The Irish Variscides: problems, perspectives and some solutions

A review is presented of interpretations made to date of the Irish Variscides, and the various models of Late Palaeozoic crustal evolution. An examination is then made of the factors controlling Variscan strain in Ireland, including lithological competence, basement fabric and basement-cover thickness. From this, the difficulties of recognizing Variscan 'fronts' and 'fold belts' are emphasized.
In order to determine whether all putative Variscan strain is indeed Variscan. in age, new time-temperature data (vitrinite reflectance; apatite fission track populations) are discussed, along with offshore data. A case is then presented for recognizing quite distinct episodes of both Variscan and post-Variscan heating and deformation. Some of the implications for the Variscides to the east are briefly examined, and the suitability of a regional 'escape tectonics' model is tested.     

The inception and early evolution of the North Alpine Foreland Basin, Switzerland

The stratigraphy of the Eocene-Miocene peripheral foreland basin in Switzerland consists of basal deposits of Nummulitic Limestones and Globigerina Marls representing a phase of deepening, followed by two shallowing-up megacycles culminating in fully continental sedimentation. The onset of sedimentation was diachronous and took place on an unconformity surface with increasing stratigraphic gap to the north and west. In the Ultrahelvetic units, which were derived from the south and have a provenance between the Helvetic shelf and the Penninic ocean, the stratigraphic gap is minimal. This restricts the initiation of erosion of the southern European margin due to emersion to post-Maastrichtian and pre-late Palaeocene. This coincides with the final closing of the Valais trough and may therefore be interpreted as the point at which continental flexure s. s. started. In the autochthon, the subcrop map of the unconformity surface shows that the regional pattern of subcropping units is oblique to both neo-Alpine tectonic structures and Helvetic (Mesozoic) passive margin structures. There are local zones of disruption to the broad regional pattern suggesting that the basal unconformity was corrugated. Both the paliaspastic restoration of the autochthon relative to the thrust front during the Palaeocene, and the regional pattern of erosion indicate that the basal unconformity may be due to erosion of a flexural forebulge. Following deposition of the shallow water Nummulitic Limestones and the deeper water Globigerina Marls, clastic sediments were shed from the orogenic wedge in the south. These turbidites, the Taveyannaz Sandstones, filled both ponded basins at the contemporaneous thrust front and the frontal trench or foredeep. Evidently, early thrusts drove at a shallow level into the embryonic basin as ‘front-runners’, whereas most shortening and uplift continued to take place within the main part of the orogenic wedge further to the south. Eventually, the frontal palaeohighs, together with the turbidite basins, were buried by the northward emplacement of surface mud-slides, and sediment depocentres were translated northwards onto the foreland. The most likely cause of the underfilled ‘Flysch’ stage is the rapid advance of a submarine thrust wedge over the flexed European plate which resulted in (i) low sediment fluxes and (ii) high subsidence rates associated with the rapid migration of the load and depocentre. Later, as the rate of advance slowed and the wedge became subaerially exposed, the basin rapidly filled with coarse-grained detritus representing the ‘Molasse’ stage.     

Phase Equilibria Constraints on the Chemical and Physical Evolution of the Campanian Ignimbrite

The Campanian Ignimbrite is a > 200 km³ trachyte–phonolitepyroclastic deposit that erupted at 39·3 ± 0·1ka within the Campi Flegrei west of Naples, Italy. Here we testthe hypothesis that Campanian Ignimbrite magma was derived byisobaric crystal fractionation of a parental basaltic trachyandesiticmelt that reacted and came into local equilibrium with smallamounts (5–10 wt%) of crustal rock (skarns and foid-syenites)during crystallization. Comparison of observed crystal and magmacompositions with results of phase equilibria assimilation–fractionationsimulations (MELTS) is generally very good. Oxygen fugacitywas approximately buffered along QFM + 1 (where QFM is the quartz–fayalite–magnetitebuffer) during isobaric fractionation at 0·15 GPa ( 6km depth). The parental melt, reconstructed from melt inclusionand host clinopyroxene compositions, is found to be basaltictrachyandesite liquid (51·1 wt% SiO₂, 9·3 wt%MgO, 3 wt% H₂O). A significant feature of phase equilibria simulationsis the existence of a pseudo-invariant temperature, 883 °C,at which the fraction of melt remaining in the system decreasesabruptly from 0·5 to < 0·1. Crystallizationat the pseudo-invariant point leads to abrupt changes in thecomposition, properties (density, dissolved water content),and physical state (viscosity, volume fraction fluid) of meltand magma. A dramatic decrease in melt viscosity (from 1700Pa s to 200 Pa s), coupled with a change in the volume fractionof water in magma (from 0·1 to 0·8) and a dramaticdecrease in melt and magma density acted as a destabilizingeruption trigger. Thermal models suggest a timescale of 200kyr from the beginning of fractionation until eruption, leadingto an apparent rate of evolved magma generation of about 10^–3km³/year. In situ crystallization and crystal settling in density-stratifiedregions, as well as in convectively mixed, less evolved subjacentmagma, operate rapidly enough to match this apparent volumetricrate of evolved magma production. KEY WORDS: assimilation; Campanian Ignimbrite; fractional crystallization; magma dynamics; phase equilibria     

Reactive transport modeling of the role of fluorine-rich groundwater in the formation of fluorite ore deposits in the Illinois-Kentucky district,USA

Hydrogeology Journal - The Illinois-Kentucky ore district hosts the largest deposits of fluorite in the USA. A long-standing hypothesis for the formation of the Illinois-Kentucky deposits is that...     

Stream temperature sensitivity to climate warming in California’s Sierra Nevada: impacts to coldwater habitat

Water temperature influences the distribution, abundance, and health of aquatic organisms in stream ecosystems, so understanding the impacts of climate warming on stream temperature will help guide management and restoration. This study assesses climate warming impacts on stream temperatures in California’s west-slope Sierra Nevada watersheds, and explores stream temperature modeling at the mesoscale. We used natural flow hydrology to isolate climate induced changes from those of water operations and land use changes. A 21 year time series of weekly streamflow estimates from WEAP21, a spatially explicit rainfall-runoff model were passed to RTEMP, an equilibrium temperature model, to estimate stream temperatures. Air temperature was uniformly increased by 2°C, 4°C, and 6°C as a sensitivity analysis to bracket the range of likely outcomes for stream temperatures. Other meteorological conditions, including precipitation, were unchanged from historical values. Raising air temperature affects precipitation partitioning into snowpack, runoff, and snowmelt in WEAP21, which change runoff volume and timing as well as stream temperatures. Overall, stream temperatures increased by an average of 1.6°C for each 2°C rise in air temperature, and increased most during spring and at middle elevations. Viable coldwater habitat shifted to higher elevations and will likely be reduced in California. Thermal heterogeneity existed within and between basins, with the high elevations of the southern Sierra Nevada and the Feather River watershed most resilient to climate warming. The regional equilibrium temperature modeling approach used here is well suited for climate change analysis because it incorporates mechanistic heat exchange, is not overly data or computationally intensive, and can highlight which watersheds are less vulnerable to climate warming. Understanding potential changes to stream temperatures from climate warming will affect how fish and wildlife are managed, and should be incorporated into modeling studies, restoration assessments, and licensing operations of hydropower facilities to best estimate future conditions and achieve desired outcomes.     

Climate change and mental health: an exploratory case study from Rigolet, Nunatsiavut, Canada

As the impacts from anthropogenic climate change are increasing globally, people are experiencing dramatic shifts in weather, temperature, wildlife and vegetation patterns, and water and food quality and availability. These changes impact human health and well-being, and resultantly, climate change has been identified as the biggest global health threat of the 21st Century. Recently, research is beginning to indicate that changes in climate, and the subsequent disruption to the social, economic, and environmental determinants of health, may cause increased incidences and prevalence of mental health issues, emotional responses, and large-scale sociopsychological changes. Through a multi-year, community-led, exploratory case study conducted in Rigolet, Nunatsiavut, Labrador, Canada, this research qualitatively explores the impacts of climate change on mental health and well-being in an Inuit context. Drawing from 67 in-depth interviews conducted between January 2010 and October 2010 with community members and local and regional health professionals, participants reported that changes in weather, snow and ice stability and extent, and wildlife and vegetation patterns attributed to climate change were negatively impacting mental health and well-being due to disruptions in land-based activities and a loss of place-based solace and cultural identity. Participants reported that changes in climate and environment increased family stress, enhanced the possibility of increased drug and alcohol usage, amplified previous traumas and mental health stressors, and were implicated in increased potential for suicide ideation. While a preliminary case study, these exploratory findings indicate that climate change is becoming an additional mental health stressor for resource-dependent communities and provide a baseline for further research.     

Relative sea‐level observations in western Scotland since the Last Glacial Maximum for testing models of glacial isostatic land movements and ice‐sheet reconstructions

Observations of relative sea‐level change and local deglaciation in western Scotland provide critical constraints for modelling glacio‐isostatic rebound in northern Britain over the last 18 000 years. The longest records come from Skye, Arisaig and Knapdale with a shorter, Holocene, record from Kintail. Biostratigraphic (diatom, pollen, dinoflagellate, foraminifera and thecamoebian), lithological and radiocarbon analyses provide age and elevation parameters for each sea‐level index point. All four sites reveal relative sea‐level change that is highly non‐monotonic in time as the local vertical component of glacio‐isostatic rebound and eustasy (or global meltwater influx) dominate at different periods. Copyright © 2006 John Wiley & Sons, Ltd.