Home – Home – Jornals – Russian Geology and Geophysics 2004 number 4

The tectonic position and ages of suprasubduction continental-margin volcanoplutonic belts controlled by their Cenozoic geodynamic environments are notably different in the Pacific and Bering Sea sectors of Kamchatka and southern Koryakia. In pre-Cenozoic time the two sectors were separated by a long-existing transform fault whose western segment now coincides with the Komandor strike-slip fault and the eastern segment is clearly traceable by satellite gravity data on the Pacific floor southeastwards on the continuation of the Komandor fault.

Late Quaternary tectonic activity in the southwestern Siberian Platform was investigated in the Kan fault system. Geodynamic analysis indicates an inherited style of faulting, mostly of oblique geometry, at the junction of the East Sayan Ridge and the Rybnaya basin. Motions on NW faults dramatically restructured the surface topography and produced horst anticlines and horsts.
Aerial and satellite imagery of the Kiyai-Arbai fault zone in the Kan system detects traces of earthquake rupture along 100 km of the fault from the Anzha to the upper Kungus River. A 9 km long segment of the fault scarp was excavated near Malinovka Village between 55.09

A typical pattern common to second-order faults in extended lithosphere was revealed based on field geological and structural data from the Obruchev fault in the Baikal rift and published evidence of rift-related faulting worldwide. The typical fault family is dominated by rift-parallel synthetic and antithetic normal faults and includes transverse faults of normal and strike-slip geometry, strike-slip faults oblique to the rift strike, and rift-parallel reverse faults and thrusts. The coexistence of faults of various types within first-order extension zones is accounted for by accommodation of shear strain and by the kinematics of large basin-parallel normal faults. The suggested typical pattern can be used for reference in interpretation of faulting histories in rifted continental lithosphere.

Distribution of recent crustal uplift at the junction of the West Siberian Plate and the Siberian craton was inferred from repeated leveling geodetic data. The new results have been applied to compile a map model of recent vertical crustal movements in the Yenisei Ridge, to correlate their rates with the modern surface topography, and to detect fault zones. Uplift rate anomalies, which represent four types of deformation, were estimated from differential rates of land uplift measured in active faults traversed by releveling profiles. The Yenisei Ridge is generally a non-deformable solid block tilted to the west towards the West Siberian Plate.

A large gap is documented in the Patom Upland, at the base of the Zherba Formation. The closely connected Tirbes Formation is now considered as its completing member. The Tinnaya Formation is separated from the Zherba and the overlying Nokhtuisk Formations by gaps. Small shelly fossils found there for the first time gave grounds for substantiation the lower boundary of the Cambrian at the bottom of the Nokhtuisk Formation and for referring the Tinnaya Formation to the Nemakit-Daldyn Stage of the Vendian. The more precise isotope chemostratigraphy of the Vendian-Cambrian deposits has shown that here, as in the other sections of the Siberian Platform, a negative marking excursion of ¹³C matches the paleontologically substantiated lower boundary of the Cambrian. The adjacent peaks helps to draw the boundaries of the regularis and lenaicus Zones and permit two conclusions. First, fossils of the trisulcatus Zone in the Siberian sections occur below the base of the Manykai Formation, whereas some representatives of the Ediacaran fauna penetrate there from below. Therefore, the Manykai Stage recognized on basis of this formation has no biostratigraphic sense and its lower boundary cannot be used for supporting the Vendian-Cambrian border. Second, the Zherba Formation corresponds to the Ediacaran Stage of the Khorbusuonka section of the Olenek uplift. The Lower Vendian deposits in the region were preserved only in paleodepressions. Their most complete section is known in the Bodaibo-Baikal pericratonic trough. The Lower Vendian is linked there, through gradual transitions, to the host rocks. Glacial formations and cap-dolomites are absent from it and, therefore, cannot be used as the bottom of the terminal Neoproterozoic of the International Scale. Quite promising for this purpose are pre-Vendian and intra-Vendian tectonic rearrangements whose ages reliably suggest that the Lower Vendian of the region formed in the interval from 650 to 612-620 Ma. Thus, the section of the Upper Vendian of the Baikal-Patom Upland is well substantiated by direct data at the Neoproterozoic III of the International Scale.

New data are reported on occurrences of calcareous algae and other concurrent groups of fossils in the transient Vendian-Cambrian deposits of the western Altai-Sayan Folded Area (East Sayans, Gornaya Shoria, Kuznetsk Alatau, and Gorny Altai). There are two successive algal complexes: lower - with Korilophyton, Gemma, Razumovskia, Girvanella, Proaulopora, and Renalcis; upper - the same, supplemented with abundant species of the genus Epiphyton and with the genera Batinevia, Tubophyllum, and Subtifloria. These complexes are compared with the Nemakit-Daldyn (Upper Vendian) and Tommotian (Lower Cambrian) associations of algae of the Siberian Platform. A conclusion is made that the lower boundary of the Cambrian in the Altai-Sayan Folded Area can be tentatively drawn where the lower complex is replaced by the upper one. These inferences receive support from rare findings of small shelly fossils.

Results of thermobarogeochemical study of bastnaesite and fluorite from the Yuzhnoe and Arshan REE deposits in western Transbaikalia are presented. The minerals have been found to bear solid, primary and secondary multiphase, and gas-liquid inclusions. The studies showed that homogenization of primary gas-liquid inclusions in fluorite from carbonatites proceeded in the temperature range 370-400

We mathematically justified a model for the formation of chemical zoning in minerals involved in exchange-diffusion reactions of the type Fe_Ol + Mg_Opx = Fe_Opx + Mg_Ol. We constructed FeO concentration profiles for garnet crystals of different sizes, using a garnet-pyroxene thermobarometer and diffusivities, and estimated the rates of burial and exhumation of low-temperature granulite facies rocks and eclogites.

The effect of high temperatures on the sedimentation of suspensions of monodisperse silica spheres (MSS) is considered. The MSS suspensions chosen as a model for our research, with ethanol as a dispersion medium, are stable to a particular temperature. The results obtained made us to study MSS suspensions in aqueous medium at much higher temperatures. With increasing temperature, the rate of sedimentation of MSS suspensions is much higher than it is on decreasing dispersion medium viscosity. This is due to the particle coagulation caused by a change in the electrolyte dissociation in the dispersion medium. With increasing temperature, the degrees of dissociation of acids and alkalies in the precritical state of solution change in the opposite manner. With decreasing temperature, the coagulation and sedimentation stabilities of positively and negatively charged particles in the dispersion media of hydrothermal solutions also show opposite tendencies. This difference must be taken into account in study of hydrothermal mineral formation involving disperse systems.

Based on the first isotope-geochronological data obtained, the age of metamorphic processes in Hercynian crystalline complexes in southeastern Mongolia has been estimated. The average ²⁰⁷Pb/²³⁵U age of monazite from synmetamorphic granites is 224 1 Ma. The Sm-Nd age for the bulk rock and its mineral inclusions is 219.2 6.2 Ma. We have established that highly metamorphosed rocks of Early and Late Hercynian (Indo-Sinian) folded structures in the South Mongolian belt cannot be fragments of the Caledonian (or more ancient) ensialic basement. They resulted from the development of deep segments in the Earth's crust during the closure of the South Mongolian and the Inner Mongolian ocean basins.

Owing to its crater-like morphology as well as to geophysical (negative gravity) and geological (superimposed setting) features, the Filippovka (Philippe) buried depression near Novosibirsk, Siberia, can be considered as a possible deeply eroded impact crater of Triassic-Early Jurassic age, with an initial diameter of ~20 km.