Strength of Faults on Mars from MOLA Topography     

David N. BarnettFrancis Nimmo 《Icarus》2002,157(1):34-42

The stresses which must be maintained on faults bounding the rift topography at Tempe Fossae—the “North Tempe Rift” (NTR)—and Valles Marineris (VM) on Mars are estimated, using a simple elastic model and topographic data from the Mars Orbiter Laser Altimeter (MOLA). The absence of rift-flank uplift at the NTR is consistent with an elastic thickness, T_e, of 20 km or greater at the time of rift formation. The maximum resolved shear stresses on bounding faults due to this topography do not therefore exceed 20 MPa, similar to the inferred strength of terrestrial faults. Elastic thickness estimates at VM are mostly around 50 km or greater. Therefore, for canyon widths of ∼400 km, the bounding faults of VM, if present, must be able to withstand stresses of up to approximately 100 MPa. However, if the fault-controlled sections of the canyons do not exceed 150 km in width, as suggested by geomorphological analysis, the fault strength required is only 20 MPa. Although the maximum resolved shear stresses required to support the topography at VM may need to be greater than the stresses which terrestrial faults can support, at least some faults on Mars are no stronger than similar features on Earth. This observation is consistent with the existence of liquid water in the shallow subsurface of Mars at the time the faults were active. On Venus, plate tectonics is probably prevented by the frictional resistance to motion across strong faults. On Mars, it is more likely that the large thickness of the elastic layer of the lithosphere and the possible positive buoyancy of the crust are responsible for the observed lack of plate tectonics.  相似文献   

Multi-wavelength observations of the 1998 September 27 flare spray     

Gallagher  Peter T.  Williams  David R.  Phillips  Kenneth J.H.  Mathioudakis  Mihalis  Smartt  Raymond N.  Keenan  Francis P. 《Solar physics》2000,195(2):367-380

We report on observations of a large eruptive event associated with a flare that occurred on 27 September 1998 made with the Richard B. Dunn Solar Telescope at Sacramento Peak Observatory (several wave bands including off-line-center H), in soft and hard X-rays (GOES and BATSE), and in several TRACE wave bands (including Feix/x 171 Å, Fexii 195 Å, and Civ 1550 Å). The flare initiation is signaled by two H foot-point brightenings which are closely followed by a hard X-ray burst and a subsequent gradual increase in other wavelengths. The flare light curves show a complicated, three-component structure which includes two minor maxima before the main GOES class C5.2 peak after which there is a characteristic exponential decline. During the initial stages, a large spray event is observed within seconds of the hard X-ray burst which can be directly associated with a two-ribbon flare in H. The emission returns to pre-flare levels after about 35 min, by which time a set of bright post-flare loops have begun to form at temperatures of about 1.0–1.5 MK. Part of the flare plasma also intrudes into the penumbra of a large sunspot, generally a characteristic of very powerful flares, but the flare importance in GOES soft X-rays is in fact relatively modest. Much of the energy appears to be in the form of a second ejection which is observed in optical and ultraviolet bands, traveling out via several magnetic flux tubes from the main flare site (about 60° from Sun center) to beyond the limb.  相似文献   

The LMDZ4 general circulation model: climate performance and sensitivity to parametrized physics with emphasis on tropical convection   总被引：3，自引：11，他引：3  

Frédéric Hourdin  Ionela Musat  Sandrine Bony  Pascale Braconnot  Francis Codron  Jean-Louis Dufresne  Laurent Fairhead  Marie-Angèle Filiberti  Pierre Friedlingstein  Jean-Yves Grandpeix  Gerhard Krinner  Phu LeVan  Zhao-Xin Li  François Lott 《Climate Dynamics》2006,27(7-8):787-813

The LMDZ4 general circulation model is the atmospheric component of the IPSL–CM4 coupled model which has been used to perform climate change simulations for the 4th IPCC assessment report. The main aspects of the model climatology (forced by observed sea surface temperature) are documented here, as well as the major improvements with respect to the previous versions, which mainly come form the parametrization of tropical convection. A methodology is proposed to help analyse the sensitivity of the tropical Hadley–Walker circulation to the parametrization of cumulus convection and clouds. The tropical circulation is characterized using scalar potentials associated with the horizontal wind and horizontal transport of geopotential (the Laplacian of which is proportional to the total vertical momentum in the atmospheric column). The effect of parametrized physics is analysed in a regime sorted framework using the vertical velocity at 500 hPa as a proxy for large scale vertical motion. Compared to Tiedtke’s convection scheme, used in previous versions, the Emanuel’s scheme improves the representation of the Hadley–Walker circulation, with a relatively stronger and deeper large scale vertical ascent over tropical continents, and suppresses the marked patterns of concentrated rainfall over oceans. Thanks to the regime sorted analyses, these differences are attributed to intrinsic differences in the vertical distribution of convective heating, and to the lack of self-inhibition by precipitating downdraughts in Tiedtke’s parametrization. Both the convection and cloud schemes are shown to control the relative importance of large scale convection over land and ocean, an important point for the behaviour of the coupled model.  相似文献   

Trace Metal Solid State Speciation in Aerosols of the Northern Levantine Basin,East Mediterranean   总被引：3，自引：0，他引：3  

Mustafa Koçak  Nilgun Kubilay  Barak Herut  Malcolm Nimmo 《Journal of Atmospheric Chemistry》2007,56(3):239-257

An established three stage sequential leach scheme was applied to a series of selected high volume aerosol samples (n = 35) collected from the Turkish Eastern Mediterranean coastline (Erdemli). Samples were selected according to their air mass back trajectory history to reflect the contrasting mixtures of aerosol material present in the Eastern Mediterranean marine aerosol. Two populations were adopted, those samples which were classed as “anthropogenic” and those which were “Saharan” dominated aerosol populations. Applying the three stage leach it was possible to define the proportion for each of the considered metals (Al, Fe, Cu, Pb, Cd, Zn and Mn) present in the (a) “exchangeable” (b) “carbonate / oxide” and (c) “refractory” phases, representing novel solid state aerosol speciation data for this marine system. Clear trends were established, conforming with data from previous studies with mainly crustal derived metals (Al and Fe) being present in the refractory phases (Al > 88%; Fe > 84%) and those influenced by anthropogenic sources being dominating in the exchangeable phase, although for these metals the variability was comparatively high (12–64%; 19–85%; 40–100% for Zn, Pb and Cd, respectively). For the majority, greater exchangeable fractions were present the lower the crustal source contribution to the aerosol population, whereas the “refractory” fraction exhibited contrasting behaviour. This was illustrated by the novel application of the mixing diagram, presenting each of the three speciation stages against the corresponding percent anthropogenic contribution to each collected sample. Zn, Pb and Cd all illustrated progressive decrease in the percent exchangeable with increasing crustal contribution to the aerosol population. The percent exchangeable was discussed in terms of its use to represent the upper limit of the bioavailable fraction of metal associated with the aerosol, post deposition. The mixing diagram approach enabled the prediction of the residual fractions for Cd, Pb and Zn (41 ± 4%; 62 ± 4% and 82 ± 5%, respectively,) in Saharan end-member material.  相似文献   

Decoupling of arsenic and iron release from ferrihydrite suspension under reducing conditions: a biogeochemical model     

André Burnol  Francis Garrido  Philippe Baranger  Catherine Joulian  Marie-Christine Dictor  Françoise Bodénan  Guillaume Morin  Laurent Charlet 《Geochemical transactions》2007,8(1):12

High levels of arsenic in groundwater and drinking water are a major health problem. Although the processes controlling the release of As are still not well known, the reductive dissolution of As-rich Fe oxyhydroxides has so far been a favorite hypothesis. Decoupling between arsenic and iron redox transformations has been experimentally demonstrated, but not quantitatively interpreted. Here, we report on incubation batch experiments run with As(V) sorbed on, or co-precipitated with, 2-line ferrihydrite. The biotic and abiotic processes of As release were investigated by using wet chemistry, X-ray diffraction, X-ray absorption and genomic techniques. The incubation experiments were carried out with a phosphate-rich growth medium and a community of Fe(III)-reducing bacteria under strict anoxic conditions for two months. During the first month, the release of Fe(II) in the aqueous phase amounted to only 3% to 10% of the total initial solid Fe concentration, whilst the total aqueous As remained almost constant after an initial exchange with phosphate ions. During the second month, the aqueous Fe(II) concentration remained constant, or even decreased, whereas the total quantity of As released to the solution accounted for 14% to 45% of the total initial solid As concentration. At the end of the incubation, the aqueous-phase arsenic was present predominately as As(III) whilst X-ray absorption spectroscopy indicated that more than 70% of the solid-phase arsenic was present as As(V). X-ray diffraction revealed vivianite Fe(II)₃(PO₄)₂.8H₂O in some of the experiments. A biogeochemical model was then developed to simulate these aqueous- and solid-phase results. The two main conclusions drawn from the model are that (1) As(V) is not reduced during the first incubation month with high Eh values, but rather re-adsorbed onto the ferrihydrite surface, and this state remains until arsenic reduction is energetically more favorable than iron reduction, and (2) the release of As during the second month is due to its reduction to the more weakly adsorbed As(III) which cannot compete against carbonate ions for sorption onto ferrihydrite. The model was also successfully applied to recent experimental results on the release of arsenic from Bengal delta sediments.  相似文献   

Geochemistry, Geochronology and Isotopic Evolution of the Chewore-Rufunsa Terrane, Southern Irumide Belt: a Mesoproterozoic Continental Margin Arc   总被引：1，自引：0，他引：1  

Johnson  Simon P.; de Waele  Bert; Tembo  Francis; Katongo  Crispin; Tani  Kenichiro; Chang  Qing; Iizuka  Tsuyoshi; Dunkley  Daniel 《Journal of Petrology》2007,48(7):1411-1441

The southern Irumide Belt (SIB) is an ENE–WSW-trending,late Mesoproterozoic orogenic belt located between the Congo–Tanzania–Bangweulu(CTB) and Kalahari cratons in central southern Africa. It isseparated from the late Mesoproterozoic Irumide Belt (IB) tothe north by Permo-Triassic graben, raising the possibilitythat the younger rifts reactivated a suture between the twobelts that has been rendered cryptic as a result of youngerKaroo cover. Both belts are dominated by calc-alkaline gneisses,but in addition the SIB contains abundant metavolcanic and metasedimentaryrocks. In this study we present detailed geochemical, isotopicand geochronological data for volcanic and plutonic lithologiesfrom the southernmost part of the SIB, the Chewore–RufunsaTerrane. This terrane comprises a wide variety of supracrustalto mid-crustal rocks that have major- and trace-element compositionssimilar to magmas formed in present-day subduction zones. Chondrite-normalizedrare earth element (REE) profiles and whole-rock Sm–Ndisotope compositions indicate that the parental supra-subductionmelts interacted with, and were contaminated by sialic continentalcrust, implying a continental-margin-arc setting. Secondaryionization mass spectrometry dating of magmatic zircon has yieldedcrystallization ages between c. 1095 and 1040 Ma, similar toelsewhere in the SIB. U–Pb dating and in situ Lu–Hfisotopic analyses of abundant xenocrystic zircon extracted fromthe late Mesoproterozoic granitoids indicate that the contaminantcontinental basement was principally Palaeoproterozoic in ageand had a juvenile isotopic signature at the time of its formation.These data are in contrast to those for the IB, which is characterizedby younger, c. 1020 Ma, calc-alkaline gneisses that formed bythe direct recycling of Archaean crust without significant additionof any juvenile material. We suggest that the SIB developedby the subduction of oceanic crust under the margin of an unnamedcontinental mass until ocean closure at c. 1040 Ma. Subsequentcollision between the SIB and the CTB margin led to the cessationof magmatism in the SIB and the initiation of compression andcrustal melting in the IB. KEY WORDS: geochemistry; Mesoproterozoic; SHRIMP zircon U–Pb dating; Sm–Nd isotopes; Southern Irumide Belt  相似文献   

Numerical simulations of protostellar encounters — II. Coplanar disc–disc encounters     

S. J. Watkins  A. S. Bhattal  H. M. J. Boffin  N. Francis  & A. P. Whitworth 《Monthly notices of the Royal Astronomical Society》1998,300(4):1205-1213

It is expected that an average protostar will undergo at least one impulsive interaction with a neighbouring protostar whilst a large fraction of its mass is still in a massive, extended disc. Such interactions must have a significant impact upon the evolution of the protostars and their discs.   We have carried out a series of simulations of coplanar encounters between two stars, each possessing a massive circumstellar disc, using an SPH code that models gravitational, hydrodynamic and viscous forces. We find that during a coplanar encounter, disc material is swept up into a shock layer between the two interacting stars, and the layer then fragments to produce new protostellar condensations. The truncated remains of the discs may subsequently fragment; and the outer regions of the discs may be thrown off to form circumbinary disc-like structures around the stars. Thus coplanar disc–disc encounters lead efficiently to the formation of multiple star systems and small- N clusters, including substellar objects.  相似文献   

Gas cooling by dust during dynamical fragmentation     

A. P. Whitworth  H. M. J. Boffin  & N. Francis 《Monthly notices of the Royal Astronomical Society》1998,299(2):554-561

We suggest that the abrupt switch, from hierarchical clustering on scales ≳ 0.04 pc, to binary (and occasionally higher multiple) systems on smaller scales, which Larson has deduced from his analysis of the grouping of pre-main-sequence stars in Taurus, arises because pre-protostellar gas becomes thermally coupled to dust at sufficiently high densities. The resulting change — from gas cooling by molecular lines at low densities to gas cooling by dust at high densities — enables the matter to radiate much more efficiently, and hence to undergo dynamical fragmentation. We derive the domain in which gas cooling by dust facilitates dynamical fragmentation. Low-mass (∼ M⊙) clumps — those supported mainly by thermal pressure — can probably access this domain spontaneously, albeit rather quasi-statically, provided that they exist in a region in which external perturbations are few and far between. More massive clumps probably require an impulsive external perturbation, for instance a supersonic collision with another clump, in order for the gas to reach sufficiently high density to couple thermally to the dust. Impulsive external perturbations should promote fragmentation, by generating highly non-linear substructures which can then be amplified by gravity during the subsequent collapse.  相似文献   

Additional Analytical Data for Gabbro, GOG-1     

Joseph L. HARRIS  Louis J. SCHWARZ  William M. D'ANGELO  Francis J. FLANAGAN 《Geostandards and Geoanalytical Research》1981,5(2):125-128

Additional data for gabbro, GOG-1, were determined by instrumental-neutron-activation analysis, atomic-absorption spectrometry, and semi-quantitative spectrographic analysis. F ratios calculated in the analysis of variance for 26 sets of data for elements determined by the three methods were not significant, and hence the elements are distributed homogeneously among the bottles. The agreement between our data and the averages previously published ranges from very good to poor. More analytical data are necessary to establish reliable estimates of the concentrations of elements in GOG-1 and in two other gabbros so that three gabbros may be available to geochemists for use as standards.  相似文献   

Crystal fractionation and partial melting in the petrogenesis of a Proterozoic high-MgO volcanic suite,Ungava, Québec     

D. M. Francis  A. J. Hynes  J. N. Ludden  J. Bédard 《Contributions to Mineralogy and Petrology》1981,78(1):27-36

There is little concensus on the relative importance of crystal fractionation and differential partial melting to the chemical diversity observed within most types of volcanic suites. A resolution to this controversy is best sought in suites containing high MgO lavas such as the Chukotat volcanics of the Proterozoic Cape Smith foldbelt, Ungava, Quebec. The succession of this volcanic suite consists of repetitive sequences, each beginning with olivine-phyric basalt (19-12 wt% MgO), grading upwards to pyroxene-phyric basalt (12-8 wt% MgO) and then, in later sequences, to plagioclase-phyric basalt (7-4 wt% MgO). Only the olivine-phyric basalts have compositions capable of equilibrating with the upper mantle and are believed to represent parental magmas for the suite. The pyroxene-phyric and plagioclase-phyric basalts represent magmas derived from these parents by the crystal fractionation of olivine, with minor chromite, clinopyroxene and plagioclase. The order of extrusion in each volcanic sequence is interpreted to reflect a density effect in which successively lighter, more evolved magmas are erupted as hydrostatic pressure wanes. The pyroxene-phyric basalts appear to have evolved at high levels in the active part of the conduit system as the eruption of their parents was in progress. The plagioclase-phyric basalts may represent residual liquids expelled from isolated reservoirs along the crust-mantle interface during the late stages of volcanic activity.A positive correlation between FeO and MgO in the early, most basic olivine-phyric basalts is interpreted to reflect progressive adiabatic partial melting in the upper mantle. Although this complicates the chemistry, it is not a significant factor in the compositional diversification of the volcanic suite. The preservation of such compositional melting effects, however, suggests that the most basic olivine-phyric basalts represent primitive magmas. The trace element characteristics of these magmas, and their derivatives, indicate that the mantle source for the Chukotat volcanics had experienced a previous melting event.  相似文献