Home – Home – Jornals – Russian Geology and Geophysics 2002 number 5

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 diffusion and mechanical stability of fluid mixtures, programs were developed to calculate isochors 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 isochors 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.

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 tonalities 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.

Altai in the Cenozoic structural framework of Central Asia is a zone of crystal 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.

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.

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.

The South Baikal earthquake of 25 February, 1999 had the main shock in the south-western 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_PSP = 6.0) in the southern Baikal region over the last 40 years after the M_LH = 6.8 Central Baikal earthquake of 1959. The intensity attained 7-8 in the epicentre, 6-7 in villages 15 to 30 km away on the north-western and south-eastern sides of Baikal, and 4-6 in neighbouring 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.