Geologiya I Geofizika, 2002, V 43, N 5, May.

PETROLOGY, GEOCHEMISTRY, AND MINERALOGY

THERMODYNAMIC PARAMETERS OF THE SYSTEM H2O-CO2-CH4-N2 FOR 
INCLUSIONS IN MINERALS 
Sverdlova V. G. and Shvedenkov G. Yu.
407

      Inclusions in minerals were studied from the point of view of 
thermobarogeochemistry. On the basis of the Peng-Robinson two-parameter equation 
of state and conditions of diffusional and mechanical stability of fluid 
mixtures, programs were developed to calculate isochores of entrapping of gas-
liquid inclusions and to estimate critical parameters of gas-liquid transition 
in the quaternary mixture H2O-CO2-CH4-N2. The difficulties related to the 
sensitivity to selection of initial conditions in Newton's method on solving 
systems of nonlinear equations have been overcome by using experimental and 
calculated data for ternary mixtures. The obtained P-T characteristics of 
critical mixtures are given in the form of sections of a multidimensional domain 
at a constant content of nitrogen and are in agreement with the known 
characteristics for triple mixtures H2O-CO2-CH4. The critical data for 
quaternary mixtures were used as initial values for finding parameters of 
homogenization, from which isochores of entrapping of gas-liquid inclusions in 
minerals have been calculated. The calculations show that ignoring even a small 
portion (relative to CO2) of water in highly carbon dioxide inclusions can lead 
to a serious error in estimation of P and T of mineral formation.

     Thermobarogeochemistry, multicomponent fluid, critical transitions

TONALITE-TRONDHJEMITE COMPLEXES OF SUPRASUBDUCTIONAL SETTINGS 
(by the example of Upper Riphean plagiogranitoids of the southwestern margin of 
the Siberian Platform) 
O. M. Turkina
418

     Within the Late Precambrian crystalline complexes of the Arzybei and 
Shumikha blocks two contrasting geochemical types of tonalite-trondhjemite 
granitoids have been distinguished - high- and low-alumina, similar in 
petrogeochemistry to continental and oceanic plagiogranitoids (according to 
classification by J.Art). The geologic position, composition, and petrogenesis 
of the granitoids evidence their formation under strongly different PT-
conditions related to the development of island-arc systems. The available data 
show that such associations of tonalites and trondhjemites of two types are also 
present in other Precambrian subductional complexes. The coexistence of high- 
and low-alumina tonalites and trondhjemites testifies to contrasting PT-
conditions of formation of their parental melts in subductional settings. The 
high-alumina varieties resulted from high-pressure melting of metabasic rocks of 
the subducting oceanic plate, whereas the low-alumina ones are the products of 
fractional crystallization of basaltic melt or low-pressure melting of metabasic 
rocks at the initial stage of development of island arc or during the back-arc 
spreading. The above association can be considered typical of Precambrian 
oceanic suprasubductional settings.

     Tonalites, trondhjemites, geochemistry, petrology, subductional settings, 
Precambrian

TECTONICS

LATE PALEOZOIC, MIDDLE MESOZOIC, AND LATE CENOZOIC STAGES OF THE ALTAI
OROGENY 
I. S. Novikov
432

     Altai in the Cenozoic structural framework of Central Asia is a zone of 
crustal breccia surrounded by more stable large blocks. The mobile zone is 
expressed in the surface topography as mountains and the stable blocks as 
plains. The sedimentary section of the latter includes Permian, Jurassic, and 
Quaternary molasse, which attests to structural inheritance from the older 
framework. The Late Paleozoic and Middle Mesozoic activity stages were 
associated with strike-slip faulting and granitoid magmatism, there is no 
evidence for granite intrusion in the Cenozoic yet.
     Altai is bounded by reactivated Paleozoic master faults, but the 
neotectonics within the province does not show such an obvious relation to 
Paleozoic structures. Spatial correlation of large Cenozoic faults in 
southeastern Russian Altai with Paleozoic faults of the same scale shows that 
the recent faults result from reactivation of older fractures over less than a 
half of their total length. The reactivation involved WE trending segments of 
most of major faults, and many blocks of metamorphic rocks underwent uplifting.

     Neotectonics, paleogeography, reactivation, Altai

FRACTURING IN THE PRIMORSKY FAULT ZONE (Baikal Rift system) 
O. V. Lunin, A. S. Gladkov, and A. V. Cheremnykh
444

     Fracturing in the Primorsky fault, a master fault in the Baikal rift 
system, is investigated using high-resolution structural data. The fault zone 
consists of several roughly parallel fault segments and elongate wedge-shaped 
blocks cut by large transversal faults that belong to a fault running along the 
Sarma valley. Fracturing parameters were used to distinguish the influence areas 
of separate fault planes having a regularly zoned structure. The Cenozoic 
activity of the Primorsky fault has been selective and is restricted to the 
influence areas of individual fault planes. Right-lateral oblique-slip and 
normal faulting occurs in NNW and NE extension stress fields along fractures 
parallel to the Primorsky and Sarma faults. The breadth of the Primorsky fault 
zone involved in the Cenozoic activity exceeds 3 km.

     Structure, fracturing, stress field, activity, Primorsky fault zone

OIL AND GAS GEOLOGY

SYNTHETIC INTERPRETATION OF POORLY CORRELATED GEOLOGICAL AND
GEOPHYSICAL DATA IN REGIONAL STRUCTURAL MAPPING 
(by the example of sedimentary cover of the West Siberian Plate) 
V. O. Krasavchikov
454

     The paper presents the outlines of a new approach and the respective 
computing techniques for regional structural mapping of the sedimentary cover of 
the West Siberian Plate on the basis of synthetic interpretation of poorly 
correlated geological and geophysical data. Processing involves log data, 
absolute altitudes on seismic exploration profiles and digitized elevation 
contour lines of the model surface (or surfaces correlated to the model 
surface), vector images of geological discontinuities and faults, etc. The new 
approach makes it possible to reveal and, under certain conditions, to eliminate 
most obvious inconsistency in raw data and to process any number of irregularly 
positioned paper or electronic maps of different scales by various techniques, 
including smooth conjugation along curvilinear boundaries. The new method has 
been tested in application to Jurassic hydrocarbon reservoirs in West Siberia.
     Regional hydrocarbon reservoir, structural map, mathematical modeling, computing

GEOPHYSICS

THE 25.02.1999 SOUTH BAIKAL EARTHQUAKE: SEISMOTECTONICS AND REGIONAL
GEODYNAMICS 
V. V. Ruzhich, R. M. Semenov, A. M. Alakshin, B. I. Mel'nikova, A. V. Chpizubov, 
S. G. Arzhannikov, O. P. Smekalin, I. A. Emel'yanova, M .G. Dem'yanovich, and N. 
A. Radziminovich
468

     The South Baikal earthquake of 25 February, 1999 had the main shock in the 
southwestern part of Lake Baikal with the normal-slip focus at the intersection 
of the Obruchev (Primorsky) and Angara faults. This was the strongest shock (M 
sub PSP = 6.0) in the southern Baikal region over the last 40 years after the M 
sub LH = 6.8 Central Baikal earthquake of 1959. The intensity attained 7-8 in 
the epicenter, 6-7 in villages 15 to 30 km away on the northwestern and 
southeastern sides of Baikal, and 4-6 in neighboring cities of Irkutsk, Angarsk, 
and Ulan-Ude. The main shock was followed by a long series of aftershocks with 
different focal mechanisms. This event occurred in a region for which pending 
seismic risk in the nearest 15 years was predicted in 1994. 
     Epicenter, earthquake focus, aftershock, fault node, focal mechanism, 
active fault, earthquake energy, intensity