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Russian Geology and Geophysics

2008 year, number 2

1.
GEOCHEMICAL DISTINCTIONS BETWEEN

V.V. Reverdatto a , A.Yu. Selyatitskiy a , D.A. Carswell b
a Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
bDepartment of Geology, University of Sheffield, Mappin Street, Sheffield, S1 3Jd, England
Keywords: High/ultrahigh-pressure metamorphic complexes; mantle-derived and «crustal» peridotites/pyroxenites; geochemical criteria for distinctions
Pages: 73-90

Abstract >>
Basic and ultrabasic rocks in high- and ultrahigh-pressure collision belts can provide important petrological information. Mantle-derived and
																								



2.
MECHANICAL ABRASION OF KIMBERLITE INDICATOR MINERALS: EXPERIMENTAL INVESTIGATIONS

V.P. Afanas'eva, E.I. Nikolenko a , N.S. Tychkov a , A.T. Titov a , A.V. Tolstov b , V.P. Kornilova b , N.V. Sobolev a
a Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
b ALROSA, 6 ul. Lenina, Mirny, 678170, Russia
Keywords: Pyrope; picroilmenite; olivine; kimberlite; diamond; mechanical abrasion; experiment
Pages: 91-97

Abstract >>
The relative abrasive stability of kimberlite indicator minerals such as pyrope, picroilmenite, olivine, and apatite as well as diamonds and kimberlite fragments was studied experimentally, and the following sequence of mineral abrasive stability was established: pyrope-olivine-picroilmenite-apatite-kimberlite fragments. Diamond did not virtually change during the experiment. Kimberlite fragments appeared to be rather stable. Their relics were preserved until the end of the experiment, whereas the other minerals acquired wearing-resistant shapes. Pyrope, olivine, and apatite were shaped into an oval. Owing to anisotropy of microhardness, picroilmenite forms hexagonal tablets, which are typical of ancient haloes of indicator minerals in all diamondiferous regions. The parity analysis of the abrasive stabilities of pyrope and picroilmenite has shown that in the haloes of
																								



3.
PLIOCENE-QUATERNARY OROGENY IN THE CENTRAL TIEN SHAN

V.G. Trifonov a , E.V. Artyushkov b , A.E. Dodonov a , D.M. Bachmanov a , A.V. Mikolaichuk c , F.A. Vishnyakov a
a Geological Institute, Russian Academy of Sciences, 7 Pyzhevsky per., Moscow, 119017, Russia
b Institute of Physics of the Earth, Russian Academy of Sciences, 10 ul. Bol'shaya Gruzinskaya, Moscow, 123995, Russia
c Central Asian Institute of Applied Research of the Earth, 73/2 ul. Timura Frunze, Bishkek, 720027, Kyrgyzstan
Keywords: Orogeny (formation of mountains); asthenosphere rise; acceleration of rise; Pliocene; Quaternary; Tien Shan
Pages: 98-112

Abstract >>
Formation of mountains of the Central Tien Shan is usually explained by the isostatic response of the crust to its shortening caused by the India-Eurasia collision. The rise of the region in the period from Oligocene to Late Pliocene (2 myr ago) reached ~700 m on average, which corresponds to the isostatic response. For the last 2 myr (Late Pliocene-Quaternary), the rate of rise increased by an order of magnitude. This is proved by the coarsening of Cenozoic molasse up the sections, acceleration of cutting of drainage systems into ridges, and formation of new ridges within basins. In the Quaternary, most of intermontane basins underwent uplifting, though not so intense. The average rate of lateral crustal shortening increased ~2-2.5 times only, and the contribution of this process to the Late Pliocene-Quaternary orogeny was no more than 10%. The acceleration of rise was caused mainly by the convective replacement of the mantle lithosphere by the less dense asthenosphere. This was due to the quick softening of the mantle lithosphere as a result of the infiltration of active fluid from the lower mantle. Such accelerations of crustal uplifting took place in the Pliocene and Quaternary in many continental regions. This evidences that mantle processes, first of all, the full or partial replacement of the mantle lithosphere by the asthenosphere, played an important role in the formation of mountains.
																								



4.
ACTIVE FAULTS AND CRUSTAL STRESS IN THE NORTHEASTERN FLANK OF THE BAIKAL RIFT SYSTEM

O.V. Lunina, A.S. Gladkov
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Active faults; stress pattern; Late Cenozoic; rift basins; northeastern flank of the Baikal rift system
Pages: 113-123

Abstract >>
We discuss the patterns of Late Cenozoic faulting and crustal stress in the northeastern flank of the Baikal rift system. The Late Cenozoic faults are mainly of NE and ENE strikes. Faults of these trends, along with W-E faults, have been principal seismogenic structures. They have normal or left-lateral oblique geometry with different amounts of horizontal motion. The N-S, NW, and NNW faults bear signature of mostly right-lateral strike slip, and those of the WNW direction are left-lateral strike-slip faults, often with a normal component. The rift basins in this part of the rift system have had different evolution dynamics, with the most rapid faulting and sedimentation in the Muya basin. According to structural and seismological data, regional stress in the area has been stable and dominated by NW extension through the Pleistocene-Holocene and the Present. The directions and obliquity angles of principal normal stresses and percentages of stress types show local lateral variations. The evolution of the northeastern flank of the Baikal rift system can be explained by a model of oblique rifting which accounts for the architecture of rift basins, the pattern of Late Cenozoic active faults, and the stress pattern derived from structural and seismicity data. The model is consistent with centrifuge models of magma emplacement during continental oblique rifting. Oblique extension associated with underplated magma can be maintained in the area by mantle plumes beneath the Kichera, Upper Angara, and Chara rift basins. The presence of subcrustal magma chambers may explain why the three basins formed before other basins in the NE flank of the rift system.
																								



5.
TRACES OF DAMMED PALEOLAKE ACTIVITY IN THE MAIN VALLEY RELIEF AND FILL IN THE SOUTH OF THE IRKUTSK AMPHITHEATER

A.V. Arzhannikova, S.G. Arzhannikov, V.V. Akulova
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Late Cenozoic tectonics; sediments; paleolake; southern Siberian Platform
Pages: 124-131

Abstract >>
Thick sand-argillaceous deposits filling the paleorelief roughness are widespread in the south of the Irkutsk amphitheater. The origin of these deposits is still debatable: Some researchers relate them to eolian processes, and others consider them sediments of a dammed paleolake. A series of sand-argillaceous sections was studied in order to refine the genesis of deposits in the area of possible flooding. Results of these studies and laboratory analyses of the deposits argue for their lacustrine genesis and a drastic change in sedimentation conditions during the early formation of the paleolake. The tectonically induced instantaneous Badarma landsliding to the Angara River valley is assumed to have caused the damming of the main rivers in the study area.
																								



6.
RADIOACTIVE POLLUTION OF THE BAIKAL AREA AS A RESULT OF NUCLEAR GROUND TESTS

B.P. Chernyago, A.I. Nepomnyashchikh
Institute of Geochemistry, Siberian Branch of the RAS, 1a ul. Favorskogo, Irkutsk, 664033, Russia
Keywords: Radiogeochemistry; 137Cs radionuclides; radioactive fallout; dose estimation
Pages: 132-137

Abstract >>
Results of analyses for 137Cs in soils sampled in more than 800 localities have shown that the Baikal area pollution is much contributed by local radioactive fallout from the Semipalatinsk nuclear ground tests. We studied the effect of the amount of atmospheric precipitation and the types of landscapes and soils on the content of 137Cs in regional soils. Local radioactive fallout is confirmed by the total beta activity of daily atmospheric precipitation measured at regional weather stations.
																								



7.
DISSIPATIVE STRUCTURES IN FAULT ZONES AND THEIR DIAGNOSTIC CRITERIA ( from physical modeling data )

S.A. Bornyakov, V.A. Truskov, A.V. Cheremnykh
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Fault zone; fracture; fault dissipative structure; physical modeling
Pages: 138-143

Abstract >>
A fault zone is considered an open nonequilibrium dynamic system in which its constituent subsystems evolve by self-organization in a nonlinear synergetic process of successive changes of structural levels (stages). A new term fault dissipative structure is suggested for an ordered 3D space-time structure produced in a fault zone by self-organization of fracturess of different scales. The approach is used in physical modeling to simulate the evolution of a strike-slip fault zone in an elastic-viscoplastic lithosphere and to recognize space and time criteria diagnostic of fault dissipative structures through fractal and entropy analyses.
																								



8.
ESTIMATING MAGNITUDES OF PREHISTORIC EARTHQUAKES FROM LANDSLIDE DATA: FIRST EXPERIENCE IN SOUTHEASTERN ALTAI

R.K. Nepop, A.R. Agatova
Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Prehistoric seismicity; earthquake-induced ground failure; earthquake-induced landslide event; Chuya earthquake of 2003; Gorny Altai
Pages: 144-151

Abstract >>
We report the first experience of deriving the magnitudes of prehistoric earthquakes in southeastern Gorny Altai from the size of largest landslides they triggered using an empirical correlation between the earthquake magnitude and the volume of associated landslides. The applicability of this relationship to the Altai region has been checked against data on the M = 7.5 Chuya earthquake of 2003 that induced a great landslide event. The paleoearthquake magnitudes derived from landslide volume are the minimum estimates; the largest magnitudes of earthquakes of about the same size and location as the Chuya event have been predicted with a simplified linear dependence of earthquake magnitude versus detachment length. The magnitudes obtained with the new approach are of a satisfactory accuracy improving proportionally for younger earthquakes. The estimates spanning magnitudes from 6.9 to largest possible in nature indicate high seismic activity of the area through the Holocene and its seismotectonic identity with the Mongolian Altai.