Fabric transitions in the Saxony granulite terrain

Quartz fabric transitions reported from the Saxony granulite terrain and from the nearby Ore Mountains are interesting because of the changes in the pattern types, and the variation in the opening angles of the fabric skeletons. A short summary is presented of salient features in research documented in the East German literature between 1961 and 1978.The changes in fabric skeleton with decreasing metamorphic grade are proposed as mainly consequent to the deactivation of prism 〈c〉 glide systems as temperature decreases. Fabrics indicative of the appearance of the 〈c〉 Burgers vector might be diagnostic for deformation in regimes of comparatively high temperatures or low strainrates, or for deformation, under conditions such that there is a relatively high ‘kinetically effective’ hydroxyl content in the quartz structure.     

The effect of the basal-prism mechanism switch on fabric development during plastic deformation of quartzite

Blacic described a transition from basal 〈a〉 to prism 〈c〉 dislocation glide systems as temperature increased or strain-rate decreased in sequences of experiments involving deformation of single crystals of quartz. In this paper theoretical aspects of competition between these systems during plastic deformation are discussed. It is concluded that the basal-prism mechanism switch should have important consequences for the development of crystallographic fabrics during plastic deformation of quartzite, and the Taylor-Bishop-Hill analysis is used in an attempt to predict expected fabric transitions.     

Post-orogenic (<1500 Ma) thermal history of the Palaeo-Mesoproterozoic, Mt. Isa province, NE Australia

We present hornblende, white mica, biotite and alkali feldspar ⁴⁰Ar/³⁹Ar data from Paleo-Mesoproterozoic rocks of the Mt. Isa Inlier, Australia, which reveal a previously unrecognised post-orogenic, non-linear cooling history of part of the Northern Australian Craton. Plateau and total fusion ⁴⁰Ar/³⁹Ar ages range between 1500 and 767 Ma and record increases in regional cooling rates of up to 4 °C/Ma during 1440–1390 and 1260–1000 Ma. Forward modelling of the alkali feldspar ⁴⁰Ar/³⁹Ar Arrhenius parameters reveals subsequent increases in cooling rates during 600–400 Ma. The cooling episodes were driven by both erosional exhumation at average rates of 0.25 km/Ma and thermal relaxation following crustal heating and magmatic events. Early Mesoproterozoic cooling is synchronous with exhumation and shearing in the Arunta Block and Gawler Craton. Late Mesoproterozoic cooling could have either been driven by increased rates of exhumation, or a result of thermal relaxation following a heat pulse that was synchronous with dyke emplacement in the Arunta, Musgrave and Mt. Isa province, as well as Grenville-aged orogenesis in the Albany–Fraser Belt. Latest Neoproterozoic–Cambrian cooling and exhumation was probably driven by the convergence of part of the East Antarctic Shield with the Musgrave Block and Western Australia (Petermann Ranges Orogeny), as well as collisional tectonics that produced the Delamerian–Ross Orogen. Major changes in the stress field and geothermal gradients of the Australian plate that are synchronous with the assembly and break-up of parts of Rodinia and Gondwana resulted in shearing and repeated brittle reactivation of the Mt. Isa Inlier, probably via the displacement of long-lived basement faults within the Northern Australian Craton.     

Strain localization behaviour in experimental shear zones

A torsional deformation apparatus has been developed which allows the evolution of microstructures and flow patterns in analogue materials to be observed to shear strains in excess of =100 while the loading conditions are kept constant. Using a computer-based video microscopy system the displacement paths of marker particles have been followed in real time, and from these the local strain states in the samples have been calculated. A series of velocity stepping experiments in octachloropropane and norbornene determined the shear localization behaviour displayed by these materials. Both of these materials underwent two phases of localization, a primary localization at the start of an experiment, and a secondary localization following increases in the imposed strain rate. The secondary localization is less pronounced, but is reversible, in the sense that lowering the imposed strain rate widens the zone. An inverse relationship between the fixed imposed strain rate and the width of the zone of localized deformation was established. This velocity dependant localization is ascribed to the softening effect of dynamic recrystallization.     

The influence of parent material on topsoil geochemistry in eastern England

The topsoil of around 10 000 km² in eastern England has recently been sampled intensely at 4609 sites to characterize its geochemistry. The parent materials, which include both solid geology and Quaternary sediments, range in age from Permian to Holocene. The distributions of the concentrations of major and trace elements have been characterized geostatistically, and the role of parent material on their spatial structure (anisotropy) and their spatial relationships (coregionalization) have been investigated. Analysis of variance with the sites grouped by major parent material type showed that this classi?cation accounted for 14 to 48 per cent of the variance for the various elements. Global variograms of 13 elements (Al, As, Ca, Cr, Cu, Fe, Mg, Mo, Ni, P, Pb, Ti, and U) have been computed and modelled. Eleven of the variograms seem to comprise two structures, both of which we modelled with spherical functions, one of short range, 3·5 to 9 km, and the other with a range of 15 to 23 km. The models included a nugget variance, which varied from 27 per cent (for As, Fe, and Mg) to 63 per cent (for P) of the total. The long‐range structures are related to the separations of the major parent materials. The variograms of several elements showed appreciable anisotropy, most notably that of Mg. Anisotropy is evident at short ranges of less than 5 km. This accords with the geological structure of the beds which dip from west to east so that their outcrops are elongated from north to south. A linear model of coregionalization ?tted to the data emphasized several important geochemical associations, which we interpret. Elements commonly associated with clay minerals (Mg, Al) and the clay size fraction (Ti) are dominated by the long‐range structure of the coregionalization, whilst several trace elements (As, Cr, Ni and U) are spatially correlated with Fe over short distances, through adsorption of the former on the surfaces of Fe oxyhydroxides. The topsoil around large urban areas is enriched in lead, but it is not clear whether anthropogenic sources are responsible for this metal's anomalous spatial relationships with other elements. Crown copyright © 2003. Reproduced with the permission of Her Majesty's Stationery Of?ce. Published by John Wiley & Sons, Ltd.     

Future Climate Impact on the Productivity of Sugar Beet (Beta vulgaris L.) in Europe

The impact of future climate change on sugar beet yields is assessed over western Europe using future (2021–2050) climate scenario data from a General Circulation Model (GCM) and the Broom's Barn simulation model of rain-fed crop growth and yield. GCM output for the 1961–1990 period is first compared with observed climate data and shown to be reliable for regions west of 24° E. Comparisons east of this meridian were less reliable with this GCM (HadCM2) and so were omitted from simulations of crop yield. Climate change is expected to bring yield increases of around 1 t/ha of sugar in northern Europe with decreases of a similar magnitude in northern France, Belgium and west/central Poland, for the period 2021–2050. Averaged for the study area (weighted by current regional production), yields show no overall change due to changed climate. However, this figure masks significant increases in yield potential (due to accelerated growth in warmer springs) and in losses due to drought stress. Drought losses are predicted to approximately double in areas with an existing problem and to become a serious new problem in NE France and Belgium. Overall west and central Europe simulated average drought losses rise from 7% (1961–1990) to 18% (2021–2050). The annual variability of yield (as measured by the coefficient of variation) will increase by half, from 10% to 15% compared to 1961–1990, again with potentially serious consequences for the sugar industry. The importance of crop breeding for drought tolerance is further emphasised. These changes are independent of the 9% yield increase which we estimate, on the basis of work by Demmers-Derks et al. (1998), is the likely direct effect of the increase in atmospheric CO₂ concentration by 2021–2050.     

The thermal signature of subducted lithospheric slabs at the core–mantle boundary

We study the thermal structure around a cold deformable lithospheric slab as it sinks to the core–mantle boundary and migrates along it. We present analytical results for the steady thermal structure established by a steady but spatially varying motion. The analysis gives a time-like criterion for the thermal signature of a cold slab to persist by the time that the slab moves along the core–mantle boundary. The model is used to assess the feasibility of a purely thermal origin for some of the observed seismic reflectors near the core–mantle boundary. Calculations of the time-like criterion show that the dynamical conditions in our model, namely the velocity and the thickness of the descending slab, are hard to reconcile with observations of subduction and seismic features. Seismic reflections and refractions from anomalously fast regions above the core–mantle boundary could be explained as thermal slabs if the thickness of the slab at subduction was larger than 200 km or somewhat less if the slab did not split at the core–mantle boundary. A simple thermal model also predicts from mineral physics a certain correlation between S- and P-wave velocity anomalies, which is not observed. However, a purely thermal origin cannot be ruled out if the slab is buckling. This process could be in agreement with the observations: the amplitude of the seismic anomalies, the vertical extent of high-gradient zones and the P versus S comparisons. Chemical heterogeneities and phase transformations remain alternative or complementary explanations.     

The simulation of fabric development during plastic deformation and its application to quartzite: fabric transitions

Fabric transitions can arise in materials such as quartz in which more than one set of symmetrically equivalent glide systems must be considered. The external conditions, such as temperature and stress, affect the relative ability of different mechanisms to operate. Adopting the Taylor-Bishop-Hill analysis allows an approximation to the resulting effects in the choice of critical resolved shear stress (CRSS) values for glide on the different dislocation systems. Different CRSS values may be appropriate to simulating fabric development in different deformational environments.For any specific set of CRSS values, for a particular deformation, a set of reorientation trajectories can be defined for differently oriented crystals with respect to the instantaneous stretching axes. There is a basic number of pattern types, and deformation leads to c-axes populating specific end-orientations.The CRSS values on different glide systems can vary smoothly relative to one another, but abrupt changes result in the deformation fabrics at critical CRSS ratios. Quartz fabrics may thus be used to delineate regions subjected to particular conditions of temperature and strain-rate in deformed metamorphic terrains, provided that allowance can be made for other factors such as trace impurity content of quartz.     

Western Palaearctic palaeoenvironmental conditions during the Early and early Middle Pleistocene inferred from large mammal communities,and implications for hominin dispersal in Europe

Large-scale fluctuations in global climate and resulting changes in ecology had a profound effect on human evolution and dispersal. Though hominin remains are scarce, studies focussing on the more abundant records of fossil land mammal communities can contribute greatly to our knowledge of the palaeoenvironmental circumstances that influenced and directed the global spread of hominins. To produce a comprehensive and accurate account of the evolution of western Palaearctic habitat diversity between 2.6 and 0.4 Ma BP, information generated from large mammal communities from 221 key sites has been included in this study.The palaeoecological conditions of the western Palaearctic during the Early and early Middle Pleistocene were principally controlled by the following key factors: (1) a widespread trend of temperature decrease, (2) the periodicity of the global temperature record, (3) the intensity of single climatic stages, (4) the temporal pattern of climatic variation, (5) geographical position, and (6) the distribution of continental water resources. A general picture of the evolution of western Palaearctic habitat diversity saw the replacement of extensive forested terrain by an alternating sequence of varied savannah-like and forested habitats during the 2.6–1.8 Ma span, as well as an alternation between different types of predominantly open habitats between 1.8 and 1.2 Ma. Both of these processes were governed by 41 ka temperature periodicity. During the 1.2–0.9 Ma time span, irregular climatic fluctuations were more common and habitat variability increased. The subsequent 0.9–0.4 Ma interval, a period controlled by 100 ka periodicity, was by comparison more stable, with longer climatic cycles alternating between open and forested landscapes. During the entire Early and early Middle Pleistocene, assemblages of large mammal communities reveal a distinct trend of decreasing continentality between Eastern and South-Eastern Europe on the one hand, and South-Western and North-Western Europe on the other. This trend was due to the effect of the Atlantic Ocean, while in Southern Europe the relatively low continentality was balanced by influences from the Mediterranean Sea.When plotted against evidence of hominin occurrence, the data on western Palaearctic habitat diversity inferred from large mammal communities indicate clear environmental stimuli for the earliest human dispersal in Europe. These are: (1) a wide range of habitats, implying a high diversity of resources; (2) mild climates with low seasonality, implying a lack of strong environmental fluctuations. Around 1.8 Ma at the latest, hominins of African origin entered the western Palaearctic for the first time, taking advantage of the diversity of habitats and resources, particularly along large river systems. Their subsequent westward spread between 1.7 and 1.3 Ma was restricted to Mediterranean-influenced areas, which offered a high variability of habitats and relatively low seasonality. The increase in environmental diversity, which occurred from 1.2 Ma onwards, opened up South-Eastern and Eastern Europe for hominin occupation. According to the available records, North-Western and Central Europe were initially colonized during late Early to early Middle Pleistocene interglacials, when these regions experienced periods of low seasonality and considerable habitat diversity.