Home – Home – Jornals – Advanced Search

We model propagation of electromagnetic waves in frequency-dependent media applying the Laguerre transform in time domain. The new algorithm is fourth-order accurate in space and computationally efficient. Maxwell's equations are reduced to a harmonic series of linear algebraic equations in which only the right side depends on the harmonic number and the inverse matrix is the same for all harmonics.
The efficiency of computation for the algebraic equations is improved by fitting the free parameter of the Laguerre transform. The value of this parameter is easy to find and is likewise the same for all harmonics.
The Laguerre scheme provides a better accuracy than the second-order accurate finite-difference solution at large path lengths.
The method is stable both in the region of the wavefield where conductivity approaches zero and the spectral Fourier method is unstable, and in the high-conductivity region where the explicit FDTD code requires a too small time step.

The reported laboratory experiments of cycling measurements investigate strain-amplitude dependence of compressional velocity and attenuation of acoustic waves ( f ~ 1 MHz) in samples of the Nivagal sandstone. Strain-amplitude growth in a range of ε_d ~ (0,3-2) · 10^-6 causes a 0.6% velocity increase and an up to 10% decrease in attenuation. Attenuation curves show a minor hysteresis in a full cycle (A_min → A_max → A_min). Amplitude dependence of velocity and attenuation can be better resolved at higher strains. Attenuation is more sensitive to strain variations than velocity. The revealed relationships are useful as an additional criterion in imaging sandstone formations.

The paper is a sketch of the fifty years-long experience in using wide-angle (postcritical) reflections in DSS. Wide-angle reflections from the Moho are commonly recorded at offsets from 60 to 200 km. Their waveform is variable but correlation against first arrivals is quite stable. Wide-angle reflections are well applicable in seismic exploration.

The Kokchetav subduction-collision zone (KSCZ) hosting ultrahigh- and high-pressure (UHP-HP) rocks underwent the multistage Vendian-Early Ordovician geodynamic evolution. The subduction of the Paleoasian oceanic lithosphere bearing blocks of continental crust and the collision of the Kokchetav microcontinent with the Vendian-Cambrian island-arc system ultimately led to the formation and exhumation of UHP-HP rocks. In the Vendian-Early Cambrian the margin of the Kokchetav microcontinent deeply subsided into the subduction zone (150-200 km), which led to UHP-HP metamorphism (the maximum at about 535 Ma) and to partial melting of its rocks. In next stage (535-528 Ma), the generated acidic melts including blocks of UHP-HP rocks quickly, at a rate of 1 m/year, ascended to depths of 90 km for 1 Myr. During subsequent 5 Myr, the UHP-HP rocks ascending at a rate of 0.6-1 cm/year reached the base of the accretionary prism (depths of 60-30 km). Then, in the period from 528 to 500 Ma, the UHP-HP rocks ascended along the faulting structures of the lower crust as a result of jamming the subduction zone by the Kokchetav microcontinent. During the period from 500 to 480 Ma, the UHP-HP rocks became part of the upper crust. This process led to the KSCZ, which comprises terranes of the Vendian-Early Arenigian subduction zone occurring at different depths, separated by zones of garnet-mica and mica schists, blastomylonites and mylonites. In the same period there was a jump of subduction zone, which led to the formation of the Ordovician Stepnyak island arc. As a result of the Late Arenigian-Early Caradocian microcontinent-island arc collisions (480-460 Ma), the KSCZ overrided upon the fore-arc trough of the Stepnyak island arc to form a thick accretion-collision orogen, which having experienced anatectic melting was intruded by collisional granites of the Zerenda complex 460-440 Ma in age.

It has been shown by extensive mapping and sampling in the central Kokchetav subduction-collision zone that it is a terrane of a Vendian-Cambrian paleosubduction zone made up of tectonic sheets of Precambrian granite gneisses, Late Precambrian garnet-kyanite-sillimanite-biotite schists with boudins of garnet and garnet-plagioclase amphibolites, and Cambrian mylonitized granite gneisses with boudins of eclogites. The fault zones between tectonic sheets are formed by garnet-quartz-muscovite and quartz-muscovite schists. Along with the central Kokchetav belt, they are isoclinally overfolded southwestward. ⁴⁰Ar/³⁹Ar dating of muscovite from five mica-schist samples yielded close plateau and isochron ages of 480-485 Ma (Early Ordovician). The geological and geochronological data confirm the Early Ordovician formation of the nappe-imbricated structure of the Kokchetav subduction-collision

The siliceous rocks from structures of the junction zone of the Kokchetav massif and Stepnyak trough that occur in turbidites of the accretionary wedge, silica-volcanogenic sequence of the Stepnyak trough and syntectonic olistostrome were dated on the basis of findings of conodonts and radiolarians within the Middle-Upper Arenigian (conodont zones O. evae, B. navis-lower Par. originalis). This range of time is marked by a powerful tectonic rearrangement, involved with the rearrangement of the accretionary wedge and overriding of the Kokchetav massif upon the Stepnyak fore-arc trough.

Based on summarized and correlated structural, paleogeographical, and geochronological data, we propose new concepts of the intricate multistage Cambrian-Middle Ordovician evolution of an island-arc system in northern Kazakhstan. It is shown that in the Early Ordovician, the Selety island arc was rebuilt to give rise to the Stepnyak island arc. This rebuilding was intimately related to the collision of the Selety island arc with the Kokchetav microcontinent, which led to the formation of a nappe-thrust structure, exhumation of high- and ultrahigh-pressure rocks, and jump of subduction zone with the emergence of a new island arc.

The Shalkar ophiolite complex consists of a series of tectonic sheets, including lavas, a complex of parallel dikes, layered complex of gabbro and gabbro-pyroxenites intruded by dikes of plagiogranites. The tectonic sheets are subsided northward and are associated with the base of a large tectonic nappe made up of rocks of the Kokchetav microcontinent. The microcontinent and ophiolites are jointly thrust upon the Kokchetav megamelange complex with HP-UHP rocks. It is inferred from the chemical composition and structural position that the Shalkar ophiolites were formed under the conditions of suprasubduction zone of extension and belong to the setting of the initial stage of formation of the Stepnyak island arc. The upper age of the ophiolites is 485 ± 6 Ma as determined for zircons from plagiogranites using the U-Th-Pb SHRIMP method. The age of the Shalkar ophiolites is within 495-485 Ma. This interval is marked by a considerable rearrangement in the subduction zone, which can be due to a jump of subduction zone to the hinterland of the Kokchetav microcontinent and to the origin of a zone of extension within its limits with the formation of ophiolites.

Diamondiferous calc-silicate rocks occur at the Kumdy-Kol' deposit as partings and lenses among biotite gneisses and schists. The most frequent alternation of these rocks is observed in crosscut 45 of the adit driven during the exploration of this metamorphic diamond deposit. Our collection of samples taken from this crosscut permitted research into the geochemistry of rocks with contrasting contents of diamonds. Study was also given to carbonate rocks with distinct banding and garnet-pyroxene rocks with abnormally high contents of diamonds taken from the adit spoil heaps. These rocks were analyzed by ICP MS and ICP AES, which ensured a simultaneous determination of a wide spectrum of trace elements with low detection limits. The determined contents of incompatible elements evidence that the diamondiferous calc-silicate rocks are not a mixture of carbonates with clays only. It is not ruled out that carbonates are also mixed with an end-member compositionally different from protoliths of garnet-biotite gneisses and schists. It is shown that at different stages of metamorphic evolution, including exhumation, subducted continental crust might have been the source of high-density fluids or melts metasomatizing the upper mantle. At the same time, the interaction of these fluids with carbonate partings present among the subducted sediments might have hindered their far migration.