Geology and Geophysics, 2004, V 45, N 12, December.
GEODYNAMICS
Fragments of oceanic islands in the Kurai and Katun' accretionary
wedges of Gorny Altai.
N.L. Dobretsov, M.M. Buslov, I.Yu. Safonova, and D.A. Kokh
1325
Paleomagnetism of Early Cretaceous volcanic rocks from
Transbaikalia: argument for Mesozoic strike-slip
motions in Central Asian structure.
D.V. Metelkin, I.V. Gordienko, and Xixi Zhao.
1349
Formation series in orogenic volcanic belts: implications for
paleoenvironments.
A.M. Kurchavov
1364
Effect of water on subduction of oceanic crust into lower mantle.
K.D. Litasov and E. Ohtani.
1373
PETROLOGY, GEOCHEMISTRY, AND MINERALOGY
Geochemistry of highly carbonaceous metasomatites of the Ospa-Kitoi
ultrabasic massif ( East Sayan ).
V.B. Savel'eva, Yu.V. Danilova, B.S. Danilov, V.V. Markova,
and S.V. Panteeva
1381
Finding of Cr-V-containing minerals in marbles and quartzites of the
Svyatoi Nos Peninsula
( Lake Baikal ). V.A. Makrygina, Z.I. Petrova, A.A. Koneva, and L.F.
Suvorova.1389
STRATIGRAPHY
The problem of post-Cambrian stratified formations in the Central
Baikal-Vitim folded area.
V.V. Buldygerov
1399
GEOPHYSICS
Seismic process and active lithospheric failure in the Baikal rift
system.
S.I. Sherman, V.M. Dem'yanovich, and S.V. Lysak
1406
Simulation of a DC field in a horizontal well drilled in a layered
conductor.
N.I. Gorbenko, Ya.L. Gurieva, V.P. Il'in, E.A. Itskovich, and M.I.
Epov.
1417
BRIEF COMMUNICATIONS
Use of potential variations in electrical prospecting of anisotropic
media.
A.V. Kuzin.
1425
FRAGMENTS OF OCEANIC ISLANDS IN THE
KURAI AND KATUN' ACCRETIONARY WEDGES OF GORNY ALTAI
N.L. Dobretsov, M.M. Buslov, I.Yu. Safonova, and D.A. Kokh
Institute of Geology , Siberian Branch of the RAS, 3 prosp. Akad.
Koptyuga, 630090, Novosibirsk , Russia
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.
Vendian-Cambrian, oceanic crust, Paleo-Asian ocean, MORB, OIB
PALEOMAGNETISM OF EARLY CRETACEOUS VOLCANIC ROCKS
FROM TRANSBAIKALIA: ARGUMENT FOR MESOZOIC STRIKE-SLIP MOTIONS
IN CENTRAL ASIAN STRUCTURE
D.V. Metelkin, I.V. Gordienko*, and Xixi Zhao**
Institute of Geology , Siberian Branch of RAS, 3 prosp. Akad.
Koptyuga, Novosibirsk , 630090, Russia
* Geological Institute, Siberian Branch of RAS, 6a ul. Sakh'yanovoi,
Ulan Ude, 670047, Russia
** University of California , Santa Cruz , USA
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.
Early Cretaceous paleomagnetic pole, Transbaikalia, magmatism,
within-plate strike-slip motions
FORMATION SERIES IN OROGENIC VOLCANIC BELTS: IMPLICATIONS
FOR PALEOENVIRONMENTS
A.M. Kurchavov
Interdepartmental Committee for Petrography, RAS Department of Earth
Sciences, 35 Staromonetny per.,
Moscow , 119017, Russia
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.
Formation series, volcanism, orogenic belts,
paleoreconstructions
EFFECT OF WATER ON SUBDUCTION OF OCEANIC CRUST INTO LOWER MANTLE
K.D. Litasov and E. Ohtani *
United Institute of Geology , Geophysics and Mineralogy, Siberian
Branch of RAS,
3 prosp. Akad. Koptyuga, Novosibirsk , 630090, Russia
* Institute of Mineralogy , Petrology and Economical Geology, Tohoku
University , Sendai , Japan
This paper reports results of experimental studies in the
MORB-H2O system at 18-26 GPa and 1000- 1800 °C . It has been found
that the pressure of garnet-to-perovskite transition in a hydrous
system is about 1 GPa lower than in a «dry» system. This implies
that at a mean temperature of the subduction slab the basalt layer
remains to be denser than peridotite at the 660 km discontinuity,
i.e., there is no density crossover between basalt and peridotite,
as it is established for «dry» MORB. Thus, hydrous basalt can
gravitationally sink into the lower mantle without separation from
peridotite of the slab. The basalt crust may be detached from
peridotite at the 660 km discontinuity under relatively «dry»
conditions of subduction or at a higher temperature of the slab.
Basalt, peridotite, subduction, lower mantle, water, 660 km
discontinuity
GEOCHEMISTRY OF HIGHLY CARBONACEOUS METASOMATITES
OF THE OSPA-KITOI ULTRABASIC MASSIF ( East Sayan )
V.B. Savel'eva, Yu.V. Danilova, B.S. Danilov, V.V. Markova, and S.V.
Panteeva
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul.
Lermontova, Irkutsk , 664033, Russia
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.
Rare-earth elements, metasomatites, carbon, fluid
FINDING OF Cr-V-CONTAINING MINERALS IN MARBLES AND QUARTZITES
OF THE SVYATOI NOS PENINSULA ( Lake Baikal )
V.A. Makrygina, Z.I. Petrova, A.A. Koneva, and L.F. Suvorova
Institute of Geochemistry , Siberian Branch of the RAS, 1a ul.
Favorskogo, Irkutsk , 664033, Russia
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.
Cr-V- containing minerals, metamorphic rocks, stratigraphy
THE PROBLEM OF POST-CAMBRIAN STRATIFIED FORMATIONS
IN THE CENTRAL BAIKAL- VITIM FOLDED AREA
V.V. Buldygerov
Irkutsk State University, 1 ul. Karla Marksa, Irkutsk , 664003,
Russia
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.
Lower Paleozoic, stratigraphy, tectonics, Baikal-Vitim folded
area
SEISMIC PROCESS AND ACTIVE LITHOSPHERIC FAILURE
IN THE BAIKAL RIFT SYSTEM
S.I. Sherman, V.M. Dem'yanovich, and S.V. Lysak
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul.
Lermontova,
Irkutsk , 664033, Russia
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.
Seismicity, failure, damage zone, zone of lithospheric failure,
earthquake prediction, Baikal rift system
SIMULATION OF A DC FIELD IN A HORIZONTAL WELL DRILLED
IN A LAYERED CONDUCTOR
N.I. Gorbenko, Ya.L. Gurieva, V.P. Il'in, E.A. Itskovich, and M.I.
Epov*
Institute of Computing Mathematics and Mathematical Geophysics,
Siberian Branch of RAS,
6 prosp. Akad. Lavrent'eva, Novosibirsk , 630090, Russia
* Institute of Geophysics , Siberian Branch of the RAS, 3 prosp.
Koptyuga, Novosibirsk , 630090, Russia
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.
Mathematical model, dc electric field, horizontal well, lateral
logging
USE OF POTENTIAL VARIATIONS IN ELECTRICAL PROSPECTING
OF ANISOTROPIC MEDIA
A.V. Kuzin
Ural Mining Academy , 30 ul. Kuibysheva, Ekaterinburg, 620144,
Russia
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.
Electrical prospecting, modeling, anisotropic layer,
Schlumberger array, Cu pyrite deposits