Archaean to Proterozoic Crustal Evolution in the Central Zone of the Limpopo Belt (South Africa-Botswana): Constraints from Combined U-Pb and Lu-Hf Isotope Analyses of Zircon   总被引：5，自引：0，他引：5  

Zeh  Armin; Gerdes  Axel; Klemd  Reiner; Barton  Jackson M.  Jr 《Journal of Petrology》2007,48(8):1605-1639

A combined set of U–Pb and Lu–Hf in situ laser ablationICP-(MC)-MS zircon analyses were obtained from orthogneissesand granitoids in the Central Zone of the Limpopo Belt, whichcomprises the Beit Bridge and Mahalapye complexes. The resultsindicate that by combining the two isotope systems primary magmaticzircon domains can be distinguished from those formed duringlater metamorphic events, even if the distinct zircon domainsunderwent multiple Pb loss and the texture–age relationships,as obtained by cathodoluminescence images and U–Pb analyses,are ambiguous. Furthermore, the applied technique allows distinctionof zircon grains formed in juvenile magmas from those generatedby melting of older continental crust or affected by substantialcrustal contamination. The combined U–Pb and Lu–Hfdata reveal that the Sand River gneiss suite of the Beit BridgeComplex was emplaced at 3283 ± 8 Ma and formed from meltingof an older Archaean crust, which was derived from a depletedmantle source at around 3·65 Ga. The hafnium model age(T_DM^Hf) is significantly older than those obtained from zirconsfrom numerous Neoarchaean granitoids of the Beit Bridge Complex,comprising the Singelele gneiss (2647 ± 12 Ma), the Bulaigranite (2612 ± 7 Ma), the Regina gneiss (2649 ±9 Ma) and two samples of the Zanzibar gneiss (2613 ±6 Ma). These granitoids show initial Hf(t) values between +0·5 and –7·1, which correspond to initialT_DM^Hf between 3·46 and 3·01 Ga. These variableT_DM^Hf_initial and Hf(t)_initial values are interpreted to be theresult of different mixtures of reworked 3·65 Ga Palaeoarchaeancrust with juvenile magmas extracted from the depleted mantleduring the Neoarchaean at 2·65 Ga. This conclusion issupported by results obtained from the Mahalapye Complex, whichwas affected by migmatization and granite intrusions duringthe Palaeoproterozoic at 2·02–2·06 Ga. TheMokgware granite (2019 ± 9 Ma) contains zircon xenocrystswith Pb–Pb ages of 2·52–2·65 Ga and2·93 Ga and hafnium model ages of 3·0–3·4Ga, indicating that this granite is derived from remelting ofArchaean crust. In contrast, uniform T_DM^Hf_initial ages of 2·61–2·67Ga obtained from a diorite gneiss (2061 ± 6 Ma) of theMahalapye Complex indicate that its protolith may have beenformed from remelting of a Neoarchaean juvenile crust. VariableHf(t)_initial values from –3·7 to +6·3 ofzircon cores (2711 ± 11 Ma) in an adjacent leucosomealso support a model of mixing of juvenile mantle derived matterwith older crust in the Neoarchaean. KEY WORDS: Archaean; Palaeoproterozoic; Limpopo Belt; zircon, U–Pb dating; Lu–Hf isotopes; LA-ICP-MS  相似文献   

Fractal clustering of induced seismicity in The Geysers geothermal area, California   总被引：3，自引：0，他引：3  

J. R. Henderson  D. J. Barton  & G. R. Foulger 《Geophysical Journal International》1999,139(2):317-324

We analyse earthquakes recorded at The Geysers geothermal field in California, an area where industrial activity induces seismicity. The seismicity is characterized by the seismic b -value and D , the fractal dimension of earthquake hypocentres measured from sliding windows containing 200 events. We study a group of events strongly clustered around an injection well. Over most of the time period examined we find a positive correlation between b and D . However, during the initiation of injection into a new well we find instead a negative correlation. The differences in correlation are statistically significant at the 1 σ level but only marginally so at the 2 σ level. These results provide evidence for a transient change in the seismic mechanisms operating, and may be explained by a change from conditions of slow stress loading to rapid loading as a result of the build-up of the rate of water injection into the reservoir.  相似文献   

Lithospheric structure north of Scotland—I. P-wave modelling, deep reflection profiles and gravity     

R. Peter LL. Morgan  Penny J. Barton  Mike Warner  Joanna Morgan  Claire Price †  Kevin Jones 《Geophysical Journal International》2000,142(3):716-736

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Ants in Labyrinths: Lessons for Critical Landscapes     

George P. Malanson  Barton E. Cramer 《The Professional geographer》1999,51(2):155-170

Percolation theory provides a starting point for models for the movement of organisms in fragmented landscapes. We simulated six general rules for movement on three general landscape patterns with different degrees of habitat fragmentation. Fragmentation pattern affects migration success. Organisms moving within habitat patches are restricted by fragmentation, but the response is less abrupt as one moves from random to hierarchical to fractal landscapes, respectively. Success for organisms moving along edges is more strongly differentiated by landscape pattern. Success and speed are usually directly related, but indirect relations may indicate rare but efficient pathways or cul-de-sacs. We propose that general rules can be applied to the identification of landscapes near critical levels of fragmentation that will respond sensitively to increases in fragmentation or efforts at restoration.  相似文献   

Vince Mannings,A. P. Boss,and S. S. Russell: Protostars and Planet,IV, Space Science Series     

Penny  Alan 《Earth, Moon, and Planets》1999,84(3):177-177

Earth, Moon, and Planets -  相似文献   

The transition zone of the Canary Current upwelling region   总被引：2，自引：0，他引：2  

E. D. Barton  J. Arístegui  P. Tett  M. Cantn  J. García-Braun  S. Hernndez-Len  L. Nykjaer  C. Almeida  J. Almunia  S. Ballesteros  G. Basterretxea  J. Escnez  L. García-Weill  A. Hernndez-Guerra  F. Lpez-Laatzen  R. Molina  M. F. Montero  E. Navarro-Prez  J. M. Rodríguez  K. van Lenning  H. Vlez  K. Wild 《Progress in Oceanography》1998,41(4):455-504

Like all the major upwelling regions, the Canary Current is characterised by intense mesoscale structure in the transition zone between the cool, nutrient-rich waters of the coastal upwelling regime and the warmer, oligotrophic waters of the open ocean. The Canary Island archipelago, which straddles the transition, introduces a second source of variability by perturbing the general southwestward flow of both ocean currents and Trade winds. The combined effects of the flow disturbance and the eddying and meandering of the boundary between upwelled and oceanic waters produce a complex pattern of regional variability. On the basis of historical data and a series of interdisciplinary field studies, the principal features of the region are described. These include a prominent upwelling filament originating near 28°N off the African coast, cyclonic and anti-cyclonic eddies downstream of the archipelago, and warm wake regions protected from the Trade winds by the high volcanic peaks of the islands. The filament is shown to be a recurrent feature, apparently arising from the interaction of a topographically trapped cyclonic eddy with the outer edge of the coastal upwelling zone. Its role in the transport and exchange of biogenic material, including fish larvae, is considered. Strong cyclonic eddies, observed throughout the year, drift slowly southwestward from Gran Canaria. One sampled in late summer was characterised by large vertical isopycnal displacements, apparent surface divergence and strong upwelling, producing a fourfold increase in chlorophyll concentrations over background values. Such intense eddies can be responsible for a major contribution to the vertical flux of nitrogen. The lee region of Gran Canaria is shown to be a location of strong pycnocline deformation resulting from Ekman pumping on the wind shear boundaries, which may contribute to the eddy formation process.  相似文献   

Global assessments of the NCEP Ensemble Forecast System using altimeter data     

Campos  Ricardo Martins  Alves  Jose-Henrique G.M.  Penny  Stephen G.  Krasnopolsky  Vladimir 《Ocean Dynamics》2020,70(3):405-419

Ocean Dynamics - Forecasts of 10-m wind (U10) and significant wave height (Hs) from the National Centers for Environmental Prediction (NCEP) Ensemble Forecast System are evaluated using altimeter...  相似文献   

Late‐glacial and Holocene record of vegetation and climate from Cynthia Bay,Lake St Clair,Tasmania     

F. V. L. Hopf  E. A. Colhoun  C. E. Barton 《第四纪科学杂志》2000,15(7):725-732

A Late‐glacial–Holocene pollen record was obtained from a 3.96 m sediment core taken from Lake St Clair, central Tasmania. Modern vegetation and pollen analyses formed the basis for interpretation of the vegetation and climate history. Following deglaciation and before ca. 18450 yr BP Podocarpus lawrencei coniferous heath and Astelia–Plantago wet alpine herbfield became established at Lake St Clair. A distinct Poaceae‐Plantago peak occurs between 18450 and 11210 yr BP and a mean annual temperature depression from ca. 6.2°C to 3°C below present is inferred for this period. The marked reduction in Podocarpus and strong increase of Poaceae suggests reduced precipitation levels during the period of widespread deglaciation (ca. 18.5–11 kyr BP). The local Late Pleistocene–Holocene non‐forest to forest biostratigraphical boundary is dated at 11.2 kyr BP. It is characterised by expansion of the subalpine taxa Athrotaxis/Diselma with Nothofagus gunnii, and by the establishment of Nothofagus cunninghamii with Eucalyptus spp. A ‘Phyllocladus bulge’ prior to the expansion of Nothofagus cunninghamii, reported at other Tasmanian sites, is not present at Lake St Clair. Nothofagus cunninghamii cool temperate rainforest peaked at 7800 yr BP, probably under wetter climatic conditions than present. The maximum development of rainforest in the early–middle Holocene may indicate that the temperature was slightly warmer than present, but the evidence is not definitive. The expansion of Eucalyptus spp. and Poaceae after 6000 yr BP may be partly a disclimax effect as a result of Aboriginal burning, but appears also to reflect reduced precipitation. The changes in vegetation and inferred climate can be explained by major changes in synoptic patterns of southern Australia and the adjacent southwest Pacific. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

Anisotropy is Everywhere,to See,to Measure,and to Model     

Nick Barton  Eda Quadros 《Rock Mechanics and Rock Engineering》2015,48(4):1323-1339

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Quantifying groundwater dependency of riparian surface hydrologic features using the exit gradient          下载免费PDF全文

Barton R. Faulkner  Scott G. Leibowitz  Timothy J. Canfield  Justin F. Groves 《水文研究》2016,30(13):2167-2177

Numerical groundwater flow models necessarily are limited to subsurface flow evaluation. It is of interest, however, to examine the possibility that, for unconfined aquifer systems, they could be used to proportionately measure the magnitude of seepage they estimate when these aquifers intersect the landscape surface. Our goal in this study was to determine the degree to which an unconfined groundwater model can estimate run‐off or seepage at the land surface during winter time wet season conditions, as well as in the dry season, when evapotranspiration is a major part of the water balance, using a lowland basin‐fill example study area in the Pacific Northwest. The exit gradient is a metric describing the potential for vertical seepage at the landscape surface. We investigated the spatial relationship of mapped surface features, such as wetlands, streams and ponds, to the model‐predicted mapped exit gradient. We found that areas mapped as wetlands had positive exit gradients. During the wet season, modelled exit gradients predicted seepage throughout extensive areas of the groundwater shed, extending far beyond mapped wetland areas (355% increase), associated with previously observed increases in nitrate‐nitrogen in streams in wet season. During the dry season, exit gradients spatially corresponded with wetland areas. The increase in in‐stream nitrogen corresponds with shorter residence times in carbon‐rich wetland zones because of the onset of saturation overland flow. We present results that suggest that the exit gradient could be a useful concept in examining the groundwater–surface water linkage that is often under represented physically in watershed flow models. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.  相似文献