Comments on the paper by G.M. Gonzaga et al.     

O. H. Leonardos  R. N. Thompson  R. Fleicher  S. A. Gibson  D. P. Svisero  R. K. Weska 《Mineralium Deposita》1996,31(4):343-344

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Late Cretaceous rift-related upwelling and melting of the Trindade starting mantle plume head beneath western Brazil   总被引：1，自引：0，他引：1  

S. A. Gibson  R. N. Thompson  R. K. Weska  A. P. Dickin  O. H. Leonardos 《Contributions to Mineralogy and Petrology》1997,126(3):303-314

High mantle potential temperatures and local extension, associated with the Late-Cretaceous impact of the Trindade mantle plume, produced substantial widespread and voluminous magmatism around the northern half of the Paraná sedimentary basin. Our previous studies have shown that, above the central and eastern portions of the postulated impact zone where lithosphere extension is minimal, heat conducted by the plume caused large-scale melting of the more fusible parts of the subcontinental lithospheric mantle beneath the margin of the São Francisco craton and the surrounding Brasilía mobile belt. Here we combine geochemical data and field evidence from the Poxoreu Igneous Province, western Brazil to show how more intense lithospheric extension above the western margin of the postulated impact zone permitted greater upwelling and melting of the Trindade plume than further east. Laser ⁴⁰Ar/³⁹Ar age determinations indicate that rift-related basaltic magmas of the Poxoreu Igneous Province were emplaced at ? 84 Ma. Our detailed geochemical study of the mafic magmas shows that the parental melts underwent polybaric crystal fractionation within the crust prior to final emplacement. Furthermore, some magmas (quartz-normative) appear to have assimilated upper crust whereas others (nepheline- and hypersthene-normative) appear to have been unaffected by open-system crustal magma chamber processes. Incompatible trace element ratios (e.g. chondrite-normalised La/Nb?=?1) and isotopic ratios (⁸⁷Sr/⁸⁶Sr?=?0.704 and ¹⁴³Nd/¹⁴⁴Nd?=?0.51274) of the Hy-normative basalts resemble those of oceanic islands (OIB). We therefore propose that these “OIB-like” magmas were predominantly derived from convecting-mantle-source melts (i.e. Trindade mantle plume). Inverse modelling of rare-earth element (REE) abundances suggests that the initial melts were predominantly generated within the depth range of ?80–100 km, in mantle with a potential temperature of ?1500 °C.  相似文献   

Observations of borehole-source amplitude reduction due to casing     

Richard L. Gibson  Walter R. Turpening  rea Born  & Roger M. Turpening 《Geophysical Prospecting》1997,45(1):1-20

Theoretical results show that the amplitude of a borehole source is reduced when the well in which it is operated is cased and cemented. This reduction is a strong function of the formation velocity and is more weakly dependent on the direction of propagation of the wave travelling from source to receiver and on the diameter of the borehole itself. We have tested these predictions with data gathered in a cross-hole seismic experiment conducted in two stages in 1990 and 1991. The source and receivers were located in the MIT/Stech 1-21A and MIT/Burch 1-20B wells at the Earth Resources Laboratory (ERL) test site in Michigan (USA). Though the source well (MIT/Stech 1-21A) was uncased in 1990, a steel casing and cement were added prior to the collection of data in 1991. Several receiver positions were reoccupied to compare data collected with the source in open and cased holes. Using a velocity model for this area and borehole diameter measurements from a calliper log, a compensation factor can be computed that will adjust the data collected in 1991 to have amplitudes comparable to that collected in the first stage of the experiment. The accuracy of the results demonstrates the validity of the theory, which can be very useful in better understanding seismic waveforms recorded in cross-hole experiments.  相似文献   

巴基斯坦北Potwar形变区地震的震源机制研究   总被引：2，自引：0，他引：2       下载免费PDF全文

MonaLisa  ShahidA．Khan  AzamA．Khwaja  许忠淮 《地震学报》2004,26(3):231-237

巴基斯坦北Potwar形变区是西北喜马拉雅褶皱逆冲带前陆区的一部分，绘制了该区的地震活动性图. 与相邻地区比较，该区地震并不活跃，没有显示出与地表地质构造相关的清晰地震活动图象. 做出了4次地震的震源机制解. 结果表明，有3次地震是左旋走滑断层活动，另一次地震是逆断层活动. 地震震源机制解的P轴方向为NW-SE和NE-SW. 现今的构造形变很可能也包括基底的形变.   相似文献   

Nature of the archean midcrust in the core of the Vredefort dome, Central Kaapvaal Craton, South Africa     

C. Lana  R.L. Gibson  S. Siegesmund 《Geochimica et cosmochimica acta》2004,68(3):623-642

Extreme uplift associated with the formation of the 2.02 Ga Vredefort dome has exposed a substantial cross section through the crystalline early Archean basement complex rocks of the Kaapvaal craton. The rocks comprise polydeformed high-grade tonalite-trondhjemite-granodiorite (TTG) gneisses, migmatites and late-tectonic intrusive granitoids that straddle the upper amphibolite-to granulite-facies transition. Field, petrographic and geochemical data indicate that compositional heterogeneity occurs on a local scale and reflects the migmatitic character of the rocks rather than crustal-scale layering as has been previously proposed. No evidence has been found to support exposure of either a melt-depleted, refractory, lower crust or an upper crustal batholithic granite layer; however, the immense volume of granitic leucosome in the rocks suggests that the exposed section represents an intermediate level between these two zones. Granitic leucosomes in the upper amphibolite-facies migmatites appear to be intrusive into the predominantly trondhjemitic host rocks, rather than of in situ derivation. Leucosome compositions in the granulite-facies migmatites are more variable, ranging from granitic and charnockitic to enderbitic, probably reflecting at least some local derivation. Leucosomes and small granitoid bodies show local-scale geochemical variation that can be explained in terms of variable amounts of melt segregation and migration, and fractionation of minerals such as K-feldspar within the melts.  相似文献   

Academic publishing as 'creative' industry, and recent discourses of 'creative economies': some critical reflections   总被引：2，自引：0，他引：2  

Chris Gibson  Natascha Klocker 《Area》2004,36(4):423-434

This paper continues recent discussions on the (geo)politics of the production of academic knowledges, in relation to the recent rise of narratives of 'the creative economy'. Creativity and the 'creative industries' are increasingly common components of urban economic development discourse, especially following the release of a set of key books – most notably Charles Landry's The Creative City (2000), and Richard Florida's The Rise of the Creative Class (2002) – that have become popular among economic development planners and cultural policy makers. This paper focuses on the traffic of these books, and their authors, beyond the Anglo-American core. It also briefly discusses policy discourses interpreted from these texts. It is principally, though, a critique of the ways in which academic knowledges circulate, stemming from theorization of academics as creative producers, and of knowledge production as part of the creative economy. The article seeks to critique the means by which particular northern economic knowledges become normative, framed as universal and 'global', and are distributed and absorbed via intellectual 'scenes' and an academic 'celebrity' circuit.  相似文献   

Resprout characteristics of creosote bush (Larrea tridentata) when subjected to repeated vehicle damage     

Arthur C. Gibson  M. Rasoul Sharifi  Philip W. Rundel 《Journal of Arid Environments》2004,57(4):411

Studies were conducted in the central Mojave Desert to quantify how creosote bushes (Larrea tridentata) respond to physical damage during large-scale military training exercises. Creosote bush possesses a resilient growth form that recovers from repeated physical damage via resprouts arising from meristems in stem bark below severed or crushed canopy units. At high levels of disturbance by heavy vehicles, nearly all individuals showed measurable breakage, but without additional damage each plant can regain a full canopy within 5 years under arid field conditions. Resprouts exhibited more vigorous growth and doubled the biomass accumulation stimulated by high rainfall of 1998, an El Niño year, vs. a normal year. New shoots of resprouted individuals were markedly different in morphological traits than canopy old growth and had slightly higher predawn shoot water potentials. The natural ability of this evergreen species to recover from cutting and crushing bodes well for re-establishment of creosote bush desert scrub communities following episodes of severe damage by vehicles.  相似文献   

Higher tritium concentrations measured in permafrost thaw lakes in northern Alberta          下载免费PDF全文

J. J. Gibson  S. J. Birks  Y. Yi 《水文研究》2016,30(2):245-249

Tritium concentrations were measured in a survey of 24 lakes, 15 wetlands, and 133 groundwaters in the oil sands region of northeastern Alberta and compared with both recent precipitation and precipitation sampled during the 1960s tritium peak caused by atmospheric thermonuclear weapons testing. Water samples from lakes included a group of 14 thaw lakes that had higher runoff attributed to melting of permafrost in peat plateaus within their watersheds. While tritium in all lakes was found to be intermediate between recent and 1960s concentrations, the thaw lakes were found to be significantly enriched in tritium compared with other lakes, as were unfrozen wetlands characterized by a thick sequence of low‐hydraulic conductivity peat. The results provide further evidence of different water sources to the thaw lakes and may indicate that melting of modern permafrost in part formed since the 1950s is occurring in these systems. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Linkages between spatio‐temporal patterns of environmental factors and distribution of plant assemblages across a boreal peatland complex          下载免费PDF全文

Jeremy A. Graham  Jeremy A. Hartsock  Dale H. Vitt  R. Kelman Wieder  John J. Gibson 《Boreas: An International Journal of Quaternary Research》2016,45(2):207-219

Here we examine the arrangement of plant species across an oligotrophic bog/poor fen peatland complex in the North American boreal plain and the relationships of these species to their physical and chemical environment. A semi‐uniform spatial sampling approach was utilized to describe the species assemblages, pore‐water chemistry and physical condition of 100 plots throughout a single peatland complex. Regardless of sharing the same ground cover of Sphagnum mosses, the remaining species separated into four distinct assemblages, each with unique indicators. These species groups along with associated chemical and physical factors are organized into four ecosites: bog, margin (edge) and two poor fen ecosites. The plant assemblages of this peatland have a complex relationship with numerous gradients, both physical and chemical, including depth to water table, shade, pH, nutrient and base cation. Rather than being homogenous across the landscape, most environmental variables exhibit distinct spatial patterns and do so in relationship to the plant assemblages, forming spatially distinct ecosites across the complex. Base cation concentrations play a smaller role than previously thought in differentiating these ecosites, and in addition to shade and depth to water table, nitrogen in the form of dissolved organic nitrogen was highly related to the placement of these ecosites. Many significant chemical factors appear related to evaporative water loss within the peatland complex, and these chemical factors are used to differentiate the ecosites. However, the mediation of evaporative water loss is due largely to self‐generated responses of the plant assemblages related to shade through plant morphology and peat acrotelm development related to depth to water table. We conclude that plant species and associated environmental gradients act together to form spatially distinct ecosites. The distribution of these ecosites within this large, environmentally complex peatland is largely controlled by differing self‐generated responses along the hydrotopographical gradient of differential water loss.  相似文献   

Attribution of extreme temperature changes during 1951–2010     

Yeon-Hee Kim  Seung-Ki Min  Xuebin Zhang  Francis Zwiers  Lisa V. Alexander  Markus G. Donat  Yu-Shiang Tung 《Climate Dynamics》2016,46(5-6):1769-1782

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