioclase is in equilibrium and remains stable during retrogression. In this case, muscovite and paragonite yield indistinguishable ages around 16·5 Ma. Above 590°C, paragonite was mostly consumed to form plagioclase >590°C, whereby the relict mica yields an age up to 5·6 Ma younger than muscovite. This partial or total resetting of the Ar clock in paragonite is interpreted to reflect plagioclase growth during decompression. Where biotite is present within this same assemblage, it systematically yields a younger age than muscovite, by 0·5-2 Ma. However, these biotites all show small amounts of retrograde chlorite formation. We conclude that even very minor chloritization of biotite is apparently a more effective process than temperature in resetting the Ar clock, as is the formation of plagioclase from paragonite decomposition. Multi-equilibrium thermobarometry is an excellent means to ensure that equilibrium in investigated samples is preserved, and this helps to obtain geologically meaningful metamorphic ages. However, even samples passing such equilibrium tests may still show retrograde effects that affect the Ar retention of micas. A more robust interpretation of such 40Ar-39Ar results may require use of a second geochronometer, such as U-Pb on monazit

An inverse modeling approach to obtain P-T conditions of metamorphic stages involving garnet growth and resorption

This contribution presents an approach and a computer program (GRTMOD) for numerical simulation of garnet evolution based on compositions of successive growth zones in natural samples. For each garnet growth stage, a new local effective bulk composition is optimized, allowing for resorption and/or fractionation of previously crystallized garnet. The successive minimizations are performed using the Nelder-Mead algorithm; a heuristic search method. An automated strategy including two optimization stages and one refinement stage is described and tested. This program is used to calculate pressure-temperature (P-T) conditions of crystal growth as archived in garnet from the Sesia Zone (Western Alps). The compositional variability of successive growth zones is characterized using standardized X-ray maps and the program XMapTools. The model suggests that Permian garnet cores crystallized under granulite-facies conditions at T>800 °C and P = 6 kbar. During Alpine times, a first garnet rim grew at eclogite-facies conditions (650 °C, 16 kbar) at the expense of the garnet core. A second rim was added at lower P (∼11 kbar) and 630 °C. In total, garnet resorption is modeled to amount to ∼9 vol% during the Alpine evolution; this value is supported by our observations in X-ray compositional maps

Derivation of Internally-Consistent Thermodynamic Data by the Technique of Mathematical Programming: a Review with Application the System MgO-SiO2-H2O

The problem of deriving an optimal set of thermodynamic properties of minerals from a diverse experimental data base is reviewed and a preferred methodology proposed. Mathematical pro-gramming(MAP) methods extend the linear programming (LIP) approach first presented by Gordon (1973), and make it possible to account for the type of information conveyed, and the uncertainties attending both phase equilibrium data and direct measurements of phase properties. For phase equilibrium data which are (in most cases) characterized by non-normal error distributions across experimental brackets, the midpoint of a bracket is no more probable than other points, and the data are best treated by considering the inequality in the change in Gibbs free energy of reaction at each half-bracket. Direct measurements of phase properties can be assumed to have approximately normal error distributions, and the MAP technique optimizes agreement with these values by using the principles of least squares in the definition of an objective function. The structure of this problem, treatment of uncertainties in various types of experimental data, and method of optimizing final solutions are discussed in some detail. The method is applied to experimental data in the MgO-SiO2-H2O system, where inconsistencies among the data are resolved and an optimal set of thermodynamic properties is presented. The derived standard state entropies and volumes agree with all direct measurements (within their uncertainties), as do enthalpies of formation from the elements except for those of talc (+16 kJ mol−1), anthophyllite (+ 14 kJ mol−1), and brucite (−1 kJ mol−1). Stable phase relations in the system have the topology predicted by Greenwood (1963, 1971), with quartz- and forsterite-absent invariant points at 683 °C-6-4 kb and 797 °C-12 kb respectively, repeating at 552 °C-120 b and 550 °C-55 b. The thermodynamic analysis indicates little remaining flexibility in the phase relations, which, when combined with suitable activity models for solid solution, should allow for accurate determination of the conditions of metamorphism of ultramafic rock

Multiple Metamorphic Stages within an Eclogite-facies Terrane (Sesia Zone, Western Alps) Revealed by Th-U-Pb Petrochronology

Convergent plate margins typically experience a transition from subduction to collision dynamics as massive continental blocks enter the subduction channel. Studies of high-pressure rocks indicate that tectonic fragments are rapidly exhumed from eclogite facies to mid-crustal levels, but the details of such dynamics are controversial. To understand the dynamics of a subduction channel we report the results of a petrochronological study from the central Sesia Zone, a key element of the internal Western Alps. This comprises two polymetamorphic basement complexes (Eclogitic Micaschist Complex and Gneiss Minuti Complex) and a thin, dismembered cover sequence (Scalaro Unit) associated with pre-Alpine metagabbros and metasediments (Bonze Unit). Structurally controlled samples from three of these units (Eclogitic Micaschist Complex and Scalaro-Bonze Units) yield unequivocal petrological and geochronological evidence of two distinct high-pressure stages. Ages (U-Th-Pb) of growth zones in accessory allanite and zircon, combined with inclusion and textural relationships, can be tied to the multi-stage evolution of single samples. Two independent tectono-metamorphic ‘slices' showing a coherent metamorphic evolution during a given time interval have been recognized: the Fondo slice (which includes Scalaro and Bonze rocks) and the Druer slice (belonging to the Eclogitic Micaschist Complex). The new data indicate separate stages of deformation at eclogite-facies conditions for each recognized independent kilometer-sized tectono-metamorphic slice, between ∼85 and 60 Ma, with evidence of intermittent decompression (ΔP ∼ 0·5 GPa) within only the Fondo slice. The evolution path of the Druer slice indicates a different P-T-time evolution with prolonged eclogite-facies metamorphism between ∼85 and 75 Ma. Our approach, combining structural, petrological and geochronological techniques, yields field-based constraints on the duration and rates of dynamics within a subduction channe

Validation of the global lung initiative 2012 multi-ethnic spirometric reference equations in healthy urban Zimbabwean 7-13 year-old school children: a cross-sectional observational study.

BACKGROUND: The 2012 Global Lung Function Initiative (GLI2012) provide multi-ethnic spirometric reference equations (SRE) for the 3-95 year-old age range, but Sub-Saharan African populations are not represented. This study aimed to evaluate the fit of the African-American GLI2012 SRE to a population of healthy urban and peri-urban Zimbabwean school-going children (7-13 years). METHODS: Spirometry and anthropometry were performed on black-Zimbabwean children recruited from three primary schools in urban and peri-urban Harare, with informed consent and assent. Individuals with a history or current symptoms of respiratory disease or with a body mass index-z score (BMI) < - 2 were excluded. Spirometry z-scores were generated from African-American GLI2012 SRE, which adjust for age, sex, ethnicity and height, after considering all GLI2012 modules. Anthropometry z-scores were generated using the British (1990) reference equations which adjust for age and sex. The African-American GLI2012 z-score distribution for the four spirometry measurements (FVC, FEV1, FEV1/FVC and MMEF) were evaluated across age, height, BMI and school (as a proxy for socioeconomic status) to assess for bias. Comparisons between the African-American GLI2012 SRE and Polgar equations (currently adopted in Zimbabwe) on the percent-predicted derived values were also performed. RESULTS: The validation dataset contained acceptable spirometry data from 712 children (344 girls, mean age: 10.5 years (SD 1.81)). The spirometry z-scores were reasonably normally distributed, with all means lower than zero but within the range of ±0.5, indicating a good fit to the African-American GLI2012 SRE. The African-American GLI2012 SRE produced z-scores closest to a normal distribution. Z-scores of girls deviated more than boys. Weak correlations (Pearson's correlation coefficient < 0.2) were observed between spirometry and anthropometry z-scores, and scatterplots demonstrated no systematic bias associated with age, height, BMI or socioeconomic status. The African-American GLI2012 SRE provided a better fit for Zimbabwean paediatric spirometry data than Polgar equations. CONCLUSION: The use of African-American GLI2012 SRE in this population could help in the interpretation of pulmonary function tests

Dynamics in the Sesia HP terrane: Combined petrochronological and structural analysis

HP terranes dominated by continental crust represent the end result of a sequence of processes that operate at lithosphere scale, i.e. rifting, subduction/accretion, return flow/exhumation. To under\uacstand the dynamics of the subduction channel in complex terranes of this kind, the effects from each stage must be investigated separately, linking the observations and data from kilometers down to micrometer scale. This task recommends an integrative approach. Here we focus on the assembly of the Sesia Zone (SZ), a key element of the internal Western Alps. This terrane comprises two main polymetamorphic base\uacment units and thin trails of a cover sequence that includes post-Permian syn- to post-rift metasediments; the latter show no pre-Alpine metamorphic imprint. The tectonic scenario of Babist et al. (2006) recognizes five main phases in the Alpine structural evolution; their model helped us select areas for detailed structural work and sampling. Our first goal was to relate the early convergent structures (D1, D2) to the P-T evolution and to establish a robust time-frame for the HP-dynamics within and between the tectonic slices. Within the subduction/extrusion channel, problems addressed include the question of tectonic mixing, i.e. temporal and spatial scales of relative and absolute movement of the slices, and the conditions and timing of their final juxtaposition prior to the rapid exhumation of the Sesia Zone as a whole. Mono- and polymetamorphic sediments from different slices display unequivocal evidence of several HP-stages separated in time. Successive stages under eclogite facies conditions occurred between 86 \u2013 65 Ma, as shown by LA-ICP-MS and SHRIMP data on growth zones in accessory allanite, monazite, zircon, and titanite. By using mutual inclusions and overgrowth relationships, the age-data on allanite and monazite can be tied to the multistage evolution of an individual sample. For different rocks, these (over)growth stages can be related to D1- and D2-deformation when micro-, meso- and megastructural observations are combined. Thermobarometry indicates intermittant decompression by ~0.8 GPa between HP phases, hence pressure cycling (aka yo-yo tectonics, Rubatto et al., 2011). This tectonic mobility occurred prior to the final juxtaposition of slices and their exhumation, which involved at least two major deformation phases and lead to widespread retrogression at amphibolite to green-schist facies conditions. Our approach combining structural, petrological, and geochronological techniques yields some field-based constraints on the duration and rates of the dynamics within a subduction channel. It may be useful to compare these to insights from numerical models, provided the latter take into account the specific conditions of the plate convergence, which turns out to have been highly oblique in the present case

Social Media Usage and Women's Empowerment in Qatar: Evidence from a National Representative Survey

Introduction Social media usage has grown in importance over the past decade, particularly in Middle East. During this period, it has come to be recognized as more than a tool for social connections between individuals, with research demonstrating how it can empowering people socially, politically and economically[1]. Past research has demonstrated the impact of social media in areas as diverse as such as healthcare provision, political participation, and marketing[2]. However, how and to what extent social media usage contributes to women's empowerment in the Arab region has not been thoroughly examined. Academic and non-academic studies alike have focused heavily on a few well-known cases of social media activism and woman's rights, such as, women driving in Saudi Arabia[3] or the Arab Spring in Egypt[4]. These studies raise a number of relevant questions. Is social media potent in the population more broadly, or is it just a tool for activists and polemics? If it does empower women, does it do so by shaping deeply rooted attitudes and norms or does it mere reflect the value of the general population? How can we understand the role of traditional education in shaping attitudes and values in light of the large volumes of readily available information provided by social media? The current paper examines social media usage in Qatar and its influences on attitudes toward women's empowerment and considers the complex role of education. We use a national representative survey data from a 2017 survey of Qatari nationals to test our expectations. In the last twenty years, Qatar has experienced a significant shift in economic development, and education accessibility. The State of Qatar has widely sought to include women in public life, particularly in the process of decision-making. Yet, the debate about the role of social media in increasing women's awareness of their equal rights in holding positions of power and contributing to their political and economic empowerment is not widely discussed in Qatar. Thus, Qatar is a case where researchers might expect to find changing attitudes in spite of the absence of major social media feminist activists. The paper also explores a number of trends emerging in the analysis of social media usage, across demographic groups such as age, gender, marital status and socio-economic differences including education levels and household income. Methodology A questionnaire was initially designed in English and then translated into Arabic by the Social and Economic Survey Research Institute (SESRI) to collect all necessary information related to the study. In this survey, the target population included people who are 18 years or older and live in residential housing units in Qatar during the survey reference period (May 6th-27th, 2017). It includes groups of Qataris and expatriates, though the analysis in this paper focuses on Qataris. It is important to note that while preparing the sample for the survey, the Qatari population sub-group was over sampled in order to ensure that Qataris were well represented in the survey. The survey was administered in CAPI (computer assisted data collection) method for face to face interviews. The interviews were conducted at the home of the respondent. In terms of data analysis, all individual interviews were merged and saved in a single BLAISE data file. This dataset was then cleaned, coded and saved in STATA formats for analysis. Analysis of the data included order logistical regression with appropriate calculation of interactive effects and predicted probabilities for interpretation. Findings and Implications Data from the 2017 survey are newly available and thus the findings discussed here are preliminary. We find that social media usage in the examined population is related to increased support for woman in community leadership among Qatari females but not males. This finding is robust to a number of statistical controls, including for the respondent's level of education. In fact, a respondent's level of formal education has much less predictive power than social media usage among female Qataris. Furthermore, we find that social media usage is only weakly related to political variables, such as interest in politics. This suggests that social media may be working to empower women first through changing gender attitudes and only secondly through engagement with the political sphere. These findings are relevant to both academics who are seeking to understand the mechanisms through which social media may empower women and policymakers in Qatar who are concerned with advancing their welfare. It shows that woman can use social media to find their voice and engage with the public sphere, even where political activism has not been common. [1] Dubai School of Government, "Arab Social Media Report, Vol 1, No. 3, November 2011. [2] TNS, "Arab Social Media Report", First Report 2015. [3] Begum, Rothna, "The Brave Female Activists Who Fought to Lift Saudi Arabia's Driving Ban", News Deeply, September 29th, 2017. [4] Tufekci, Zeynep and Wilson, Christopher, "Social Media and the Decision to Participate in Political Protest: Observations From Tahrir Square", Journal of Communication, 62 (2012) 363-37.qscienc