Formation of lunar mare domes along crustal fractures: Rheologic conditions, dimensions of feeder dikes, and the role of magma evolution

In this study we examine a set of lunar mare domes located in the Hortensius/Milichius/T. Mayer region and in northern Mare Tranquillitatis with respect to their formation along crustal fractures, their rheologic properties, the dimensions of their feeder dikes, and the importance of magma evolution processes during dome formation. Many of these domes display elongated summit vents oriented radially with respect to major impact basins, and several dome locations are also aligned in these preferential directions. Analysis of Clementine UV/VIS and Lunar Prospector gamma ray spectrometer data reveals that the examined mare domes formed from low-Si basaltic lavas of high FeO and low to moderate TiO₂ content. Based on their morphometric properties (diameter, height, volume) obtained by photoclinometric and shape from shading analysis of telescopic CCD images, we derive rheologic quantities (lava viscosity during eruption, effusion rate, duration of the effusion process, magma rise speed) and the dimensions of the feeder dikes. We establish three rheologic groups characterised by specific combinations of rheologic properties and dike dimensions, where the most relevant discriminative parameter is the lava viscosity η. The first group is characterised by and contains the domes with elongated vents in the Milichius/T. Mayer region and two similar domes in northern Mare Tranquillitatis. The second group with comprises the very low aligned domes in northern Mare Tranquillitatis, and the third group with the relatively steep domes near Hortensius and in the T. Mayer region. The inferred dike dimensions in comparison to lunar crustal thickness data indicate that the source regions of the feeder dikes are situated within the upper crust for six of the domes in northern Mare Tranquillitatis, while they are likely to be located in the lower crust and in the upper mantle for the other examined domes. By comparing the time scale of magma ascent with the time scale on which heat is conducted from the magma into the host rock, we find evidence that the importance of magma evolution processes during ascent such as cooling and crystallisation increases with lava viscosity. We conclude that different degrees of evolution of initially fluid basaltic magma are able to explain the broad range of lava viscosities inferred for the examined mare domes. The spectral data reveal that differences in TiO₂ content may additionally account for the systematic difference in lava viscosity between the two examined lunar regions. We show that the described mechanisms are likely to be valid also for other lunar mare domes situated near Cauchy and Arago, regarded for comparison. On the other hand, we find for the Gruithuisen and Mairan highland domes that despite their inferred high lava viscosities of , no significant magma cooling in the dike occurred during ascent, supporting previous findings that the highland domes were formed during a specific phase of non-mare volcanism by highly silicic viscous lavas.     

新疆祁漫塔格找矿远景区重要矿产整装勘查

新疆祁漫塔格地区成矿地质条件优越,找矿潜力巨大,是新疆十大重要找矿远景区之一。通过开展重要矿产资源整装勘查,有望在该区快速实现重大找矿突破。笔者首先简要阐述了本区地质矿产特征、以往工作程度和已取得的重要找矿进展,在此基础上,提出了实施矿产资源整装勘查部署的初步方案和预期取得的找矿成果,并从组织管理、运行机制、矿业权管理以及经费来源、队伍组织等多方面探讨了实现整装勘查的具体保障措施。     

关于地理国情监测的探讨

地理国情监测作为一项日常的、长期的工作,是一项主动性、超前性的系统服务工程,是践行科学发展观的重大举措,本文在分析传统地形图局限性的基础上,根据地理国情监测的概念,提出地理国情监测的特点、内容以及陕西省地理国情监测的任务。     

Transmogrifying Regions

Abstract

Teacher Education Models in Geography: An International Comparison by William Marsden, ed. Kalamazoo, Ml: Commission on Geographical Education, 1984. v and 155 pp., figs., tables, and refs. NP

Principles of Thematic Map Design by D.B. Dent Reading, MA: Addison-Wesley, 1984. xv and 398 pp., figs., tables, photos, maps, refs., and index. $29.95 cloth.

Health Care in Developing Countries by Wilbert M. Gesler Resource Publication in Geography, edited by C.G. Knight. Washington, DC: Association of American Geographers, 1984.88 pp., figs., graphs, tables, maps, and biblio. $5.00 paper.

Cotton Fields No More: Southern Agriculture 1865–1980 by Gilbert C. Fite Lexington: University Press of Kentucky, 1984. xiv and 273 pp., tables, photos, app., refs., and index. $28.00 cloth; $10.00 paper.

Population: Patterns, Dynamics, and Prospects by James Newman and Gordon Matzke Englewood Cliffs, NJ: Prentice-Hall, 1984. xi and 306 pp., figs., graphs, tables, maps, biblio., and index. $27.95 cloth.     

基于SCS和USLE的程海总磷总氮参照状态推断

将流域入湖的氮磷负荷分为点源和非点源两部分,基于SCS模型和USLE模型,分别计算流域入湖的溶解态和颗粒态非点源负荷,根据工业、城镇生活及规模养殖废水排放,计算了流域入湖的点源负荷。通过计算的入湖负荷和现有的湖泊氮磷浓度数据,建立了湖泊氮磷质量守恒模型。根据计算的逐年入湖氮磷负荷和建立的湖泊氮磷质量守恒模型,逆推逐年的湖泊氮磷浓度,确定湖泊氮磷参照状态。应用建立的模型推断程海的TP、TN参照状态分别为0.014 mg/L和0.247 mg/L。构建的湖泊氮磷参照状态推断模型,所需资料少,概念清晰,在资料缺乏的其他湖泊流域具有推广应用价值。     

季节性积雪区不同遮挡条件下深霜发育比较

以中国科学院天山积雪雪崩研究站为研究区,在2009～2010年冬季观测期利用体视显微镜(XTZ-E)及拍照设备和雪特性分析仪(Snow Fork),对3种遮挡条件的开阔地(0遮挡)、树缘(50%遮挡)和树下(90%遮挡)的积雪深霜进行连续观测,比较和分析西北季节性积雪区不同遮挡条件下的深霜发育特征。研究表明:1)深霜发育主要受温度制约,其次是温度梯度。由不同遮挡条件引起积雪累积和太阳辐射差异而导致雪深不同,从而形成的温度环境差异,是深霜发育差异的根本原因。2)深霜发育厚度与雪深呈正相关关系,有开阔地(0遮挡)>树缘(50%遮挡)>树下(90%遮挡),融雪期深霜的消减速率为树下>开阔地>树缘。3)深霜冰晶粒径呈先减小(稳定累积期-过渡期)再增大(-融雪期)的变化,积雪稳定累积期后,深霜粒径开阔地>树缘>树下。4)2009～2010年冬季雪深大,因而圆角深霜(DHxr)和圆角刻面冰晶(FCxr)在深霜中发育最多,二者共占70%～80%。开阔地易发育杯型深霜(DHcp),树缘和树下则易发育柱状条纹深霜(DHla)、棱柱状深霜(DHpr)和刻面冰晶(FCso)。深霜中胶结态冰晶约占10%～30%,其比例在开阔地深霜中递减,而在树缘和树下处递增。     

Correlation of icing events with meteorological variables, and two statistical models to discriminate freezing days

In this paper, the characteristics of meteorological variables are statistically correlated with icing events (i.e., glaze and rime) in China, using daily observations of air temperature, relative humidity, wind speed, and weather phenomena from 700 stations in China from 1954 to 2008. The weather conditions most favorable for icing events are investigated and two statistical models are developed to discriminate potential freezing days. Low air temperature, high relative humidity, and low wind speed are shown to be important conditions for occurrence of icing events; also, the favorable daily mean air temperature is shown to have a decreasing trend from north to south in China, while the favorable relative humidity and wind speed varies little across the country. The statistical model developed with the daily mean temperature combined with precipitation, fog, and mist weather phenomena proved to be well able to determine the possible occurrence of freezing days. The accuracy of model outputs is well above 60% for northwestern Yunnan, Guizhou, northern Guangxi, southern Hunan, and southern Jiangxi, among other regions where icing events are more frequent, and the average false alarms are few. Using observations or forecast products of conventional meteorological variables, this model has high performance and is practical and applicable for early warning and monitoring of icing events.     

Endogenic heat from Enceladus' south polar fractures: New observations, and models of conductive surface heating

Linear features dubbed “tiger stripes” in the south polar region of Enceladus have anomalously high heat fluxes and are the apparent source of the observed plume. Several explanations for the observed activity have been proposed, including venting from a subsurface reservoir of liquid water, sublimation of surface ice, dissociation of clathrates, and shear heating. Thermal modeling presented in this work, coupled with observations from the Cassini Composite Infrared Spectrometer (CIRS) instrument, seeks to elucidate the underlying physical mechanism by constraining vent temperatures and thermal emission sources, using a model in which the observed thermal signature results primarily from conductive heating of the surface by warm subsurface fractures. The fractures feed surface vents, which may themselves contribute to the observed thermal emission. Model variables include vent temperature, presence of a surface insulating layer, vent width, time-variable heat input, and heat sources other than the central vent. Results indicate that CIRS spectra are best fitted with a model in which the surface is heated by narrow vents at temperatures as high as 223 K. Although equally good fits can be obtained for vent temperatures in the range of 130 to 155 K if the vents are wider (180 m and 22 m respectively) and dominate the emission spectrum, these models are probably less realistic because vents with these temperatures and widths cannot supply the observed H₂O vapor flux. The lack of emission angle dependence of the thermal emission when July 2005 and November 2006 CIRS observations are compared also argues against thermal emission being dominated by the vents themselves. Thus, results favor high-temperature models, possibly venting from a subsurface liquid water reservoir. However, a fracture filled with liquid water near the surface would produce significantly higher radiances than were detected unless masked by a thermally insulating surface layer. Models that best match the CIRS data are characterized by small fractions of the surface at high temperatures, which strengthens the case for the vents and/or their conductively-heated margins being the primary heat source. Models where the thermal emission is dominated by conductive heating of the surface from below by a laterally-extensive buried heat source cannot reproduce the observed spectrum. Models with a 10 cm thick upper insulating layer produce a poor match to the CIRS spectra, suggesting high thermal inertias near the tiger stripes. Finally, tiger stripe thermal emission measured by CIRS varied by less than 15% over the 16 month period from July 2005 to November 2006.     

Spatial distribution of putative water related features in Southern Acidalia/Cydonia Mensae, Mars

Many putative water-related features exist in the northern lowlands of Mars. These features may provide clues to the abundance and timing of water or ice that existed there in the past. The Cydonia Mensae and Southern Acidalia area was chosen as the study area owing to the abundance of two of these features: giant polygons and pitted cones. In addition a section of the Deuteronilus shoreline is located there. The abundance and close proximity of the features makes this area an excellent place to study the spatial relationships between these landforms, as well as the morphological characteristics of pitted cones. The features were mapped into a GIS for spatial analyses. The highest densities of pitted cones and giant polygons are adjacent but distinctly separated by a knobby ridge that is surrounded by the Deuteronilus putative shoreline. Pitted cones were measured and examined to determine if a classification by morphology is possible, but the results were inconclusive. Statistical tests on pit-to-cone diameter ratios and tests of surface temperatures of cone material suggest, but do not verify, a single cone origin. The various shapes, sizes, and putative ages of pitted cones may be attributed to temporal variation in emplacement and spatial variation in material properties. Among the possible scenarios put forth for pitted cone genesis on Mars two are likely candidates in Cydonia Mensae: (1) the sublimation of a cold-based glacier, and (2) a buried lens of methane and/or CO₂ clathrates.     

Evaluation of carbonate abundance in putative martian paleolake basins

Carbonate deposits have not been found so far on Mars, although there appears to have been sufficient water to have supported their formation. Many hypotheses have been proposed in order to explain this. In the present work we explore the possibility that the missed detection of carbonate deposits on the martian surface could be simply due to the fact that the concentration of carbonates, when mixed with other materials present in the sedimentary deposits, may be below the detection limit of the various instruments used so far in this search. In the present study we consider 21 putative paleolacustrine basins and use a sediment transport model to estimate the abundance of carbonates which could be present in the sediments deposited on the basin floor. In this way we find that for all the selected basins the estimated carbonate abundances are in general less than a few percent, and such values are below (or at best comparable to) the detection limits of the spectrometers flown around Mars during the recent space missions. Furthermore, applying the sediment transport model to the well studied Eberswalde crater, we conclude that the fluvio-lacustrine activity in this basin should have lasted for a period on the order of 10³–10⁴ years, in good agreement with previous work. Our results suggest that a hydrological cycle, able to move large volumes of water and to create relatively stable lakes, could have been active intermittently on Mars in the past, producing carbonate deposits that could escape detection by the instruments that have flown to date.