Formation of Paleoproterozoic eclogitic mantle, Slave Province (Canada): Insights from in-situ Hf and U–Pb isotopic analyses of mantle zircons

In-situ Hf isotope analyses and U–Pb dates were obtained by laser ablation-MC-ICP-MS for a zircon-bearing mantle eclogite xenolith from the diamondiferous Jericho kimberlite located within the Archean Slave Province (Nunavut), Canada. The U–Pb zircon results yield a wide range of ages (2.0 to 0.8 Ga) indicating a complex geological history. Of importance, one zircon yields a U–Pb upper intercept date of 1989 ± 67 Ma, providing a new minimum age constraint for zircon crystallization and eclogite formation. In contrast, Hf isotope systematics for the same zircons display an intriguing uniformity, and corresponding Hf depleted mantle model ages range between 2.1 ± 0.1 and 2.3 ± 0.1 Ga; the youngest Hf model age is within error to the oldest U–Pb date.

The Jericho eclogites have previously been interpreted as representing remnants of metamorphosed oceanic crust, and their formation related to Paleoproterozoic subduction regimes along the western margin of the Archean Slave craton during the Wopmay orogeny. Hf isotope compositions and U–Pb results for the Jericho zircons reported here are in good agreement with a Paleoproterozoic subduction model, suggesting that generation of oceanic crust and eclogite formation occurred between 2.0 and 2.1 Ga. The slightly older Hf depleted mantle model ages (2.1 to 2.3 Ga) may be reconciled with this model by invoking mixing between ‘crustal’-derived Hf from sediments and more radiogenic Hf associated with the oceanic crust during the 2 Ga subduction event. This results in intermediate Hf isotope compositions for the Jericho zircons that yield ‘fictitiously’ older Hf model ages.     

Segmented seismicity of the M w 6.2 Baladeh earthquake sequence (Alborz Mountains, Iran) revealed from regional moment tensors

The M _w 6.2 Baladeh earthquake occurred on 28 May 2004 in the Alborz Mountains, northern Iran. This earthquake was the first strong shock in this intracontinental orogen for which digital regional broadband data are available. The Baladeh event provides a rare opportunity to study fault geometry and ongoing deformation processes using modern seismological methods. A joint inversion for hypocentres and a velocity model plus a surface-wave group dispersion curve analysis were used to obtain an adapted velocity model, customised for mid- and long-period waveform modelling. Based on the new velocity model, regional waveform data of the mainshock and larger aftershocks (M _w ?≥3.3) were inverted for moment tensors. For the Baladeh mainshock, this included inversion for kinematic parameters. All analysed earthquakes show dominant thrust mechanisms at depths between 14 and 26 km, with NW–SE striking fault planes. The mainshock ruptured a 28° south-dipping area of 24 × 21 km along a north-easterly direction. The rupture plane of the mainshock does not coincide with the aftershock distribution, neither in map view nor with respect to depth. The considered aftershocks form two main clusters. The eastern cluster is associated with the mainshock. The western cluster does not appear to be connected with the rupture plane of the mainshock but, instead, indicates a second activated fault plane dipping at 85° towards the north.     

Comparison of single-domain and dual-domain subsurface transport models

Subgrid modeling of some type is typically used to account for heterogeneity at scales below the grid scale. The single-domain model (SDM), employing field-scale dispersion, and the dual-domain model (DDM), employing local hydrodynamic dispersion and exchange between domains having large hydraulic conductivity contrasts, are well-known examples. In this paper, the two modeling approaches are applied to tritium migration from the H-area seepage basins to a nearby stream--Fourmile Branch--at the Savannah River Site. This location has been monitored since 1955, so an extensive dataset exists for formulating realistic simulations and comparing the results to data. It is concluded that the main parameters of both models are scale-dependent, and methods are discussed for making initial estimates of the DDM parameters, which include mobile/immobile porosities and the mass exchange coefficient. Both models were calibrated to produce the best fit to recorded tritium data. When various attributes of the dataset were considered, including cumulative tritium activity discharged to Fourmile Branch, plume arrival time, and plume attenuation due to closure of the seepage basins in 1988, the DDM produced results superior to the SDM, while causing no unrealistic upgradient dispersion. A sensitivity analysis showed that only the DDM was able to accurately produce both the instantaneous activity discharge and cumulative activity with a single parameter set. This is thought to be due to the advection-dominated nature of transport in natural porous media and the more realistic treatment of this type of transport in the DDM relative to the SDM.     

Recent trends in vegetation dynamics in the African Sahel and their relationship to climate

Contrary to assertions of widespread irreversible desertification in the African Sahel, a recent increase in seasonal greenness over large areas of the Sahel has been observed, which has been interpreted as a recovery from the great Sahelian droughts. This research investigates temporal and spatial patterns of vegetation greenness and rainfall variability in the African Sahel and their interrelationships based on analyses of Normalized Difference Vegetation Index (NDVI) time series for the period 1982–2003 and gridded satellite rainfall estimates. While rainfall emerges as the dominant causative factor for the increase in vegetation greenness, there is evidence of another causative factor, hypothetically a human-induced change superimposed on the climate trend.     

Controls on the lateral channel-migration rate of braided channel systems in coarse non-cohesive sediment

Lateral movements of alluvial river channels control the extent and reworking rates of alluvial fans, floodplains, deltas, and alluvial sections of bedrock rivers. These lateral movements can occur by gradual channel migration or by sudden changes in channel position (avulsions). Whereas models exist for rates of river avulsion, we lack a detailed understanding of the rates of lateral channel migration on the scale of a channel belt. In a two-step process, we develop here an expression for the lateral migration rate of braided channel systems in coarse, non-cohesive sediment. On the basis of photographic and topographic data from laboratory experiments of braided channels performed under constant external boundary conditions, we first explore the impact of autogenic variations of the channel-system geometry (i.e. channel-bank heights, water depths, channel-system width, and channel slope) on channel-migration rates. In agreement with theoretical expectations, we find that, under such constant boundary conditions, the laterally reworked volume of sediment is constant and lateral channel-migration rates scale inversely with the channel-bank height. Furthermore, when channel-bank heights are accounted for, lateral migration rates are independent of the remaining channel geometry parameters. These constraints allow us, in a second step, to derive two alternative expressions for lateral channel-migration rates under different boundary conditions using dimensional analysis. Fits of a compilation of laboratory experiments to these expressions suggest that, for a given channel bank-height, migration rates are strongly sensitive to water discharges and more weakly sensitive to sediment discharges. In addition, external perturbations, such as changes in sediment and water discharges or base level fall, can indirectly affect lateral channel-migration rates by modulating channel-bank heights. © 2019 The Author. Earth Surface Processes and Landforms published by John Wiley & Sons, Ltd. © 2019 The Author. Earth Surface Processes and Landforms published by John Wiley & Sons, Ltd.     

A new Jurassic cow shark (Chondrichthyes,Hexanchiformes) with comments on Jurassic hexanchiform systematics

A new taxon, ?Crassodontidanus gen. nov. of Hexanchiformes (cow sharks) from the Jurassic of Germany is described. It is characterized by peculiar teeth combining apomorphic (serrated mesial cutting edge of the main cusp) and plesiomorphic features (deep root with convex mesial and distal margins in labial and lingual views; protruding lingual root bulge). This character combination readily distinguishes members of the new taxon from all other known extant (Heptranchias, Hexanchus, Notorynchus) and extinct (?Gladioserratus, ?Notidanodon, ?Notidanoides, ?Pachyhexanchus, ?Paraheptranchias, ?Weltonia) hexanchiforms. Currently, two species, ?C. serratus (type species; Late Jurassic, Late Kimmeridgian of Nusplingen, South Germany) and ?C. wiedenrothi (Early Jurassic, Early Pliensbachian of Gretenberg (Hanover), North Germany) are assigned to this taxon. ?Crassodontidanus gen. nov. is member of ?Crassonotidae fam. nov. and sister to ?Notidanoides Maisey, 1986 and ?Pachyhexanchus Cappetta, 1990. We consider ?Notidanus amalthei Oppel, 1854 from the Pliensbachian of South Germany, ?Notidanus insignis Seguenza, 1887 from the Oxfordian of Sicily (Italy) and ?Notidanus wagneri Agassiz, 1843 from the Early Tithonian of Solnhofen (South Germany) nomina dubia and nomina nuda, respectively. The family ?Crassonotidae comprises plesiomorphic hexanchiforms ranging from the Sinemurian (Early Jurassic) to the Hauterivian (Early Cretaceous).     

Probabilistic life cycle analysis model for evaluating electric power infrastructure risk mitigation investments

One effect of climate change may be increased hurricane frequency or intensity due to changes in atmospheric and geoclimatic factors. It has been hypothesized that wetland restoration and infrastructure hardening measures may improve infrastructure resilience to increased hurricane frequency and intensity. This paper describes a parametric decision model used to assess the tradeoffs between wetland restoration and infrastructure hardening for electric power networks. We employ a hybrid economic input–output life-cycle analysis (EIO-LCA) model to capture: construction costs and life-cycle emissions for transitioning from the current electric power network configuration to a hardened network configuration; construction costs and life-cycle emissions associated with wetland restoration; and the intrinsic value of wetland restoration. Uncertainty is accounted for probabilistically through a Monte Carlo hurricane simulation model and parametric sensitivity analysis for the number of hurricanes expected to impact the project area during the project cycle and the rate of wetland storm surge attenuation. Our analysis robustly indicates that wetland restoration and undergrounding of electric power network infrastructure is not preferred to the “do-nothing” option of keeping all power lines overhead without wetland protection. However, we suggest a few items for future investigation. For example, our results suggest that, for the small case study developed, synergistic benefits of simultaneously hardening infrastructure and restoring wetlands may be limited, although research using a larger test bed while integrating additional costs may find an enhanced value of wetland restoration for disaster loss mitigation.