Low-magnetic crust underlying South Province of Mars

The lack of distinct magnetic signatures observed by Mars Global Surveyor (MGS) over the impact craters and impact-related Quasi-Circular-Depressions (QCDs) with diameters greater than 200 km located on South Province, south of 30S and from almost the west of Hellas to Argyre basins, implies a weakly magnetized crust. Using MOLA topography and the recent JPL gravity model of Mars we determine the structure of the crust beneath the craters and impact-related QCDs, and show that the impacts that have created these features were capable of strongly disturbing the crust directly beneath. On the basis of theoretical magnetic anomaly modeling and shock demagnetization models, we demonstrate that the impacts are capable of demagnetizing the entire crust beneath and creating distinct magnetic anomalies at the satellite altitude of 400 km in case the crust was appreciably magnetized prior to the impacts. We derive the magnetic anomalies of these features using the radial component of the high-altitude nighttime MGS data. An upper limit of <2 × 10⁴ A for the bulk magnetization of the crust beneath South Province is estimated, which is about 30 times less than that underlying Terra Cimmeria and Terra Sirenum. Similar weak bulk magnetization is obtained for part of the crust surrounding Hellas, Isidis, and Argyre basins.     

Hazards of mechanized tunnel excavation in H<Subscript>2</Subscript>S bearing ground in Aspar tunnel,Iran

Occurrence of hydrogen sulfide gas (H₂S) is one of the most important engineering geological hazards during tunneling. Its hazards and consequent challenges are very difficult and costly to solve. During site investigation, one of the tasks for engineering geologists is prediction and evaluation of the risk of H₂S gas in the underground spaces. In this study, water conveyance tunnel of Aspar, which was excavated in H₂S-bearing environments, is discussed. The tunnel is excavated in the hydrocarbon formations. Applied experiments suggest that geological formations pertaining to hydrocarbon resources are crucial in formation and reservation of H₂S gas. This paper briefly discusses hazards and geological sources of H₂S, as well as remedial measures for decreasing the risks and problems in excavation of the tunnel. To predict the risk of H₂S gas in the underground spaces, it is possible to use some precursors such as: sulfur springs, organic traces, organic argillaceous rocks, exposure of H₂S odor from fresh surface of rock and smell of H₂S during boreholes drilling. Controlling the inflow of groundwater into the excavation, diluting the concentration of H₂S, training the personnel and utilization of some proper safety equipment have been used to mitigate risks and problems in tunnel excavation.     

Integrating GIS and DSS for identification of suitable areas for artificial recharge,case study Meimeh Basin,Isfahan, Iran

Flood spreading is an inexpensive method for flood mitigation and artificial recharge of aquifers that results in a large budget return for relatively small investment.It is necessary to study some regional characteristics in order to determine the appropriate areas for artificial groundwater recharge by flood spreading in Meimeh Basin, Isfahan Province, Iran. Necessary regional characteristics to be studied are: slope, infiltration rate, sediment thickness, transmissivity, and water quality. In this research to identify suitable areas for artificial recharge several thematic layers were prepared, assigning each layer to one of the mentioned characteristics. The thematic layers were classified to several classes based on the existing criteria. All of the classes of the thematic layers were integrated and analyzed using a decision support system (DSS) in a geographical information system (GIS) environment. Finally suitability of the integrated classes for artificial recharge was identified in which the following classes were separated:(i) Very suitable, (ii) suitable, (iii) moderate suitability, and (iv) unsuitable.The validity of the generated model was verified by applying the model to a number of successful floodwater spreading stations throughout Iran. The verified model showed satisfactory results for all of the stations. The results for Meimeh Basin showed that about 70% of the Quaternary sediments in the studied area are suitable and moderately suitable for artificial recharge by flood spreading.     

Lunar mascons as consequences of giant impacts

A model is proposed for the formation of lunar mascons which explains persistence of lunar mascons for more than 3 b.y., evidence for the volcanic activity 3.7-3.2 b.y. ago, and negative gravity anomalies surrounding the mascons. It is concluded that mascons have resulted from the perturbations introduced by the giant impacts into an otherwise spherically symmetric Moon; a giant impact enhances the rate of cooling beneath the impact site by introducing releatively low temperature to a deeper part of the Moon through forming a basin and also by removing substantial amount of radioactive material by means of ejecta. On the other hand, it reduces the rate of cooling beneath the surrounding highland by thermal insulation through extensive fracturing and covering by an ejecta blanketing. Consequently, the base of the lithosphere (100 km thick) beneath the highland remelts to a depth of about 80 km and this creates thermal stresses strong enough to open the fractures in the overlying region and to cause magmatization and volcanic activity. Persistence of the molten phase around 100 km depth for about 1 b.y. probably provides further differentiation and an upward concentration of low density material, giving rise to the observed negative gravity rings. On the other hand, the relatively cold lithosphere beneath the basin forms a layer strong enough to support the associated mascon.     

Density and stress distribution in the moon

A model is presented for the lateral variations of density within the Moon. The model gives rise to a gravitational potential which is equal to the observed potential at the lunar surface, moreover, it minimizes the total shear-strain energy of the Moon. The model exhibits lateral variations of about ±0.25 g cc^–1 within 50 km depth. The variations, however, reduce to ±0.06 and ±0.008 g cc^–1 within layers at 50–135 and 135–235 km respectively, and they become negligible below this region. The associated stress differences are found to be about 50 bar within 600 km depth, having their maximum values of about 90 bars at a depth of about 250 km. On the basis of these stress differences a strength of about 100 bar is concluded for the upper 400 km of the lunar interior for the last 3.3 b.y.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April, 1973.The Lunar Science Institute is operated by the Universities Space Research Association under Contract No. NSR 09-051-001 with the National Aeronautics and Space Administration. This paper is Lunar Science Institute Contribution No. 117.     

Contribution of serpentinized ultramafics to marine magnetic anomalies at slow and intermediate spreading centres: insights from the shape of the anomalies

Detailed characteristics of marine magnetic anomalies 33r and 20r suggest that the magnetization of the deeper magnetic layers, including the lower crust and possibly the uppermost mantle, is horizontally displaced with respect to that of the upper crust. We examine the possibility that serpentinization of ultramafics in the lower crust and possibly the uppermost mantle delays the acquisition of magnetization and introduces a shift between the upper- and lower-crustal magnetization patterns. Thermal evolution models and the resulting magnetization patterns of the oceanic lithosphere are calculated for a wide range of physical parameters such as the Nusselt number and the depth of hydrothermal circulation in the crust, and the temperature range of serpentinization. The models with moderate hydrothermal cooling of the whole crust and serpentinization temperatures ranging between 200 and 300 C successfully explain the anomalous skewness and the 'hook shape' of observed sea-level magnetic anomalies created at slow and intermediate spreading rates.     

Environmental management of coastal regions in the Caspian Sea

Considering rapid population growth and migration, higher accumulation of communities is noticed in coastal areas. This is especially true with the coastal areas of the Caspian Sea. In the present investigation coastal areas between Jouybar to Behshahr region is selected for their special geographical and ecological locations. Further, adverse impacts of human, agriculture and industrial activities was examined along side the above mentioned coasts. It should be pointed out that protected Miankaleh Wildlife zone which is an internationally recognized wetland, falls within area of study. In the present study strengths, weaknesses, opportunities and threats method is used for the evaluation of environmental management status. In this regard, internal and external factors gained 2.28 and 2.58 scores. This is indicative of the abundance of weaknesses over strengths and it also shows that opportunities are more than threats. Subsequently 27 strategies were developed and quantitative strategic planning matrix method was also used to score each strategies. The results of quantitative strategic planning matrix method analysis was programmed in strategic position and action evaluation matrix. The present situation falls within “competitive” classification. This is indicative of weakness in coordinating development and environmental strategic plans. The result of present investigation strongly emphasis on compilation of strategic environmental plans for the control of population, pollution emission and land use planning changes. The most important strategies include development of environmental regulations and better supervision on enforcement of laws.     

Geophysical implications of the Martian Gravity Field

The undulations of the Martian gravitational potential indicate lateral density variations in the Mars interior. A gravitating and solid Martian model deforms under the influence of these variations, producing stress differences of about 125 bars at a depth of about 200 km. Introduction of a partially molten core of 1300 km radius does not affect the stress distribution in the mantle significantly, whereas the assumption of a partially molten asthenosphere umderlying a solid lithosphere of about 300 km increases the stress differences appreciably. A strong linear correlation of the gravitational potential and the surface topography indicates that the extensive volcanism at the Tharsis region is a recent phenomenon. The high stresses associated with this region imply that there has been no extensive molten region within the upper 300 km since the volcanism.