Home – Home – Jornals – Russian Geology and Geophysics 2006 number 9

Detailed geochemical studies of buried andesites, supposedly of Permo-Triassic age, from the West Siberian Plate have revealed their specific features. The andesites differ drastically in petro- and geochemical compositions from similar rocks of active continental margins and island arcs and are very similar in geochemistry to continental latites. These data do not contradict the model of andesite formation through crystallization differentiation of subalkalic basaltic melt.

The high gold potential of Cu-ore (Cu-skarn, porphyry Cu-Mo, and pyrite-polymetallic) deposits widespread in the Altai-Sayan folded area is shown. The ore formation processes at these deposits included multistage mineralization with the gradually decreasing temperature of ore-forming solutions. At the early high-temperature stages, mainly the profile ores (often, of zonal structure) of the deposits were produced. Their mineral composition is closely related to the composition of the corresponding ore-magmatic system and is regularly repeated in ore objects of different sizes and ages. At the final stages, low-temperature mineral assemblages formed, which often have a similar set of ore and vein minerals. A distinctive feature of these assemblages is the presence of Ag, Pb, and Au tellurides, Bi minerals, native bismuth, and Hg-containing minerals. Gold in the minerals is of varying fineness; the main trace elements in it are Ag and Hg. The low-temperature mineral assemblages are close to ores of epithermal deposits (including Carlin-type ones) developed in the same areas. These ores might be the products of the final-stage evolution of the ore-magmatic systems.

In this paper we consider products of natural pyrolysis of lignite, which resulted from the high-temperature spontaneous combustion of spoil heaps of the Chelyabinsk coal basin. These products were studied by pyrolysis, element and petrographic analyses, chromato-mass spectrometry, and X-ray diffraction method. We have established that under reducing conditions, the degree of pyrogenic coal transformation and the composition of pyrolysis products vary greatly, from graphite-like phases to bitumens, and depend on the temperature and degree of the system openness.

Analysis of a sample of 18 relatively well-explored sedimentary basins of North America shows that the distribution of oil fields with regard to amount of resources depend on the characteristics of the sedimentary filling of the basins. In rather young and intensely sagged basins, which abound in objects with high rates of accumulation, the deviations from the truncated Pareto distribution can be quite significant. The revealed dependences permit the parameters of size distribution of oil accumulations to be estimated at the very beginning of study of basins, without sufficient information on sizes of largest fields. The obtained empirical regularities qualitatively agree with results of analysis of the available simplest theoretical model for size distribution of hydrocarbon accumulations in petroliferous systems.

We investigated signature of Middle Pleistocene soft sediment deformation in lacustrine deposits of the Kok-Moinok basin (a bay of Lake Issyk-Kul', western Issyk-Kul' region) and its implications for past seismicity. Convolute structures are exposed in interfingering shallow lacustrine, beach and fluvial sediments. Other deformational structures we encountered exist as marl intrusions in coarse pebbly sand whereby the latter progresses upward making a sort of pillars. We used seven field criteria for relating soft sediment deformation to paleoseismic triggering which provide strong evidence for the seismic origin of the structures: (1) proximity to active seismic zones; (2) potentially liquefiable composition of sediments which are soft metastable sand and low-cohesion clay; (3) similarity to structures formed experimentally and those produced by natural seismic shaking; (4) absence of detectable effects of slope instability (gravity sliding); (5) stratigraphic position of deformation units between undeformed sediments; (6) lateral extent and regional abundance of deformational structures correlated over large areas; (7) cyclic repetition of structures in the section. We infer from empirical relationships between earthquake magnitude and the maximum epicentral distance to liquefaction sites that deformation was most likely associated with events in the highly seismic active epicentral zone north of Lake Issyk-Kul'. The earthquake-induced deformational structures record high seismic activity already in the Middle Pleistocene.

MTS data from five profiles across the zone of the Talas-Fergana Fault (TFF), interpreted in geoelectrical models, reveal a 10-15 km thick crustal conductor (consisting of several conductors) traceable to a depth of 45 km. The conductor rises to the upper crust along TFF and flattens out towards the Southwestern and Northeastern Tien Shan. The anomaly attributed to the TFF zone is divided into the Talas, Central, and Southern segments in the map view. TFF, interpreted as a high-angle strike-slip fault from geological data, is recorded in the electromagnetic field as a nearly vertical conductor only in its Central segment and shows a listric geometry in the Talas segment.
Interpretation of the MTS results correlated to geological data suggests that the modern geodynamic framework of the southwestern Tien Shan includes a structure of an arcuate plan view and a listric geometry which deepens to 40-45 km toward the center of the Fergana block. This pattern of the active zone agrees well with structural, paleomagnetic, and GPS data on the rotation of the Fergana block under the effect of pressure from the Pamirs.

Studies of deposition processes in Bay Proval on the eastern side of Lake Baikal allow us to approach reconstruction of natural and man-caused events through the past 150 years on the basis of reliable sedimentological tracers. The tracers include fossil soil (1862), chlorinated organic compounds (since the 1950s), ¹³⁷Cs (since 1953), and modern soil (1959). The project for investigation of recent sedimentation in the region around the Selenga delta offers an opportunity to decipher the unique natural microchronicle of natural and man-caused events in the Late Holocene.