Spectral and Spatial Variations of Flare Hard X-ray Footpoints     

Fletcher  L.  Hudson  H.S. 《Solar physics》2002,210(1-2):307-321

In a sample of strong RHESSI M-class flares we have made a study of the relationship between the `hardness' of the HXR spectrum and the intensity in the 30–50 keV energy range. In all events we find clear evidence for a `soft–hard–soft' pattern of correlation between hardness and flux, on time scales as short as 10 s. We investigate whether or not this pattern is intrinsic to the acceleration mechanism. The RHESSI images in this energy range are dominated by footpoint brightenings, and we have searched for a correlation between footpoint separation velocity and spectral hardness, to be compared qualitatively with theoretical flare models. We find quite systematic footpoint motions, and also note that episodes in which footpoint separation varies rapidly often correspond with episodes of significant change in the flare spectral index, though not as the simplest flare models would predict. We report also on one of our events, on 14 March 2002, which exhibits highly sheared HXR footpoint ribbons extending over a scale of 100 arc sec. For this flare we find a correlation between footpoint motion and hard X-ray flux. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022479610710  相似文献   

Crustal evolution of Archaean granitoids in the Murchison Province, Western Australia     

Keith P. Watkins  Ian R. Fletcher  John R. de Laeter 《Precambrian Research》1991,50(3-4)

Four suites of granitoids intruded the supracrustal greenstone sequence in the Murchison Province of the Archaean Yilgarn Craton during a 300 million year period. The earliest granitoid suite intruded the base of the developing greenstone sequence as a series of thin subhorizontal tabular plutons of monzogranite and granodiorite at 2.9Ga. This suite has been deformed and metamorphosed, and is now a pegmatite-banded gneiss. At about 2.7 Ga, thick, subhorizontal, tabular plutons of monzogranite intruded the base of the greenstone sequence. This suite, which now forms much of the regions between greenstone belts, was folded and recrystallized during regional deformation and metamorphism. Two distinct but contemporaneous suites of post-folding granitoids intruded the greenstone belts at 2.6 Ga, largely post-dating regional metamorphism. One suite of post-folding granitoids comprises tonalite, trondhjemite, granodiorite and monzogranite plutons, confined mainly to the north of the Province. The other suite comprises quartz-rich monzogranite and syenogranite plutons, confined mainly to the south of the Province.Pegmatite-banded gneiss, recrystallized monzogranite, and the northern suite of post-folding granitoids were all derived by partial-melting of mafic crustal rocks. Most post-folding granitoids from the southern suite were derived by partial-melting of siliceous crustal material at least as old as basal greenstones. The modes and sites of intrusion of all granitoid plutons were controlled by active tectonic processes or by structural features of the crust. Widespread 2.6 Ga Rb---Sr ages of pegmatite-banded gneiss and recrystallized monzogranite reflect post-metamorphic cooling which was contemporaneous with intrusion of post-folding granitoids.  相似文献   

Ropy pahoehoe: Surface folding of a viscous fluid     

Jonathan H. Fink  Raymond C. Fletcher 《Journal of Volcanology and Geothermal Research》1978,4(1-2)

The regularly spaced surface structure observed on ropy pahoehoe basalt flows may be interpreted as folds which develop at the surface of a fluid whose viscosity decreases with depth. Folds form by the selective amplification of an irregular waviness in surface shape during shortening of the flow surface. The development of a regular fold arc length, predicted by folding theory, is reflected in the length scale of pahoehoe ropes. Pahoehoe fold arc lengths and the strength of the folding instability are determined by: (1) the ratio of the surface viscosity to the interior viscosity; (2) the thickness of the thermal boundary layer across which the viscosity changes; and (3) the ratio of the surface compressive stress to a stress related to the weight of the lava.The braided appearance of many ropy pahoehoe flows can be explained by a superposition of two episodes of folding.  相似文献   

Folding of a single viscous layer: Exact infinitesimal-amplitude solution     

Raymond C. Fletcher 《Tectonophysics》1977,39(4):593-606

The folding of a single viscous layer embedded in an uniformly shortening medium is analyzed mathematically by superposing the mean flow corresponding to uniform shortening and a perturbing flow corresponding to folding. The mean flow is coupled to the perturbing flow in the boundary conditions applying at the layer—medium interfaces. These are expanded according to the surface-wave approximation in hydrodynamics, and the solution applies when the amplitude of folding is small. Folding with interfacial adherence and interfacial slip are both treated. The dominant wavelenghts obtained do not agree with those presented in Biot (1959a) and Remberg (1963a, 1970b).  相似文献   

Coupling of diffusional mass transport and deformation in a tight rock     

Raymond C. Fletcher 《Tectonophysics》1982,83(3-4)

Simple governing equations coupling diffusional transport at a macroscopic scale and deformation in a tight rock are formulated. The deformation alone is described by a linear viscous rheology. Several deformations of geological interest are analyzed, including the folding of an embedded layer. A basic feature is the length-scale dependence of diffusional effects. Interpretation of natural structures permits the estimation of parameters governing the diffusional transport and rate of deformation.  相似文献   

Precise determination of initial ϵNd from Sm-Nd isochron data     

I.R. Fletcher  K.J.R. Rosman 《Geochimica et cosmochimica acta》1982,46(10):1983-1987

Current methods for evaluating Sm-Nd isochron data determine initial ?_Nd (?_Ndⁱ) values by extrapolation to ${}^{147}\text{Sm}{}^{144}\text{Nd = 0}$, resulting in unnecessarily large uncertainties. It is shown that values of ?_Ndⁱ can be optimised by determining the present ${}^{143}\text{Nd}{}^{144}\text{Nd}$ composition of a hypothetical sample having chrondritic $\text{Sm}\text{Nd}$. This method of determination requires only a simple data translation for its implementation using standard line-fitting techniques, and is particularly advantageous in providing precise ?_Ndⁱ values for sample suites of limited compositional range close to chondritic $\text{Sm}\text{Nd}$.A method of constructing error (uncertainty) polygons appropriate to the optimised (?_Ndⁱ, T) values is discussed, and optimum?_Ndⁱ values are tabulated for all published Precambrian Sm-Nd isochrons.  相似文献   

Wastage of the Klutlan ice-cored moraines,Yukon Territory,Canada     

Fletcher G. Driscoll 《Quaternary Research》1980,14(1):31-49

A series of ice-cored Neoglacial moraines at the terminus of the Klutlan Glacier covers an area of 90 km². Studies were made to determine empirically how long ice persisted in the Klutlan moraines and to develop models that can accurately predict wastage rates under current climatic conditions. A meltout curve based on climatological data reflects the sum of three melting processes: surficial melting, melting by lake water, and melting by geothermal heat. About 950 yr are required to melt 180 m of ice with a debris concentration of 1%, or about 1200 yr for a 1.5% debris load. Another meltout curve, based on seismic data, suggests total meltout in about 875 yr. When all geologic factors are considered, the empirical meltout curve is remarkably similar to that derived by considering the major heat-flux parameters. Meltout rates can be predicted if (1) the fundamental climatic parameters can be ascertained, and (2) the sediment concentration in the ice is known.  相似文献   

The effect of oil pollution in Bantry Bay     

John P. Cullinane  Patrick McCarthy  Anthony Fletcher 《Marine pollution bulletin》1975,6(11):173-176

The biological damage caused by the large oil spill in Bantry Bay and the clean-up measures adopted to deal with it (already reported in the Marine Pollution Bulletin) has been followed up in the months following the oil spill. This report refers to damage to algae and lichens.  相似文献   

SHRIMP U–Pb zircon geochronology of Neoproterozoic crustal granitoids (Southern Brazil): A case for discrimination of emplacement and inherited ages     

Luiz Carlos da Silva  Neal Jesse McNaughton  Ian R. Fletcher 《Lithos》2005,82(3-4):503-525

SHRIMP U–Pb zircon studies on two post-collisional granitic plutons and reassessment of the data previously reported for two anatectic gneissic granites are used to assess the late Neoproterozoic history of the Florianópolis Batholith, southern Brazil. The results, supported by SEM backscattered and cathodoluminescence imagery, identify inherited zircon populations and confirm the long-lived, crustal recycling processes responsible for the accretion of the batholith. The study casts new lights on the timing of the processes involved in the generation and modification of the internal structure of distinct zircon populations, and enables discrimination to be made between inherited cores and melt-precipitated overgrowths. New dating of two post-tectonic plutons (samples 1 and 2) revealed crystals showing magmatic-textured cores sharply bounded by melt-precipitated overgrowths. The U/Pb isotopic results from both samples spread along concordia by ca. 40 m.y. (sample 1) to 100 m.y. (sample 2), clustering in two closely spaced (bimodal), partially overlapping peaks. Melt-precipitated rims and homogeneous new grains, dated at ca. 600 Ma, furnish the crystallisation age of the plutons. The magmatic textured cores and xenocrysts dated at ca. 630–620 Ma are interpreted as inherited restitic material from supposedly short-lived (meta)granitic sources. The reassessment of previous SHRIMP data of two banded anatectic granitoids (samples 3 and 4) revealed more complex morphological patterns, in which the overgrown inherited cores are sharply bounded against large melt-precipitated rims, dated at ca. 600 Ma and 592±2 Ma, respectively. Major populations of magmatic-textured inherited cores dated at 2006±3 Ma and 2175±13 Ma characterise samples 3 and 4, respectively. The latter additionally shows metamorphic and magmatic inherited cores with a large range of ages (ca. 2900–620 Ma), suggesting partial melting of metasedimentary components. The main magmatic Paleoproterozoic core populations are interpreted as inherited restite from partial melting of the adjacent (meta)tonalitic gneiss and amphibolitic country-rock (paleosome). The recognition of the (melt-precipitated) Neoproterozoic overgrowths and new crystals, and the restite provenance of the cores, supplants a previous interpretation of Paleoproterozoic magmatism (cores) and Neoproterozoic (solid-state) metamorphic overprint. As a major consequence of the former interpretation, the unit was mistakenly considered part of major Paleoproterozoic gneissic remnant within the Neoproterozoic Florianópolis Batholith/arc.  相似文献   

Effects of valley and local channel morphology on the distribution of gold in stream sediments from Harris Creek, British Columbia, Canada     

S.J. Day  W.K. Fletcher 《Journal of Geochemical Exploration》1989,32(1-3)

Large (60 kg, minus 2 mm) sediment samples collected from a 5-km reach of a mature highland stream were used to investigate the effects of bar morphology and channel slope on the downstream dispersion of Au. Results are compared with those for an abundant heavy mineral (magnetite) and are discussed in terms of differential entrapment of light and heavy minerals by gravels and differences in supply of magnetite and Au to the study reach. Gold is selectively trapped in gravels; therefore, highest Au concentrations, a long dispersion train and lowest between-sampling-location variability were obtained with heavy-mineral concentrates of the minus 105+74 μm fraction from sandy gravels. In contrast, sands in bar-tail eddy pools gave the least reliable Au concentrations, due to random (Poisson) sampling errors and local post-depositional winnowing, and show rapid downstream anomaly decay.  相似文献