Home – Home – Jornals – Russian Geology and Geophysics 2005 number 7

The position of mantle plumes detected from geophysical data has been correlated to Late Cenozoic and prerift geology and to the present-day lithospheric structure. Quantitative estimates of horizontal lithospheric stress were obtained for viscous drag produced by plume material spreading laterally over the asthenosphere and for the potential energy stored in the asthenospheric upwarp beneath the Baikal rift. The development of the upwarp is responsible for tensile stress in the lithosphere sufficient to induce normal faulting and rifting. The plumes apparently supply hot mantle material into the upwarp, otherwise it would inevitably degrade by cooling and would be unable to evolve as a gravity instability. The Baikal rift owes its origin to both the activity of plumes and the existence of prerift lithospheric structures whose orientation relative to far-field forces associated with the India-Eurasia collision did not conflict with rifting-related extension.

The 16-20 km deep Barents basin is among world deepest sedimentary basins. Its eastern part occupied by the East Barents subbasin develops upon very thin consolidated crust with high P velocities. It is often interpreted as oceanic crust, though the thickness of sediments (up to 10-12 km) accumulated after the subsidence of oceanic crust would have completed far exceeds the amount necessary to fill an oceanic basin. The Moho is underlain by an ~20 km thick layer of heavy mafic rocks with crustal chemical signatures and P velocities typical of garnet granulite and eclogite, intermediate between crustal and mantle velocities. Thus, the total thickness of consolidated rocks, including those above and below the Moho, amounts to 35-40 km and indicates its continental origin. A lithospheric extension of 10-15% can be hypothesized but cannot be verified because the available penetration of seismic profiling is limited to 12-14 km. Most of subsidence in the East Barents basin was caused by phase-change consolidation of the lower crust anyway.
A granitic layer is present in the thin crust over the greatest part of the Barents basin. Its large-scale subsidence is commonly explained by strong lithospheric extension but this explanation contradicts the seismogeological evidence that extension was weak almost throughout the basin. It can allow for only 5 % of subsidence which was to the greatest extent caused by phase change in the lower crust. Rapid subsidence at the Early/Middle Permian boundary and in the Late Jurassic produced relatively deep seas providing a favorable environment for the formation of the Barents petroleum province.

New Rb-Sr dates have been obtained for metabasalts of the Tyya Formation of the Olokit structure-lithological complex (SLC) (927±10 Ma), as well as model Nd ages for the Purpol (Patom SLC) and Anai (Sarma SLC) Formations (2.54 and 2.61-2.82 Ga, respectively). The Tyya metabasalts are intermediate in geochemical composition between flood basalts and N-MORB and were produced from a primitive source ((⁸⁷Sr/⁸⁶Sr)₀ = 0.7025-0.7032). They are similar to metabasalts of the Medvezhevka and Anai Formations that occur at the same stratigraphic level. High-alumina schists and quartzites of the Purpol and Anai Formations are compositionally identical to redeposited weathering crusts and, together with the Tuluokit Formation of the Olokit SLC, form a single basal level of the Patom SLC, which frames the Chuya and Cisbaikalian uplifts of the Precambrian basement of the Siberian Platform. We have summarized data on the ages of all formations of the northern Baikal area published for the last 15 years and have considered the evolution history of these objects.

Dike belts of NE strike, up to 100 km long and 12-15 km wide, have been recognized in the central area of western Transbaikalia. These are subparallel near-vertical dikes of trachybasalts, syenite-porphyries, trachydacites, and trachyrhyodacites. According to chemical composition, the dike rocks form a bimodal alkaline magma series. Rb-Sr isotope dating of the syenite-porphyry dikes showed that the dike belts formed in the Late Carboniferous-Early Permian (300-285 Myr ago), nearly simultaneously with the formation of massifs of alkali A -type granites and volcanic fields composed of bimodal trachybasalt-trachyte and comendite associations of the North Mongolian-Transbaikalian rift belt.

By the example of the Altai-Sayan area, a more detailed classification of nepheline syenites by the type of alkalinity (the degree of their K-Na enrichment) is proposed. Six groups of nepheline syenites have been recognized, which formed under different geologic conditions. Zonal distribution of syenites of different alkalinity types throughout the Siberian craton has been established. The K-richest varieties occur along the craton boundary.

Spinel geothermometer, based on proton microprobe determination of Zn content (and in qualitative analysis, Ni content as well), was reproduced by applying electron microprobing. The analyses were carried out on Camebax Micro and JXA-8100 microprobes. The accuracy of determination, estimated as the difference between the data obtained by proton and electron microprobing, was 0.004-0.005%, and the repeatability standard deviation estimated from replicate measurements is ca. 0.003%. The temperature standard deviation was 20-30

Recurrence time and probability for large and great earthquakes in the Baikal region and Mongolia were estimated within the limits of an inhomogeneous faulting model using the maximum entropy principle and assuming Poisson distribution of events. The predicted recurrence times of K_P = 18 earthquakes are 220 years for the Baikal region, 370, 470, and 430 years for the southwestern, central, and northeastern subregions, and 210 years for Mongolia, if the maximum energy of characteristic events is taken as K_{P max} = 19. At K_{P max} = 19, the events that may occur in the future 50 years within these territories to a 10% probability are expected to release the energies corresponding to classes 18.39, 18.15, 18.00, 18.06, and 18.42; the estimated probabilities ( P ) for K_P = 18.0 events within 50 years are 0.20, 0.13, 0.10, 0.11, and 0.21, respectively.

Numerical modeling using the semblance parameter has demonstrated high efficiency and noise immunity of emission tomography in reconstructing the spatial distribution of small seismic sources in the crust at single-component acquisition. High noise of additive (low signal/noise ratio) and parametric (lateral inhomogeneities) origin deteriorates the quality of source imaging but does not rule out the possibility to create the image. The modeling implies signals limited within a certain estimated frequency bandwidth.