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171.
Over the last 15 years there have been dramatic shifts in the consensus over how best to cope with natural hazards in economically developing regions such as South America. One very positive outcome of the United Nations sponsored International Decade for Natural Disaster Reduction (IDNDR 1990–2000) has been that there is now greater interchange between the work of earth scientists examining the processes and mechanics of hazard occurrence and impact, and social scientists exploring the causes of human vulnerability to hazard – and thereby disaster. This paper traces the development of this new understanding with reference to earthquakes and volcanic eruptions in Peru, one of the most hazardous countries in South America. Particular focus is placed on the excellent progress currently being made by scientists in better understanding the physical dimensions of natural hazard exposure, and the ground-breaking work by social scientists in promoting new approaches to understanding and mitigating human vulnerability to disaster. The paper concludes by emphasizing the need to build on this research to produce more inclusive, incultured and unified strategies of disaster mitigation at the local, national and international levels. 相似文献
172.
Silicate, Hydrous and Carbonate Metasomatism at Lherz, France: Contemporaneous Derivatives of Silicate Melt-Harzburgite Reaction 总被引:4,自引:0,他引:4
BODINIER JEAN-LOUIS; MENZIES MARTIN A.; SHIMIZU NOBUMICHI; FREY FREDERICK A.; McPHERSON ELAINE 《Journal of Petrology》2004,45(2):299-320
Complex multi-stage models involving silicate, hydrous and carbonatemelts of distinct provenance have been invoked to explain themetasomatism observed in mantle rocks. In contrast, relativelysimple models requiring polybaric crystallization of alkalinesilicate melts have been proposed to explain the occurrenceof veined mantle rocks. To address the spatial and temporalrelationships between veins and wall-rocks, a sequence of drillcores was obtained from Lherz, France. In outcrop the vein (amphibolegarnetpyroxenite dyke) is spatially associated with hornblendite veinlets(lherzite), and proximal amphibole-bearing and distal apatite-bearingwall-rock peridotite. Considerable elemental and isotopic heterogeneityexists in these wall-rock peridotites, in many instances equivalentto, or greater than, that observed in mantle xenoliths fromworldwide localities. A single stage of reactive porous flowbest explains the elemental and isotopic heterogeneity in thewall-rock. In essence it is proposed that emplacement of thesilicate melt (dyke) was inextricably linked to chromatographicfractionation/reaction of derivatives which led to the coexistence,in space and time, of silicate, hydrous and carbonate melts.This model elegantly and simply describes the formation of complexmetasomatic aureoles around mantle veins and negates the need,in the case of basalt-hosted (and kimberlite-hosted) xenoliths,for complex multi-stage models involving several episodes ofmelt influx with each melt being of different provenance. KEY WORDS: mantle metasomatism; trace-element enrichment; isotopic contamination; wall-rock peridotite; Lherz peridotite 相似文献
173.
MARTIN PERRY 《New Zealand geographer》2003,59(2):54-63
Labour markets are generally thought to have become more integrated across international boundaries. Employees that move between branches of transnational organisations are a potential contributor to this integration. Interviews with human resources managers in 33 New Zealand‐based organisations indicate that the number of employees transferred is small and likely to decrease as corporate functions are transferred from New Zealand to regional offices. 相似文献
174.
Anastomosing river plains of the Channel Country, central Australia, have aggraded slowly over the past 100 ka. Channel sediments accumulate mainly as accretionary benches of mud and sand, sandy channel-base sheets and vegetation-shadow deposits. The channels are laterally stable and the sediments have aggraded locally against erosional banks of tough floodplain muds. Channel sediments are profoundly affected by desiccation during dry periods and by bioturbation caused by within-channel trees and burrowing invertebrates, especially crayfish. Excavations show that mud-dominated channel bodies of low width:thickness ratio are generated by a combination of vertical and lateral accretion. Levees and braided surfaces, composed mainly of mud aggregates, border the channels and are activated during valley-wide floods which lay down distal mud sheets. Floodplain muds are converted to vertisols with gilgai, deep desiccation cracks, and impregnations of carbonate and gypsum. A fixed-channel facies model is applicable to the Channel Country river deposits. Anastomosis apparently results from the need for the system to move large volumes of water and moderate sediment loads across low-gradient interior basins.
Channels distant from upland source areas receive an abundant supply of pedogenic, sand-sized mud aggregates generated on adjacent floodplains and reworked into braid bars during valley-wide floods. Some quartz sand is provided from excavation of subsurface Pleistocene sands in deep channels and waterholes and from aeolian dunes on the floodplains. Adjacent gibber plains supply some gravel to the system. 相似文献
Channels distant from upland source areas receive an abundant supply of pedogenic, sand-sized mud aggregates generated on adjacent floodplains and reworked into braid bars during valley-wide floods. Some quartz sand is provided from excavation of subsurface Pleistocene sands in deep channels and waterholes and from aeolian dunes on the floodplains. Adjacent gibber plains supply some gravel to the system. 相似文献
175.
Variation in Metamorphic Style along the Northern Margin of the Damara Orogen, Namibia 总被引:2,自引:0,他引:2
The northern margin of the Inland Branch of the Pan-AfricanDamara Orogen in Namibia shows dramatic along-strike variationin metamorphic character during convergence between the Congoand Kalahari Cratons (M3 metamorphic cycle). Low-P contact metamorphismwith anticlockwise PT paths dominates in the westerndomains (Ugab Zone and western Northern Zone), and high-P Barrovianmetamorphism with a clockwise PT path is documented fromthe easternmost domain (eastern Northern Zone). The sequenceof M3 mineral growth in contact aureoles shows early growthof cordierite porphyroblasts that were pseudomorphed to biotitechloritemuscoviteat the same time as an andalusitebiotitemuscovitetransposed foliation was developed in the matrix. The peak-Tmetamorphic assemblages and fabrics were overprinted by crenulationsand retrograde chloritemuscovite. The KFMASH PTpseudosection for metapelites in the Ugab Zone and western NorthernZone contact aureoles indicates tight anticlockwise PTloops through peak metamorphic conditions of 540570°Cand 2·53·2 kbar. These semi-quantitativePT loops are consistent with average PT calculationsusing THERMOCALC, which give a pooled mean of 556 ± 26°Cand 3·2 ± 0·6 kbar, indicating a high averagethermal gradient of 50°C/km. In contrast, the eastern NorthernZone experienced deep burial, high-P/moderate-T Barrovian M3metamorphism with an average thermal gradient of 21°C/kmand peak metamorphic conditions of c. 635°C and 8·7kbar. The calculated PT pseudosection and garnet compositionalisopleths in KFMASH, appropriate for the metapelite sample fromthis region, document a clockwise PT path. Early plagioclasekyanitebiotiteparageneses evolved by plagioclase consumption and the growthof garnet to increasing XFe, XMg and XCa and decreasing XMncompositions, indicating steep burial with heating. The developedkyanitegarnetbiotite peak metamorphic parageneseswere followed by the resorption of garnet and formation of plagioclasemoats, indicating decompression, which was followed by retrogressivecooling and chloritemuscovite growth. The clockwise PTloop is consistent with the foreland vergent foldthrustbelt geometry in this part of the northern margin. Earlier formed(580570 Ma) pervasive matrix foliations (M2) were overprintedby contact metamorphic parageneses (M3) in the aureoles of 530± 3 Ma granites in the Ugab Zone and 553514 Magranites in the western Northern Zone. Available geochronologicaldata suggest that convergence between the Congo and KalahariCratons was essentially coeval in all parts of the northernmargin, with similar ages of 535530 Ma for the main phaseof deformation in the eastern Northern Zone and Northern Platformand 538505 Ma high-grade metamorphism of the CentralZone immediately to the south. Consequently, NNESSW-directedconvergent deformation and associated M3 metamorphism of contrastingstyles are interpreted to be broadly contemporaneous along thelength of the northern margin of the Inland Branch. In the westheat transfer was dominated by conduction and externally drivenby granites, whereas in the east heat transfer was dominatedby advection and internally driven radiogenic heat production.The ultimate cause was along-orogen variation in crustal architecture,including thickness of the passive margin lithosphere and thicknessof the overlying sedimentary succession. KEY WORDS: Pan-African Orogeny; PT paths; pseudosections; low-P metamorphism; contact metamorphism; Barrovian metamorphism 相似文献
176.
Spinel lherzolite and pyroxenite inclusions from the Geronimovolcanic field, Arizona (and Dish Hill, California) record,in their constituent minerals, a chronology of diverse mantledepletion and enrichment events. Certain portions of the lithosphericmantle have remained relatively isolated for considerable periodsof time(14 b.y.) while wall rock adjacent to conduitsof basanite has been recently (< 0-2 b.y.) modified. Evidenceexists for a widespread ancient (14 b.y.) partial meltingresidue, now recognizable as MORB-like mantle below the southwesternU.S.A. Trace element enrichment (0?9 b.y.) has increased thelight rare earth element (LREE) and Sr content of many refractoryperidotites without any mineralogical changes to the host rock.The fluids/melt responsible for this enrichment have a complexhistory involving heterogeneous mantle sources. In contrast,modal metasomatism of the mantle (< 0.2 b.y.) in aureolesaround evolved derivatives of basanite has petrographicallyand chemically transformed this ancient partial melting residue.The metasomatic fluids responsible for such metasomatism havean asthenospheric mantle source identical to the host magma.It is proposed that modal metasomatism occurs in contact metamorphicaureoles that surround apophyses of basanitic silicate meltin the lithospheric mantle. The gradient in CO2/(CO2 ? H2O)ratio that must surround such veins in the upper mantle (<20 kb) may encourage the development of enrichment fronts. Immediatelyadjacent to the vein, a wet zone with a relatively low CO2/(CO2? H2O) ratio would allow a precipitation of mica ? amphibole.Beyond this a dry zone with a higher CO2/(CO2 ? H2O) ratio wouldhasten chemical but not petrographic transformation of the wallrock. 相似文献
177.
STEFANIE HARWART BIRGIT HAGEDORN MARTIN MELLES ULRICH WAND 《Boreas: An International Journal of Quaternary Research》1999,28(1):167-180
In order to evaluate the sensitivity of aquatic and terrestrial ecosystems to climatic changes, lithological (sediment structure and color, grain size, physical properties) and biochemical (TOC, TOC/TN, δ13 C of TOC and carbonates) investigations were carried out on an 11.12 m-long sediment core from Lama Lake (Central Siberia, Putorana Plateau). According to the pollen data, the sequence represents the termination of the Pleistocene, and the entire Holocene. It is composed of highly terrigenous and stratified clays and silts. Sediment structure, grain-size distribution, carbonate contents and physical properties of the sediment indicate that glaciers were present in the catchment area of Lama Lake during the period Oldest Dryas to AllerØd. For the same time period, δ13 C values of TOC give indications of a perennial ice cover. Since the AllerØd, organic matter accumulation has increased, caused by an increasing input of land vegetation and aquatic primary production as revealed by relations TOC contents, TOC/TN ratios and δ13 C values of TOC. During the Holocene climatic optimum, in late Preboreal and Boreal times, biogenic primary production in Lama Lake reached its maximum and the vegetation in the catchment area changed from grassy tundra to a dense forest. From the Atlantic period to the present, small variations in δ13 C values of TOC and TOC contents are probably related to the location of Lama Lake on the border between grass and wooded steppe, leading to responses of environmental conditions to even small changes in climate. 相似文献
178.
Fenitizing Processes Induced by Ferrocarbonatite Magmatism at Swartbooisdrif, NW Namibia 总被引:1,自引:0,他引:1
The southeastern margin of the anorthositic Kunene IntrusiveComplex, NW Namibia, has been subsequently invaded by Mesoproterozoicsyenite, nepheline syenite and ferrocarbonatite dykes alongNE- and SE-trending faults. The first generation of carbonatiteintrusions frequently contains fenitized anorthositic wall-rockfragments set in a ferrocarbonatite matrix; later, subordinateveins of massive ferrocarbonatite are almost xenolith-free andcut through the main carbonatite dykes. A mantle source forboth carbonatite generations is constrained by their respectiveoxygen and carbon isotope compositions of ankerite (18OSMOW8·919·73; 13CPDB 6·98 to6·76). Na-rich fluids, released from the meltparental to the ferrocarbonatites, caused the fenitization ofboth the incorporated anorthosite xenoliths and the borderinganorthosite, syenite and nepheline syenite. This process ismainly characterized by the progressive transformation of Ca-richplagioclase, K-feldspar and nepheline into albite and/or sodalite.The changing mineral modes indicate that the fenitizing fluidswere sodium-rich and strongly Si-deficient solutions, whichalso contained significant amounts of Sr, Ba, Nb and the lightrare earth elements. On the basis of mineral equilibria studies,it is possible to reconstruct the temperature conditions forcarbonatite emplacement (c. 830 ± 200°C) and recrystallization(c. 480 ± 130°C), and for the metasomatic formationof sodalite (c. 700 ± 70°C). KEY WORDS: anorthosite; fenitization; ferrocarbonatite; sodalite; stable isotopes 相似文献
179.
180.