The impact of upland land management on flooding: results from an improved pasture hillslope

In response to growing concern about impacts of upland agricultural land management on flood risk, an intensely instrumented experimental catchment has been established at Pontbren, a sheep‐farmed headwater catchment of the River Severn, UK. Primary aims are to develop understanding of the processes governing flood generation and the associated impacts of land management practices, and to bridge the gap between process understanding and ability to predict effects on downstream flooding. To achieve this, the experiment is designed to operate at plot (～100 m²), hillslope (～0·1 km²) and small catchment scale (～10 km²). Hillslope‐scale data, from an under‐drained, agriculturally ‘improved’ pasture, show that drain flow is a dominant runoff process. However, depending on antecedent moisture conditions, overland flow may exceed drain flow rates and can be an important contributor to peak flow runoff at the hillslope‐scale. Flow, soil tension data and tracer tests confirm the importance of macropores and presence of perched water tables under ‘normal’ wet conditions. Comparisons of pasture runoff with that from within a 10 year‐old tree shelterbelt show significantly reduced overland flow due to the presence of trees and/or absence of sheep. Comparisons of soil hydraulic properties show significant increases in hydraulic conductivity and saturated moisture content of soil under trees compared to adjacent improved pasture. Copyright © 2008 John Wiley & Sons, Ltd.     

Large-amplitude ground-motion recordings and their interpretations

Empirical recordings of ground motions featuring large amplitudes of acceleration or velocity play a key role in defining the maximum levels of ground motion required for the design of engineering projects, in view of their potentially very destructive nature. They also provide valuable insights as to the nature of the tails of the ground-motion distribution. The present paper reviews the interpretations put forward for a dataset of recordings selected for their large value of PGA (?1 g) or PGV (?100 cm/s), and attempts to classify causative physical processes according to a uniform nomenclature. The aim of the study is to identify to what extent large-amplitude ground motions are predictable, by separating scenarios that could generally be encountered and foreseen using current knowledge of the processes involved in earthquake ground-motion generation and propagation, such as particular features of the recording site, from occurrences that are related to unforeseeable characteristics of the source and path.     

Tropical drought patterns and their linkages to large-scale climate variability over Peninsular Malaysia

Ocean–atmosphere modes of climate variability in the Pacific and Indian oceans, as well as monsoons, regulate the regional wet and dry episodes in tropical regions. However, how those modes of climate variability, and their interactions, lead to spatial differences in drought patterns over tropical Asia at seasonal to interannual time scales remains unclear. This study aims to analyse the hydroclimate processes for both short- and long-term spatial drought patterns (3-, 6, 12- and 24-months) over Peninsular Malaysia using the Standardized Precipitation Index, Standardized Precipitation Evapotranspiration Index, and Palmer Drought Severity Index. Besides that, a generalized least squares regression is used to explore underlying circulation mechanisms of these spatio-temporal drought patterns. The tested drought indices indicate a tendency towards wetter conditions over Peninsular Malaysia. Based on principal component analysis, distinct spatio-temporal drought patterns are revealed, suggesting North–South and East–West gradients in drought distribution. The Pacific El Nino Southern Oscillation (ENSO), the South Western Indian Ocean (SWIO) variability, and the quasi-biennial oscillation (QBO) are significant contributors to the observed spatio-temporal variability in drought. Both the ENSO and the SWIO modulate the North–South gradient in drought conditions over Peninsular Malaysia, while the QBO contributes more to the East–West gradient. Through modulating regional moisture fluxes, the warm phases of the ENSO and the SWIO, and the western phases of the QBO weaken the southwest and northeast monsoon, leading to precipitation deficits and droughts over Peninsular Malaysia. The East–West or North–South gradients in droughts are related to the middle mountains blocking southwest and northeast moisture fluxes towards Peninsular Malaysia. In addition, the ENSO and QBO variations are significantly leading to short-term droughts (less than a year), while the SWIO is significantly associated with longer-duration droughts (2 years or more). Overall, this work demonstrates how spatio-temporal drought patterns in tropical regions are related to monsoons and moisture transports affected by the oscillations over the Pacific and Indian oceans, which is important for national water risk management.     

Response spectrum to strong aftershocks of the Lancang-Gengma earthquake

Near-field records of two strong aftershocks with magnitudeM _S=6.7 andM _S=6.3 in the Lancang-Gengma earthquakes sequence, Yunnan Province, November 1988, are used to calculate the response spectrum. The instruments, site conditions and the methods for computing are also illustrated in this paper. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 338–343, 1991. This project is supported by The Chinese Joint Seismological Science Foundation, SSB and the West Yunnan Earthquake Prediction Test Field, Yunnan Seismological Bureau.     

Basic lavas of the Archean La Grande Greenstone belt: Products of polybaric fractionation and crustal contamination

Despite the fact that some greenstone belts preserve the record of contemporaneous komatiitic and tholeiitic volcanism, a genetic link between the two is not widely accepted. The significance of a compositional gap seperating these magma types and differences in their respective degree of light rare earth element (LREE) enrichment, cited as evidence against a derivative relationship, are complicated by the possibility of crustal assimilation by magmas of komatiitic affinity. In the Archean La Grande Greenstone belt of northern Quebec a succession of metamorphosed tholeiitic basalts and younger, high-Mg, LREE-enriched andesites are preserved. The tholeiites are differentiated basaltic rocks whose chemical compositions appear to have been controlled by low pressure, gabbroic fractional crystallization and are similar to Type 1 MORB. Parental magmas were probably high-Mg liquids of compositions similar to komatiitic basalts which also occur in the greenstone belt. These high-Mg liquids are believed to be themselves the product of high pressure, OLIV+OPX fractional crystallization of more magnesian primary liquids of komatiitic composition. The higher La/Sm ratios of komatiitic basalts and tholeiites relative to komatiites in this belt, can be explained by small degrees of crustal assimilation. In the central part of the belt, late-stage, mafic igneous rocks have chemical compositions similar to Archean examples of contaminated volcanic rocks (e.g., Kambalda, Australia). These late-stage lavas consist of basalts and andesites with high-Mg, Ni and Cr abundances, LREE-enriched profiles and low Ti abundances. They are believed to be the products of crustal assimilation and crystallization of OPX-PLAG-CPX from high-Mg liquids of komatiitic affinity. The volcanic stratigraphy records the progressive effects of crustal contamination through time. A light sialic crust may have initially acted as a density barrier, preventing the eruption of primary high-Mg liquids and forcing fractionation at depth which produced more buoyant compositions. With subsequent thinning of the crust, the density barrier presumably failed, and primary liquids migrated directly toward the surface. Reaction of these liquids with tonalitic crust produced contaminated differentiates.     

The genesis of Variscan (Hercynian) plutonic rocks: Inferences from Sr,Pb, and O studies on the Maladeta igneous complex,central Pyrenees (Spain)

The Maladeta plutonic complex formed during the latest stages of the Variscan orogeny. It was emplaced into the Paleozoic sedimentary sequence of the Pyrenees. The eastern part, investigated in the present study, consists of an early intrusion of cumulate gabbronorites followed in order of emplacement by the main biotite-hornblende granodiorite, which was itself intruded by two small stocks of two-mica cordierite granite. An ⁸⁷Rb-⁸⁷Sr isochron dates the granodiorite at 277±7 m.y. with an initial (⁸⁷Sr/⁸⁶Sr)_o ratio of 0.7117±3. Gabbroic rocks have lower strontium initial ratios, down to 0.7092, while those of granite range from that of the granodiorite up to about 0.715. The three rock types have distinctive ¹⁸O values: 8.7 to 9.6 for the gabbronorites, 9.4 to 10.4 for the granodiorites and 10.3 to 11.8 for the granites. Lead isotopic compositions of rocks and feldspars are all radiogenic. Feldspars give consistent Pb model ages around 280 m.y., with and values of about 9.7 and 4.05, respectively. No pristine, mantle-derived magma was found among the investigated samples and the rocks cannot be related to one another by any simple mechanism of fractional crystallization. Some type of mixing process involving two end members seems to be required: a high-¹⁸O, high-⁸⁷Sr material that is clearly of crustal origin, and a lower-¹⁸O, lower-⁸⁷Sr end member derived from the mantle. Examination of various mixing models does not support magma mixing nor assimilation of crustal rocks by a mafic magma. The most acceptable model involves melting at different levels of a vertically-zoned source in the continental crust; this source was formed by mixing between mantle-derived magmas and crustal metasediments. This material was apparently thickened, tectonically downwarped and partially melted. None of the Maladeta magma-types appear to have been derived at a consuming plate boundary.Contribution Number 3280, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA     

Relative oxidation states of magmas inferred from Ce(IV)/Ce(III) in zircon: application to porphyry copper deposits of northern Chile

Major- and trace-element compositions of zircons and whole rocks from 14 barren and seven ore-bearing calc-alkaline intrusions from the Chuquicamata-El Abra porphyry copper belt of northern Chile have been measured in situ by excimer laser ablation (ELA) ICP-MS. These data permit the Ce(IV)/Ce(III) ratio within zircon to be calculated using a lattice-strain model for mineral-melt partitioning of Ce(IV) and Ce(III). Zircon Ce(IV)/Ce(III) and Eu_N/Eu_N* ratios, and by inference magmatic oxidation states, generally increase from older, mafic to younger, felsic units. Within this sequence, porphyry copper mineralization is directly associated only with intrusions with zircon Ce(IV)/Ce(III)>300 and Eu_N/Eu_N*>0.4. Such trends can be understood in terms of interdependent relations between oxygen fugacity, sulfur speciation and solubility, and chalcophile element partitioning in silicate magmas. Because zircon occurs in most calc-alkaline intrusions and is resistant to subsolidus alteration, zircon Ce(IV)/Ce(III) ratios provide a useful tool for evaluating the economic potential of such rocks for magmatic-hydrothermal Cu-Au mineralization. The approach is general and may provide a means to infer relative oxidation state in a wide range of intermediate to felsic igneous rocks.     

M25 roadworks reveal earliest UK Neanderthal occupation at Dartford

As anyone travelling on the roads of southeast England between 2006 and 2008 has probably been aware, major improvements have recently been made at the junction of the M25 and A2 motorways, south of the main Dartford crossing of the River Thames ( Fig. 1 ). The roadworks, funded by the UK Highways Agency, with the main contractors Jacobs Babtie and Costain, were accompanied by archaeological investigations carried out by Oxford Archaeology between 2003 and 2006. The archaeological programme had a major Palaeolithic/Pleistocene element under my direction ( Fig. 2 ) that has produced evidence of Neanderthal occupation early in the last glaciation (the Devensian, which lasted from 115 000 to 10 000 bp ), during a period when Britain had until now, been thought to have been entirely deserted.

Figure 1 Open in figure viewer PowerPoint Site location and areas of investigation.     

Petrogenetic implications of Ti,Zr, Y,and Nb variations in volcanic rocks

Data from experimental runs, coexisting phases in ultramafic rocks and phenocryst-matrix pairs in volcanic rocks have been used to compile a table of mineral-liquid distribution coefficients for Ti, Zr, Y, and Nb for basic, intermediate and acid melt compositions. These values have then been used to interpret variations of these elements, first in basalts and second, during fractional crystallization from basic to acid magmas. For basalts, petrogenetic modelling of Zr/Y, Zr/Ti, and Zr/Nb ratios, when used in conjunction with REE, Cr and isotopic variations, suggests that: (1) the increase in Zr/Y ratio from mid-ocean ridge to within plate basalts and the low Zr/Nb ratios of alkalic basalts are due to (fluid controlled) source heterogeneities; (2) the low Zr and Zr/Y ratio of volcanic arc basalts results from high degree of partial melting of a depleted source; and (3) the high Zr and similar Zr/Y ratio of basalts from fast spreading relative to slow spreading ridges results from open-system fractional crystallization. Modelling of fractionation trends in more evolved rocks using Y-Zr, Ti-Zr and Nb-Zr diagrams highlights in particular the change in crystallizing mafic phases from island arcs (clinopyroxene-dominated) to Andean-type arcs (amphibole±biotite-dominated). These methods can be applied to altered lavas of unknown affinities to provide additional information on their genesis and eruptive environment.