Timing of UHP metamorphism in the Hong’an area,western Dabie Mountains,China: evidence from zircon U–Pb age,trace element and Hf isotope composition

The Hong’an area (western Dabie Mountains) is the westernmost terrane in the Qinling-Dabie-Sulu orogen that preserves UHP eclogites. The ages of the UHP metamorphism have not been well constrained, and thus hinder our understanding of the tectonic evolution of this area. LA-ICPMS U–Pb age, trace element and Hf isotope compositions of zircons of a granitic gneiss and an eclogite from the Xinxian UHP unit in the Hong’an area were analyzed to constrain the age of the UHP metamorphism. Most zircons are unzoned or show sector zoning. They have low trace element concentrations, without significant negative Eu anomalies. These metamorphic zircons can be further subdivided into two groups according to their U–Pb ages, and trace element and Lu–Hf isotope compositions. One group with an average age of 239 ± 2 Ma show relatively high and variable HREE contents (527 ≥ Lu_N ≥ 14) and ¹⁷⁶Lu/¹⁷⁷Hf ratios (0.00008–0.000931), indicating their growth prior to a great deal of garnet growth in the late stage of continental subduction. The other group yields an average age of 227 ± 2 Ma, and shows consistent low HREE contents and ¹⁷⁶Lu/¹⁷⁷Hf ratios, suggesting their growth with concurrent garnet crystallization and/or recrystallization. These two groups of age are taken as recording the time of prograde HP to UHP and retrograde UHP–HP stages, respectively. A few cores have high Th/U ratios, high trace element contents, and a clear negative Eu anomaly. These features support a magmatic origin of these zircon cores. The upper intercept ages of 771 ± 86 and 752 ± 70 Ma for the granitic gneiss and eclogite, respectively, indicate that their protoliths probably formed as a bimodal suite in rifting zones in the northern margin of the Yangtze Block. Young Hf model ages (T _DM1) of magmatic cores indicate juvenile (mantle-derived) materials were involved in their protolith formation. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Quantitative textural analysis of packings of elongate crystals

The spatial distribution of grains in a solidifying igneous rock controls the physical properties of the crystal mush, and in turn is controlled by the rate of crystal growth and accumulation. A predominant non-spherical habit for igneous minerals brings into question the use of spherical particles in reference packings used for quantification of spatial distribution. Furthermore, variations of crystal clustering/ordering with length scale require spatial statistics which take into account the distribution of particles beyond nearest neighbours. Using random close packings of spherocylinders, we demonstrate the importance of aspect ratio for the aggregation index (usually known as R) and show that packings of spherical particles have more structure than packings of rods. The spatial distribution functions demonstrate that the plagioclase grains in the colonnade from the Holyoke basalt are clustered on a length scale of 0.5 mm. Understanding the controls on grain spatial distribution in igneous rocks will depend on the application of these techniques to well-understood environments.     

Seismic Hazard Analysis — Quo vadis?

The paper is dedicated to the review of methods of seismic hazard analysis currently in use, analyzing the strengths and weaknesses of different approaches. The review is performed from the perspective of a user of the results of seismic hazard analysis for different applications such as the design of critical and general (non-critical) civil infrastructures, technical and financial risk analysis. A set of criteria is developed for and applied to an objective assessment of the capabilities of different analysis methods. It is demonstrated that traditional probabilistic seismic hazard analysis (PSHA) methods have significant deficiencies, thus limiting their practical applications. These deficiencies have their roots in the use of inadequate probabilistic models and insufficient understanding of modern concepts of risk analysis, as have been revealed in some recent large scale studies. These deficiencies result in the lack of ability of a correct treatment of dependencies between physical parameters and finally, in an incorrect treatment of uncertainties. As a consequence, results of PSHA studies have been found to be unrealistic in comparison with empirical information from the real world. The attempt to compensate these problems by a systematic use of expert elicitation has, so far, not resulted in any improvement of the situation. It is also shown that scenario-earthquakes developed by disaggregation from the results of a traditional PSHA may not be conservative with respect to energy conservation and should not be used for the design of critical infrastructures without validation. Because the assessment of technical as well as of financial risks associated with potential damages of earthquakes need a risk analysis, current method is based on a probabilistic approach with its unsolved deficiencies.

Traditional deterministic or scenario-based seismic hazard analysis methods provide a reliable and in general robust design basis for applications such as the design of critical infrastructures, especially with systematic sensitivity analyses based on validated phenomenological models. Deterministic seismic hazard analysis incorporates uncertainties in the safety factors. These factors are derived from experience as well as from expert judgment. Deterministic methods associated with high safety factors may lead to too conservative results, especially if applied for generally short-lived civil structures. Scenarios used in deterministic seismic hazard analysis have a clear physical basis. They are related to seismic sources discovered by geological, geomorphologic, geodetic and seismological investigations or derived from historical references. Scenario-based methods can be expanded for risk analysis applications with an extended data analysis providing the frequency of seismic events. Such an extension provides a better informed risk model that is suitable for risk-informed decision making.     

An experimental study of the grain-scale processes of peridotite melting: implications for major and trace element distribution during equilibrium and disequilibrium melting

The grain-scale processes of peridotite melting were examined at 1,340°C and 1.5 GPa using reaction couples formed by juxtaposing pre-synthesized clinopyroxenite against pre-synthesized orthopyroxenite or harzburgite in graphite and platinum-lined molybdenum capsules. Reaction between the clinopyroxene and orthopyroxene-rich aggregates produces a melt-enriched, orthopyroxene-free, olivine + clinopyroxene reactive boundary layer. Major and trace element abundance in clinopyroxene vary systematically across the reactive boundary layer with compositional trends similar to the published clinopyroxene core-to-rim compositional variations in the bulk lherzolite partial melting studies conducted at similar P–T conditions. The growth of the reactive boundary layer takes place at the expense of the orthopyroxenite or harzburgite and is consistent with grain-scale processes that involve dissolution, precipitation, reprecipitation, and diffusive exchange between the interstitial melt and surrounding crystals. An important consequence of dissolution–reprecipitation during crystal-melt interaction is the dramatic decrease in diffusive reequilibration time between coexisting minerals and melt. This effect is especially important for high charged, slow diffusing cations during peridotite melting and melt-rock reaction. Apparent clinopyroxene-melt partition coefficients for REE, Sr, Y, Ti, and Zr, measured from reprecipitated clinopyroxene and coexisting melt in the reactive boundary layer, approach their equilibrium values reported in the literature. Disequilibrium melting models based on volume diffusion in solid limited mechanism are likely to significantly underestimate the rates at which major and trace elements in residual minerals reequilibrate with their surrounding melt. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Framing in geographical analysis of environmental conflicts: Theory, methodology and three case studies

When applied to environmental concerns, framing offers a rigorous conceptual and analytic approach with potentially practical significance for dealing with complex issues relevant to geography. The purpose of this article is to introduce framing concepts, typology and modes of analysis to address issues which geographers commonly treat: disputes over land ownership and uses, competition for water resources, cultural clashes over control of territory and resources and the impact of spatial patterns and structures when siting noxious sites. Framing analysis is applied to three cases of environmental conflict in Israel and, more generally, for understanding conflicts revolving around the management of human habitats in relation to the physical environment.     

Magma Ascent and Crustal Accretion at Ultraslow-Spreading Ridges: Constraints from Plagioclase Ultraphyric Basalts from the Arctic Mid-Ocean Ridge

Plagioclase ultraphyric basalts (PUBs) with up to 54% plagioclasephenocrysts were dredged in the rift valley and adjacent flanksof the ultraslow-spreading Mohns and Knipovich ridges. The PUBsshow large variations in crystal morphologies and zoning. Thelarge variations suggest that single basalt samples containa mixture of plagioclase crystals that aggregated at differentlevels in the magma conduits. Resorbed crystals and repeatedreverse zones suggest that the magma reservoirs were replenishedand heated several times. Thin concentric zones with melt inclusions,and sharp reductions in the anorthite content of 3–7%,are common between the reverse zones. These zones, and skeletalcrystals with distinctly lower anorthite contents than massivecrystals, are interpreted to be the result of rapid crystalliztionduring strong undercooling. The changes between short periodsof cooling and longer periods with reheating are explained bymultiple advances of crystal-rich magma into cool regions followedby longer periods of gradual magma inflow and temperature increase.The porphyritic basalts are characterizd by more depleted andmore fractionated compositions than the aphyric basalts, withlower (La/Sm)_N, K₂O and Mg-numbers. This relationship, and theobservation that PUBs are sampled only close to segment centresalong these ridges, suggests that the PUBs formed by higherdegrees of melting and evolved in more long-lived magma reservoirs.We propose that the zoning patterns of plagioclase crystalsand crystal morphologies of these PUBs reflect the developmentand flow of magma through a stacked sill complex-like conduitsystem, whereas the aphyric equivalents represent later flowof magma through the conduit. The formation of voluminous higher-degreemelts may trigger the development of the magma conduits andexplain the generally depleted compositions of PUB magmas. KEY WORDS: basalt; mineral chemistry; MORB; magma mixing; magma chamber; major element     

Correlation of the Jurassic-Cretaceous siliceous-volcanogenic sediments in northwestern surroundings of the Pacific (Koryak Upland)

In distribution areas of the Pekul’neiveem and Chirynai formations customary distinguishable in the Koryak Upland, complicated tectonostratigraphic units are composed of alternating thrust sheets of different lithologic composition and age, which are juxtaposed because of widespread thrust faulting, as is proved by the radiolarian analysis. Nineteen radiolarian assemblages of different age are first established here in the Lower Jurassic-Hauterivian succession of siliceous-volcanogenic sediments. In the Lower Jurassic interval, the lower and upper Hettangian, lower and upper Sinemurian, and Pliensbachian beds are recognized. Paleontological characterization is also presented for the Aalenian (or Toarcian?-Aalenian), upper Bajocian, lower and upper Bathonian, and Callovian beds of the Middle Jurassic. Within the Upper Jurassic, the Oxfordian-early Kimmeridgian, late Kimmeridgian-early Tithonian, Tithonian, and late Tithonian-early Berriasian radiolarian assemblages are distinguished. The late Berriasian-early Valanginian, middle-late Valanginian, and Hauterivian radiolarian assemblages are first recognized or compositionally revised. Radiolarians and lithofacies data are used to correlate the tectonostratigraphic units and individualize the jasper-alkali basaltic (lower Hettangian), chert-terrigenous (Hettangian-Sinemurian), jasper-cherty (Pliensbachian-Aalenian), jasper (Bajocian-Hauterivian), jasper-basaltic (upper Bajocian-Valanginian), Fe-Ti basaltic (upper Bajocian-Bathonian), tuffitejasper-basaltic (Bathonian-Hauterivian), and terrigenous-volcanogenic (Bajocian-Valanginian) sequences. The correlation results are extrapolated into other continental areas flanking the Pacific, i.e., to the western Kamchatka, northern and northwestern coastal areas of the Sea of Okhotsk, where the analogous radiolarian assemblages are characteristic of comparable allochthonous units of terrigenous-siliceous-volcanogenic sediments.     

Feasibility of pre-earthquake strengthening of buildings based on cost-benefit and life-cycle cost analysis, with the aid of fragility curves

There are two fundamental questions this article aims to deal with. First, whether a pre-earthquake strengthening of a large and heterogeneous building stock (the emphasis here is on building types common in S. Europe), is economically feasible or not, and second what is the optimal retrofit level for mitigating the seismic risk. To this purpose contemporary decision making tools, namely cost-benefit and life-cycle cost analyses, are tailored to the needs of the present study, and implemented with the aid of an ad-hoc developed new software application (COBE06). A method for estimating the reduction in structural vulnerability due to retrofit is proposed, as well as a methodology to determine the optimum retrofit level using the fragility curve approach. Finally, the proposed methodology is used in a pilot application that concerns the city of Thessaloniki, and results are drawn for the feasibility of strengthening the reinforced concrete building stock in this city.