Home – Home – Jornals – Russian Geology and Geophysics 2004 number 11

It is commonly recognized that sea level experienced strong global-scale systematic changes in the geologic past. Its 20-100 m fluctuations in 1-10 Myr (third-order) cycles are most often detected using Fischer plots based on thicknesses of meter-scale cycles in sections of shallow-marine carbonate facies. However, the classical Fischer plots constructed for different regions of East Siberia show considerable mismatch though eustatic changes are supposed to be globally synchronous. This mismatch is caused by time-dependent variations in durations of elementary cycles asynchronous in different sections. Therefore, a great number of earlier inferred global eustatic events apparently never existed in reality. We obtained more reliable accommodation plots with thicknesses of 0.5 Myr synchronous chronozones as elementary units in the Silurian sections of East Siberia. According to these plots, eustatic fluctuations never exceeded 5-7 m throughout the Silurian, which is far below the accepted magnitude of 30-130 m. The Silurian stratigraphic subunits and intervals between tectonic events were constrained due to stable durations of the chronozones. Our plots reveal strong lateral variations of crustal subsidence rates over East Siberia. These variations were controlled neither by the flexural response of lithosphere to changes in horizontal stresses nor by the mantle topography dynamics, but rather by metamorphic phase change and related consolidation of the mafic lower crust. Subsidence rates occasionally became times faster or slower within ~0.5 Myr intervals, the contrasts surprisingly great for platforms. Phase change rates may have been controlled by deviatoric stresses in addition to temperature and fluid migration. This mechanism can be responsible for subsidence rate variations reported for many intracratonic basins.

Fission-track ages were determined from detrital zircons from sandstones of the Drozdov and Stanislavskaya Formations and Tyushev Series of the Kumroch Range, eastern Kamchatka. The age of the young zircon population in the sandstones of the Drozdov Formation (Kumroch Range) is late Late Paleocene (from 55.9 4.4 to 57.7 3.5 Ma). The Drozdov Formation was deposited in the late Paleocene and continued to accumulate in the Early Eocene. The age of young zircon populations in the coarse-clastic sequence of the Stanislavskaya Formation (40.7 3.1; 40.9 3.9; 42.4 1.9 Ma) indicates that it accumulated as late as middle Bartonian. The age of young zircon populations in the sandstones of the Tyushev Series (from 50.0 2.9 to 38.1 3.4 Ma) is much older than the age of the sandstones themselves (Early-Middle Miocene, 24-11 Ma). Therefore, in the Early Miocene, the Tyushev Series accumulated a distance from central Kamchatka, volcanic activity was intense at that time.
There are three tectonic zones within the East Kamchatkan accretionary prism: Stanislavskaya, Vetlovaya, and Tyushev. The Stanislavskaya zone, represented by terrigenous units, are interpreted as deposits of the accretionary prism that originated after the Ozernoe-Valagin arc collided with the Eurasian margin. The Vetlovaya zone was formed by offscraping cover strata from the oceanic crust that separated the Ozernoe-Valagin and Kronotsky arcs in the middle Paleocene to late Miocene. Deposits of the Tyushev zone formed the Oligocene-Miocene cover of the

Neotectonics and orography of southern Great Altai are controlled by a system of dextral shears of northwestern strike, which are completed in the north by systems of upthrows and divergent faults. The recentmost structure of the northern part of Altai is more complicated. It does not result, however, from chaotic hummocking of the continental lithosphere but follows the same structural regularities as the shear structure of the southern part of the Mongolian Altai mountains to which it is spatially and genetically related. It is more complicated because, unlike the Mongolian Altai, it formed at the boundary of three rather than two framing walls which approach each other slipping along the strike. Within the Mongolian Altai the strikes of fractures of the Paleozoic basement and the recentmost fractures generally coincide in space so that the recentmost disjunctions often renew the lines of old tectonic contacts, whereas in northern Altai the recentmost ruptures occur along the Paleozoic faulting zones only at some segments and more frequently cut them at different angles.

Alumina-bearing phases with orthorhombic (phase CF) and hexagonal (phase NAL) structures have been identified in average MORB composition at pressures above 25 GPa. These phases are very similar in structure and composition and can coexist in a single experimental sample. In this paper, we report data on the chemical composition, structure, and stability fields of the CF and NAL phases and study their relationship in the lower mantle. It is shown that these phases are stable at a pressure of 20 GPa and temperatures below 1300

Compositions of initial magmas for some well-explored troctolite-anorthosite massifs of North America were estimated and numerically modeled using the COMAGMAT-3.3 program. The objects of study were the Kiglapait, Michikamau, and Harp Lake massifs of the Nain plutonic series of the Labrador Peninsula (Canada), the Adirondack massif of the Grenville orogen of Proterozoic age (New York, USA), and the Duluth massif of the North American rift (Minnesota, USA). These massifs have been characterized in short, with petrochemistry of their rock components estimated statistically. It has been established that in norm composition the initial magma corresponded to olivine-orthopyroxene-bearing basalts for the Harp Lake massif, and to olivine basalts, for the other massifs. On the basis of the Albarede regression equations, it has been shown that the magmas were extracted by melting at pressures of 9-13 kbar (Harp Lake, Michikamau, and Kiglapait massifs) to 19-22 kbar (Duluth and Adirondack massifs) and at temperatures of 1200 to 1400

We carried out a comparative analysis of the distribution of background contents of PGE and gold in rocks of ophiolite associations of the Altai-Sayan area (ophiolites of Kuznetsk Alatau, Gorny Altai, and West Sayan) and the Mid-Atlantic Ridge (MAR). The sections of ophiolites and modern oceanic lithosphere (ultrabasites-gabbros-dikes-effusions) have been studied. The contents of gold, PGE, and accompanying elements in the rocks were determined by the radiochemical neutron activation, atomic absorption, and X-ray fluorescence methods, and the contents of volatiles, by gas chromatography. Modern and ancient structures show similar PGE and gold patterns. The studied rocks have contents of low-melting PGE of the palladium association commensurate with those in primitive mantle and are drastically depleted in high-melting PGE of the iridium association. In PGE contents ultrabasites of all studied regions of the Altai-Sayan area are close to primitive mantle. Modern oceanic basalts (like N-MORB of the MAR) are most similar to magmatic ophiolite complexes of Gorny Altai and West Sayan. Island-arc ultrabasites of Kuznetsk Alatau are depleted in Ir and are enriched in Au. Both in the ancient ophiolite associations and in the modern oceanic lithosphere of the MAR, the water content increases with the contents of PGE of the iridium association. Fractionation of PGE and gold occurs both as a result of magmatic differentiation of matter and under the effect of fluids at postmagmatic stages.

The values of were measured in phosphorites and dolomites composing productive formations of the Hövsgöl and Karatau phosphorite basins (Hesen and Chulaktau Formations, respectively) and in dolomites overlying these formations (Hövsgöl and Tamdy series, respectively). The plots of both phosphorites and dolomites from the Hesen Formation demonstrate a clearly expressed negative anomaly with of up to -7

The method of refracted (head) waves rarely used in petroleum prospecting has a high exploratory potential and can be applied to regional- and local-scale problems. Special experimental work in petroleum basins will be useful to investigate the influence of oil and gas fields on the behavior of refracted waves.

A new approach to estimating the wall thickness and magnetic permeability of well casing strings for flaw detector designing implies simulation of the TEM response of a current loop in a cylindrically layered ferromagnetic conductor using analytical expressions of emf partial derivatives. Wall thicknesses and magnetic permeabilities of metal casing found as an iteration solution to the inverse problem show good agreement with the exact parameters in theoretical curves and with experimental data.

GPS and paleoseismological studies of Holocene and recent slip rates in the zone of the Main Sayan Fault show that the fault is currently locked, and the accumulated stress may release in a large earthquake. The southeastern flank of the fault generated at least five large earthquakes for the past 10 kyr. For the time elapsed since the ultimate event, the fault has accumulated pending displacement of 1.4-3.1 m, given its 3.1 mm/yr average slip, which can be accommodated in an M = 7.1-7.5 earthquake. Thus the densely populated industrial areas around the southeastern fault flank in the southern Baikal region are exposed to high seismic risk.