Home – Home – Jornals – Russian Geology and Geophysics 2004 number 12

The paper presents new data on the structure and composition of the Vendian-Early Cambrian oceanic crust of the Paleo-Asian ocean, which has been fragmentally preserved in Early Cambrian accretionary prisms of Gorny Altai. Mid-oceanic ridge and oceanic island basalts are recognized on the basis of their geochemical characteristics and the composition and geological relationships of volcanogenic and sedimentary units. Our data indicate that hot-spot and mid-ocean ridge volcanism was active in the early stages of the Paleo-Asian oceanic evolution, from Vendian to Early Cambrian. The study of paleo-oceanic islands incorporated in accretionary-collisional belts is important for the reconstruction of ancient oceans and understanding of endogenic processes that resulted in the formation of the oceanic lithosphere.

New paleomagnetic data have been obtained from Early Cretaceous volcanosedimentary sections of the Chikoi-Khilok, Borgoi, and Uda depressions of Transbaikalia. A paleomagnetic pole has been positioned to satisfy the modern criteria of reliability so that it can be used in paleotectonic reconstructions. The Early Cretaceous age of the recognized directions of natural remanent magnetization receives support from reversal, fold, and conglomerate paleomagnetic tests as well as from the coincidence of magnetozones of normal and reversed polarity with the magnetochronological scale. The obtained paleomagnetic data evidence that there are strike-slip deformations of sinistral kinematics in the south of the Siberian Platform, which are responsible for the regime of intracontinental extension and formation of graben structures filled with products of intraplate magmatism.

Orogenic volcanic belts form in various tectonic settings including active continental margins, mature island arcs upon continental crust, or zones of continental collision. Orogenic belts of all environments share common evolution trends and go through the same stages of volcanism changing progressively in composition from basalt-andesite or successively differentiated basalt-andesite-dacite-rhyolite series (early orogenic stage) to mostly rhyodacite-rhyolite series (middle stage), and finally to late orogenic high-alkali mono- or bimodal series. Each evolution stage of orogenic belts produced a general lateral sequence of rock compositions from low-K rocks in the belt front to high-K and alkalic varieties at the back. Comprehensive investigation into formation series and related rock chemistry variations in space and time can provide clues to paleogeodynamic environments of volcanism associated with orogenic belts.

This paper reports results of experimental studies in the MORB-H₂O system at 18-26 GPa and 1000-1800

Distribution of trace elements in highly carbonaceous metasomatites of the Ospa-Kitoi ultrabasic massif (East Sayan) was studied by mass spectrometry with inductively coupled plasma (ICP-MS). Though the ultrabasites are strongly depleted in lithophile elements, the metasomatites developed after these rocks and serpentinites are slightly enriched in Rb, Cs, Ba, Y, Nb, Pb, Sn, and REE. The increase in contents of light REE and trace lithophile and chalcophile elements in the metasomatites is the result of the activity of reduced graphite-forming fluid. Carbonatization of serpentinites is accompanied by an increase in contents of MREE and HREE in them.

Accessory inclusions of Cr-V-garnets, Cr-V-Ba-micas, Cr-V-diopsides, zoisites, and titanites have first been found in exposed member of alternating marbles, quartzites, and diopside schists on the shore of Chivyrkui Bay on the Svyatoi Nos Peninsula (Baikal). The minerals show a strongly uneven distribution of Cr and V. Their composition fields overlap the fields of similar minerals of the Ol'khon and Slyudyanka series and expand the spectra of mineral compositions. Along with the typical alternation of metasedimentary rocks, the presence of these rare minerals unambiguously evidences that the deposits of the Ol'khon series extend to the Svyatoi Nos Peninsula.

In the central Baikal-Vitim folded area, a rock complex has been revealed which was earlier considered to be part of the thick Vendian Padrokan Formation. The complex lies with erosion over the Lower-Middle Cambrian Yanguda Group. It is of volcanoterrigenous composition and Early Ordovician(?) age. The complex is recognized as the Kaalu Formation, which is more than 3200 m in visible depth. It formed in graben-like troughs. The trough periphery is made up mainly of proluvial-alluvial and olistostrome deposits, and the central parts, of alluvial- lacustrine sediments. Volcanogenic formations are abundant traces of tuffaceous matter in terrigenous sediments and less frequent tuffs, lavas, and crater facies. The volcanics are of contrasting rhyolite-basaltic composition. Similar formations in other zones of the central Baikal-Vitim folded area, which were earlier dated to the Late Cambrian, might be in fact of post-Middle Cambrian age.

Statistical distribution of instrumental seismicity in the Baikal rift system was used to outline persistent earthquake clusters which together make up a zone of lithospheric failure existing as a single active seismic and tectonic unit. Large earthquakes regularly delineate its axis and the pattern of smaller events is controlled by its separate fragments. Earthquake sources show space and time oscillations along and across the strike of damage zones around the failure zone and its fragments. Earthquake migration and the hierarchic association of events of different magnitudes with active faults reflect the general features of faulting in the rifted lithosphere. Faulting-seismicity interaction is approached at comparable scales of lithospheric failure as rare large events mark the evolution of the whole active unit and more frequent smaller earthquakes record processes in its subunits. Time-dependent migration of earlier large events along the axis of the failure zone thus can be a guide to predict large earthquakes in the region, and smaller events can be predicted proceeding from the patterns of comparable seismicity within zone fragments. The available collection of data is extensive enough to approach new composite tectonophysical modeling of seismicity accompanying lithospheric failure in different geodynamic environments. These models will open up new avenues in medium-term earthquake prediction on the geological and geophysical basis.

The paper presents a mathematical model, numerical solutions and results for 3D fields from dc sources in a horizontal well drilled in a layered conductor. The model simulates the behavior of apparent resistivity in the well and its sensitivity to instrument parameters and variations in formation  resistivity and thickness in terms of a boundary-value problem. BVP approximation is obtained as finite volume and iteration solutions to systems of grid algebraic equations using fast incomplete factorization solvers.

The study compares electrical prospecting in anisotropic media by Schlumberger and classical (Lundberg) arrays. Schlumberger symmetric arrays with source electrodes positioned at an angle to a steeply bedded anisotropic layer provides more prominent bending of field lines over conductors than arrays with the bedding-parallel position of electrodes. The reason is in higher effective resistivity of anisotropic layers at field direction across their bedding.