Home – Home – Jornals – Russian Geology and Geophysics 2011 number 4

The magmatic characteristics of the Levant were investigated by combining recompiled regional scale magnetic maps, revised petrophysical and gravity databases, and previous interpretations. In the study area, the inclination of the Earth's magnetic field vector is low and the total magnetic intensity map is, therefore, difficult to interpret for location and strike of magnetic bodies. Hence, a pole-reduced magnetic map was compiled. The integrated interpretation of magnetic and local gravity anomalies allowed for the estimation of depth and size of magmatic bodies and, in some cases, permitted identification of their composition. Reliability and accuracy of the results were approximated by forward modeling and by comparison with seismic reflection and drillhole data.
The study area was divided into five regions of consistent regional-scale magnetic patterns with more or less distinct boundaries. This was based not only on recognition of formal magnetic patterns but also on correlation with known (or expected) geology. The strong coincident magnetic and positive gravity anomalies probably correspond to ophiolite massifs in the northern part of the study area and basic magmatic intrusions in the southern part. Strong magnetic anomalies, which are not associated with positive gravity anomalies, most likely correspond to Mesozoic and Cenozoic volcanics located mainly in the central part of the region. The absence of magnetic anomalies in some areas suggests the lack of basic magmatic bodies (volcanic or plutonic) but provides no information on the presence of acid magmatics (granite, etc.).

We have modeled central-loop and coincident-loop transient responses of a magnetically viscous layer sandwiched between two nonmagnetic ones. The coincident-loop transients show exponential voltage decrease (at a fixed delay time), at any thickness h ₂ of the magnetic layer, with an increasing depth to the latter ( h ₁) or the loop height if the layer is exposed on the ground surface. The patterns of central-loop transients are different from those of the coincident-loop ones and from one another for thin and thick magnetic layers. Namely, the voltage first rises to its maximum and then falls as the depth to the magnetic layer ( h ₁) increases, if it is thin: The thinner the layer, the more prominent the peak. If the layer is thick, the voltage decreases monotonically with its depth (or with loop height above the ground). Voltage grows, first rapidly and then progressively more slowly, at ever greater thicknesses of the magnetic layer in both loop configurations. At large h ₂, the effect from the magnetic layer becomes similar to that from a magnetically viscous half-space. These features of the transient responses have to be taken into account in planning and conducting TEM surveys, as well as in a geological interpretation of the TEM data affected by natural and/or man-caused magnetically viscous ground. In the general case, the turn-off of the transmitter current induces eddy current in the ground beneath the loop, which decays at a rate proportional to the ground resistivity. The eddy current decay and magnetic relaxation processes being independent at conductivities (resistivities) common to the real subsurface, the effect of the former can be allowed for using the superposition principle. This principle implies that the total response of a magnetically viscous conductor is a sum of the magnetic relaxation and eddy current components.

The large earthquake that shook the southern part of Lake Baikal on August 27, 2008 has received much scientific and public interest. The occurrence of the event in the high-seismicity Baikal rift was not surprising, but its origin time was, as usual, unexpected. We discuss the geological conditions of the earthquake, its macroseismic effects, and some data interpreted as its precursors. A special focus is made on preseismic changes in dissolved helium in Lake Baikal. Studying helium patterns in a deep freshwater lake in an active seismic area is a pioneering experience that appears to have no match in the world earthquake prediction practice.

We consider the geologic occurrence and PT -conditions of the generation and crystallization of siliceous melts that formed volcanic structures within the Altai collisional system of Hercynides. The data on the geologic occurrence, internal structure, and petrogeochemical age of rocks are presented, as well as results of the thermobarogeochemical studies of inclusions in minerals. The performed investigations showed the presence of siliceous dacitic melts in the studied volcanic structures. These melts were generated in the lower crust (~10 kbar, 1000-1200 ?C) as a result of the partial melting of crustal substrates under the influence of high-temperature mantle melts.

The paper presents new data on geochemistry (major and trace elements, isotopes) and petrology (composition of phenocrysts and mineral thermometry) of basalts from the Early Cambrian Katun' accretionary complex of the Kuznetsk-Altai island arc (Gorny Altai). We also discuss the geodynamic settings of the formation of basalts based on petrological, geochemical, and isotope data taking into account their relationships with associated sedimentary rocks of oceanic origin and with terrigenous sedimentary rocks of the accretionary complex. The Late Neoproterozoic basalts associated with siliceous sediments are characterized by medium TiO₂ and Zr/Nb, flat REE patterns and Nb/La_PM < 1 (MORB). The oceanic floor basalts are represented by high- and low-magnesium varieties. The Early Cambrian basalts (Manzherok Formation) formed in an oceanic island setting (OIB). They occur in sections consisting of paleo-oceanic island basalts alternating with siliceous-terrigenous-carbonate sediments of slope facies. The Manzherok basalts are characterized by high crystallization temperatures (1175? C) and are enriched in incompatible elements (LREE, Ti, Nb), whose variations suggest a mantle plume source and variable degrees of partial melting in the spinel and garnet stability fields. The low- to high-Mg Middle Cambrian basalts of the Ust'-Sema Formation occur as subparallel dikes and lava flows, which cut and overlap the accretionary complex. The basalts are characterized by low TiO₂, Nb, LREE, high Zr/Nb and, compared to the OIB, lower temperatures of crystallization, 1047? C and 1138? C for the low- and high-Mg varieties, respectively. Geochemically, the low-Mg basalts of the Ust'-Sema Formation are close to MORB, and the high-Mg basalts, to island-arc tholeiites. Such a combination of geochemical and geological data suggest that the low-Mg basalts of the Ust'-Sema Formation formed during ridge subduction, and the high-Mg basalts formed in a suprasubduction setting, at the crust-mantle boundary.

Gold and silver sulfides were synthesized from melts Ag_{2- x} AuxS ( x = 0.1, 0.4, 0.8, 1.6) in the system Au-Ag-S with Au/Ag = 0.1-7.4. Optical microscopy, X-raying, electron probe microanalysis, and scanning electron microscopy were used to identify and analyze the solid products. It has been established that aсanthite, solid solutions Ag_{2- x} Au _x S ( x < 0.5), and electrum are produced in the Ag-rich binary system Ag₂S-Au₂S. Petrovskaite, high-fineness gold, and sulfur are produced in the systems with low Ag contents ( x > 1). The systems with an intermediate composition produce petrovskaite, uytenbogaardtite, and solid solutions Ag_{2- x} Au _x S (0.5 < x < 1). The results of experimental studies together with data on the compositions of natural gold and silver sulfides suggest that the wide variations in Au and Ag contents in uytenbogaardtite and petrovskaite from different deposits are, most likely, due to the inhomogeneous structure of mineral grains and the presence of solid solutions Ag_{2- x} Au _x S. It is assumed that gold and silver sulfides crystallizing from Au-Ag-containing sulfide melts might be of magmatic genesis. Poorly extractable gold in sulfide ores might be present as microinclusions of Au-Ag sulfides.

Experimental results concerning the radon exhalation rate from samples of building materials which were collected from the districts of Muzaffarabad and Neelum Valley, Azad Kashmir, Pakistan are presented. The study aims at assessing the contribution of building materials toward the total indoor radon exposure to the inhabitants of the studied area. In this context, samples of building materials, namely, soil, sand, gravel aggregates, and bricks, were collected from different parts of the districts of Muzaffarabad and Neelum Valley. After processing, the samples were placed in plastic containers, and box type radon detectors were installed in it at heights of 25 cm above the surface of the samples. The containers were then hermetically sealed. After 80 days of exposure to radon, CR-39 detectors were etched in 25% NaOH at 80? C for 16 h and counted under an optical microscope. The measured track densities were related to radon concentrations. Radon exhalation rate from soil, gravel aggregates, sands, and bricks varied from 171 ± 11 to 344 ± 11, 168 ± 17 to 322 ± 11, 366 ± 8 to 649 ± 8 and 184 ± 14 to 231 ± 14 mBq/(m²·r), respectively. Present data have been compared with the published data for other parts of the world.

Pollen and AMS¹⁴C analyses of bottom sediments from the upper nine meters of the Lake Kotokel' section were carried out. The regional climate and landscape dynamics during the Late Glacial and Holocene have been reconstructed with an average time resolution of ~120 years. It is shown that the climatic conditions in the Kotokel' basin during Termination I (~15.5-11.0 ka) were characterized by short drastic changes resulting in reorganization of landscapes and vegetation. Five short (400-1200 years) intervals have been recognized: 15.5-14.7, 14.7-14.3, 14.3-13.2, 13.2-12.5, and 12.5-11.7 ka. In the early Holocene (~11 ka), the climate became less continental and stayed such till ~7 ka. Later on, it again became more continental, which led to a significant decrease in average annual precipitation and winter temperatures and an increase in average summer temperatures.
The pollen record from Lake Kotokel' agrees with the general climatic trend for the Northern Hemisphere. The amplitude of vegetation and climate variations during the Late Glacial is best expressed as compared with the previous regional pollen records. This is probably because the ecosystem of the small lake localized deep inside the continent, at the boundary of two large ecotones (forest and steppe), is highly sensitive to moisture deficit.
The new dated detailed pollen record from the Kotokel' bottom sediments might be regarded as a key for the reconstruction of variations in regional vegetation and climatic dynamics for the last 15-15.5 kyr. The results obtained refined the character of changes in regional vegetation, and the reliable age model permitted intra- and interregional correlations of environmental changes.

The paper presents magnetostratigraphic data on Paleogene-Neogene continental sediments stripped by borehole 8 on the southern periphery of the Om' basin, in the Ishim-Irtysh interfluve, on the border with northern Kazakhstan. Pollen assemblages and zones have been distinguished, and a paleomagnetic section of borehole 8 has been compiled. The section consists of 11 magnetozones, which are associated with pollen zones and paleocarpological data. The absence of some paleomagnetic and pollen zones from the section suggests that the geological record is incomplete, because the formations in the studied sediments are partly eroded. A dinoflagellate community was first recorded in the Upper Oligocene-Lower Miocene sediments on the periphery of the Om' basin. The composition and ecologic characteristics of this community cannot give an unambiguous answer to the question whether seawater could penetrate southwestern Siberia through the Turgai Sea. The composition of pollen and spores from the Lower Oligocene sediments (Novomikhailovka Formation) and from the Chilikty Formation (northern Kazakhstan) is similar and reflects the development of Turgai flora on a huge territory. Later, in the second half of the Miocene, broad-leaved/coniferous forests were replaced by small-leaved ones. In the late Miocene, the latter were replaced by forest steppes with some arid flora.